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Early Days of Pacing Indications for and Results of Implanting Cardiac Pacemakers Management of Cardiac Tachyarrhythmias with Cardiac Pacemakers Pre-Excitation: Recognition and Intervention Atrioventricular and Ventriculoatrial Excitation in Wolff-Parkinson-White Syndrome (Type B) Studies on the Spread of Excitation through the Ventricular Myocardium Action Potentials Supporting the Presence of Specialized Conduction Pathways in the Dog's Ventricle The Value of Esophageal Electrocardiogram in the Elucidation of Postinfarction Intraventricular Block Clinical Electrocardiography Observations on Additional Instances of a Supernormal Phase in the Human Heart Sino-Auricular Block, Interference Dissociation, and Different Recovery Rates of Excitation in the Bundle Branches Analogy of Electronic Pacemaker and Ventricular Parasystole with Observations on Refractory Period, Supernormal Phase, and Synchronization Vectorcardiography Experiments with AV Cushion Defects Further Investigations The Stress ECG in Hypoxemia and Coronary Insufficiency Circulatory Adjustment to Hypoxemia in Cyanotic Congenital Heart Disease Partial Heart Block Treated with an External Pacemaker and Intravenous Isoproterenol after Ventricular Septal Defect Repair Medical History Hemopericardium in Britain's King George II Influence of Sir Thomas Lewis Digitalis Poisoning

See Also: Biography of Howard Burchell

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Clinical Electrocardiography

Observations on Additional Instances of a Supernormal Phase in the Human Heart

During his clinical work Burchell looked for human illustrations of the phenomena that he and others saw in the animal lab. He would devote considerable attention to individual cases, teasing out whatever lessons they could give.

In 1942 he reported 2 cases of interference dissociation, outlining the interval, during post-excitation recovery, of supernormal conduction where interference occurred. The first case was one of high-grade AV block and the interval during which conduction occurred is illustrated in the accompanying figure.

Burchell HB. Observations on Additional Instances of a Supernormal Phase in the Human Heart. J Lab Clin Med, 1942;28:7-11

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Sino-Auricular Block, Interference Dissociation, and Different Recovery Rates of Excitation in the Bundle Branches

He extended these clinical observations in a 1949 case report of a 60 year-old woman with intermittent SA block and interference dissociation. There was bundle branch block present in the majority of the interference beats. The BBB was either right or left, dependent on the time relationships of the auricular beat to be conducted and the preceding R wave of the idioventricular rhythm. He postulated that the ECG findings could be explained by different recovery rates in excitation of the two bundle branches.

Burchell HB. Sino-Auricular Block, Interference Dissociation, and Different Recovery Rates of Excitation in the Bundle Branches. Brit Heart J, 1949;11:230-236

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Analogy of Electronic Pacemaker and Ventricular Parasystole with Observations on Refractory Period, Supernormal Phase, and Synchronization

Naturally Burchell was quickly attracted to the natural experiment afforded by the artificial pacemaker. The early fixed rate units mimicked parasystole and permitted detailed observations on refractory period, supernormal phase and synchronization. In a 1963 paper he elaborated on these findings using ECGs from 3 cases who had epicardial pacemakers implanted at the Mayo Clinic in April and May of 1962.

Characteristic of his diligence he squeezed as much as he could out of these 3 cases, describing retrograde conduction in one and the effect, or lack of, of reserpine withdrawal on refractory periods, as well as accrochage, in another.

The accompanying figure relates to one case where occasional AV conduction occurred in a patient with high-grade AV block. The interval where interpolated beats occurred identified a zone of 'supernormal' conduction.

Burchell HB. Analogy of Electronic Pacemaker and Ventricular Parasystole with Observations on Refractory Period, Supernormal Phase, and Synchronization Circulation, 1963;27: 878-889

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Vectorcardiography

Experiments with AV Cushion Defects

Another natural experiment investigated by Burchell was that provided by congenital AV cushion defects. He, DuShane and Brandenburg hypothesized that the basic VCG abnormality in these cases was related to an anomolous left bundle branch system, producing left axis deviation (LAD) with an early frontal plane vector in the 0 to -60 degree zone. The rSR in the right precordial leads represented right ventricular volume overloading.

Using a bipolar exploring electrode during open heart surgery they found a delay in activation near the anterior interventricular groove consistent with their theory that there was a congenital absence of a portion of the left bundle branch system.

Similarly he and Tuna investigated VCG loops of 94 patients with gross LAD who had been fully investigated at the University of Minnesota (Eur J Cardiol, 1979;10:259-277). So diverse was the variation in voltage and direction of the initial and maximal vectors that no reliable criteria could distinguish between those who had established heart disease and those who did not. A further theory, that some VPBs in such cases were a product of reentry, could not be supported by analysis of early VPB vectors (Eur J Cardiol, 1976;4:71-78).

Burchell HB, DuShane JW, Brandenburg RO. The Electrocardiogram of Patients with Atrioventricular Cushion Defects (Defects of the Atrioventricular Canal). Am J Cardiol, 1960;6:575-588.

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