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Ventricular hypertrophy (VH) is
thickening A thickening agent or thickener is a substance which can increase the viscosity of a liquid without substantially changing its other properties. Edible thickeners are commonly used to thicken sauces, soups, and puddings without altering their ...
of the walls of a ventricle (lower chamber) of the
heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...
. Although
left ventricular hypertrophy Left ventricular hypertrophy (LVH) is thickening of the heart muscle of the left ventricle of the heart, that is, left-sided ventricular hypertrophy and resulting increased left ventricular mass. Causes While ventricular hypertrophy occurs ...
(LVH) is more common, right ventricular hypertrophy (RVH), as well as concurrent hypertrophy of both ventricles can also occur. Ventricular hypertrophy can result from a variety of conditions, both adaptive and maladaptive. For example, it occurs in what is regarded as a physiologic, adaptive process in pregnancy in response to increased blood volume; but can also occur as a consequence of
ventricular remodeling In cardiology, ventricular remodeling (or cardiac remodeling) refers to changes in the size, shape, structure, and function of the heart. This can happen as a result of exercise (physiological remodeling) or after injury to the heart muscle (patho ...
following a
heart attack A myocardial infarction (MI), commonly known as a heart attack, occurs when Ischemia, blood flow decreases or stops in one of the coronary arteries of the heart, causing infarction (tissue death) to the heart muscle. The most common symptom ...
. Importantly, pathologic and physiologic remodeling engage different cellular pathways in the heart and result in different gross cardiac
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
s.


Presentation

In individuals with eccentric hypertrophy there may be little or no indication that hypertrophy has occurred as it is generally a healthy response to increased demands on the heart. Conversely, concentric hypertrophy can make itself known in a variety of ways. Most commonly, chest pain, either with or without exertion is present, along with shortness of breath with exertion, general fatigue, syncope, and
palpitations Palpitations occur when a person becomes aware of their heartbeat. The heartbeat may feel hard, fast, or uneven in their chest. Symptoms include a very fast or irregular heartbeat. Palpitations are a sensory symptom. They are often described as ...
. Overt signs of heart failure, such as edema, or shortness of breath without exertion are uncommon.


Physiology

The ventricles are the chambers in the heart responsible for pumping blood either to the lungs (right ventricle) or to the rest of the body (left ventricle). Ventricular hypertrophy may be divided into two categories: concentric hypertrophy and eccentric hypertrophy. These adaptations are related to how the cardiomyocyte contractile units, called sarcomeres, respond to stressors such as exercise or pathology. Concentric hypertrophy is a result of pressure overload on the heart, resulting in parallel sarcomerogenesis (addition of sarcomere units parallel to existing units). Eccentric hypertrophy is related to volume overload and leads to the addition of sarcomeres in series. Concentric hypertrophy results from various stressors to the heart including hypertension,
congenital heart defect A congenital heart defect (CHD), also known as a congenital heart anomaly, congenital cardiovascular malformation, and congenital heart disease, is a defect in the structure of the heart or great vessels that is present at birth. A congenital h ...
s (such as
Tetralogy of Fallot Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are: * Pulmonary stenosis, which is narrowing of the exit from the r ...
), valvular defects (aortic coarction or
stenosis Stenosis () is the abnormal narrowing of a blood vessel or other tubular organ or structure such as foramina and canals. It is also sometimes called a stricture (as in urethral stricture). ''Stricture'' as a term is usually used when narrowing ...
), and primary defects of the myocardium which directly cause hypertrophy (
hypertrophic cardiomyopathy Hypertrophic cardiomyopathy (HCM, or HOCM when obstructive) is a condition in which muscle tissues of the heart become thickened without an obvious cause. The parts of the heart most commonly affected are the interventricular septum and the ...
). The underlying commonality in these disease states is an increase in pressures that the ventricles experience. For example, in tetralogy of Fallot, the right ventricle is exposed to the high pressures of the left heart due to a defect in the septum; as a result the right ventricle undergoes hypertrophy to compensate for these increased pressures. Similarly, in systemic hypertension, the left ventricle must work harder to overcome the higher pressures of the vascular system and responds by thickening to deal with increased wall stress. Concentric hypertrophy is characterized by an addition of
sarcomere A sarcomere (Greek σάρξ ''sarx'' "flesh", μέρος ''meros'' "part") is the smallest functional unit of striated muscle tissue. It is the repeating unit between two Z-lines. Skeletal striated muscle, Skeletal muscles are composed of tubular ...
s (the contractile units of cardiac cells) in parallel. The result is an increase in thickness of the myocardium without a corresponding increase in ventricular size. This is maladaptive largely because there is not a corresponding proliferation of the vasculature supplying the myocardium, resulting in
ischemic Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems ...
areas of the heart. Ultimately, this response can be compensatory for a duration, and allow for improved cardiac function in the face of stressors. However, this type of hypertrophy can result in a dilated ventricle which is unable to effectively pump blood, leading to
heart failure Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome caused by an impairment in the heart's ability to Cardiac cycle, fill with and pump blood. Although symptoms vary based on which side of the heart is affected, HF ...
. When stressors that encourage this concentric hypertrophy are reduced or eliminated (either surgically corrected in the case of cardiac defects, or hypertension is reduced from diet and exercise) it is possible for the heart to undergo ' reverse remodeling', returning to a somewhat more 'normal' state instead of progressing to a dilated, pathologic phenotype. This reversion may even go beyond muscle mass, and repair abnormalities in cardiac
connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
. Eccentric hypertrophy is generally regarded as healthy, or physiologic hypertrophy and is often termed " athlete's heart." It is the normal response to healthy exercise or pregnancy, which results in an increase in the heart's muscle mass and pumping ability. It is a response to 'volume-overload', either as a result of increased blood return to the heart during exercise, or a response to an actual increase in absolute blood volume as in pregnancy. This increase in pumping ability is the result of the addition of sarcomeres in series, which enables the heart to contract with greater force. This is explained by the Frank Starling mechanism, which describes the sarcomere's ability to contract with greater force as more of the elements of its contractile units become engaged. This response can be dramatic; in trained athletes have hearts that have left ventricular mass up to 60% greater than untrained subjects. Rowers, cyclists, and cross-country skiers tend to have the largest hearts, with an average left ventricular wall thickness of 1.3 centimeters, compared to 1.1 centimeters in average adults. Though eccentric hypertrophy is termed 'athlete's heart' it is typically only found in individuals who are aerobically conditioned. For example, weight lifters tend to undergo remodeling which more closely resembles concentric hypertrophy, as the heart does not experience a volume-overload, but instead responds to transient pressure overload as a consequence of increased vascular resistance from pressures exerted on arteries by sustained muscular contraction. Though it is the case that eccentric hypertrophy is largely considered to be a healthy response to increased cardiac demand, it is also associated with risks. For example, in athletes with significantly increased left ventricular weight there is also a corresponding increased risk for conduction abnormalities and sudden cardiac death. Additionally, in pregnant individuals, a subpopulation progress to peripartum cardiomyopathy, characterized by a dilation of the left ventricle and a corresponding deficit in heart function. There are suggestions that this progression is partially determined by underlying metabolic derangement (
diabetes Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes is due to either the pancreas not producing enough of the hormone insulin, or the cells of th ...
) and hypertension which may result in a more maladaptive cardiac response to pregnancy. As such, though it is convenient to consider clear cut distinctions between pathologic and physiologic cardiac hypertrophy, there may be a broader range of phenotypes than may be accounted for by gross cardiac phenotypes alone. The development of pathologic states in LVH is complex. Electrical abnormalities are commonly found in individuals with LVH, both ventricular and super-ventricular tachycardia. Additionally, cytoarchitecture and the extracellular environment of the myocardium are altered, specifically genes typically expressed in the fetal heart are induced, as are collagen and other fibrotic proteins. LVH may interfere with heart functionality in a number of ways. Before progression to a dilated phenotype, mechanical obstruction of the outflow tract can occur, leading to reduced cardiac output. Additionally, increased fibrosis of the ventricle can result in a failure to relax appropriately which impairs cardiac filling and may lead to diastolic dysfunction or heart failure with preserved ejection fraction.


Research

Androgens An androgen (from Greek ''andr-'', the stem of the word meaning ) is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes ...
, especially
dihydrotestosterone Dihydrotestosterone (DHT, 5α-dihydrotestosterone, 5α-DHT, androstanolone or stanolone) is an endogenous androgen sex steroid and hormone primarily involved in the growth and repair of the prostate and the penis, as well as the production o ...
(DHT), are active in the ventricle and promote hypertrophy. Researchers are investigating the potential for
finasteride Finasteride, sold under the brand names Proscar and Propecia among others, is a medication used to treat pattern hair loss and benign prostatic hyperplasia (BPH) in men. It can also be used to treat hirsutism, excessive hair growth in women. It ...
—a drug that inhibits the synthesis of DHT—to reduce hypertrophy.


Mechanics of cardiac growth

As described in the previous section, it is believed that the eccentric hypertrophy is induced by volume-overload and that the concentric hypertrophy is induced by pressure-overload. Biomechanical approaches have been adopted to investigate the progression of cardiac hypertrophy for these two different types. In the framework of
continuum mechanics Continuum mechanics is a branch of mechanics that deals with the deformation of and transmission of forces through materials modeled as a ''continuous medium'' (also called a ''continuum'') rather than as discrete particles. Continuum mec ...
, the volumetric growth is often modeled using a multiplicative decomposition of the deformation gradient \mathbf into an elastic part \mathbf^e and a growth part \mathbf^g, where \mathbf = \mathbf^e\mathbf^g. For the generic orthotropic growth, the growth tensor can be represented as \mathbf^g = \vartheta^f \mathbf_0\otimes\mathbf_0+\vartheta^s \mathbf_0\otimes\mathbf_0+\vartheta^n \mathbf_0\otimes\mathbf_0, where \mathbf_0, \mathbf_0 and \mathbf_0 are normally the orthonormal vectors of the microstructure, and \mathbf = vartheta^f,\vartheta^s,\vartheta^n/math> is often referred as growth multipliers, which regulates the growth according to certain growth laws. In eccentric growth,
cardiomyocyte Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of ...
lengthens in the direction of the cell's long axis, \mathbf_0. Therefore, the eccentric growth tensor can be expressed as \mathbf^g = \mathbf + vartheta^-1mathbf_0\otimes\mathbf_0, where \mathbf is the identity tensor. The concentric growth, on the other hand, induces parallel deposition of the sarcomeres. The growth of
cardiomyocyte Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of ...
is in the transverse direction, and thus the concentric growth tensor is expressed as: \mathbf^g = \mathbf + vartheta^-1mathbf_0\otimes\mathbf_0, where \mathbf_0 is the vector perpendicular to tangent plane of the cardiac wall. There are different hypothesis on the growth laws governing the growth multipliers \vartheta^ and \vartheta^. Motivated by the observation that eccentric growth is induced by volume-overload, strain-driven growth laws are applied to the \vartheta^. For the concentric growth, which is induced by pressure-overload, both stress-driven and strain-driven growth laws have been investigated and tested using computational
finite element method Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat tran ...
. The biomechanical model based on continuum theories of growth can be used to predict the progression of the disease, and therefore can potentially help developing treatments to pathological hypertrophy.


Diagnosis


Quantification

Hypertrophy of the ventricle can be measured with a number of techniques. Electrocardiogram (EKG), a non-invasive assessment of the electrical system of the heart, can be useful in determining the degree of hypertrophy, as well as subsequent dysfunction it may precipitate. Specifically, an increase in Q wave size, abnormalities in the
P wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
, as well as giant inverted
T wave In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the ''absolute refractory period''. The last half of the T wav ...
s, are indicative of significant concentric hypertrophy. Specific changes in repolarization and depolarization events are indicative of different underlying causes of hypertrophy and can assist in the appropriate management of the condition. Changes are common in both eccentric and concentric hypertrophy, though are substantially different from one another. In either condition fewer than 10% of patients with significant hypertrophy display a normal EKG. Transthoracic
echocardiography Echocardiography, also known as cardiac ultrasound, is the use of ultrasound to examine the heart. It is a type of medical imaging, using standard ultrasound or Doppler ultrasound. The visual image formed using this technique is called an ec ...
, a similarly non-invasive assessment of cardiac morphology, is also important in determining both the degree of hypertrophy, underlying pathologies (such as aortic coarction), and degree of cardiac dysfunction. Important considerations in echocardiography of the hypertrophied heart include lateral and septal wall thickness, degree of outflow tract obstruction, and systolic anterior wall motion (SAM) of the
mitral valve The mitral valve ( ), also known as the bicuspid valve or left atrioventricular valve, is one of the four heart valves. It has two Cusps of heart valves, cusps or flaps and lies between the atrium (heart), left atrium and the ventricle (heart), ...
, which can exacerbate outflow obstruction. It is not uncommon to undergo cardiopulmonary exercise testing ( CPET), which measures the heart's response to exercise, to assess the functional impairment caused by hypertrophy, and to prognosticate outcomes.


In other animals

In most situations, described above, the increase in ventricular wall thickness is a slow process. However, in some instances hypertrophy may be "dramatic and rapid." In the
Burmese python The Burmese python (''Python bivittatus'') is one of the largest species of snakes. It is native to a large area of Southeast Asia and is listed as Vulnerable on the IUCN Red List. Until 2009, it was considered a subspecies of the Indian pyth ...
, consumption of a large meal is associated with an increase in metabolic work by a factor of seven and a 40% increase in ventricular mass within 48 hours, both of which return to normal within 28 days.


See also

* Athletic heart syndrome * Cardiac fibrosis *
Cardiology Cardiology () is the study of the heart. Cardiology is a branch of medicine that deals with disorders of the heart and the cardiovascular system. The field includes medical diagnosis and treatment of congenital heart defects, coronary artery di ...
* Cardiomegaly *
Cardiovascular disease Cardiovascular disease (CVD) is any disease involving the heart or blood vessels. CVDs constitute a class of diseases that includes: coronary artery diseases (e.g. angina, heart attack), heart failure, hypertensive heart disease, rheumati ...
*
ECG Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of ...
– See diagnosis * Right ventricular hypertrophy


References


External links

{{DEFAULTSORT:Ventricular Hypertrophy Exercise physiology Cardiomegaly