Haemodynamic basis for the development of left ventricular failure in systolic hypertension and for its logical therapy

Abstract

In youth, properties of the human arterial system are such that pulse pressure generated by ventricular ejection is low, and the major component of wave reflection returns to the heart after the aortic valve has shut, so making no contribution to ventricular load, but boosting pressure throughout diastole and so aiding coronary perfusion. That constitutes optimal arterial function and optimal vascular/ventricular interaction. With ageing, the aorta and elastic arteries stiffen, so that aortic pulse pressure is markedly increased. This is a consequence of a direct stiffening effect on the aorta itself, and of an indirect effect caused by early return of wave reflection consequent upon stiffening of the whole arterial system with an increase in its pulse wave velocity. There is a change in contour of the aorta pressure wave with wave generation of a late systolic peak and disappearance of the diastolic wave; the reflected wave moves from diastole and systole. Because the lowest diastolic pressure remains relatively constant [1,10], increased pulse pressure causes a substantial increase in aortic systolic pressure. Increased aortic systolic pressure is associated with increased left ventricular pressure and leads to left ventricular hypertrophy. Sustained elevation in systolic pressure and persistent left ventricular hypertrophy are associated with progressive degenerative changes in the hypertrophied myocytes such that these weaken, developing less force with each contraction. The weakened, hypertrophied fibres lengthen and the ventricle dilates, with force and cardiac output initally being maintained at greater muscle length and ventricular volume through the Frank-Starling mechanism. Ultimately compensation is lost. The hypertrophied ventricle normally functions as a flow source, which is capable of generating flow even against very high pressure. With the development of cardiac failure through muscle weakening, the ventricle comes to act as a pressure source, with ventricular output very sensitive to pressure and to changes in pressure. The normal ventricle functions in an intermediate position, even though it is closer in behaviour to a flow than to a pressure source. Wave reflection adds to pressure but subtracts from flow. In youth, wave reflection returns to the heart during diastole when the aortic valve is shut. Negative flow is not possible, so wave reflection is apparent only as a secondary pressure wave in the ascending aorta. In older subjects, when the left ventricle is beating powerfully, return of wave reflection during systole has less obvious an effect on the ascending aortic flow wave, but causes an obvious secondary boost to pressure in the ascending aorta and left ventricle. Hence, under normal circumstances, wave reflection at the heart is apparent as a positive secondary pressure wave, either because the aortic valve is shut when this wave returns, or because the ventricle possesses enough power that it virtually overcomes any negative influence on flow when reflection returns during systole. When the myocardium weakens and the heart fails, the heart starts to behave like a pressure source, and wave reflection starts to have a far greater effect on flow; wave reflection is manifested more as a negative influence on flow than as a positive influence on pressure. As heart failure develops, there is a progressive change in flow wave contour, with early deceleration of aortic flow and ultimately, abbreviation of systolic ejection duration with fall in stroke volume. Early wave reflection is the major factor in the genesis of systolic hypertension. Early wave reflection remains a major factor when heart failure develops, although its effect is apparent in reduction of late systolic flow rather than as a boost to late systolic pressure. Reduction in wave reflection through use of vasodilatory agents is a logical strategy in treatment of systolic hypertension. That type of therapy is equally logical in treatment