Fundamentals of Anaesthesia

Pregnancy

Normal pregnancy involves major physiological and anatomical adaptations by maternal organs. It is important that anaesthetists involved in the care of the pregnant woman understand these changes, to provide safe maternal anaesthetic care which is compatible with safe delivery of the baby.

Cardiovascular system

There are multiple changes in the cardiovascular system, many of which are compensatory changes designed to cope with the growing fetus, uterus and placenta. These are summarised in Figure PN1. Although the majority of changes occur during pregnancy, significant changes also occur during labour and immediately following delivery of the baby.

Cardiac output

Patient posture has been found to influence cardiac output measurements significantly during pregnancy (Figure PN2). Measurements performed in the lateral position, to avoid aortocaval compression, demonstrate an increase in cardiac output by 5 weeks gestation. Cardiac output continues to increase from this time, resulting in a rise of 35–40% by the end of the first trimester, increasing to 50% by the end of the second trimester (Figure PN3). Cardiac output then remains at 50% above non-pregnant levels throughout the third trimester.

A further transient rise in cardiac output occurs at delivery, as a result of labour and uteroplacental transfusion into the maternal intravascular volume.

Heart rate and stroke volume

The increase in cardiac output in pregnancy is produced by a combination of increased heart rate, reduced systemic vascular resistance (SVR) and increased stroke volume.