Ventricular Outflow Dynamics in the Lizard, Varanus Niloticus: Responses to Hypoxia, Hypercarbia and Diving

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Abstract
Blood flow and blood pressure have been measured in the right aorta and left pulmonary artery of the semi-aquatic lizard, Varanus niloticus, during normal breathing, during hypercarbic and hypoxic breathing and during voluntary diving. Mean pulmonary blood pressure during normal breathing was 19 · 5 ± 2 · 0 cmH2O while right aortic pressure was 118 · 0± 3·0 cmH2O. The high systemic blood pressure and high ratio of systemic to pulmonary vascular resistance (4·0−6·0) stand out among reptiles and approach values in homeotherm vertebrates. Pulmonary pressure rise preceded right aortic pressure rise by 120 msec at a heart rate of 25/min. Pulmonary ejection lasted 50% of the cardiac cycle compared to 25 % for aortic ejection during normal breathing. CO2 breathing increased right aortic vascular resistance by 120% while pulmonary resistance increased moderately by 30%. Carotid vascular resistance decreased during CO2 breathing. The pulmonary blood pressure increase was however much higher than the systemic, but at no time did systemic and pulmonary blood pressures overlap. Hypoxic breathing increased pulmonary blood pressure to 36·0 ± 4·0 cmH2O while right aortic pressure fell to 100·0± 10·0 cmH2O. Ejection time remained unchanged in the right aorta while pulmonary flow became continuous. Overall pulmonary vascular resistance increased markedly while systemic resistance changed little. Voluntary diving increased pulmonary blood pressure while the systemic blood pressure fell markedly. The results obtained are discussed in the light of ventricular outflow distribution in reptiles. Directional shunting of blood inside the heart of V. niloticus during cardiac systole must be reduced or absent. Intracardiac shunting during cardiac filling or by systolic residual volumes is small, placing varanid lacertilians haemodynamically closer to homoetherm vertebrates than other reptiles studied.