Influences on the efficiency of cardiac work.

The efficiency of cardiac work measured in isolated guinea pig atria can be influenced by change of the heart rate or the stroke work due to electric stimulation or drug actions: An increased cardiac performance caused by enhanced heart rate needs considerably more oxygen than an increase of the total cardiac work to the same degree due solely to a positive inotropic action. Oxidation of long and short chain fatty acids lowers the efficiency of cardiac work connected with negative inotropic effects. An increase of the heart rate is followed by a shift to the oxidation of carbohydrates, at rest to the oxidation of FFA. Long chain fatty acids are not sufficient for the increased demand of energy in the cardiac muscle at high heart rates and are then poorly oxidized. The problems connected with the Ca++ binding of FFA in the cardiac cytoplasm after inhibition of their oxidation well be discussed.