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Cardiac Work and Contractility J Br Heart J: first published as 10.1136/hrt.30.4.443 on 1 July 1968. Downloaded from Brit. Heart 7., 1968, 30, 443. Cardiac Work and Contractility J. HAMER* It has been customary to regard the heart as a fibres is needed to maintain the stroke volume in a pump maintaining the flow of blood, and to assess larger ventricle, and ventricular work is correspon- the work done by the heart from the pressure and dingly reduced (Gorlin, 1962). Simple calcula- volume of blood leaving the ventricles. While cor- tions suggest that there is, in fact, little change in rect in physical terms, measurement of external work as the ventricle dilates (Table). The more cardiac work in this way is a poor index of the myo- forceful contraction produced by increased stretch- cardial oxygen consumption which is related to the ing of the muscle fibres through the Starling work done by the ventricular muscle. Systolic mechanism probably gives the dilated ventricle a pressure seems to be a more important determinant functional advantage. of ventricular work than stroke volume, and under Estimates of ventricular work based on force some conditions myocardial oxygen consumption measurements still give an incomplete picture of can be predicted from the systolic pressure, duration myocardial behaviour, as a rapid contraction needs of systole, and heart rate (Sarnoff et al., 1958). more energy than a slow one. The velocity of This relationship does not hold in other situations, contraction of the muscle fibres is an important as ventricular work depends on the force of the con- additional determinant of myocardial oxygen con- traction in the ventricular wall rather than on the sumption (Sonnenblick, 1966). The increase in systolic pressure. The force developed for a given myocardial contractility due to inotropic agents such http://heart.bmj.com/ pressure increases as the cavity becomes largert, so as digitalis or noradrenaline is associated with an ventricular work is affected by the size of the cham- increase in the velocity as well as the force of con- ber as well as by the pressure and volume of blood traction, and the converse effect is seen in heart ejected. A better prediction of myocardial oxygen failure where contractility is reduced. Ventricular consumption follows if the calculation is based on volume measurements are needed before the velocity ventricular wall force instead of pressure, as altera- of contraction in the ventricular wall can be calcu- tions in ventricular volume are then taken into con- lated. However, if the ventricle does not alter in sideration (Britman and Levine, 1964). Some work size, changes in the rate of ejection or in the rate of is wasted at the early stages of contraction in stretch- rise of pressure are indications of variations in on September 24, 2021 by guest. Protected copyright. ing non-contractile tissue in the ventricular wall, velocity. but this factor becomes relatively unimportant once A fundamental consideration in the assessment ejection has begun. Accurate measurement of ven- of ventricular performance is the inverse relation tricular wall force requires a detailed knowledge of between the force and the velocity of contraction. the changes in ventricular volume during systole. Velocity becomes less as force increases, suggesting The ventricles can be regarded as spherical in shape without producing serious error in these calcula- tions. TABLE EFFECT OF VENTRICULAR DILATATION ON The dilatation of the ventricle in heart failure VENTRICULAR WORK might be expected to lead to an increase in ventricu- lar work, but the effect of greater force in the ven- End-diastolic Mean-systolic Change in Ventricular volume force (F) radius work tricular wall is offset by the mechanical advantage (ml.) (mm. Hg cm.2) (zr) (cm.) (W=4F.4r) a of larger chamber. Less shortening of the muscle 75 1650 0-81 5350 100 2150 0 59 5050 * Address: St. Bartholomew's Hospital, London E:C.1 200 3800 0 33 5000 500 7350 0-17 5000 t The force per unit circumference is given by f= {Pr, and the total force as F = +Pr x 27rr= 7Tr2P, where P is the pressure (Theoretical calculations assuming a mean systolic pressure of and r the radius of the ventricle. 100 mm. Hg and a stroke of 50 ml. in a spherical ventricle.) 1 4L43 Br Heart J: first published as 10.1136/hrt.30.4.443 on 1 July 1968. Downloaded from AAA J. Hamer that there is only a limited amount of energy avail- reduction in blood pressure produced by proprano- able which can be used in either way. Similarly, lol is mediated entirely by a fall in the cardiac out- the extent of shortening of the fibres is reduced as put. The finding of a reduced rate of ejection of the force of contraction increases, and the duration blood into the aorta under these circumstances sug- of contraction is reduced as velocity increases. The gests that a decrease in the velocity of contraction interdependence of these various aspects of myo- of the heart muscle is a major factor in the response cardial contraction is evident in heart disease. In (Shinebourne, Fleming, and Hamer, 1967). The aortic stenosis, for instance, ejection is prolonged effect is not dependent on the reduction in heart rate because of the mechanical obstruction, and the sus- produced by the drug. The suspicion that these tained systole is accompanied by a great reduction changes might be due to a direct depressant action in the velocity of contraction, with a corresponding of propranolol on the heart has been eliminated by economy in myocardial oxygen consumption. The the finding that a beta-sympathetic blocking drug metabolic requirements for the slow prolonged sys- without local anaesthetic or quinidine-like action tole of aortic stenosis may not be very different from (ICI 50, 172) has a similar effect to propranolol. those needed for the fast but brief contraction of The fall in velocity of contraction produced by aortic incompetence. Although contractility is im- sympathetic blockade would be expected to reduce paired in myocardial disease, a reduced velocity of myocardial oxygen consumption and so increase contraction in the intact heart does not indicate that exercise tolerance in patients with angina pectoris. the behaviour of the heart muscle is abnormal if the The consistent fall in cardiac output found after ventricle is increased in size. The greater force propranolol is difficult to explain on the basis of needed to maintain the systolic pressure under these reduced myocardial contractility alone. A similar circumstances may in itself lead to a fall in velocity fall in output is seen supine and erect, in normal (Gorlin, 1962), offering a further possible advantage subjects and in heart disease, and is not due to slow- of ventricular dilatation in heart failure. ing of the heart as the effect persists when the heart An increase in the activity of the sympathetic rate is maintained by pacing (Bloomfield, Redwood, nervous system might be expected in heart failure in and Sowton, 1966). There is evidence in normal an attempt to maintain contractility. The finding subjects and in aortic stenosis of an increase in left of an increase in urinary catecholamine excretion ventricular size after propranolol. The larger appeared to confirm this hypothesis, but direct volume will produce a more forceful contraction by measurement has shown a reduction in catechol- the Starling mechanism, and give a mechanical ad- amine concentration in myocardial tissue removed vantage which will help to maintain the stroke vol- http://heart.bmj.com/ at operation in patients with heart failure, which ume, but an increase in stroke volume to restore the parallels the impairment of contractility (Braunwald cardiac output to the previous level in spite of and Chidsey, 1965). It has been suggested that bradycardia does not seem possible. The patients excessive sympathetic stimulation may produce are usually capable of an increase in heart rate and depletion of the myocardial catecholamines and a cardiac output on exercise in spite of the action of loss of response to sympathetic stimulation. How- propranolol, though they seem unable to compensate ever, other studies do not support this view. Sym- for the reduction in contractility produced by the pathetic blocking drugs such as propranolol tend drug in the absence of an increase in venous return. on September 24, 2021 by guest. Protected copyright. to accentuate heart failure, and our own work in The apparent interrelation between the force, patients with severe aortic stenosis (Hamer and duration, and velocity of myocardial contraction is Fleming, 1967) has shown that the deleterious of interest in view of recent evidence as to the effect on myocardial function includes a reduction mechanism by which contraction is produced. The in the velocity of contraction of the left ventricular sarcoplasmic reticulum, a system of tubules running muscle. These changes after sympathetic blockade alongside the muscle fibrils within the myocardial point to a considerable degree of sympathetic acti- cell, has been shown to take up calcium ions very vity in patients with left ventricular failure. The actively. It is postulated that electrical activation inotropic effect of sympathetic stimulation seems to of the myocardial cell is associated with the release play an important part in maintaining cardiac per- of calcium ions from the sarcoplasmic reticulum, and formance under these conditions, though we cannot that each calcium ion initiates contraction at one rule out exhaustion of the mechanism under more site in the actomyosin complex (Weber, 1966). The prolonged and severe load. long period of depolarization, which is such a strik- Interference with the natural inotropic effect of ing feature of the active state in heart muscle, may sympathetic stimulation has been shown to have an be needed to allow the release of a sufficient number important clinical application in the treatment of of calcium ions.
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