Ventricular fibrillation thresholds in Langendorff perfused rabbit hearts: all or none effect of low potassium concentration

SUMMARY The median single pulse ventricular fibrillation threshold of 56 determinations in eight isolated rabbit hearts perfused with a Krebs-Henseleit solution was 36 mA, with a range from 12.5 to above 50 mA. The fibrillation threshold remained in the same range when the potassium concentration in the perfusing fluid was decreased from its normal 5.6 mmol.litre- ' to as low as 1.8 mmol.litre-'. A further decrease in potassium concentration reduced the fibrillation threshold to a median value of 1.6 mA in nine determinations on six hearts. With a 1.6 mmol.litre- ' potassium concentration, thresholds were in either the high or the low range depending on the individual heart. A pronounced shortening of refractory period and an earlier vulnerable period were associated with the sharp change in fibrillation threshold; the diastolic extrasystolic threshold, cycle length, and RT interval duration increased in a more linear manner as the potassium concentration was lowered. Ventricular fibrillation is generally held to be more readily precipitated under hypokalic conditions. MacConaill and Murnaghan4 and Murnaghan ' have shown that in isolated rabbit hearts perfused with McEwen's solution,6 alterations in potassium concentration in the perfusing solution could produce changes in the single pulse electrical ventricular fibrillation threshold, raised potassium concentrations increasing this threshold and lowered potassium concentrations decreasing it. I therefore studied the range of potassium concentrations over which a graded change in fibrillation threshold could be produced in a given heart, at the same time replacing the hypertonic McEwen's solution with a more physiologically constituted modified Krebs-Henseleit solution in the hope of lessening the drift in fibrillation thresholds that had been seen in many of the earlier experiment^.