Hypotension on initiation of cardiopulmonary bypass (CPB) occurs commonly. Causal factors are complex; however, it is thought that the sudden decrease in systemic vascular resitance due to abrupt haemodilution which decreases blood viscosity, is a significant cause (1). There is a direct relationship between haematocrit and blood viscosity and the perfusion pressure falls in proportion to the change in viscosity (1, 2). This shock-like state conferred by CPB is commonly met with a stress response which spontaneously increases the
mean arterial pressure (MAP).