CARDIAC EFFICIENCY

Efficiency is defined as the ratio of the energy delivered by a system to the energy supplied to it. Depending on the particular question be ing addressed, there exist a plethora of definitions of efficiency in medical texts, thus hampering their comparison. If only the ventricula r work seen by the arterial system is under investigation, pressure-vo lume work will serve as a useful numerator. If, on the other hand, ext ernal and internal work together, i.e. the total mechanical work, is o f interest, the pressure-volume area might be employed. Total myocardi al oxygen consumption (MVO(2)) will be a useful denominator in the cas e of aerobic energy production. The MVO(2) for the unloaded contractio n must be assessed if, as in other energy transfer systems, net effici ency is to be addressed. If even smaller steps in the chain of energy transfer are to be investigated MVO(2) for the arrested heart must be assessed. With an appropriate therapy, hemodynamic determinants can be varied, to improve cardiac efficiency. Nonetheless, measurement of al l variables necessary for the calculation of efficiency remains a chal lenge, in particular in the clinical setting. Separation of the direct effects of drugs on efficiency is even more difficult, since hemodyna mic conditions can hardly be controlled throughout the observation per iod, and changes in efficiency might be secondary to changes in hemody namics. Whether the heart by itself employs mechanisms to improve its efficiency is still a matter of discussion: there is evidence that whe n oxygen supply decreases, the heart can switch from one substrate to a less costly one, or possibly can improve efficiency through better u se of oxygen. Moreover, the heart seems to ''sense'' an even more decr eased oxygen supply and reduce function in response, Myocardial stunni ng could be regarded as a protective mechanism as well, with function remaining depressed and the oxygen supply being normal or close to nor mal. One may conclude from the decreased efficiency that the excess ox ygen consumption is used up for repair processes. The improved efficie ncy found in hypertrophied hearts represents another adaptive process. The underlying mechanism is unclear: a shift towards isomyosin V-3 or some undefined shift in metabolic pathway is discussed. It is also st ill a moot question towards which objective the efficiency of the hear t is adjusted. It has been described that under physiologic conditions , the efficiency of both the left and the right ventricle ought to be maximized. The alternative finding that the heart and the arterial sys tem are adjusted to maximize stroke work is only a serious contrast if both maxima are narrow and clearly separated from each other.