I Hypotension in obstetric spinal anaesthesia : a lesson from pre-eclampsia

Vasopressor use to prevent hypotension occurs after 80% of spinal anaesthetics for Caesarean section. The problem was first recognized 50 yr ago when it was attributed to caval compression. This theory became accepted as the basis for clinical management, and it remains current today. 4 However, using this theory as a basis for the management of hypotension has proved disappointingly ineffective. 6 Important information from the ‘natural experiment’ of pre-eclampsia was overlooked, and fresh information from vascular biology now calls for a reconsideration of our management of hypotension in these circumstances. Previously, the concept of compression of the vena cava and the aorta was linked to three features, which can coexist and are often considered together. However, each probably has a different mechanism. First, spinal anaesthesia almost always causes hypotension in normal pregnancy, and we will consider the reasons for this phenomenon later. Secondly, cardiac output can be reduced by aortocaval compression when some mothers lie in the supine position, although this is not necessarily clinically evident. Thirdly, a marked bradycardia with a reduction in cardiac output and severe hypotension can occur suddenly in a few subjects at some time after the mother moves to the supine position. This reflex effect is the relatively uncommon supine hypotensive syndrome of pregnancy (SHSP). Holmes proposed that compression of the inferior vena cava by the gravid uterus caused hypotension after spinal anaesthesia because venous return was reduced and thus cardiac output decreased. Marx developed the concept that blood was trapped in the legs, and introduced the treatment strategy of ‘acute hydration’ supported by a widely cited illustrative case history. Subsequently, fluid administration before spinal anaesthesia became the putative ‘prophylaxis’ and an almost universal therapy. The theory of caval compression and supine hypotension was based largely on studies by Scott and colleagues, who measured cardiac output by dye dilution in eight patients. Overall, cardiac output was 12% less in the supine compared with the lateral position. In three subjects, the mean reduction was only 6% and the investigators suggested that vena caval compression was relieved because the fetal head was engaged. However, in two patients, there was sudden bradycardia, hypotension, and a decrease in cardiac output by more than 50%, suggesting a reflex response. Clearly, this study reported a heterogeneous group of patients, and the patients with bradycardia developed the supine hypotensive syndrome, which is a different phenomenon from the hypotension seen in the other patients in that study. An extensive review of SHSP found a wide range of case selection, clinical features, definitions, and degrees of hypotension. Severe hypotension was reported in 2.5–20% of these patients. In some patients, hypotension only occurred after 20 min in the supine position. The possible reasons given for hypotension in these patients were either vena caval obstruction or a vagal reflex bradycardia, which is a well-known phenomenon associated with poorly filled heart. Later studies found much less difference between supine and lateral positions. Using transcutaneous Doppler, a maximum change of cardiac output of 6% occurred with moving from supine to a left 158 tilt, and fetal head engagement made no difference. Undoubtedly, the vena cava is affected by the gravid uterus. Femoral venous and distal inferior caval pressures were greater in the supine position. In the lateral position, venous pressure was less, but still not as low as nonpregnant levels. Angiography showed occlusion of the inferior vena cava and distension of the collateral azygos circulation in 12 supine patients having Caesarean section under general anaesthesia. The abdominal vena cava remained partly occluded in the lateral position. However, in these studies, the link between changes in venous behaviour and hypotension was inferred rather than directly proved. No early studies involved spinal anaesthesia because general anaesthesia and increasingly epidural anaesthesia had, by that time, largely replaced spinals for Caesarean section in the UK.