The pregnant patient
Obstetric anaesthesia requires detailed knowledge of the physiological changes associated with pregnancy. Whilst these are covered thoroughly in Section 2, Chapter 14, the salient points are outlined below to aid the reader.
As pregnancy progresses, the maternal blood volume increases and, although total haemoglobin increases, the haemoglobin concentration falls by dilution. The concentration of clotting factors increases, causing a tendency to deep vein thrombosis exacerbated by pressure on the pelvic veins from the increasingly bulky uterus. Cardiac output increases throughout pregnancy due to increases in stroke volume and heart rate. Thoracic volume rises so that although tidal volume remains comparable to pre-pregnancy values, there becomes an impression of hyperinflation. At the end of pregnancy PaCO2 is reduced to 4 kPa. The hormonal changes of pregnancy cause relaxation of smooth muscle and ligaments, resulting in a reduction in lower oesophageal sphincter tone which, combined with increasing intra-abdominal pressure, leads both to functional hiatus herniae and oesophageal reflux. Gastric contents are more voluminous than usual and gastric emptying is slowed. In labour, gastric emptying virtually ceases.
Patients in the third trimester of pregnancy should not be allowed to lie in the supine position for any reason without left lateral tilt to displace the uterus, because the weight of the uterus compresses the inferior vena cava. The substantial reduction in venous return to the heart that follows may produce fainting. If compensatory vasoconstriction is abolished by epidural blockade, serious falls in cardiac output may result.