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First published online October 21, 2004
Journal of Experimental Biology 207, 4111-4120 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01279

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Elevated intra-abdominal pressure limits venous return during exercise in Varanus exanthematicus

Suzanne L. Munns^*, Lynn K. Hartzler, Albert F. Bennett and James W. Hicks

Department of Ecology and Evolutionary Biology, University of California, 321 Steinhaus Hall, Irvine, CA 92697, USA

View larger version (22K): [in a new window]   Fig. 1. A representative recording from one varanid lizard (post exercise) demonstrating that costal ventilation caused phasic alterations in post caval blood flow (PC) and intra-abdominal pressure (PIA). A 25.0 ml expiration increased PC by 6.91 ml min-1 and PIA by 2.21 mmHg. A 23.3 ml inspiration decreased PC by 3.61 ml min-1 and PIA by 2.97 mmHg. The effect of respiratory cycle variations in PIA was a net increase in PC, comparable to the respiratory pump in mammals. Note: PC blood flow was pulsatile with a rate that was strongly correlated with heart rate. Gular pumps (*) decrease PIA slightly, but do not alter PC beyond fluctuations due to heart rate variability. Trace taken 10 min after exercise at 30°C in V. exanthematicus (mass 502 g). Scale bar, 15 s. 1 mmHg=133.3 Pa.

View larger version (23K): [in a new window]   Fig. 2. The effects of treadmill exercise on (A) inferior vena cava blood flow (PC), (B) mean arterial blood pressure (a), (C) intra-abdominal pressure (PIA) and (D) heart rate in Varanus exanthematicus. *Indicates significance relative to rest (Dunnett's test, P<0.05). Indicates significance relative to 2.0 km h-1 (Student's paired t-test, P<0.05). Data presented are means ± S.E.M. (N=6). Note: statistical significances are shown only for mean arterial blood pressure for visual clarity; the statistical results for systolic and diastolic blood pressure follow the same trend as those for mean arterial blood pressure. 1 mmHg=133.3 Pa; 1 cmH2O=98.1 Pa.

View larger version (31K): [in a new window]   Fig. 3. The effects of treadmill exercise on (A) minute ventilation (E), (B) tidal volume (TI), (C) breathing frequency (fb), (D) oxygen consumption and carbon dioxide production, in Varanus exanthematicus. *Indicates significance relative to rest (Dunnett's test, P<0.05). Indicates significance relative to 2.0 km h-1 (Student's paired t-test, P<0.05). Data presented are means ± S.E.M. (N=6).

View larger version (26K): [in a new window]   Fig. 4. Post cava blood flow (PC) and mean arterial blood pressure (a) both peak during the recovery period after exercise. Maximal PC occurs at 40 s after exercise ceases, while a peaks at 140 s. Data presented are mean percentage change relative to end exercise ± S.E.M. (N=6).

View larger version (13K): [in a new window]   Fig. 5. A twofold increase in intra-abdominal pressure (PIA) caused a decrease in post caval vein transmural pressure (PTRANS) and a 52% decrease in flow through the post cava vein (PC). Despite inter-animal variation, a threefold increase in PIA significantly decreased PTRANS but did not alter PC. The gray panel indicates negative PTRANS which theoretically correspond to the partial or complete collapse of the post cava vein. The open square represents resting PIA, the black square represents double PIA and the gray square represents triple PIA. The dotted line indicates resting PC. *Indicates a significant decrease in PTRANS and indicates a significant decrease in PC (Bonferroni's corrected paired t-tests, P<0.05). Data presented are means ± S.E.M. (N=5).