Maximum heart rate does not limit cardiac output at rest or during exercise in the American alligator ( Alligator mississippiensis)

In most vertebrates, increases in cardiac output result from increases in heart rate (f ) with little or no change in stroke volume (V ), and maximum cardiac output (Q̇) is typically attained at or close to maximum f . We therefore tested the hypothesis that increasing maximum f may increase maximum Q̇. To this end, we investigated the effects of elevating f with right atrial pacing on Q̇ in the American alligator ( Alligator mississippiensis) at rest and while swimming. During normal swimming, Q̇ increased entirely by virtue of a tachycardia (29 ± 1 to 40 ± 3 beats/min), whereas V remained stable. In both resting and swimming alligators, increasing f with right atrial pacing resulted in a parallel decline in V that resulted in an unchanged cardiac output. In swimming animals, this reciprocal relationship extended to supraphysiological f (up to ~72 beats/min), which suggests that maximum f does not limit maximum cardiac output and that f changes are secondary to the peripheral factors (for example vascular capacitance) that determine venous return at rest and during exercise.