The Physiology of Pregnancy

All major body systems undergo adaptation during pregnancy and many of these changes have a significant impact on anaesthetic management.The ability to choose appropriate anaesthetic protocols and successfully manage the pregnant animal depends on understanding the physiologic changes that occur in the cardiovascular, respiratory, central nervous and gastrointestinal systems. Some data is extrapolated from humans and some data has been published based on dogs studies, but much less is known about cats.

The cardiovascular system.
Significant cardiovascular changes, which continue to progress until term, have been documented as early as mid-pregnancy in dogs. There is a decrease in blood pressure and an increase in heart rate and cardiac output(1). Although total blood volume increases, red blood cells do not keep pace with plasma expansion resulting in anaemia, the severity of which is correlated with the number of puppies (2).

The foetus is vulnerable to changes in the mother’s cardiovascular system because foetal blood flow is not autoregulated and uterine perfusion is pressure- dependent. The consequences of maternal changes are obvious when it is understood that uterine blood flow is directly proportional to the arterial-venous blood pressure difference and inversely proportional to systemic vascular resistance. Anaesthetic drugs, dehydration and intra-operative fluid losses will result in maternal hypotension. Fear, stress, excitement and pain can all cause increased sympathetic nervous system activity resulting in vasoconstriction and increased systemic vascular resistance; all of which will adversely affect the foetus. [...]