Learning Objectives

 

• Describe the infrastructure and anatomy of the heart.

• Relate Poiseuille’s law to hemodynamics.

• Explain the basic embryology of heart formation.

• Recall the pump cycle of the heart.

• Discuss the size, shape, and position of the heart.

• Summarize imaging methods for determining the normalcy or disease state of the heart.

Principles

Simple concepts govern the interactions between the heart and body. The unique structure and function defines disease processes, diagnosis and treatment for each organ. The format used in this tutorial uses a specific format, conforming to the principles outlined for each organ.

Each structure though a unique biological unit, is part of a system that is bigger and more powerful than itself. The heart has to be connected to the other organs and systems in order to survive.

As Biological Unit
The heart is a hollow organ with a muscular wall consisting of an inner endothelial lining, a thick muscular layer, and a double outer layer called the pericardium.

Links and Connections
The heart has an intimate connection to all the tissues and cells of the body from the tip of the toes to the crown of the head. The systemic arterial connection originates from the aorta with connections via the brachiocephalic, mesenteric, renal and peripheral arterial systems. The systemic veins join to form the inferior vena cava that drains the lower part of the body and superior vena cava that drains the upper part. The lymphatic system flows from the heart into the thoracic and right lymphatic ducts. The nerve supply is via connections to the sympathetic (celiac axis) and parasympathetic (vagus) systems. Hormonal connections include interaction with epinephrine, nor epinephrine, and thyroid hormones, but the atrial muscles also produce hormones.   The atrial natriuretic peptide (ANP)  is one hormone which helps maintain homeostasis of water, sodium, potassium and adipose tissue.

Units to Unity
The concept of units to unity reflects the importance of each unit within the system being an important part of something larger. The cardiovascular system contains the heart, which in turn is part of every system in the body. This vital organ contains pumps and a tubular system of vessels and capillaries.

Dependence and Independence
The heart cannot function alone and requires centrally based innervations and signaling of the neurohormonal axis to instruct it as to the needs of each of the other systems of the body.

Time Growth and Aging
The heart arises from the primitive mesenchyme as a single tube in the embryo. The single tube becomes a two chambered system by septation, growth and resorption, and selective apoptosis. The heart ages well, though atherosclerosis of the coronary arteries is part of the aging process and is a common cause of morbidity and mortality in the elderly.

Space
The heart lies within the thoracic cavity of the chest, in close association with the lungs. It lies in an eccentric position, slightly toward the left of middle, and divides the chest into right and left sides.

Forces
The forces that govern cardiac function are mostly mechanical in origin. The forces created by contraction and relaxation enable blood to flow. The heart fills with passive and active mechanisms by ongoing forward forces (pushing force from behind) and forces that also pull the blood (pulling forces from in front).  Valves between the atria, ventricles, and the great vessels, prevent backflow, allowing forward flow into the pulmonary and systemic circulations.

Interactions
Simple concepts govern the interactions between the heart and body.The heart acts as the transport system of required vital nutrients allowing function of all parts of the body.  In particular it transports oxygen and carbon dioxide. Transportation of metabolic substrates to the capillaries and diffusion across pressure gradients into the intercellular spaces, and transportation through the cell membrane occur through circulation of nutrient rich arterial flow. Waste products transport in the opposite direction, from the cell to the interstitium, into the capillaries, and finally into the venous system for transport to the excretory organs.

The heart responds with beat to beat accuracy to the bodies needs, balances right and left circulations, as well as the differential needs within the vast range of physiologic conditions from sleep to intense exercise.

States of Being – Health and Disease
By virtue of its function as a pump, diseases related to the heart are of a mechanical nature. The most common malady of the heart is ischemic disease. Myocardial failure results in venous congestion due to output reduction by the chambers to the arterial circulation. Often the mechanical causes of disease such as stenosis of the coronary arteries, acute coronary thrombosis, or valvular stenosis, require minimally invasive or surgical procedures to open blockages. When the myocardial forces have been reduced to levels where function is impaired, the range of therapies relate to enhancing myocardial contractility, or reducing the workload.  There is a point where conventional therapy fails and the only hope for survival is heart transplantation.

Principles of Circulation

In a tube, the volume of the flow rate changes due to the pressure differences, length and diameter of the tube and the viscosity of the fluid.

The heart is a muscular organ specifically designed for the transport of blood by acting as a pump. At the terminal end of the arterial circulation is a capillary system structurally designed in network formation to increase the surface area available for rapid and efficient metabolic exchange.

Multiple tubular systems exist throughout the body. Classical examples include the airway, venous, lymphatic, ductal, gastrointestinal, and genitourinary systems. Flow occurs when a pressure difference exists between the two sides of the tubes.

The velocity of flow relates to diameter, resistance, friction, and pressure. Velocity of flow also depends on whether the flow is laminar or turbulent. Poiseuille’s law states that the flow rate depends on the pressure differences between one end of the tube and the other. Thus, if the driving pressure doubles, the flow rate also doubles, as long as the radius remains constant. The radius of the tube, however, is very important. Poiseuille found that if either the length or viscosity doubled there was a reduction of the flow rate by half. He found that changes in the radius also affected the flow rate, but not as little as expected. Due to the inverse relationship of the flow volume to flow viscosity and the direct relationship to tube radius, doubling the radius increases the flow rate by sixteen times!

Though a significant finding, Poiseuille dealt with uniform tubes with the same radius throughout the length. Due to the elasticity of arterial walls, which expand and contract with the heart, flow in the body does not adhere to this law exactly. Remember, viscosity changes in the body due to disease processes or hydration.

Key Points of Poiseuille’s Law

1. An increase in pressure difference across the vessel results in an increase in flow rate.
2. Increasing diameter also increases flow rate.
3. Flow rate decreases with an increase in length.
4. Viscosity increases result in flow rate decrease.

            Reynold’s number = inertia (drag) forces/viscous force

Inertia forces are directly proportional to the density of fluid, velocity of fluid and length of tube

Laminar flow occurs with low drag forces and high viscous forces and is characterized by smooth constant fluid motion. Turbulent flow on the other hand occurs with high Reynolds numbers, is characterized by chaotic patterns of flow and occurs when inertial (drag) forces dominate. These changes occur in the normal individual under certain states, such as during exercise, or with disease for example with stenosis of vessels.

Causes of Reynolds Number Increases

1. Increases in flow speed.
2. Diameter increases.
3. Density increases.
4. Viscosity decreases.

 

 

 

Factors Affecting Velocity Flow Profiles

Diameter

Mechanical properties of blood

Flow velocity

Time

 

 

The cardiovascular system is beautifully designed to enable rapid and efficient delivery of blood to the organs. Not only is the delivery rapid, but the design is such that differential needs are accommodated. Thus, during exercise the skeletal muscle is hyperactive while the gastrointestinal tract is not active. Thus, blood is shunted to the skeletal muscle and away from the gastrointestinal tract.

What a system! If we could only have such a system in the world that delivers adequate amount of supplies efficiently to the regions that most need the substrates at a particular moment in time.

 

Quiz Me

The volume of flow has a direct relationship to the length of a tube.

True  False

Principles of Circulation: The Heart as Pump

All functional devices work on the principal that they receive, process and export.

A pump is a mechanical device that moves gases or fluids. In order for a pump to be effective it must pump out what it receives.

In order for the heart to be effective in the body it needs to pump out what the body needs and be able to receive what is returned to it.

The heart is a positive displacement pump, meaning that it causes the blood to move by first trapping a fixed amount of blood in the chambers, and then forcing the blood into the outflow system by contraction. It accomplishes this by a reciprocal motion of relaxation and contraction. The pumping chamber reduces its volume by muscular contraction, while the receiving chamber increases its volume by relaxation. The contraction forces the blood to move forward, and the relaxation creates a relative vacuum allowing blood to be sucked into the heart.

The heart consists of two such pumps that are connected in series, allowing for a pulmonary circulation and a left-sided systemic circulation.

The heart is made of a special type of muscle which has the innate properties of contractile function.

 

The blood flows from a high to low pressure system via arterioles, capillaries, venules and then into the venous system, which returns the blood to the heart.

In an average lifetime, the heart beats more than two and a half billion times. Think about this number a few times – imagine what an unbelievable engineering feat nature has accomplished. In fact it is not just a pump that moves in and out. There is second to second, and beat to beat variation so that it continuously changes its patterns of contraction and relaxation, always accommodating bodily needs.  The heart circulates the body’s blood supply about 1,000 times each day, circulating 5-6000 liters of blood through 62,000 miles (length of two and half times around the earth).  An average adult body contains about five liters of blood.

The beat to beat to beat variation allows the output from the left to be reconciled with what is coming back in the return, so that congestion does not occur.  In addition, physiology is a dynamic process and the demands of each system in the body changes based on special needs and activities.  The heart also has to accommodate to these changing needs distributing a fixed amount of blood to the regions that need it most at any one time. During exercise for example, the skeletal muscle requires increased flow due to the increased metabolic demand, while the gastrointestinal system uses less flow due to a resting state. Blood shunts from the one system to the other.  The controlling mechanisms at both the local and systemic levels are simply elegant and complex, but effective.

Structure of the Heart

Structural principles are grounded in the notion that biological units have characteristic and defining size, shape, position and character, with connections via neurovascular bundles, ducts and lymphatics to the system at large.

The size of heart is characteristically described as being fist-sized and can be measured in many ways including centimeters (cm), cubic centimeters (cc), or grams (g).  The most useful, functionally, is the chamber volume at end diastole and end systole. The end diastolic volume of each ventricle is about 150 ccs and end systolic volume is about 70 ccs.  Conceptually the four chambers of the heart have been described as two upper receiving chambers and two lower pumping chambers but the structural characteristics of each is unique.  In general though, the atria, as receiving chambers, tend to be round and the ventricles oval, or conical.

Traditionally, the position of the heart is considered to be on the left side and true to tradition, it is mostly left-sided with the apex of the heart pointed to the left.  The right atrium and ventricle are dominantly positioned to the right of midline. The left ventricle has a central position while the ventricle has a left-sided location.
The heart is demarcated by:

• • • A point nine centimeters to the left of the midsternal line (apex of the heart);
    • the seventh right, sternocostal articulation; the upper border of the third right, costal cartilage one centimeter from the right sternal line;
    • The lower border of the second left, costal cartilage; two and a half centimeters from the left lateral, sternal line.

The heart is made of striated, syncytial muscle with a surrounding membrane (pericardium) consisting of a membranous, inner, serous layer and an outer, sero-fibrous compose layer.

The connections of the heart to the rest of the cardiovascular system are via the arteries, veins, nerves and endocrine system. Coronary arteries provide the heart blood supply, venous drainage via the cardiac veins, and lymphatics via the pericardial system.  Parasympathetic and sympathetic systems connect it to the autonomic nervous system, while the hormonal control is via natriuretic hormone, adrenaline, and nor adrenaline.

The main component parts of the heart include the right atrium, tricuspid valve, right ventricle, left atrium, mitral valve, and left ventricle.  The aortic valve and the pulmonary valve are part of the great vessels and vascular system that emanate from the heart.  They are included in part in the following discussion since they are the link to the systemic and pulmonary circulations, and their competence is necessary for normal function of the heart.

Structure of the Heart: The Basic Infrastructure of the Heart

From the outset, it is reasonable to conceptually visualize the infrastructure of the heart as a four-chambered box centered around a scaffolding that has a cross-like formation, with two smaller upper chambers and two larger lower chambers.

The heart chambers do not lie in a vertical orientation.  The atria are relatively posteriorly and rightwardly placed and the ventricles are relatively anterior and leftward.  The right-sided structures tend to be anteriorly placed while the left are posteriorly placed.

The vascular system and conduction system are organized around the scaffolding of the heart.

The aorta which is centered at the crux gives rise to two main arteries; the right and left coronary arteries whose tributaries course along the vertical and horizontal axis of the of the scaffolding .

 

Quiz Me

What is the approximate volume change of the ventricles at end diastole and end systole?
(Note: You will be given 2 tries to answer this question, then the answer will be provided.)
70 ccs
150 ccs
80 ccs
220 ccs

Structure of the Heart: Histology

The layers of the heart conform to the basic pattern seen in the histology of other tubular structures except that the muscle layer dominates.

Tubular structures in the body have a basic structural makeup of an inner layer lined with an epithelial layer abutting the lumen, a middle functional layer and an outer protective layer or skin.  In the heart the inner layer is called the endocardium, the middle muscular layer is called the myocardium, and the outer layer is called the pericardium.

 

The layers are complex in their makeup.  The endocardium consists of an endothelial layer which rests on a subendothelial layer and basement membrane.  Between the endocardium and myocardium there is a subendocardial layer. The myocardial layer is complex in the manner in which the myocardial cells are oriented and organized.  The pericardial layer consists of two basic layers between which is the pericardial space, and each of which is made of unique tissue.

Structure of the Heart: Principles

The endocardium consists of a single layer of squamous endothelial cells that rests on a layer of fine collagen fibers and a basement membrane.

The squamous endothelial cells are shaped like the scales of a fish and have oval or round nuclei.  The squamous lining of the entire heart is continuous with the endothelial cells lining all the vessels in the body forming one large sheet of tissue spread throughout the entire cardiovascular system. Perhaps one can think of it as a tubular table cloth that extends from the toes to the head –one long, extensive, and remarkable sheet of tissue.

The endothelium rests on a continuous layer of fine collagen fibers, which is itself anchored by a basement membrane.  These two structures (fine collagen layer and basement membrane) form the subendothelium.  Beneath the subendothelium is a thicker layer of dense connective tissue forming the subendocardium. It is composed of collagen fibers, elastic fibers, smooth muscle cells, small blood vessels, and in the ventricles may contain specialized cardiac muscle cells of the conduction system. The connective tissue in this region binds the endocardium to the myocardium.

The endocardium is variable in thickness, being thickest in the atria (left > right) and thinnest in the ventricles particularly the left ventricle. Localized areas of endocardial thickening (jet lesions) are common, particularly in the atria, as a result of turbulent blood flow within the chamber.

Macroscopically the endothelial surface is shiny, smooth, and moist.

There are certain diseases that cause thickening of the endocardium usually caused by subendocardial ischemia.  Endocardial fibroelastosis (EFE) which is seen in the endocardium of the left ventricle in hypoplastic left heart syndrome is shown below.  The entity is thought to result from ischemic changes induced by the suprasystemic pressures in the left ventricle caused by severe left ventricular outflow obstruction and resulting in subendocardial ischemia and secondary fibrosis.

Principles : Myocardium

The myocardium is made up of layers of cardiac myocytes (75% of the total volume of the myocardium), attached to each other by connective tissue fibers, and arranged in spiral bundles. The myocardium is the contractile layer of the heart and hence the major function of myocardial muscle cells is to execute the cardiac contraction-relaxation cycle.  The cell is striated, with a single nucleus, with dark staining intercalated discs.  It is thickest in the left ventricle and thinnest in the atria.  The overall architecture is one of muscles arranged in spiral and circular bundles.

 

Principles : Ventricular Myocytes

The muscles of the ventricles are arranged as:

(1) external longitudinal layer

(2) middle circular and

(3) inner longitudinal layer.

Isolated bundles of cardiac muscles project into the lumen of the ventricle forming the trabeculae carneae.

The overall architecture of the myocardium of the left ventricle is in the form of a figure of 8 with one side attached to the subendocardium and then a gradual helical twist, almost to 180 degrees as it extends to the subepicardial surface.

A myofiber or myocyte is the basic structural and functional unit of the myocardium.  The cells are held together by surrounding collagenous, connective tissue with strands of collagen also connecting the myofibers to each other.  Within the myofiber there are myofibrils.

Principles : Pericardium

The pericardium consists of two basic layers.  There is an inner layer called the visceral pericardium that is made of a layer of fibroelastic connective tissue, blood vessels, lymphatics, nerve fibers, and a variable amount of adipose tissue that merges with the myocardium.  The second layer called the parietal pericardium is a more superficial layer, and is made up of a tougher fibroelastic layer, called the fibrous pericardium and a more delicate mesothelial, serous lining.

 Visceral Pericardium

The epicardium (or visceral pericardium) forms the outer covering of the heart. (Cormack).   Its free surface is composed of a single layer of flat to cuboidal mesothelial cells, resting on a layer of connective tissue rich in elastic fibers.  The subepicardial space adjacent to the myocardium contains blood vessels, nerves and an abundance of fat cells.

 Parietal Pericardium

The parietal pericardium has two parts: an inner portion that has a serous lining and an outer fibrous component.  The latter is made from collagen with a small amount of elastin.  The collagen is organized in a criss-cross pattern.

When the inner, single cell, thick, cuboidal, mesothelial surface of the linings of both the parietal and visceral pericardium are reviewed under the electron microscope, they reveal microvilli and long cilia which secrete fluid and provide a low friction surface.

Below the mesothelial epithelia a layer of elastic fibrils supports the epithelium.

There is a rich network of lymphatics that provide the substrate for the pericardial fluid.

Quiz Me

What cell type results in the contractility of the heart?
(Note: You will be given 2 tries to answer this question, then the answer will be provided.)
Endocardium
Myocytes
Myocardium
Syncytium

Structure of the Heart: Fibrous Skeleton

The fibrous skeleton of the heart, formed of connective tissue, separates the atria from the ventricles and includes the annuli fibrosi – (rings of dense connective tissue around the valves), trigona fibrosi (masses of  fibrous connective tissue connecting the annuli) and the septum membranaceum (membranous spot at the top of the interventricular septum). It is composed of dense collagen fibers, scattered elastic fibers and occasional fat cells (less regular than tendon, more regular than dermis). The cardiac skeleton serves as the place of origin and attachment of muscles of the atria and ventricles.

Structure of the Heart: Valve Leaflets

Valve leaflets are composed of three layers (ventricularis, fibrosa and spongiosa) and two general cell types (1) endothelium – non thrombogenic external lining and (2) valvular interstitial cells  – heterogeneous cell population composed of fibroblast and smooth muscle cells.  The cardiac valves are lined with endothelium and all have a similar layered architecture consisting predominantly of a dense collagenous core, and the fibrosa layer, close to the outflow surface and continuous with the valvular supporting structures. Towards the ventricular/atrial cavity the ventricualis/atrialis layer is found and it is rich in elastin. Between these two layers is the centrally located core of loose connective tissue, the spongiosa layer. The collagen of the fibrosa layer is responsible for the mechanical strength of the valve, whereas the connective tissue of the spongiosa layer works as a shock-absorber. Normal leaflets and cusps have only scant blood vessels and are avascular structures.

Structure of the Heart: Histology of the Cardiac Conduction System

The conduction system is made of myocardial fibers designed for rhythm rather than contraction. They include the nodal cells (AV nodal, SA nodal) and the AV bundle along with its branches (Purkinje cells).

Nodal Cells:  Nodal cells are smaller than ordinary cardiac myocytes, contain fewer and poorly organized myofibrils and no intercalated discs.

Purkinje cells:  They are specialized myocytes found in the subendocardium of the ventricles.  Purkinje cells are rich in glycogen and mitochondria and often have two more nuclei centrally placed.

Quiz Me

Select the tissue layer responsible for the mechanical strength of a valve.
(Note: You will be given 2 tries to answer this question, then the answer will be provided.)
Collagen of the fibrosa layer
Connective tissue of the Spongiosa
Ventricularis
Purkinje cells

Anatomy

The macroscopic appearance of the heart follows, with attention paid to the elements that allow the pathologist, clinician, cardiologist, radiologist and surgeon to evaluate the heart.

The universal structural descriptors that are used by all professionals who analyze structure include the size, shape, position and character, as well as evaluation of the connections that enable integration into the body.

Size

Size is one of the most important descriptors of the heart.   It is a measurable entity, and determining whether the heart is normal or abnormal can be verified against a normal standard.   Size and weight of many organs differ between the sexes and among age groups, and it is no different for the heart.  Measurements may be linear, and portray a volume, mass, rate or frequency.  Size may be reflected as a ratio, such as the size of the left atrium and the base of the aorta that should have close to a 1:1 ratio.

The analogy of the heart being fist-sized is helpful conceptually, but has limited use in the clinical realm.  The evaluation of the size of the heart in clinical medicine is of the utmost importance.  Although the global size of the heart is important, the size of the individual chambers is even more so, and in fact essential.

Size of the heart is one of the key determinants in the diagnosis of heart disease.  Each of the chambers is so unique that separate criteria have been developed for each of the chambers.  Linear measurements are still the standard for most evaluations.

Anatomy of the Heart: Anatomical Evaluation

The post-mortem evaluation of the heart historically, has been the manner in which the measurements of the heart evolved, but cannot be accurately transposed to the living and beating heart that is filled with blood and interstitial fluid.

The adult heart measures about 12 cm in length by 8 to 9 cm in width at the broadest part, by approximately 6 cm in thickness. Translated into inches, it would approximate 4.5 by 3.5 by 2.5. The weight of the heart varies from 280 to 340 grams in the adult male and from 230 to 280 grams in the adult female. Thus, for the adult male it is just over half a pound and for the adult female it is about half a pound.

At post-mortem LV thickness of the adult heart ranges from about 8mms to 1.3cms and is usually less than 1.5cms.  The RV wall thickness ranges from 4 mm-8mms., being thinnest on its free wall.

The volumes of the left ventricle have been well studied.  The left ventricular end diastolic volume ranges from 55ccs to 190ccs, and end systolic volume from about 15 ccs to 70 ccs with a stroke volume of 35-130 ccs, and ejection fraction ranging from 55% to 75%.  For males, the volumes are slightly higher and for females slightly lower.

The heart increases in weight and size as human’s age. At birth, the heart appears large in proportion to the diameter of the chest cavity. Between puberty and 25 years of age the heart attains its adult shape and weight. The size of the heart is governed by its overall mass, which reflects ventricular thickness, as well as the volume of the cavities.

Principles in Disease

Tubular structures remain normal in size if the pressure, volume, flow, and integrity of the walls are maintained.  The heart is in effect a tube with modifications and therefore will follow the principles that govern tubular function.

In general tubular structures enlarge as a result of:

increased pressure
increased volume
turbulent flow
loss of wall integrity

Enlargement of the heart may result from hypertrophy or dilatation.  Hypertrophy is caused by increased pressure and it usually results from stenotic valves, or increased afterload as a result of hypertension.

Increased volume is caused by an increased preload.  An incompetent aortic valve, left to right shunt or a failing heart that cannot pump out what it receives will result in an increased preload.  Loss of structural integrity, caused by a myocardial infarct for example will cause a dilated heart for multiple reasons.  The weakened tissue will bulge, and the accumulation of blood in the chambers as a result of a failing pump may both contribute to the enlargement.

Causes of a small heart are much less common, though smallness of individual structures such as stenotic valves, hypoplasia, aplasia, and dysplasia commonly occur in the congenital disorders of the heart.  There are usually compensatory mechanisms in the heart that will attempt to overcome the deficiency so that the overall size of the heart increases.  For example, in tricuspid hypoplasia or atresia, the right atrium will enlarge as it attempts to push blood through the narrowed tricuspid valve, so that the overall size of the heart may increase to the extent that the child presents with a gigantic heart caused by a very dilated right atrium.

When the heart enlarges in disease, the cause of the enlargement can be approached in many ways. An anatomical differential diagnosis is one place to start.  First, the parts of the structure being analyzed must be identified.  Next, the enlargement should be localized to one or more of the components of the heart.  If a heart is enlarged on a chest X-ray an important branch point depends on determining whether the left side is abnormally big, the right side or both.  Left-sided enlargement allows deduction that the left atrium and or left ventricle is enlarged.  If it is finally established that the left ventricle is normal and the left atrium is enlarged, then stenosis of the mitral valve becomes the most likely candidate for the cause of the disease.  Since mitral stenosis in an adult is almost always caused by rheumatic fever, one can further deduce that the cause of mitral stenosis was a pediatric infection with Lancefield type A beta hemolytic streptococcus bacteria.  The patient probably acquired cardiac involvement from streptococcus pharyngitis when he or she was between 5-15 years.  The polymers in the wall of the organism, called M proteins, are highly antigenic and at the time forced the patient’s defense mechanism to create antibodies.  While responding to the foreign organism, these antibodies also inadvertently reacted to the patients own tissues, including the myocardium, and or mitral valve, and possibly the pericardium, brain and even joints.  The depth of the detail in the diagnosis depends on the physician’s knowledge, but the story would not have been a story, and the diagnosis would not have been made, and treatment would not have been instituted, had a diagnostic finding not been identified – in this case the change in size of the heart.   The beginning of diagnosis, even if it involves complex disease, starts with simple observations of aberrant structure or function.  In this case it started with the simple observation of a change in size in the heart.

Clinical Medicine

In clinical medicine palpation of the heart is the most valuable tool to assess heart size.  Most of the right ventricle is positioned anteriorly while only a small portion of the left ventricle, dominantly the apex, is anterior.  The position of the apex enables the assessment of left ventricular size.  It is normally palpated with the tips of the index and middle fingers in the midclavicular line in the 5th intercostal space.  In left ventricular enlargement the apex is projected down and out (laterally).  The right ventricle lies anteriorly and is not usually felt but when enlarged a prominent parasternal heave is felt with the palm of the hand.

Clinical Medicine: EKG

EKG is better able to evaluate individual chamber enlargement with atrial evaluation identified in the p wave and ventricular enlargement evaluated in the QRS complex.  Right-sided structures are evaluated in the leads overlying the right side of the heart, and left sided enlargement similarly on the leads on the left side of the heart. When the right atrium is enlarged the p wave becomes enlarged.  The characteristic  “p pulmonale” characterized by a high amplitude, (greater than 2.5mm) peaked, and narrowed p wave is present.  When the left atrium enlarges the p wave, “p mitrale” results which is broader (>.11 msecs) and notched so that the shape of the wave becomes shaped like an “M’ .  Right ventricular hypertrophy is characterized by an R wave in leads V1 > 7mm, or if the R wave is larger than the T wave in V1.  Right axis deviation is also a sign of right ventricular dominance.

Left ventricular hypertrophy has a number of criteria:

• • • QRS height
    • R in lead I plus S in lead III greater than 25 mm.
    • Increased precordial QRS voltage: S in lead V1 plus R in either V5 or V6 greater than 35 mm.
    • Large leftward voltage: R wave in lead L greater than 11 mm.
    • Typical ST and T abnormalities:
      • ST depression or T wave inversion (or both) in the “lateral” leads (I, L, V4-V6)

Clinical Medicine: Imaging

There are a wide range of technologies that are able to measure cardiac size with variable accuracy.  The CXR was the first imaging technique to evaluate heart size and was the only imaging technique for about 70 years until modern technology evolved.  It is still commonly used but is not as accurate as echo CT or MRI.

Imaging: Chest Radiograph (CXR)

The cardiothoracic ratio is the ratio between the transverse diameter of the heart and the widest diameter of the chest.  The normal cardiothoracic ratio is less than 50%.

The actual measurement of the cardiothoracic ratio is not used in clinical practice, but a gestalt of the approximate cardiothoracic ratio is commonly used.  The distinction between lung disease and heart disease for example in a patient with shortness of breath may rest in identifying cardiomegaly.  The A-P examination magnifies the heart so evaluation of cardiomegaly with this projection should be used with caution.

Imaging: Applied Anatomy

In order to fully evaluate heart size on the chest X-ray (and any other imaging technology for that matter) knowledge of the relative position of the structures is important.  A useful overlay is shown below.

Imaging: Echocardiogram

The echocardiogram provides a real time, safe, accurate, and relatively inexpensive method of evaluating the structures of the heart at any phase of the cycle.  Measurements of size including volume, ejection fraction, wall thickness, valve areas, velocity and direction of flow are relatively easy to measure.  This technique is the bread and butter of structural cardiac evaluation.

 

Due to the real-time nature of sonography and the echocardiographic examination, the clinician is able to observe the flow through the normal and abnormal heart.

Imaging: Computerized Tomography (CT)

CT scan is not used routinely for imaging of the heart because it is not real time, is expensive, requires gating for accurate measurements, requires contrast injection, and involves radiation exposure.  On the other hand it is used routinely for examinations of the chest, and although this examination is not tailored for cardiac evaluation, evaluation of the size and an estimate  of the thickness of component structures is possible.

Volume measurements and wall thickness measurements requires a diastolic frame in order to standardize the measurement and also because there is thickening of the myocardium during systole as it shortens. This requires a gated study in order to determine the phase of the heart

However, the size of the atria and to some extent the ventricles can be fairly accurately and subjectively assessed often by reviewing the shape of the chambers.

Imaging: Magnetic Resonance Imaging (MRI)

MRI is not a routine method for the evaluation of heart size due to the accurate capabilities of echocardiography which is safe, inexpensive and accurate.  However, there are many advantages that gated cardiac MRI offers in the evaluation of the heart.

Imaging: Angiography

Overall heart size is not easily evaluated by angiography since only the cavity is visible.  However, the hypercontractile ventricle is an indirect sign of ventricular hypertrophy.

Imaging: Clinical Examples

 

.

Clinical Medicine: Shape

The shape of the heart is quite complex and can be described in many ways based on which professional is evaluating the heart and from which angle it is viewed. The shape is of extreme importance in imaging since a subtle change may be the key to the diagnosis.  Right ventricular dysplasia, for example can cause life threatening arrhythmias and sometimes, the only clue to the diagnosis is a subtle bulge of the right ventricular outflow tract.  Thus although simple in concept, shape of structure is a key element to diagnosis.

The shape of the heart has been variably described as conical, triangular and ovoid.  It is rather difficult to provide a single descriptor that accurately describes the shape of the heart mostly because of the variety of facets or angles from which  it can be evaluated.

The anatomist, and pathologist will have a sense of the shape with the heart devoid of life and blood while the surgeon and the imager will have a different perspective with a heart filled with life and blood that changes in shape every millisecond.  On the other hand the detail of muscle bundles, chordae, as well as either micro components are part of the secrets of the heart available only to the anatomist and pathologist, but not yet visible to the imager at this time.

The angle from which the heart is viewed also affects the perspective.  The frontal view is different from the lateral which will be different from all the other projections that are used to view this organ including the oblique, coronal, long axis, and short axis.  In each of these many views the shape will invoke different subjective descriptions, and most importantly will help define whether the heart is normal or not.

Each of the chambers is quite unique in individual structural characteristics and shape is no exception.    The first general description will relate to the anatomical view- how it appears to anatomist or pathologist.

The overall external surface is smooth.

The shapes on the inner surface of the heart are unique to each chamber.  The right atrium has a combination of smooth areas, with both large and smaller muscular bundles; the left atrium is mostly smooth, while the left ventricle is smooth but for its set of papillary muscles, and the right ventricle tends to be heavily trabeculated.

Shape: Clinical and Imaging

The shape of the heart cannot be evaluated by clinical examination. However, in imaging it is a key and major factor in diagnosis.

In vivo, during life, the heart on a chest X-ray is usually imaged during inspiration and the heart tends to be more elongated.

Others perceive the overall shape in this view as an oblong, perhaps an egg on its side, or a football waiting to be punted.

The perception as to whether the heart is shaped like a  triangle or an oval depend on multiple variables including the viewers perception, the phase of the respiration, the body habitus of the patient and the overall structure of the individual heart.

In truth though, the heart has both triangular and ovoid features.  The right ventricle tends to be triangular and the left ventricle more oval, and this theme will be revisited a number of times throughout this module.

The CXR and EKG used to be standard and first line methods of evaluation of the heart.  The shape of the heart on CXR was a means of evaluating its size.  Although it is still important, and practical, it is subjective.   The enlarged right ventricle is reflected in triangular cardiomediastinal silhouette while the enlarged  left ventricle  is reflected as an ovoid shape on the frontal view.

Clinical and Imaging: Shapes of the Heart in Congenital Heart Disease

A coronal view of the heart as viewed by CT or MRI imaging portrays the deeper coronal shapes of the heart, its muscle and cavities.  In these views the triangle of the RV and the oval of the LV are maintained.

Clinical Medicine: Axial Imaging

In this projection the triangular nature of the RV is mostly retained, but because of the non orthogonal nature of routine cross sectional imaging, the apex of the LV is not usually imaged orthogonally, so the oval appearance of the LV is not consistent. The shape of the atria are best defined and assessed in the axial projection.

The Atria

In the axial projection the atria have flattened free borders- the lateral border of the right atrium and the posterior border of the left atrium.  When it enlarges the borders become rotund indicating an increase in pressure or volume.

Clinical Medicine: Position

The heart lies between the lungs in the middle mediastinum and is enclosed in the pericardium. It is placed obliquely in the chest behind the body of the sternum and adjoining parts of the rib cartilages, and projects farther into the left than into the right half of the thoracic cavity. About one-third of it is situated on the right and two-thirds on the left of the median plane. Boundaries of the heart are clinically important as an aid in diagnosing heart disorders.

The heart does not lie upright with atrial chambers totally superiorly.  Rather it is tipped with the atria rightward and backward and ventricles leftward and forward.   In addition, the right sided chambers are more anterior and superior than the left sided chambers. It is of extreme importance for you to spend some time on this section trying to visualize in your mind’s eye how the heart lies, since once the template is captured, you can turn the heart in any which way and you will be able to find your way around it.

 

The relevance of the anatomy is introduced in the evaluation of the chest X-ray.

The right ventricle is anterior to the left ventricle and is also superior. The right atrium (blue ring overlay) is slightly superior and anterior to the LA i.e. blue right sided structures are superior and more anterior than red left sided structures . The atria are superior and posterior to the ventricles as shown in the diagram.
Ashley Davidoff MD

Position: Embryologic Considerations in Positioning of Cardiac Structures

The fetal heart begins beating at 22 to 23 days. The position of the heart in the chest, and the position of the chambers are defined in the embryological development of the heart.

There are 4 major events in embryology of the heart.

Situs development
Looping
Septation
Growth and regression

Positioning of the apex is a minor independent event separate from situs and looping.

Of these events situs, looping, and apex positioning play a major role in the positioning of the heart and its components.

Innate in the genetic instruction is positioning of the atria, lungs, tracheobronchial tree and some of the abdominal organs.  Normal situs instruction would result in the positioning of the right atrium on the right, left atrium on the left, right lung, liver, pancreatic head, and gallbladder on the right, and left lung, spleen, and pancreatic tail on the left.

Looping of the heart is also under genetic instruction, and normal looping would bring the right ventricle to the right and left ventricle to the left.  This usually places the aorta to the right of the pulmonary artery.

Positioning of the apex is an independent event and is usually the result of the final positioning of the left ventricle.

Position: Levocardia

Levocardia is the normal positioning of the heart dominantly in the left chest as a result of the normal placement of the apex by the left ventricle

Position: Dextrocardia

Dextrocardia is defined as a malposition of the heart that can be congenital or acquired resulting in the apex of the heart being in the right chest.  There are two congenital forms; one is associated with situs solitus, and the second associated with situs inversus.  The form that is associated with situs solitus is thought to be due to an embryonic arrest resulting in the right ventricle (rather than the left) forming the apex.  In this instance the right ventricle is still on the right side and the left ventricle is on the left side.  This form of dextrocardia is associated with a higher incidence of congenital abnormalities.  The second form of the congenital version is associated with situs inversus. In this situation the incidence of associated congenital abnormalities is lower. Mesocardia is also an atypical position, with the heart in a central position in the thorax; neither a right nor leftward pointing apex.

Acquired dextrocardia may be due to either loss of volume in the right chest or a push due to mass effect from the left side.

Position: Situs Inversus

Situs inversus is a congenital disorder in which the organs of the chest and abdomen are inverted in position.  Situs inversus is an abbreviated term for situs inversus viscerum.

The cause of the genetic aberration is usually an autosomal recessive but situs inversus is associated with entities such as the asplenia syndrome, polysplenia syndrome, Kartagener’s syndrome (primary ciliary dyskinesia).

The result of the aberration is either a partial form (situs inversus partialis) or a total form (situs inversus totalis).

Structurally,  situs inversus totalis the atria, tracheobronchial tree, and lungs are inverted, while in the abdomen the  right sided organs including the liver gallbladder, head of the pancreas, and left sided structures such as the spleen, stomach, tail of the pancreas are inverted.

Situs inversus is not necessarily associated with dextrocardia. isolated dextrocardia without situs inversus is associated with a much higher incidence of congenital heart defects.

Functionally, situs inversus in the absence of other congenital defects can be totally asymptomatic, but is usually fatal in asplenia syndrome.  In this entity situs is usually ambiguous.

     What type of heart orientation does this radiograph demonstrate?
(Note: You will be given 2 tries to answer this question, then the answer will be provided.)
Dextrocardia
Levocardia
Mesocardia
L – TGA

Position: Character

The heart has a smooth, glistening, external surface surrounded by a variable amount of epicardial fat.  It is red to maroon in color, muscular, rubbery, and distensible,  but less so than organs of the gastrointestinal tract. Its inner surface is covered by a smooth white glistening surface endothelium which has an underlying fibrovascular endocardium and subendocardium.

The atria have thin muscular walls, while the ventricles, particularly the left ventricle are much thicker.

The heart is distensible, and on occasion in order to examine the heart, the anatomist or pathologist distends the heart in a closed system with fixatives that include formalin, followed by embedding the heart in wax.  The distensibility of the heart enables this process to display a distended heart, as it might appear in vivo.

Character: Characterizing the Heart on Imaging

On plain X-ray the the heart is characterized by its soft tissue density which is quite limited except when calcium is present in any of the components, best exemplified by calcific pericarditis.

Character: CT Imaging

CT scan is an advanced digital form of X-ray evaluation of the body. Characterization of the tissues is by attenuation of the X-ray as well, except that digital acquisition and cross sectional technique have allowed for significant advance in the characterization of tissue. The density of each structure is reflected and measured by its Hounsfield units.

Character: CT Density Scale in Hounsfield Units

CT Density Scale in Hounsfield Units Air -1000        black Fat -100HU    light black Water 0HU         dark gray Soft Tissue about 20-60HU  light gray Contrast +130       white Calcification +1000      white Cancellous bone +400HU  white

Character: Echocardiographic Imaging of Heart Contractibility
The typical method to assess left ventricular function is through the use of M-mode, two-dimensional (2D) imaging and conventional Doppler techniques. The addition of the relatively new techniques of tissue Doppler imaging, strain and strain-rate offers a quantitative method of evaluating systolic ventricular function that is without some of the limitations of traditional methods such as load-dependency and the need for adequate endocardial definition.

This section includes a brief overview of a few echocardiographic methods of assessing contractibility of the heart wall.  For an in depth discussion of these topics, visit the Assessment of LV Systolic Function with TDI, Strain and Strain Rate, Basic Cardiac Color Flow Imaging, Basic Cardiac Doppler, Cardiac Doppler: Normal Flow Patterns, and Echo Assessment of LV Diastolic Function tutorials.

2D and M-mode

2D and M-mode allow for systolic and diastolic linear measurement of ventricular internal dimensions and wall thickness. These measurements allow for LV fractional shortening, ejection fraction and mass calculations. M-mode measurements allow for precise timing of cardiac events. Ejection fraction is after-load dependent. This means that it is affected by the pressure and impedance against which the ventricle must contract.

2D / M-mode Limitations
Causes	Results
Oblique orientation	Overestimation or underestimation
LV shape alteration	Incorrect ejection fraction results
Regional wall motion anomalies
Poor endocardial definition

 

The modified Simpson’s method or disc summation method allows for calculation of LV volumes from 2D linear and area measurements. These measurements, obtained from the apical 4-chamber and 2-chamber views, provide quantitative evaluation of global LV systolic function with:

adequate systolic and diastolic endocardial definition,
visualization of the entire ventricle without foreshortening,
normal wall motion, and
a normal geometric shape.

To assist in the semi-quantitative 2D evaluation of global systolic function and regional wall motion the LV has been divided into into 3 levels and 16 segments.

This qualitative assessment is through a visual analysis of the function of the ventricle as a whole and of individual wall segments. Transthoracic imaging allows for imaging of all segments; however, accurate assessment requires adequate endocardial definition and complete visualization of the apex without foreshortening.

Doppler

Conventional Doppler assessment of global LV systolic function includes stroke volume and dP/dt.

Stroke volume is the amount of blood pumped by the heart with each beat. 2D measurement of the left ventricular outflow tract (LVOT) cross-sectional area multiplied by the Doppler velocity time integral of flow through that area, yields stroke volume. A slight error in measuring the LVOT area can cause a significant miscalculation of flow volume. Stroke volume is after-load dependent.

dP/dt (the rate of change in pressure over time) represents the rate of rise of pressure within the ventricle. This measurement requires a clearly delineated mitral regurgitation velocity curve to obtain instantaneous pressure measurements. It is relatively load-independent.

The Doppler Effect
The Doppler effect was first described by the Austrian physicist Christian Johann Doppler in 1842 and refers to a:

“Change in frequency of sound, light, or other waves caused by  the motion of the source or the observer.”

One of the classic examples of this effect is the change in pitch of a train whistle as it moves past a stationary observer.

The train whistle sends out a constant pitch that is perceived by the stationary listener, to increase or decrease upon the speed and direction that the train is traveling.

 

The Doppler Effect

Likewise, as the train moves away from the receiver, the wavelengths are elongated and the frequency of waves, or pitch, is perceived as being decreased.

How does this apply to Doppler imaging of the heart?

The frequency of the reflected ultrasound waves increases when the red blood cells are moving toward the transducer.

 

 

Conversely, the frequency of reflected ultrasound waves decreases when the red blood cells are moving away from the transducer.

 

 

In the train example above, a frequency shift is the change in frequency between the transmitted sound and the reflected sound.

In the Heart

In the heart, a frequency shift is induced when a sound wave strikes moving red blood cells.

 

 

Character: Tissue Doppler Imaging

Tissue Doppler Imaging
The previous description applies to general Doppler principles which we generally apply to flow. Tissue Doppler Imaging (TDI) is a technique for the evaluation of global and regional LV systolic function. TDI employs Doppler principles to display the velocity of myocardial motion at any point in the cardiac cycle.

High velocity and low amplitude signals characterize the Doppler signals obtained from fast moving blood flow. However, Doppler signals of relatively dense, slower moving myocardial motion exhibit low velocity and high amplitude signals.

For conventional Doppler, instrument settings maximize the high velocity (>20cm/s), low amplitude blood flow signal. TDI evaluation of myocardial motion requires the suppression of low amplitude signals and the optimization of low velocity (<20cm/s), high amplitude tissue signals. Current ultrasound systems have automatic settings for tissue Doppler interrogation.

Tissue Doppler Techniques: Normal Pulsed Wave 

Pulsed wave TDI allows for the measurement of peak myocardial, ventricular ejection. The myocardial motion of any wall segment can be interrogated with TDI. We know the best angle of incidence is from 0 – 60 degrees.  Due to this angle dependence, the apical views position the myocardium parallel to the transmitted beam.

The mitral annular descent towards the apex represents the left ventricular longitudinal contraction. Decreased global LV systolic function results in diminished mitral annular motion.  Good correlation has been demonstrated between annular velocity and left ventricular ejection fraction over a wide range of ventricular function. Rather than placing the sample volume in the area of flow with TDI, place the sample volume in the myocardium immediately adjacent to the septal or lateral mitral annulus. When imaged in the apical-4 chamber view, this records the longitudinal shortening and lengthening of the mitral annulus. Another location for sample volume placement is the anterior and inferior walls in the apical-two-chamber view.

The motion of the ventricular myocardium throughout the cardiac cycle produces three waveforms:

S – systolic myocardial velocity reflected above the baseline as the annulus moves toward the apex and towards the transducer.

E – early diastolic myocardial relaxation velocity reflected below the baseline as the annulus moves away from the apex and the transducer.

A – myocardial velocity associated with atrial contraction, reflected below the baseline.
Note: the prime symbol (‘) may used to differentiate tissue Doppler velocities (E’ and A’) from mitral inflow velocities (E and A). Alternatively, lower case “a” for annulus (Ea or Aa) or “m” for myocardial (Em or Am) may be used.

The normal functioning myocardium contains inherent regional variations.  These changes depend on the sampled level.  Higher TDI pulsed-wave TDI peak velocities occur in the basal segments with much lower velocities obtained from the apical segments. In addition, interrogation of the normal heart at the four basal segments (septal, lateral, anterior and inferior) also demonstrate variation in velocities. PW TDI sampling at the apical level in the apical views demonstrates slight longitudinal motion in the opposite direction of the mid and basal levels.

 

Case Study

Acute infarction is demonstrated by increase water content in the infarcted region characterized by a bright region on T2 weighted sequences.  Myocardial thinning in acute infarction is also present.  However the most specific characterization is manifest by the accumulation of gadolinium in the infarcted region 5-10 minutes after injection.  Delayed enhancement of infarcted myocardium, differentiates normal myocardium from ischemic myocardium.  The area first involved is the subendocardial region, but if the entire wall is involved, transmural delayed enhancement occurs.

Scar tissue is characterized by administering a contrast agent and using a sequence called inversion recovery resulting in black normal myocardium and white scar tissue.

Summary

The heart is an incredible organ.  Its asymmetric symmetry both in structure and function is a remarkable feat of nature.  The right sided structures are subjected to lower pressures and tend to function as the volume ventricles. Hence structurally they are less muscular, serve the systemic and jugular venous systems to collect blood and provide the lungs with sufficient output to ensure oxygenation of all the cells of the body.  The left sided structures and particularly the left ventricle are built for pressure to ensure that the oxygenated blood gets to all the cells from the head to the toes.  The integration of volume and pressure requirements, are enabled by the delicate second to second, and beat to beat integration of both sides of the heart and both the pulmonary and systemic circulations.

Disease of the heart is very common in Western society, most commonly as a result of coronary artery disease.  The combination of advancing imaging and treatment techniques that have blossomed in the last 20 years has enabled the field to advance understanding and decrease morbidity and mortality .