Perspectives on physiological monitoring: junctional-type potentials in the food ventricle

1. Many toads monitored throughout survival with no support other than protection against drying, pass terminally through a remarkable evolution which is described here in the full details of a single experiment lasting some 40 hours. 2. The essential features of this particular sequence is block of the Luciani-Wenckebach type affecting SA, AV, and intraventricular conduction. SA block was apparently the major cause of periods of arrest and of cycles of heart beats. Periodically PR delay based on progressive AV block was observed but it was not an outstanding feature. 3. Progressive, rate-determined intraventricular block during the cycles of ventricular beats was the first new feature of these observations. 4. As intraventricular block progressed, an initial ventricular deflection separated itself from the rest of QRS. 5. This initial deflection diminished in amplitude throughout each cycle of ventricular beats, its rate of rise diminished, and the interval separating it from the rest of the ventricular complex increased until the whole initial deflection was revealed. 6. Thereafter, with a small decrease in amplitude of the initial deflection, the remainder of the ventricular electrogram failed to follow and the complex stood alone. 7. Its polarity indicated its origin at the base of the ventricle, the interval separating it from the origin of P indicated that it was downstream from the AV conduction mechanism. 8. This deflection, now a local ventricular potential (LVP) then progressively declined in amplitude and disappeared. 9. The possibility has been discussed that the potential represents (a) a true action potential localized by block or (b) a local, nonpropagated potential akin to junctional potentials like: (1) end-plate potentials, (2) generator potentials, (3) excitatory postsynaptic potentials (EPSPs), or (c) a pacemaker potential. The experiments that have revealed the phenomenon have not provided other than suggestive but inconclusive information about its nature. 10. The observations are new or certainly not well known and further study should shed light on the problem of intracardial impulse formation and conduction.