Overall myocardial energetics in physiological conditions and in acute volume overloading

Left ventricular heat production (PH,lv) was compared with myocardial O2 consumption (P met O2) and mechanical parameters in intact anaesthetized dogs before and after acute volume overload induced by a perfusion of LMW dextran. From myocardial blood flow and coronary arterio-venous temperature differences, the coronary heat loss was determined. Thermodilution techniques were used to estimate the ratio between heat carried away by the coronary system and total heat production as the left ventricular volumes. Compared with P met O2, PH,lv bore a linear correlation. Heat production per beat was highly correlated to mean systolic wall force, midwall circumferential length, midwall fibre shortening and mean circumferential shortening rate (MCSR). Mechanical efficiency computed from heat production (EH) appeared to be very close to the efficiency computed from oxygen consumption (EO). Acute overload was characterized by a very large increase in both stroke volume and MCSR. The dramatic increase observed in heart external power was accompanied by a moderate increase in PH,lv and mechanical efficiency. Nevertheless, the changes in efficiency appeared to be strongly dependent upon the changes observed in heart rate (HR). When HR was increased, the heat production per beat was moderately decreased and efficiency was drastically improved. Conversely, dogs in which HR remained stable exhibited a marked increase in the energetic cost of each beat, with efficiency being practically unmodified. Thus, since myocardial energetics is involved in acute volume overload, two possible means of adaptation may be considered: (a) tachycardia may alleviate the energetic cost of each beat, thus improving heart mechanical efficiency and (b) conversely, if HR remains stable, it may do so at the expense of the energetic cost of the myocardium for each beat, with efficiency not actually being enhanced.