Protective zone and the determination of vulnerability to ventricular fibrillation

LOWN. Protective zone and the determination of vulnerabiLity to ventricuZar fibrillation. Am. J. Physiol. 234(5): H592H596, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 3(5): H592-H596, 1978.-Delivery of a train of stimuli across the vulnerable period (VP) is a widely used method for assessing vulnerabil-ity to ventricular fibrillation (VF). We have found that a protective zone (PZ) closely adjoins the VP. When a fibrilla-tory stimulus is followed by a second stimulus that engages the PZ, VF is prevented. We therefore examined whether the VF threshold is altered by a train traversing the PZ. In 53 chloralose-anesthetized dogs, bipolar catheters were placed in the right ventricle for pacing and delivering test stimuli. VF thresholds were determined by scanning the VP with a 5-ms stimulus and a 100.Hz train. The timing of the trains was adjusted to occur at various intervals in relation to the VP