Table of Contents
See Also: Biography of Howard Burchell
Early Days of Pacing
Indications for and Results of Implanting Cardiac Pacemakers
Cardiac pacing intrigued Burchell, offering, as it did, a clinical equivalent of the rhythms modification he used in the experimental laboratory. In 1964 he, Connolly and Ellis reported on the first 38 patients who had completely implantable units (Electrodyne and Medtronic) placed at the Mayo Clinic during 1962-1964. Prior to that time (1961-1962) it appears that external generators were used in 2 patients at Mayo, connected to implanted leads, one myocardial and one transvenous. It is remarkable that so early in the experience of cardiac pacing Burchell clearly recognized the hemodynamic benefit of maintaining AV synchrony and the potential of using AV ablation for PAT. He also included, for comparison, a contemporaneous control group of complete heart block patients who did not receive a pacemaker. Interestingly the majority of these admittedly less symptomatic patients did quite well on thiazides or sublingual Isuprel.
Burchell HB, Connolly DC, Ellis, FH. Indications for and Results of Implanting Cardiac Pacemakers. Am J Med, 1964;37:764-777.
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Management of Cardiac Tachyarrhythmias with Cardiac Pacemakers 
By the late 1960s it was known that pacemakers could be therapeutic in cases of ventricular tachycardia. Burchell and Merideth presented clear illustrations of successful overdrive pacing and also discussed the notion of anti-tachycardia pacing using random extra stimuli from a fixed rate mode, non-invasive function changes newly available at that time. Just as carefully portrayed was the inconsistency of such approaches with arguments made for continued growth in the sophistication of pacemaker technology.
Burchell HB, Merideth J. Management of Cardiac Tachyarrhythmias with Cardiac Pacemakers. Ann New York Acad Sci, 1969; 167:546-556. Back to Top
Pre-Excitation: Recognition and Intervention Ventricular Pre-Excitation in the Isolated Perfused Heart 
In 1939 Burchell spent six months at the National Heart Hospital in London under the guidance of Dr (later Sir) John Parkinson, the P in WPW. Thereafter he maintained a strong interest in the hypotheses used to explain WPW ECG patterns. Around 1951, still early days of cardiac catheterization, he made intracardiac recordings in a WPW case as had been reported by his friend Charles Kossmann in 1950. Interestingly, when poring over electrical recordings, he occasionally noticed the WPW phenomenon in his isolated perfused dog heart preparation. Because early septal activation still occurred after the ventricular free walls had been removed, he postulated the existence of a conducting pathway from the atrium to the right side of the septum.
Burchell HB. Ventricular Pre-Excitation in the Isolated Perfused Heart. Am J Physiol, 1952;169:721-725. Back to Top
Atrioventricular and Ventriculoatrial Excitation in Wolff-Parkinson-White Syndrome (Type B) 
For some years Burchell had talked with his friend. Dirk Durrer in Amsterdam, about interrupting the bundle of Kent at surgery. The ideal case to try this on would be a patient who had WPW and was coming to open-heart surgery for some other reason. It was not until 1966 that he and surgeon Dwight McGoon had the opportunity of testing this idea. The 43 year old man had an ASD, paroxysmal tachycardia, and WPW Type B. At operation early activation was found at the base of the right ventricle near the AV groove. Injection of procaine into the base of the right ventricle temporarily abolished the pre- excitation of the ventricle. Burchell HB, Frye RL, Anderson MW, McGoon DC. Atrioventricular and Ventriculoatrial Excitation in Wolff-Parkinson-White Syndrome (Type B): Temporary Ablation at Surgery. Circulation, 1967; 36:663-672 Back to Top
Studies on the Spread of Excitation through the Ventricular Myocardium 
Burchell was introduced to experimental medicine by Arlie Barnes, at the Mayo Clinic, in the late 1930s. Together they studied the ECG effects of chromic catgut-induced localized pericardial injury, comparing it with the changes seen after ligating a coronary artery (Am Heart J 1939, 18: 133-144). He also put together a detailed analysis of the heart's movements seen in ultra high-speed (1,200 frames/sec) cines made by Maurice Visscher and his colleagues at the University of Minnesota (Am Heart J, 1941, 22: 794-803). When he returned to the Mayo Clinic after war service, in 1946, he subjected many of his clinical ideas to testing in the experimental lab. In the realm of electrophysiology his most active interest was in the spread of excitation.
He and his long-time collaborator. Ray Pruitt, wanted to better understand the sequence of activation of the septal and free wall myocardium. In a series of studies published in the then new journal, Circulation, in the early 1950s, they mapped activation delays at multiple sites in the Langendorff-perfused dog heart. By removing sections of the heart or by making judicious incisions they deduced the pathways of impulse propagation.
They found that the mean pathway in the muscular mass of the dog's ventricular septum is from apex to base. The left side of the septum is activated before the right side, and the apical part of the right side is activated before its base. Incisions in either side of the septum usually produced on the homolateral side delays in excitation of the septal surface above the injury as well as below it. The incision on the left side of the septum usually resulted in an increased duration of the P-R interval (Circulation, 1952, 6:161-171).
Burchell HB, Essex HY, Pruitt RD. Studies on the Spread of Excitation through the Ventricular Myocardium: II. The Ventricular Septum. Circulation, 1952, 6:161-171. Back to Top
Action Potentials Supporting the Presence of Specialized Conduction Pathways in the Dog's Ventricle
In a second report, in Circulation Research the next year, and using the same model, Burchell described early potentials of short duration preceding the main muscle excitatory potentials. These were frequently encountered on the upper left surface of the septum and preceded the main muscle potentials by 5-20 milliseconds. He attributed these early potentials to specialized conducting tissues or to activation of bundle branch fibers. Burchell HB, Essex HE, Lambert EH. Action Potentials Supporting the Presence of Specialized Conduction Pathways in the Dog's Ventricle. Research in Progress, 1952; : 186-188.
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The Value of Esophageal Electrocardiogram in the Elucidation of Postinfarction Intraventricular Block
These laboratory investigations naturally led to efforts to better map the sequence of ventricular activation in man. His earliest efforts used an esophageal electrode, an approach he had earlier evaluated for detecting healed posterior infarcts, where information from the standard leads was equivocal (Am J Med Sc, 1948, 216:492-500). Mapping with the esophageal lead was the basis of a 1951 paper where late posterior LV activation was attributed to peri-infarction block complicating prior posterior (inferior) myocardial infarction. Burchell recognized similar, but not identical, findings made earlier by Frank Wilson and S.R. First. Burchell HB, Pruitt RD. The Value of Esophageal Electrocardiogram in the Elucidation of Postinfarction Intraventricular Block. Amer Heart J, 1941; 42:81-87.
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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 Back to Top
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 Back to Top
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. Back to Top
Further Investigations
The Stress ECG in Hypoxemia and Coronary Insufficiency
Burchell and his colleagues, Ray Pruitt and Arlie Barnes, developed a large experience of the first stress test used for the diagnosis of CAD. Proposed as the "Anoxemia Text" by Levy in 1941, it required the patient to breathe a mixture of 10% oxygen and 90% nitrogen for 20 minutes or until diagnostic ECG changes occurred.
By 1947 the Mayo group had performed 730 such tests and, in a number, had additional ventilation and oximetry data. They analyzed the electrocardiographic variations induced and examined the interpretation pitfalls produced by LVH and WPW. They proposed physiologic explanations for many of the complex ECG findings seen and, after clinical follow-up, emphasized the test's lack of sensitivity.
Burchell HB, Pruitt RD, Barnes AR. The Stress and the Electrocardiogram in the Induced Hypoxemia Test for Coronary Insufficiency. Am Heart J, 1948;36:373-389. Back to Top
Circulatory Adjustment to Hypoxemia in Cyanotic Congenital Heart Disease
With the development of cardiac catheterization in the mid-1940s Burchell and his colleagues quickly found many new questions to investigate. One related to what circulatory mechanisms did cyanotic congenital heart disease patients use to increase their oxygen delivery during exercise.
Twenty subjects were exercised on a treadmill while arterial oxygen saturations were obtained by ear oximetry and from arterial line samples. In 4 of them mixed venous samples were also taken from a catheter in the right atrium. Arterial oxygen saturations dropped with exercise, but even so, there was usually still an increase in the A-V oxygen difference. This, together with an increase in cardiac output, provided the necessary oxygen delivery. Variations in exercise tolerance, degree of A-V shunting and anatomic defects limited further conclusions.
Burchell HB, Taylor BE, Knutson JRB, Wood EH. Circulatory Adjustments to the Hypoxemia of Congenital Heart Disease of the Cyanotic Type. Circulation, 1950;1:404-414. Back to Top
Partial Heart Block Treated with an External Pacemaker and Intravenous Isoproterenol after Ventricular Septal Defect Repair
The pump-oxygenator was first used clinically at the Mayo Clinic on March 22nd 1955. The ensuing rush of patients sent for repair of intracardiac defects generated many fresh issues for investigation. For example, when was pre-operative catheterization needed, how reversible was pulmonary hypertension and how should patients be monitored post-operatively? A major issue was the not infrequent development of complete heart block. In his 1957 Henry Jackson Lecture to the New England Cardiovascular Society, Burchell emphasized the catastrophic nature of post-operative complete heart block and the inadequacy of the treatment options then available. As imperfect as it was, in the absence of any realistic alternative, he described the Zoll external pacemaker as a "boon to adequate management".
Burchell HB. Clinical Problems Related to Surgical Repair of Intracardiac Defects with the Aid of an Extracorporeal Pump-Oxygenator. Circulation, 1957;16:976-987. Back to Top
Medical History
Hemopericardium in Britain's King George II Burchell had developed a scholar's love of history in his medical school days in Toronto and he nourished it throughout his career. In a 1942 paper he, and Mayo historian Thomas Keyes, reviewed possible causes for the hemopericardium that, in 1760, ended the life of Britain's George II. He concluded that, for lack of careful documentation at autopsy, no definite conclusion would ever be possible. Burchell HB, Keys TE. Bull Med Library Assn, 1942; 30: 198-202.
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Influence of Sir Thomas Lewis 
When in London in 1939 Burchell met Sir Thomas Lewis and attended several of his clinics. Burchell was impressed by Lewis's belief in clinical science as a distinct and respectable discipline and he subsequently became an outspoken advocate and dedicated practitioner himself. His 1981 editorial on Lewis's impact on American cardiology might be seen as a tribute to an old role-model and influential friend of so many of his lab-based compatriots. Burchell H. Brit Heart J, 1981; 40: 1-4
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Digitalis Poisoning
Burchell was a early Member of the American Osler Society and reveled in the learned dissertations that Society fostered, such as his 1983 critical review of digitalis poisoning. Reflecting Burchell's wide knowledge of literature this report includes many references to the homicidal use of digitalis in forensics and in the fiction of Mary Webb, Dorothy Sayers and Agatha Christie.
Burchell HB. J Amer Coll Cardiol, 1983; 1: 506-516.
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