Hypertensive Heart Disease
Uncontrolled and prolonged elevation of blood pressure (BP) can lead to a variety of changes in the myocardial structure, coronary vasculature, and conduction system of the heart. These changes can lead to the development of left ventricular hypertrophy (LVH), coronary artery disease, various conduction system diseases, and systolic and diastolic dysfunction of the myocardium, which manifest clinically as angina or myocardial infarction, cardiac arrhythmias (especially atrial fibrillation), and congestive heart failure (CHF). Thus, hypertensive heart disease is a term applied generally to heart diseases, such as LVH, coronary artery disease, cardiac arrhythmias, and CHF, caused by direct or indirect effects of elevated BP. Although these diseases generally develop in response to chronically elevated BP, marked and acute elevation of BP can also lead to accentuation of an underlying predisposition to any of the symptoms traditionally associated with chronic hypertension.
The pathophysiology of hypertensive heart disease is a complex interplay of various hemodynamic, structural, neuroendocrine, cellular, and molecular factors. On one hand, these factors play integral roles in the development of hypertension and its complications; on the other hand, elevated BP itself can modulate these factors. Elevated BP leads to adverse changes in cardiac structure and function in 2 ways: directly by increased afterload and indirectly by associated neurohormonal and vascular changes. Elevated 24-hour ambulatory BP and nocturnal BP have been demonstrated to be more closely related to various cardiac pathologies, especially in African Americans. The pathophysiologies of the various cardiac effects of hypertension differ and are described in this section.
Left ventricular hypertrophy
Of patients with hypertension, 15-20% develop LVH. The risk of LVH is increased 2-fold by associated obesity. The prevalence of LVH based on ECG findings, which are not a sensitive marker at the time of diagnosis of hypertension, is variable. Studies have shown a direct relationship between the level and duration of elevated BP and LVH.
LVH, defined as an increase in the mass of the left ventricle (LV), is caused by the response of myocytes to various stimuli accompanying elevated BP. Myocyte hypertrophy can occur as a compensatory response to increased afterload. Mechanical and neurohormonal stimuli accompanying hypertension can lead to activation of myocardial cell growth, gene expression (Some of the genes are given expression primarily in fetal cardiomyocytes.), and, thus, LVH. In addition, activation of the renin-angiotensin system, through the action of angiotensin II on angiotensin I receptors, leads to growth of interstitium and cell matrix components. Thus, the development of LVH is characterized by myocyte hypertrophy and by an imbalance between the myocytes and the interstitium of the myocardial skeletal structure.
Various patterns of LVH have been described, including concentric remodeling, concentric LVH, and eccentric LVH. Concentric LVH is an increase in LV thickness and LV mass with increased LV diastolic pressure and volume, commonly observed in persons with hypertension. Compare this with eccentric LVH, in which LV thickness is increased not uniformly but at certain sites, such as the septum. Concentric LVH is a marker of poor prognosis in the presence of hypertension. While the development of LVH initially plays a protective role in response to increased wall stress to maintain adequate cardiac output, later it leads to the development of diastolic and, ultimately, systolic myocardial dysfunction.
Left atrial abnormalities
Frequently underappreciated, structural and functional changes of the left atrium (LA) are very common in patients with hypertension. The increased afterload imposed on the LA by the elevated LV end-diastolic pressure secondary to increased BP leads to impairment of the LA and LA appendage function plus increased LA size and thickness. Increased LA size accompanying hypertension in the absence of valvular heart disease or systolic dysfunction usually implies chronicity of hypertension and may correlate with the severity of LV diastolic dysfunction. In addition to these structural changes, these patients are predisposed to atrial fibrillation. Atrial fibrillation, with loss of atrial contribution in the presence of diastolic dysfunction, may precipitate overt heart failure.
Although valvular disease does not cause hypertensive heart disease, chronic and severe hypertension can cause aortic root dilatation, leading to significant aortic insufficiency. Some degree of hemodynamically insignificant aortic insufficiency is often found in patients with uncontrolled hypertension. An acute rise in BP may accentuate the degree of aortic insufficiency, with return to baseline when BP is better controlled. In addition to causing aortic regurgitation, hypertension is also thought to accelerate the process of aortic sclerosis and cause mitral regurgitation.
Heart failure is a common complication of chronically elevated BP. Hypertension as a cause of CHF is frequently underrecognized, partly because at the time heart failure develops, the dysfunctioning LV is unable to generate the high BP, thus obscuring the etiology of the heart failure. The prevalence of asymptomatic diastolic dysfunction in patients with hypertension and without LVH may be as high as 33%. Chronically elevated afterload and resulting LVH can adversely affect both the active early relaxation phase and late compliance phase of ventricular diastole.
Diastolic dysfunction is common in persons with hypertension. It is usually, but not invariably, accompanied by LVH. In addition to elevated afterload, other factors that may contribute to the development of diastolic dysfunction include coexistent coronary artery disease, aging, systolic dysfunction, and structural abnormalities such as fibrosis and LVH. Asymptomatic systolic dysfunction usually follows. Later in the course of disease, the LVH fails to compensate by increasing cardiac output in the face of elevated BP and the left ventricular cavity begins to dilate to maintain cardiac output. As the disease enters the end stage, LV systolic function decreases further. This leads to further increases in activation of the neurohormonal and renin-angiotensin systems, leading to increases in salt and water retention and increased peripheral vasoconstriction, eventually overwhelming the already compromised LV and progressing to the stage of symptomatic systolic dysfunction.