Haemodynamic assessment in pregnancy and pre-eclampsia: A Guytonian approach

Hypertensive disorders of pregnancy continue to be amongst the leading causes of maternal morbidity and mortality. There is debate about the optimal monitoring and treatment for these women, particularly in regard to circulatory and fluid management. A restrictive fluid strategy is advocated, which conflicts with the prevailing view that the circulating volume is contracted in pre-eclampsia. This belief has been erroneously reinforced by use of the central venous pressure (CVP) as a measure of the volume state.

We used a Guytonian model of the circulation involving the mean systemic filling pressure (Pms) to review published data using a cohort of normal pregnant/post partum women and a pre-eclamptic cohort. The Pms is the pressure left in the circulation when the heart is stopped, arguably the true volume state measure. An analogue of the Pms (Pmsa) can be calculated using commonly measured haemodynamic variables.

Our results show the Pmsa to be elevated in normal pregnancy versus post partum (10.79 vs. 9.58, a 12.6% difference) and elevated further in pre-eclamptic pregnancy (13.86, 29% higher than the normal pregnant group).

There is scope to challenge the long held belief that the volume state is contracted in pre-eclampsia. This approach indicates that the maternal volume state in pre-eclampsia is often elevated. When viewed in combination with recent echocardiographic insights this model helps to explain some of the haemodynamic management paradoxes that these women present. Most importantly, it provides a sound physiological basis for the restrictive fluid strategy that is currently recommended.