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The technique of recording electrograms of the His bundle, using a tripolar electrode catheter positioned across the tricuspid valve, was applied in a physiological study of atrioventricular conduction in man. Increasing the heart rate to 160 beats/min by right atrial pacing produced progressive prolongation of the P-H interval while the H-Q interval remained constant. At any given paced heart rate, digitalis caused a prolongation of the P-H interval. Isoproterenol and atropine markedly shortened the P-H interval at any given heart rate. Neither drug had any significant effect on the H-Q interval.

The Boston Marathon has been run for 115 years during which there were three sudden cardiac arrests. The most recent was a near death avoided by rapid cardiopulmonary resuscitation (CPR) and defibrillation. Awareness of the dangers of participating in a marathon, the risk factors associated with sudden death during competition, and the life-saving importance of rapid CPR and defibrillation are essential for participants and event organizers. Available records and reports of the three known cases of cardiac arrest during the Boston Marathon were examined. These cases were identified by representatives of the Boston Athletic Association, which has organized each marathon since its inception. Pertinent literature was reviewed and new information was obtained during interviews of witnesses and rescuers. The data were analyzed in search of shared risk factors for cardiac arrest, death, and the optimal requirements for survival. In 115 years, there were two cardiac deaths and one near death from cardiac arrest. A history of coronary artery disease, advanced age, and prolonged race time are risk factors for sudden cardiac arrest. Rapid application of CPR and defibrillation are essential for survival. Prevention or reduction of life-threatening cardiac incidents during marathon races might be achieved if participants of advanced age or with a history of coronary artery disease seek medical clearance prior to entering an event. Those with coronary risk factors should have a discussion with their physician. Availability of trained personnel and defibrillators are important considerations in marathon planning.