Effect of pacing rate and intensity on mechanical alternans amplitude

The aim of this paper is to report the effect of variations in stimulus frequency (SF) and intensity (SI) of right ventricular pacing on the left ventricular mechanical alternans (MA) amplitude, (MAA) which is defined as 100 (S-W)/S, where S is the ventricular pressure of the strong beat and W that of the weak beat. In 30 dogs the right ventricle was paced at SF ranging from the sinus frequency up to 400/min and at SI ranging from threshold up to 270 mA, while the left ventricular pressure was recorded. Increasing SF at near threshold values of SI resulted in an increase of MAA up to a maximal value. Further increase in SF might cause a diminution of MAA. A slight raise in SI than could increase abruptly MAA. Further increase in SI while keeping a constant SF caused a reduction in MAA which was an approximate logarithmic function of SI. Digoxin, adrenaline or calcium reduced or abolished MA, even in experiments which left atrial and mean aortic pressure were kept constant by suitably placed open-air reservoirs. Pindolol increased MAA. Raising the height of the atrial constant pressure reservoir induced the MAA, while lowering its height increased the MAA. Changing the height of the aortic constant pressure reservoir did not produce consistent changes in MAA. The effects of SF and SI on the MAA could possibly be explained on the basis of the hypothesis that the MA is a complex oscillatory phenomenon. A different number of myocardial fibers contracting at each systole depending on SF and SI may play a role in the genesis of MA and may explain the present findings. It is concluded that, in addition to hemodynamic and inotropic factors, stimulating the ventricles at a high SI may ameliorate the MA at a given heart rate.