The sources of error in determination of the beginning of QRS and the end of T during measurement of the Q-T duration are analyzed. An important error is confusion of an elevated U wave with the T wave, resulting in the diagnosis of a prolonged Q-T. In such cases, some of the precordial leads usually show a notch or kink between T and U which indicates approximately the end of T. If these criteria are used, the true corrected Q-T duration in hypopotassemia without hypocalcemia is not prolonged, but normal or shortened, corresponding to an earlier appearance of the second heart sound.

The action potentials recorded from heart muscle with a suction electrode have been compared to those recorded with an intracellular microelectrode. It has been found that if the suction electrode is properly used the monophasic potentials recorded with it may be taken as a reliable index of the time of arrival of excitation at the electrode and as a reliable index of the shape of the action potential during the entire phase of repolarization. The suction electrode potentials differ from the microelectrode potentials in showing a lower rise velocity, a smaller amplitude, a quantitatively different reversal or overshoot and, in the beating heart, ‘afterpotentials’ caused by mechanical effects. When the shape of the action potential, as observed with the microelectrode, is changed by ions such as K+ or Ca++ a similar change is observed in the potential recorded with the suction electrodes.

Myocardial cells isolated from 8-day chick embryos were grown in monolayer culture under conditions that produce "standard embryonic" and "adult-type" cells. These cells were subjected to electric field stimulation that had a waveshape and intensities similar to those used in clinical electric countershock procedures. Photocell mechanograms obtained before, during, and after stimulation were correlated with simultaneously measured transmembrane potentials to determine the relationship between membrane polarization and arrhythmia production that occured after the stimulus. The results of these experiments demonstrate that a predictable sequence of mechanical responses occurs after stimuli ranging in intensity from 6 to 200 V/cm. This sequence, which closely resembles that observed in vivo after similar stimulation intensities, consists of a single response (activation), tachyarrhythmia, relaxed arrest followed by transient tachyarrhythmia, arrest with contracture, and cellular fibrillation. This diverse pattern of arrhythmias is associated with a prolonged depolarization of the cell membrane which increases with the intensity of the applied stimulus. It is probable that this depolarization is caused by a transient electromechanical deformation of the cell membrane during the shock. These findings contribute to a better understanding of the causes of the arrhythmias that appear after clinical and experimental electric countershock procedures.

In some cases of complete A-V block the curve of the duration of P-P intervals showed a sudden dip with a gradual return, during or after the T wave. This positive chronotropic (accelerating) effect of ventricular systole is probably caused by the traction exerted on the right auricle by the contracting ventricle. Other cases showed a sudden rise in the curve of P-P intervals late in diastole,followed by a gradual return. This negative chronotropic (slowing) effect is probably caused by a vagal reflex precipitated by stimulation of the arterial pressoreceptors by the pulse wave. The interplay of these two effects determines whether the P-P intervals containing QRS will be shorter than those not containing it, or whether the relation is reversed (paradox effect).

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Isolated rabbit hearts were perfused with Krebs-Henseleit solution deficient in K, Ca or both. Ventricular monophasic action potentials registered with the suction electrode, electrocardiograms and intraventricular pressures were recorded simultaneously for periods up to 20 minutes. Perfusion with K-deficient solutions resulted in a brief initial prolongation of the entire descending limb of the action potential and of the T wave, followed by progressive increase in slope of the initial portion and decrease in slope of the terminal portion of the action potential. During this time the T wave became shorter and a U-wave-like deflection occupying the entire diastole developed. QRS showed progressive prolongation. Finally, A-V conduction disturbances, ectopic beats and ventricular fibrillation developed. The force of contraction increased slightly. Perfusion with Ca-deficient solutions decreased the slope of the initial portion while increasing the slope of the terminal portion of the action potential. This was accompanied by prolongation of the ST- segment and reversal in the direction of the T wave. The duration of the action potential reached a maximum after 40–80 seconds of perfusion, while the force of contraction decreased progessively. Perfusion with solutions without potassium and calcium prolonged the initial as well as terminal portion of the action potential.

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