It is assumed that patients who need anaesthetising for emergency surgery will not have an empty stomach, which poses an increased risk of regurgitation and aspiration into the lungs of acidic stomach contents. The greatest risk is during induction of anaesthesia but some patients are also at risk during extubation and recovery. The incidence of complications appears to be related to both the volume (>25 mL) and pH (<2.5) of the material aspirated. Factors predisposing to aspiration include:

• A full stomach. An inadequate period of starvation (emergency patients), increased gastrointestinal contents secondary to bowel obstruction, distension following face-mask ventilation.
  
• Delayed gastric emptying. Drugs (especially opiates), trauma (particularly head injury), peritoneal irritation, blood in the stomach, pain and anxiety.
  
• Obstetric patients (see below).
  
• Other causes. A history of gastro-oesophageal reflux, hiatus hernia, obesity, head-down position, presence of a bulbar palsy, oesophageal pouch or stricture.

Consequently, in these patients measures will be taken to prevent aspiration and the majority will be intubated in order to secure and protect their airway. In order to achieve this as safely as possible, the technique used for induction of anaesthesia is slightly modified and referred to as rapid-sequence induction or RSI.

A variety of methods are used, alone or in combination.

1. Reduction of residual gastric volume:

an adequate period of preoperative starvation;

avoidance of drugs that delay gastric emptying;

insertion of a nasogastric tube and aspiration of gastric contents;

use of pro-kinetic drugs, such as metoclopramide.

2. Increase pH of gastric contents:

sodium citrate to neutralize gastric acid;

H₂ antagonists, such as ranitidine;

proton pump inhibitors, for example omeprazole, lansoprazole (conveniently available in an oro-dispersible preparation).

3. Use of cricoid pressure (see below).

Cricoid Pressure (Sellick’s manoeuvre)

Obstetric patients may require anaesthesia for a variety of surgical procedures but the commonest is for a caesarean section, either electively or as an emergency, usually when the mother is already in labour. The following is an outline of the principles of anaesthesia. It is important to note that, whichever technique is used, adequate prophylaxis against acid aspiration is mandatory:

• elective caesarean section – an H2 antagonist or proton pump inhibitor the night before and morning of surgery;
  
• emergency caesarean section – an H2 antagonist and 30 ml 0.3 M sodium citrate immediately before going to theatre.

There are two main anaesthetic techniques for a caesarean section: regional (epidural or spinal anaesthesia) and general anaesthesia. Current recommendations are that caesarean section should wherever possible be performed under regional anaesthesia as this is associated with lower maternal and foetal mortality.

Regional anaesthesia is now the choice for almost all elective (90%) and most emergency caesarean sections, and the majority of these are spinal anaesthetics as this provides rapid, reliable and intense anaesthesia. It is recommended that women are offered intrathecal diamorphine as part of the technique, as this improves postoperative pain control and reduces the need for further analgesia.

The principles of spinal anaesthesia are as described in Chapter 5. Most anaesthetists perform the spinal with the patient sitting as this makes the midline easier to identify and is associated with slightly faster onset of block. The main problems associated with this technique are that unlike general and epidural anaesthesia, it is time limited and hypotension is more common. The latter is usually managed with a combination of an IV fluid preload and an infusion of phenylephrine (30–60 μg/min).

Epidurals are predominantly used to provide analgesia during labour. The extent and intensity of the block can be increased (anaesthesia) to allow caesarean section to take place. However, this is a relatively slow process and there is a risk of inadequate anaesthesia due to inadequate or absent block of some nerve roots. The technique and other problems are as described in Chapter 5.

The commonest reasons for the use of general anaesthesia are the urgency of the caesarean section (usually because of an immediate threat to the life of the mother or foetus), refusal of a regional technique by the patient, failure or contraindication of the regional technique. There are a number of problems specifically associated with general anaesthesia:

• There is an increased risk of regurgitation and aspiration. This is due to the progesterone-induced relaxation of the lower oesophageal sphincter and increased intra-abdominal pressure from the presence of the gravid uterus. This is exacerbated by the fact that in labour, gastric emptying is very slow. All pregnant women requiring general anaesthesia are regarded as having a full stomach and receive antacid prophylaxis as described above and anaesthesia is induced using an RSI with cricoid pressure. During emergency surgery, a gastric tube is passed to try and empty the stomach, and patients should be extubated once there is evidence of return of their laryngeal reflexes (e.g. coughing) and sat up at 30°.
  
• Failed intubation is more common in obstetric patients (1:300 compared to 1:3000 non-obstetric patients). This is primarily due to anatomical factors, in particular enlargement of breast tissue, engorgement of the airway mucosa and the fact that most women have a full set of teeth. When combined with the fact that the FRC is reduced and oxygen consumption increased, the pregnant woman will desaturate and become hypoxic remarkably quickly during repeated attempts at intubation. Attention must be paid to ensuring full preoxygenation, head and neck position must be optimized, and intubation must only be attempted at the point of maximal action of suxamethonium. If intubation fails, institute a failed intubation drill. Oxygenation is more important than intubation.
  
• Maternal awareness as a result of the use of inadequate doses of the induction and inhalational drugs in an attempt to avoid oversedating the baby. Adequate doses of drugs must be given; ‘flat’ babies can be resuscitated by a paediatrician.

Aortocaval Compression

As the gravid uterus enlarges through the pregnancy, it compresses the inferior vena cava, reducing venous return to the heart and therefore cardiac output and blood pressure. The effect is maximal by 36 weeks gestation, worse in the supine position and exacerbated by the sympathetic block produced by epidurals and spinals. In addition, compression of the aorta may occur, reducing blood pressure and flow in the uterine arteries that may cause foetal hypoxia. Both of these effects can be prevented by using a 15° left lateral tilt in the supine patient and it is essential that, whichever technique of anaesthesia is used for a caesarean section, the mother is placed in this position.

Surgery to the chest contents or to the anterior thoracic spine via a thoracotomy poses significant challenges for the anaesthetist – in particular the need to isolate and ventilate both lungs independently. This allows one lung to be deflated whilst ventilation is maintained via the other. The indications for this are to:

• facilitate surgical access, for example to the oesophagus, thoracic spine, the deflated lung;
  
• avoid contamination from one lung to the other, for example infection, bleeding;
  
• allow differential ventilation of both lungs, for example in the presence of a large leak due to a bulla.