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Thanks Ken.
Those PR intervals seem to be in line with the
dimensions of the hearts mentioned and known conduction rates of the specialized
cells of the heart that transmit. Cardiac output is equal to heart rate
times stroke volume. Stoke volume is determined by the
Frank-Starling mechanism. I would assume sauropods had hearts large
enough to sustain CO over a wide range of activities. Otherwise they would
not have been successful. Mechanisms for regulating CO would have been,
well....dinosaurian, but likely much the same for all erect animals in a
non-equatic environment.
Thanks,
Michael
----- Original Message -----
Sent: Thursday, July 22, 1999 5:41
PM
Subject: Re: sauropod necks again
tons wrote: What
is the upper limits of normal for the PR interval in a giraffe or large whale?
Best, Michael Teuton
Michael: Thanks for an
interesting discussion. I don't have data on the upper limits of the PR
intervals, but I do have some readings which I assume indicate rough
averages. The resting heart rate of an anesthetized giraffe is 70 beats
per minute with a PR interval of 0.18 sec. Some lost soul actually did
an EKG on a humpback whale (trapped in a net on the ocean's surface), 10
meters long with a weight of 30,000kg, and whose estimated heart weight was
150 to 180kg. The heart rate was 30 to 35 beats/min and the PR interval
did not exceed 0.4 sec. For those who don't know what a PR interval is,
it is a measurement found on EKGs that represents the time it takes for the
electrical impulse generated by the heart's pacemaker (sinoatrial node) to
travel through the atria, into the atrioventricular node, and down the
vetricular conduction system into the muscle (myocardium) of the
ventricles. In humans it ranges from 0.14 to 0.22 sec.--Ken Clay,
M.D.
REFERENCES
1. Rossof A.H. An electrocardiographic study
of the giraffe. Am Heart J 1972 Jan;83(1):142-3.
2. Meijler F.L. et al.
Electrocardiogram of the humpback whale J Am Coll Cardiol
1992;20:475-9.
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