Anatomical and Physiological Changes in Pregnancy Relevant to Anaesthesia

Abstract

The hormonal changes that occur from very early on in pregnancy cause a complex series of physiological and anatomical changes that affect every system of the body. To illustrate how all these changes may alter or affect anaesthetic management, it is useful to imagine performing a general anaesthetic for caesarean section and list some key points:

Careful attention to the assessment of the airway and any necessary preparation to deal with a potentially difficult airway in the pre operative period.
When positioning the patient on the table, remember to use either a left tilt of between 15 – 30 degrees on the table or a wedge under the right buttock to minimize aorto-caval compression.
Venous access if often easier due to engorgement of the venous system.
Pre oxygenation is essential and should be with a tight fitting mask for at least 3 minutes.
Rapid sequence induction with the application of cricoid pressure is mandatory. Intubation may be difficult and so adjuncts for difficult intubation should be available. The trained anaesthetic assistant should be careful when placing cricoid pressure if there is left tilt on the operating table as the temptation is to place the cricoid pressure straight down thus distorting the view at laryngoscopy.
Once the airway is secured, ventilation should be aimed to keep the PCO2 in the normal range for pregnancy.
The MAC of volatile anaesthetic is slightly reduced.
Volatile agents cause relaxation of the uterus (uterine atony) and may result in haemorrhage after delivery of the fetus.
There is decreased sensitivity to endogenous and exogenous catecholamines and so if vasopressors are required to maintain adequate blood pressure, the amounts needed may be greater.
Extubation should be done with the patient awake and on their side to reduce the risk of aspiration of gastric contents.
Using the article, answer True or False to the following questions about what occurs during a normal pregnancy:

A decrease in red cell mass causing a fall in the haematocrit
Increased oxygen carrying capacity of blood
Increased oxygen delivery
Decreased fibrinolysis and decreased level of coagulation factors
Decreased cardiac output in the third trimester
Slightly increased cardiac output during labour
Unchanged diastolic blood pressure
Decreased peripheral vascular resistance
Increased myocardial contractility
There is a marked respiratory alkalosis
There is a fall in arterial PCO2
Arterial PO2 is increased
Minute ventilation is increased
Tidal volume is decreased
Vital capacity is increased
The respiratory quotient is increased
Arterial bicarbonate concentration is increased
Increased glucose filtration into glomerular fluid