Printed in Great Britain VENTRICULAR HAEMODYNAMICS IN THE MONITOR LIZARD VARANUS EX ANTHEMATICUS: PULMONARY AND SYSTEMIC PRESSURE SEPARATION

The haemodynamics of the anatomically undivided ventricle of the monitor lizard Varanus exanthematicus have been examined by measurements of blood pressure and flow. Central blood POi, PGOt and pH were also measured. Intracardiac pressure measurements show the ventricle to be functionally divided throughout systole into a high pressure pump (cavum arteriosum: 'mean ' pressure 89 cm H2O) perfusing the systemic circulation, and a low pressure pump (cavum pulmonale: 'mean ' pressure 40 cm H2O) perfusing the pulmonary circulation. Hypoxia produced by asphyxia or N2 breathing changed systolic pressures in the ventricular cava, but never resulted in superimposable pressure waveforms which would have indicated a break-down from ventricular division into two pressure pumps. Diastolic pressures were superimposable in the ventricular cava under all conditions. Analysis of blood Po and O2 content revealed the potential for nearly complete separation of left and right atrial blood in the ventricle, but both left-to-right and right-to-left shunts of considerable magnitude could also develop. The varanid heart with its systolic pressure separation allows the develop-ment of high blood pressure gradients capable of driving a large cardiac output through the high impedance systemic vascular beds. Concurrently, the low impedance pulmonary circuit is perfused at a much reduced blood pressure, circumventing filtration of plasma into the lungs and impairment of gas exchange. Haemodynamically this situation resembles that present in crocodilians and the homeothermic vertebrates