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

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-1 to as low as 1.8 mmol.litre-1. 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-1 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.