The pacemaker story: a cold heart spinoff.

It is 1949. Room 64 in the hasement of the Banting Institute in Toronto was the home of the fledgling cardiovascular surgical lahoratory. Experiments on the microcirculation and on hypothermia were initiated as two different projects. We were astonished to find as work progressed, that they were closely interrelated: Cooling an animal helow a certain body temperature level produced serious obstructions to flow in the capillary circulation. Furthermore, the excitement of work in hypothermia was overshadowed by a great cloud of frustration; When the laboratory animal was cooled below 24°C, the heart would arrest— and yet infant animals could be safely cooled and resuscitated from body temperatures near freezing [5°C). We were in a hurry to solve these problems so that we could study deep hypothermia, which we hoped would allow direct vision correction of heart defects. Why did the cold heart suddenly stop? We pondered, discussed, and researched the subject, but three years passed without an answer. One morning, a standard experiment was planned. As I entered the laboratory, an anesthetized dog was already being cooled in refrigeration blankets with ice bags. At a body temperature of 22°C, with regular heart action and adequate blood pressure, the cooling was discontinued, the top blanket removed, and the chest opened. Cardiac arrest was not expected to occur until 20°C. so the pericardium was opened. We were now ready to make certain physiologic observations regarding hypothermia, after which the pericardium and chest would be closed and the animal rewarmed. Just as we were about to begin these tests, the