What are the key pathophysiological principles of MOH?
Total circulating blood volume increases by 50% to approx. 100 ml/kg of ideal body weight during late pregnancy (7000 ml if 70 kg). This is due to an increase in both plasma and red cell mass (plasma > red cells resulting in the physiological anaemia of pregnancy).
The uteroplacental unit is a high flow, low resistance vascular bed, which receives 25% of the cardiac output. Consequently, large volumes of blood can be lost rapidly from this unit.
There is an increase in coagulation factors during pregnancy, providing some physiological protection against haemorrhage.
Additionally, this physiological anaemia offers an evolutionary advantage in which less red cell mass is lost per ml of blood during haemorrhage. It is important to remember healthy pregnant women can compensate very well for a prolonged period during haemorrhage, and so initial observations may be falsely reassuring. Furthermore, a higher resting heart rate and lower mean arterial pressure is physiological during pregnancy.
Up to 30% on the circulating blood volume (2100 ml in 70 kg woman) may have been lost before significant tachycardia (HR < 120 bpm) or hypotension (systolic BP of 80) are present.
