Differences between directly measured and calculated values for cardiac output in the dogfish: a criticism of the Fick method.

Cardiac output has been measured directly, and calculated by the Fick method, during normoxia and hypoxia in six artificially perfused dogfish (Scyliorhinus canicula) in an attempt to estimate the accuracy of this method in fish. The construction and operation of a simple extra-corporeal cardiac bypass pump is described. This pump closely mimics the flow pulse profiles of the fish's own heart and allows complete control of both cardiac stroke volume and systolic and diastolic periods. During normoxia (PO2 = 21 kPa) there was no significant difference between directly measured and calculated values for cardiac output. However, some shunting of blood past the respiratory surface of the gills may have been obscured by cutaneous oxygen uptake. In response to hypoxia (PO2 = 8.6 kPa) there is either a decrease in the amount of blood being shunted past the respiratory surface of the gills and/or an increase in cutaneous oxygen uptake such that the Fick calculated value for cardiac output is on average 38% greater than the measured value. It is proposed that the increase in the levels of circulating catecholamines that is reported to occur in response to hypoxia in this species may play an important role in the observed response to hypoxia. The results are discussed in terms of their implications for the calculation of cardiac output by the Fick principle in fish.