Kay Millar

The sequence of normal ventricular recovery

Abstract Functional refractory periods (FRPs) were measured at epicardial, intramural, and septal sites in pentobarbital anesthetized dogs. The sinus node was crushed and the atria were driven at a fixed rate. Activation times at the test sites were measured from electrograms recorded from closely spaced bipolar electrodes. The test stimuli were cathodal “make” stimuli delivered to one pole of the pairs of electrodes. FRPs at the base of the free wall of the left ventricle and of the septum were shorter than FRPs at the apex. FRPs on the epicardium were shorter than those on the endocardium, and FRPs on the right side of the septum were shorter than those on the left side of the septum. The findings indicate that normal ventricular recovery properties are systematically distributed and inversely related to activation sequence. Areas of the ventricle activated early have the longest FRPs, and areas activated late have the shortest FRPs. This distribution of recovery properties tends to make all portions of the ventricles complete recovery at about the same time, and may play a protective role in the prevention of reentrant arrhythmias. This distribution of recovery properties is also applicable to an explanation of the configuration of normal T waves.

Diagnosis of old inferior myocardial infarction by body surface isopotential mapping

Body surface isopotential maps obtained from 28 patients with old inferior wall myocardial infarction were compared with maps from 120 normal subjects. The 12 lead electrocardiogram of 8 of the 28 patients (29 percent) with inferior wall infarction was normal or showed only nondiagnostic ST-T wave abnormalities at the time the isopotential maps were obtained. In all patients with inferior wall infarction the isopotential map showed a minimum (area of negative potentials) on the inferior or right thoracic surface during the early portions of the QRS complex. This finding was observed in patients with normal or nonspecific abnormalities in the 12 lead electrocardiogram as well as those with QRS abnormalities. By contrast, the minimum during the early QRS complex in normal subjects was located on the right upper back and shoulder region...

Cancellation of electrocardiographic effects during ventricular recovery

Summary Theoretical estimates of the degree to which ventricular recovery is not expressed in the ECG (cancellation) have been made. These estimates were based on T waves derived from available data on excitation sequence, ventricular functional refractory periods, and a recently developed T wave model. The percent cancellation during ventricular recovery for a limited number of recovery patterns was estimated to be 92% to 99%. These estimates provide insights into the lability of T waves, and provide an explanation of the apparently paradoxical occurrence of QT prolongation in states such as ischemia and increased adrenergic tone that are known to shorten ventricular refractory period.

Time course of vulnerability to fibrillation after experimental coronary occlusion

Abstract The early time course of vulnerability of the ventricles to fibrillation after coronary artery occlusion was measured in 6 dogs. The duration of a train of 60 Hz stimuli with currents of 2 to 3 times the diastolic threshold was used as the measure of vulnerability to fibrillation. Within 2 minutes of coronary occlusion the duration of the train of 60 Hz stimuli inducing fibrillation fell by an average of 25 percent of control values. Five minutes after coronary occlusion the duration of the train was an average of 34 percent shorter than control values. In all animals, the duration of the train required to produce fibrillation began to increase 30 minutes after coronary occlusion and approached control values. The relation of these findings to the time course of arrhythmias in experimental infarction and the early high mortality rate in cases of infarction in man is discussed.

Accelerated repolarization as a factor in re-entry—stimulation of the electrophysiology of acute myocardial infarction

Abstract A laboratory animal model empolying thermal lesions was developed to demonstrate that re-entrant activity made possible by prolongation of conduction time could be enhanced by shortening of recovery properties. Cooling was used to prolong conduction time and warming to shorten recovery time. The cardiac cycle was scanned with premature stimuli of three times diastolic threshold intensity. Re-entrant activity could not be elicited in the presence of shortened recovery times alone. However, abbreviation of recovery time enhanced re-entrant activity, permitting re-entrant beats to occur during a greater period of the cardiac cycle, and to occur at cold temperatures that, when present alone were not associated with re-entry. During some re-entrant beats, right preceded left ventricular activation despite evidence that re-entrant activity originated in the left ventricle in the area of the cold-spot. This was an unexpected finding and suggested that the Purkinje net played a role in the re-entrant loop. The findings of this study indicate that, while prolongation of conduction time is necessary for re-entrant activity, it is enhanced by shortening of recovery times. Both of these electrophysiologic features are present in the early phases of acute myocardial infarction and may be a factor in the very high incidence of ventricular ectopic activity at that time

Characteristics of ventricular recovery as defined by the vectorcardiographic T loop

Abstract The normal T loop of the vectorcardiogram is inscribed in the same direction as the QRS loop with which it is associated. By separate consideration of the T loop components that are secondary to the activation sequence and primary components that depend on intrinsic ventricular recovery properties, new insights concerning the recovery properties have been obtained from the direction of the T loop inscription. The graphic studies presented in this report indicate that normal ventricular recovery properties must be so organized that they would yield a primary T loop inscribed in the same direction as the actual QRS and T loops. An organization of recovery properties in layers with geometry similar to that of activation boundaries would meet this requirement and, in view of other established features of recovery, is the most likely organization of these properties.

Observations concerning the validity of the ventricular gradient concept

Abstract The QRS, ST-T, and combined QRS-T area in multiple bipolar chest leads was measured during normal and abnormal activation in a patient with intermittent left bundle branch block. QRS and ST-T areas were plotted as functions of angular lead axis and compared with sine waves which would be the expected form of such plots with a single central dipole in a circular homogeneous conducting medium. Total QRS-T areas during normal and abnormal activation were most nearly alike in leads in which QRS and ST-T areas during both activation states corresponded to sine waves. These results suggest that the gradient concept of QRS-T area independent of activation order is applicable to electrocardiographic leads only when these are equally sensitive to activation and recovery patterns and that it is applicable to single leads.

The geometry of ventricular repolarization boundaries

Abstract The effects of altered recovery time in areas of controlled geometry on the T-wave form were studied. The pericardial sacs of 6 dogs were irrigated with saline at various temperatures in the range of 20 to 45° C. T-wave form was dependent on the geometry of recovery boundaries prior to alteration of recovery times and the geometry of the area of altered recovery. Since the latter was controlled, the findings permitted some conclusions about the former. In all experiments the termini of mean T vectors fell along a straight line and their positions were systematically related to temperature. This indicated that the area of altered recovery times remained constant during the course of each experiment. In addition, the T vectors fell along the same axis during the course of 5 of the 6 experiments. This indicated that the geometry of the area of altered recovery times was the same as repolarization boundary geometry prior to pericardial irrigation. The one exception may represent a normal variant, an abnormality of repolarization, or may be due to loculation of pericardial fluid. The results indicated that recovery boundaries are sufficiently near parallel to the cardiac surface to account for T-wave form.