AJP - Regulatory, Integrative and Comparative Physiology, Vol 262, Issue 6 986-R992, Copyright © 1992 by American Physiological Society
Mechanism of intracardiac shunting in the turtle Pseudemys scripta
J. W. Hicks and G. M. Malvin
Oxygen Transport Program, Lovelace Medical Foundation, Albuquerque, New Mexico 87108.
Two principal hypotheses account for right-left (R-L) intracardiac shunting
in reptiles. The "pressure shunting" hypothesis proposes no functional
separation between the ventricular cava during systole. The "washout
shunting" hypothesis suggests that during systole, the cavum pulmonale (CP)
is functionally separate from the rest of the ventricle. The purpose of
this study was to test these hypotheses during control, after acetylcholine
(ACh) administration, after epinephrine (Epi) administration, and during
apnea. Anesthetized (pentobarbital) turtles (Pseudemys scripta) were
mechanically ventilated and three nonocclusive catheters were implanted
into the right atrium (RAt), left atrium (LAt), and CP. In addition, three
blood gas catheters connected to a mass spectrometer were implanted into
the RAt, LAt, and right or left aortic arch for measurement of PO2. A
tracer gas, He dissolved in saline, was sequentially injected into the
three cardiac chambers and was detected by the mass spectrometer. The
presence of R-L shunting was assessed with the blood PO2 and PHe
measurements. ACh produced R-L shunting in all animals. No R-L shunting
occurred after Epi administration and 20 min of apnea. In all animals after
ACh administration, He was detected in an aorta after He infusion in the
RAt and LAt, but never after infusion of He into the CP. These results from
this preparation are inconsistent with the pressure hypothesis and support
the washout hypothesis of intracardiac shunting.
