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The harmful effect of tachycardia in patients with cardiac disease is well recognized. A sustained increase in rate is not infrequently an important precipitating cause of a break in compensation. Occasionally one observes symptoms of congestive failure occurring during an attack of paroxysmal auricular tachycardia in a person whose heart is normal in every other respect. Such a case has been reported by Barcroft, Bock and Roughton. 1 However, some animals normally have heart rates which are considerably faster than those of patients with paroxysmal tachycardia. Why is a heart rate of 200 per minute likely to be attended by grave symptoms in men while the resting heart rates of guinea-pigs are more than 220? Again, why is it that the heart beats 600 times per minute in the mouse and 40 per minute in the horse (Clark 2 )? If the optimum (i.e., the normal) heart rate is different in
Tinsley R. Harrison (Fri,) studied this question.