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P ROLONGED OPERATIONS in the open heart at normal body temperature require heart-lung machines as a substitute for cardiopulmonary function. The principle of all these is the same: they withdraw blood from the venae cavae, oxygenate it, and return it into the aorta. Thus the patient's heart is completely bypassed (Fig. 1). It was believed for a long time that the machine would have to pump and oxygenate blood in amounts equal to the normal resting cardiac output-at least 100 ml. per kg. of body weight per minute. Accordingly, several elaborate machines have been devised to pump and oxygenate 5 liters of blood per minute, for example by Dennis, 1 Jongbloed, 2 and Kolffand Dubbelman. 3 ' 4 The most successful design was that of Miller, Gibbon, and Gibbon. 5 A similar machine is in use at the Mayo Clinic 6-7 with outstanding success; its complexity and cost have prevented its wider use.

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