Left Ventricular Function During Epinephrine Stimulation and Hypothermia: Effects at Spontaneous and Paced Heart Rates in a Porcine Model

Abstract

Postcardiac arrest patients treated with hypothermia, frequently require vasopressors and inotropic medication. The aim of this experimental study was to investigate the effect of epinephrine on left ventricular (LV) function during hypothermia. In an open-chest porcine model, seven animals were equipped with LV micromanometer and epicardial ultrasound transducers to provide LV pressure, Tau, and wall thickness and thickening velocities in systole (S') and early diastole (e'). Arterial, central venous, and pulmonal artery pressures were recorded. Cardiac output (CO) was measured by transit-time flow probe on the ascending aorta. Hypothermia was induced using a cooling catheter through the femoral vein. Pacemaker leads were attached to the right atrium for pacing. LV volumes were obtained by two-dimensional echocardiography. Measurements were made at normothermia (38°C) and hypothermia (33°C), without and with epinephrine infusion (0.03 μg/kg/min), at spontaneous and paced heart rates (HRs) 120 and 140 beats/min. Hypothermia reduced LV stroke volume (SV). Epinephrine during hypothermia increased the SV with reduced end-systolic volumes. LV dP/dt_max and wall-thickening velocity increased. During normothermia, epinephrine increased CO mainly due to accelerated HR, but during hypothermia, the increased CO resulted from augmented SV and, to a lesser degree, elevated HR. The incomplete relaxation and shortened diastolic filling time and the following reduction in SV seen in hypothermic animals, was repealed by epinephrine. The CO remained elevated also due to a shortened systolic duration, which gave time for complete relaxation during higher HRs. Epinephrine infusion improved systolic and diastolic function during hypothermia, and thereby reversed the effects induced by hypothermia considerably. Epinephrine augmented CO at hypothermia through increases in both SV and HR, in contrast to a mainly HR-dependent effect during normothermia. Systolic duration was shortened, which gave sufficient diastolic duration for complete relaxation. This allowed diastolic filling and maintained CO at elevated HRs.

Keywords: cardiac arrest; epinephrine; hypothermia.