3. Physiological Changes of Late Pregnancy Relevant to General Anaesthesia

12 kg gain Half of this is accounted for by an increase in plasma volume and interstitial fluid. Plasma volume increases by up to 40% and total body water by around 7–8 litres mild cardiac dilatation, and so heart murmurs (for example, that of mitral regurgitation) are common. Cardiac output increases by 40–45% to near maximal at 32 weeks’ gestation. The resting heart rate increases by 15%, and tachyarrhythmias are more common. The ECG shows left axis deviation caused by mechanical displacement by the gravid uterus, and minor T wave and ST segment changes may be seen. Blood pressure falls, with the diastolic drop of 10–15 mmHg making a bigger contribution than the systolic, and there is a decrease in systemic vascular resistance There is reduced sensitivity to circulating vasopressors, although it appears that the uterine circulation may be more sensitive to these than the systemic.

Aortocaval compression (supine hypotension syndrome): this is of potential importance because it occurs to some degree in all women, although modern methods of assessment using, for example, femoral vein ultrasound, suggest that it is significant in around only 30%. The remainder are able to compensate by an increase in sympathetic tone and diversion of venous return through the effective collateral circulation of the azygos veins and the vertebral venous plexus. Compression by the gravid uterus of the great vessels affects mainly venous return, but it can also compromise aortic and uterine blood flow. Turning from the lateral to the supine position at term in some women may decrease cardiac output by up to 30%.

Anaesthetic implications: because it is not possible to predict those mothers in whom aortocaval compression will reduce cardiac output, there should still be appropriate positioning to avoid the possibility. Cardiac output and systemic blood pressure must be maintained to ensure continued perfusion of the uteroplacental unit, but equally the anaesthetist must be aware of the consequences of fluid-loading a mother who is in effect already waterlogged.

• There is an increase in minute volume by 40% at term, but this is initiated early in pregnancy when progesterone-induced hyperventilation reduces PaCO2 by around 1 kPa • This would shift the oxygen– haemoglobin dissociation curve downwards and to the left were it not for an increase in maternal 2,3-DPG, which offsets this effect • Increased metabolic demand for oxygen increases by around 50%, along with an increase in the work of breathing and a decrease in both chest wall and lung compliance • The increased demand for oxygen is more than compensated by the increase in cardiac output, and so there is a small rise in PaO2 of about 1 kPa • There are anatomical changes which influence the upper airway; -general fluid retention and oedema of pregnancy may complicate laryngoscopy and intubation. • With regard to pulmonary volumes, the most important change is the 20% decrease in FRC, which, by the third trimester, may fall in the supine position to half its predicted value

the FRC must be filled with oxygen prior to induction to minimize risk of desaturation. This can be achieved either by preoxygenating the mother for 3 minutes with 100% O2, or by asking her to take three vital capacity breaths. Slight head-up or ramped positioning will reduce encroachment of the closing volume on the FRC. The reduced FRC means that the onset of the effect of volatile anaesthetic agents will be more rapid, as will maternal desaturation. Relative hyperventilation and low-normal PaCO2 should be maintained, although it is not until the PaCO2 falls below about 2.7–3.3 kPa (20–25 mmHg) that uterine blood flow is compromised. The congested and more oedematous upper airway may be traumatized during instrumentation. A smaller tracheal tube (7.0) may be required.

by the third trimester some 70% of mothers have symptoms of gastro-oesophageal reflux and heartburn. Oesophageal barrier pressure decreases with the loss of lower oesophageal sphincter tone, and there is also a fall in intestinal transit time and some duodenal gastric reflux. Gastric emptying itself, however, is not delayed in late pregnancy. Gastric residual volumes are increased, as is placental gastrin secretion. Whether this translates into maternal gastric hyperacidity remains disputed.

the airway must be protected against the risk of pulmonary aspiration of gastric contents by antacid prophylaxis (H2 antagonists, proton pump inhibitors and sodium citrate). Effective cricoid pressure applied during a rapid sequence induction is also considered essential.

under the influence of progesterone and endogenous β-endorphins, the MAC of anaesthetic agents decreases by about one-third, and there is an increased sensitivity to all drugs which act centrally. (Requirements for local anaesthetics also decrease, which may be related to an increased availability of free drug and to hormonally enhanced neural sensitivity.)

reduction in the doses of anaesthetic agents, sedatives and analgesics may be possible. Interpatient variability, however, is so great that it would be unwise to assume that anaesthetic awareness or severe postoperative pain are less likely.

: pregnancy increases ligamentous laxity owing to the rises in the hormones progesterone and relaxin. There is also an increased lumbar lordosis which helps to accommodate the enlarging uterus. There is also an increased lumbar lordosis which helps to accommodate the enlarging uterus.

Anaesthetic implications: scrupulous positioning of the patient with appropriate supports and protection may minimize the risk of postoperative backache or other joint problems.

: pregnancy is a hypercoagulable state. There is an increase in all clotting factors, except for Factor XI, and fibrinolysis is impaired by a plasminogen inhibitor that is derived from the placenta.

— Anaesthetic implications: the risk of venous thromboembolism is increased fivefold and routine preventative measures should be used Should a mother have additional risk factors, then pharmacological intervention may be necessary, although in many units low molecular weight heparin is given routinely after caesarean section or other surgery.

there is a 30% fall in the levels of plasma cholinesterase. — Anaesthetic implications: this fall has the greatest implications for those patients with atypical cholinesterases. It is often claimed that this decrease does not produce a clinically important increase in the duration of suxamethonium. Clinical experience would suggest, however, that the actions of suxamethonium are prolonged in many pregnant patients and that rapid offset with the resumption of spontaneous respiration is by no means guaranteed.

Drug handling: increased renal blood flow and glomerular filtration enhances the clearance of drugs excreted renally. The reduction in maternal albumin may increase the amount of free drug present in plasma, which may enhance its effects.

Discussion of these various factors is likely to take up much of the time available. If you have covered many of the points discussed here, then the oral may move on to related topics, such as traditional rapid sequence induction and the role of cricoid pressure. A rocuronium/sugammadex technique has its advocates, but others cite the rapidity of suxamethonium’s action (up to 35 s quicker) as the prime reason for its continued use in obstetrics. You would be brave to argue against the routine use of cricoid pressure in obstetric anaesthesia. It has often been argued that it is not used commonly in France, but this is something of an urban myth. By 1998, some 88% of French anaesthetists were employing the technique in obstetric general anaesthesia. Certainly, cricoid deformation can make intubation more difficult, but equally the backwards, upwards and rightwards pressure manoeuvre may well improve the view. Take a balanced approach in any discussion, enough to show that you are at least aware of the opposing arguments.