Simultaneously halving aortic pressure and doubling cardiac output, or vice versa, result in the same value for cardiac work. However, the O2 requirements are greater for any given amount of cardiac work when a major proportion of the work is pressure work, as opposed to volume work. An increase in cardiac output at a constant aortic pressure (volume work) is accomplished with only a small increase in left ventricular O2 consumption, whereas increased
CHAPTER 16 Elements of Cardiac Function arterial pressure at constant cardiac output (pressure work) is accompanied by a large increase in myocardial O2 consumption. Thus myocardial O2 consumption may not be well correlated with overall cardiac work. The magnitude and duration of left ventricular pressure are correlated with left ventricular O2 consumption. The work of the right ventricle is one-seventh that of the left ventricle because pulmonary vascular resistance is much less than systemic vascular resistance.
The efficiency of the heart may be calculated as the ratio of the work accomplished to the total energy used. If the average O2 consumption is assumed to be 9 mL/minute/100 g for the two ventricles, a 300-g heart will consume 27 mL of O2 per minute. This value is equivalent to 130 small calories when the respiratory quotient is 0.82. Together, the two ventricles do approximately 8 kg-m of work per minute, which is equivalent to 18.7 small calories. Therefore, the gross efficiency of the heart is approximately 14%:
Equation 16.9
Thegreaterenergydemandofpressureworkthanofvolumeworkisclinicallyimportant,especiallyinaorticstenosis.Inthiscondition,leftventricularO2consumptionisincreased,mainlybecauseofthehighintraventricularpressuredevelopedduringsystole.However,coronaryperfusionpressure(andhenceO2supply)iseithernormalorreducedbecauseofthepressuredropacrossthenarroworificeofthediseasedaorticvalve.
The actual gross mechanical efficiency of the heart is slightly higher (18%) than the value calculated and is determined through subtracting the O2 consumption of the nonbeating (asystolic) heart (≈2 mL/minute/100 g) from the total cardiac O2 consumption in the calculation of efficiency. The efficiency of the heart as a pump is relatively low. During physical exercise, efficiency improves because mean blood pressure does not change appreciably, whereas cardiac output and work increase considerably, without a proportional increase in myocardial O2 consumption. Of interest is that the chemical efficiency of the heart is rather high, as indicated by the estimate of 60% for the efficiency of generating ATP from oxidative phosphorylation. The energy expended in cardiac metabolism that does not contribute to the propulsion of blood through the body appears in the form of heat. The energy of flowing blood is also dissipated as heat.
Myocardial Adenosine Triphosphate and Its Relation to Mechanical Function
The chemical energy that fuels cardiac contractile work and relaxation is derived from ATP hydrolysis (