Objective: This study was designed (1) to investigate the effects of normothermic and hypothermic perfusion on the median frequency of the fibrillating myocardium, and (2) to elucidate whether frequency-guided countershock therapy improves countershock success during the reperfusion phase of cardiac surgery.
Design: Prospective, randomized study.
Setting: University hospital cardiac surgery room.
Patients: Thirty patients (first part of the study) and 38 patients (second part of the study) scheduled for elective coronary artery bypass surgery.
Methods and results: During cardiopulmonary bypass, ventricular fibrillation (VF) was induced at a core body temperature of 34.1±0.2°C (normothermia) (n=15) or at a core body temperature of 29.8±0.2°C (hypothermia) (n=15). Using fast Fourier transformation of the ECG signal, median fibrillation frequency was recorded continuously for a period of 120 s. At the end of surgery, countershock was performed as soon as VF was recognized on the ECG monitor (X Hz group; n=19) or countershock was not performed until median fibrillation frequency had increased to the threshold of at least 5 Hz (5 Hz group; n=19). Median fibrillation frequency in the normothermic fibrillation group was statistically higher than in the hypothermic group. In the X Hz and 5 Hz countershock group, median fibrillation frequency before the first countershock attempt was 3.6±0.2 Hz and 5.4±0.1 Hz (p<0.0001), respectively. In the X Hz group, six countershocks resulted in supraventricular rhythm, 10 in VF, two in electromechanical dissociation, and one in asystole. In the 5 Hz group, 16 countershocks resulted in supraventricular rhythm, two in VF, and one in asystole (p=0.008).
Conclusions: During normothermia, median fibrillation frequency is significantly higher than during hypothermic perfusion conditions. During the reperfusion phase of cardiac surgery, countershock success rate is significantly higher when a threshold of at least 5 Hz had been reached before the first countershock attempt.