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.