Heart rate and electrocardiogram characteristics of a young California gray whale (Eschrichtius robustus)

Electrocardiogram (ECG) analyses of Holter monitor recordings from a young California gray whale were performed to determine ECG waveform characterist ics, evaluate the heart rate pattern for sinus arrhythmia, obtain resting h eart rates at known body masses as the whale increased in size, and compare those heart rates with predicted heart rates from allometric equations. Th e PR and QRS intervals (475 +/- 35 msec, 208 +/- 24 msec, respectively, n = 20) support the concept (Meijler et al. 1992) that atrioventricular transm ission and ventricular excitation times do not increase linearly in very la rge mammals. A sinus arrhythmia pattern at rest (apneic heart rates of 15-2 5 beats per min [bpm] and eupneic heart rates of 34-40 bpm) is consistent w ith a relative eupneic tachycardia and apneic bradycardia during diving act ivity of whales. The heart rate-body mass measurements (35-24 bpm at body m asses of 3,531-8,200 kg) in this study (1) extend the range of allometric h eart rate and body mass data in mammals a full order of magnitude, to almos t 10,000 kg, (2) support the use of allometric equations (based primarily o n mammals <1,000 kg in body mass) in estimating resting heart rates in whal es, and (3) demonstrate that previously reported heart rates in large whale s are not representative of resting heart rate, probably secondary to circu mstances during measurement.