Protective zone and the determination of vulnerability to ventricular fibrillation
Verrier, R.L.; Brooks, W.W.; Lown, B.
American Journal of Physiology 234(5): H592-H596
1978
ISSN/ISBN: 0002-9513 PMID: 645925 Document Number: 135896
Delivery of a train of stimuli across the vulnerable period (VP) is a widely used method for assessing vulnerability to ventricular fibrillation (VF). A protective zone (PZ) closely adjoins the VP. When a fibrillatory stimulus is followed by a second stimulus that engages the PZ, VF is prevented. Whether the VF threshold is altered by a train traversing the PZ was examined. 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 nadir. When the train terminated at the nadir, the VF threshold was nearly identical to that obtained with a single stimulus. When the train originated before the nadir and transversed it, the VF threshold was reduced by 35%. When the train originated at the nadir and activated the PZ the VF threshold was increased by 37%. In 6 dogs, the VF threshold determined by a single stimulus was increased by 35% when a train was delivered following the fibrillatory stimulus. A train of stimuli can activate the PZ and thereby alter the measurement of cardiac vulnerability.