Andersons pediatric cardiology 1738

effect of an increase in contractility during inotropic stimulation The pressure-volume relationship can also provide important information regarding the energetic state of the ventricle In many critically ill patients with myocardial disease, the relationship between myocardial oxygen demand and supply is already precarious; it is therefore imperative that any potentially desirable augmentation of ventricular performance should not be offset by adverse effects on myocardial metabolism and energetics Suga demonstrated that the total energy consumption of the ventricle can be quantified by the specific area in the pressure-volume diagram that is bounded by the end-systolic and end-diastolic pressure-volume relations and the systolic pressure-volume trajectory.13 The scope of the pressure-volume diagram therefore extends beyond cardiac mechanics to include cardiac energetics and mechanoenergetic coupling under varying contractile conditions The use of the pressure-volume relationship to assess the diastolic properties of the ventricle is based on the assumption that throughout the period during diastole when both volume and pressure are increasing, the ventricle exhibits elastic behavior As a result, at any point during this time, the slope of the relationship between pressure and volume represents ventricular compliance As the normal pressure-volume relation at this time is curvilinear, chamber compliance becomes lower as filling proceeds, indicating that the cavity has become stiffer The pressure-volume curve during this part of diastole is usually assumed to be exponential and to show behavior characteristic of Lagrangian stress, so that, if pressure is plotted logarithmically and volume linearly, then a linear relationship will be obtained; it is then possible to calculate its slope and intercept There are few data addressing the changes that occur in the ventricular pressure-volume relationship in children with myocardial failure However, studies in adults have shown that assessment of the pressure-volume relationship can be used to determine the effects of progressive myocardial failure on integrated cardiovascular performance Studies investigating the matching of ventricular properties to arterial load are particularly important in this respect In normal subjects with an ejection fraction of 60% or more, ventricular elastance is nearly double arterial elastance This condition affords an optimal coupling between ventricular work and oxygen consumption In patients with moderate heart failure and ejection fractions of 40% to 59%, ventricular elastance is almost equal to arterial elastance, a condition affording maximal stroke work from a given end-diastolic volume However, in patients with severe heart failure, with ejection fraction of less than 40%, ventricular elastance is less than half of arterial elastance, which provides a suboptimal relationship between ventricular work and either oxygen consumption or stroke volume These studies suggest that ventriculoarterial coupling is normally set toward maximizing work efficiency in terms of the relationship between left ventricular work and oxygen consumption As heart function becomes impaired in patients with moderate cardiac dysfunction, ventricular and arterial properties are initially matched in order to maximize stroke work at the expense of work efficiency However, as cardiac dysfunction becomes severe, the ventricle and vasculature become uncoupled, so that neither the stroke work nor work efficiency are near maximum for patients with severe cardiac dysfunction.14,15 Whereas up to this point we have considered the function of the left ventricle as a whole, regional dyssynchrony of left ventricular function is frequently observed in patients with heart failure, in whom it results in inefficiencies in the contraction of left ventricle, a decreased cardiac output, and increased risk of sudden cardiac death (Fig 65.4) Recently, new therapies aimed at restoring mechanical synchrony in such patients have been shown to result in improvements in symptoms and outcomes.16 FIG 65.4 Dyssynchrony of left ventricular function, demonstrated with tissue Doppler imaging The time to the peak of inward movement of the lateral part of the annulus occurs 150 ms after the peak inward movement of the central fibrous body