Emergency Anaesthesia
PREOPERATIVE ASSESSMENT
The likely surgical diagnosis, and the extent and urgency of the proposed surgery must be discussed with surgical and medical colleagues preoperatively. The urgency for surgery is most helpfully conveyed using a recognized classification system, such as the one created by the National Confidential Enquiry into Patient Outcome and Death (NCEPOD) (Table 37.1). The nature and urgency of the planned surgery dictate the extent of preoperative preparation and anaesthetic technique. They also influence plans for postoperative care, which may include transfer to a HDU/ICU facility.
During the preoperative visit a past medical and drug history is elicited. In particular, the patient’s degree of cardiorespiratory reserve should be established, even if there is no formal diagnosis of cardiovascular or respiratory disease. The presence and severity of symptoms suggestive of reduced reserve such as angina, productive cough, orthopnoea or paroxysmal nocturnal dyspnoea should be sought. The patient’s functional capacity is of useful prognostic value and can be simply quantified in terms of metabolic equivalents (METs). 1 MET is a unit of resting oxygen consumption and appropriate questioning can allow an estimate of the patient’s maximal oxygen consumption capacity (VO2 max) (Table 37.2 and Ch 18 [Table 18.1]). A patient who is unable to perform activity at 4 METs or more is at increased risk of perioperative cardiac complications.
TABLE 37.2
MET Score | Approximate Level of Activity |
1 | Dress, walk indoors |
2 | Light housework, slow walk |
4 | Climb one flight of stairs |
6 | Moderate sport eg golf or dancing |
10 | Strenuous sport or exercise |
Preoperative evaluation of the airway is always important. The standard clinical tests of airway assessment should be used (see Ch 21: The practical conduct of anaesthesia) and any previous anaesthetic charts consulted if available. A history of difficult intubation is of considerable significance; however, a past record of easy tracheal intubation does not guarantee future success. In emergency anaesthesia, airway difficulties may be caused by the patient’s usual anatomy, but also surgical pathology such as dental abscesses, trauma and bleeding or haematoma. If a rapid-sequence induction is contemplated, then contingency plans are required for management of the patient in the event of failure to intubate the trachea. If a high degree of difficulty in tracheal intubation is anticipated then an awake technique may be necessary.
Assessment of Circulating Volume
Intravascular Volume Deficit
Blood loss may be assessed using the patient’s history and any measured losses, but more commonly the anaesthetist has to rely on clinical evaluation of the patient’s current cardiovascular status. Profound circulatory shock with hypotension, poor peripheral perfusion, oliguria and altered cerebration is easy to recognize. However, a more careful assessment is needed to recognise the early manifestations of haemorrhage, such as tachycardia and cutaneous vasoconstriction. Useful indices include heart rate, arterial pressure (especially pulse pressure), the state of the peripheral circulation, central venous pressure and urine output. Table 37.3 describes approximate correlations among these clinical indices and the extent of haemorrhage, but it should be stressed that these refer to the ‘ideal’ patient. In young, healthy adults, arterial pressure may be an unreliable guide to volume status because compensatory mechanisms can prevent a measurable decrease in arterial pressure until more than 30% of the patient’s blood volume has been lost. In such patients, attention should be directed to pulse rate, skin circulation and a narrowing pulse pressure. Tachycardia in the presence of a normal arterial pressure should never automatically be attributed to pain or anxiety if there is a clinical history consistent with the potential for intravascular volume loss. In elderly patients with widespread arterial disease, limited cardiac reserve and a rigid vascular tree (fixed total peripheral resistance), signs of severe hypovolaemia may become evident when blood volume has been reduced by as little as 15%. However, as baroreceptor sensitivity decreases with age, elderly patients may exhibit less tachycardia for any degree of volume depletion.
Extracellular Volume Deficit
Assessment of extracellular fluid volume deficit is difficult. Guidance may be obtained from the nature of the surgical condition, the duration of impaired fluid intake and the presence and severity of symptoms associated with abnormal losses (e.g. vomiting). At the time of the earliest radiological evidence of intestinal obstruction, there may be 1500 mL of fluid sequestered in the bowel lumen. If the obstruction is well established and vomiting has occurred, the extracellular fluid deficit may exceed 3000 mL. Table 37.4 describes some of the clinical features seen with varying degrees of severity of extracellular fluid losses. It is clear that considerable losses must occur before clinical signs are apparent, and that these signs are often subjective in more minor degrees of extracellular fluid deficit.
TABLE 37.4
Indices of Extent of Loss of Extracellular Fluid
Percentage Body Weight Lost as Water | mL of Fluid Lost per 70 kg | Signs and Symptoms |
Over 4% (mild) | Over 2500 | Thirst, reduced skin elasticity, decreased intraocular pressure, dry tongue, reduced sweating |
Over 6% (mild) | Over 4200 | As above, plus orthostatic hypotension, reduced filling of peripheral veins, oliguria, low CVP, apathy, haemoconcentration |
Over 8% (moderate) | Over 5600 | As above, plus hypotension, thready pulse with cool peripheries |
10–15% (severe) | 7000–10 500 | Coma, shock followed by death |
Once the extent of blood volume or extracellular fluid volume deficit has been estimated, deficits should be corrected with the appropriate intravenous fluid. The overall priority is to maintain adequate tissue perfusion and oxygenation, therefore correction of intravascular deficit takes precedence – hypovolaemia due to blood loss should be treated with either a balanced crystalloid solution (such as Hartmann’s solution) or a suitable colloid until packed red cells are available (see Ch 12: Fluid, electrolyte and acid–base balance). Resuscitation is usually guided by clinical indices of circulating volume status and organ perfusion. Central venous pressure (CVP) measurement has often been used to guide fluid therapy but CVP has limitations when used to predict intravascular volume status and responsiveness to infused fluids. High-risk surgical patients may benefit from the use of (non-invasive) cardiac output measuring devices to direct fluid resuscitation towards predetermined goals for cardiac output and systemic oxygen delivery.
THE FULL STOMACH
Gastric Emptying
Gastric emptying results from peristaltic waves sweeping from cardia to pylorus at a rate of approximately three per minute, although temporary inhibition of gastric motility follows recent ingestion of a meal. The gastric emptying of clear fluids is an exponential process, i.e. the rate of emptying at any given time is proportional to the volume of liquid in the stomach. The half-time for this process is about 20 min, so less than 2% of ingested clear fluid remains in the stomach at 2 h. The gastric emptying of solids is roughly linear, i.e. occurs at a constant rate, and usually begins about 30 min after ingestion of a meal. The rate of emptying varies depending on the composition of food ingested. Typically, about 50% of food reaches the duodenum within 2 h although meals high in fat content may take considerably longer. The rate of gastric emptying is also significantly delayed if the mixture reaching the duodenum is very acidic or hypertonic (the inhibitory enterogastric reflex), but both the nervous and humoral elements of this regulating mechanism are still poorly understood. Many pathological conditions reduce gastric emptying (Table 37.5). In the absence of any of these factors, it is reasonably safe to assume that the stomach is empty provided that solids have not been ingested within the preceding 6 h, or clear fluids consumed in the preceding 2 h, and provided that normal peristalsis is occurring. This is the usual case for elective surgical patients. However, in emergency surgery it may be necessary to induce anaesthesia urgently before an adequate period of starvation occurs. In addition, the patient’s surgical condition is often accompanied by delayed gastric emptying or abnormalities of peristalsis. In these circumstances, even if the usual period of fasting has been observed it cannot be assumed that the patient’s stomach is empty.
TABLE 37.5
Situations in Which Vomiting or Regurgitation May Occur
Full stomach
With absent or abnormal peristalsis
Peritonitis of any cause
Postoperative ileus
Metabolic ileus: hypokalaemia, uraemia, diabetic ketoacidosis
Drug-induced ileus: anticholinergics, those with anticholinergic side-effects
With obstructed peristalsis
Small or large bowel obstruction
Gastric carcinoma
Pyloric stenosis
With delayed gastric emptying
Diabetic autonomic neuropathy
Fear, pain or anxiety
Late pregnancy
Opioids
Head injury
Other causes
Hiatus hernia
Oesophageal strictures – benign or malignant
Pharyngeal pouch
TECHNIQUES OF ANAESTHESIA
Phase I – Preparation
Although not completely effective, insertion of a nasogastric tube to decompress the stomach and to provide a low-pressure vent for regurgitation may be helpful. Aspiration through the tube may be useful if gastric contents are liquid, as in bowel obstruction, but is less effective when contents are solid. Cricoid pressure is still effective at reducing regurgitation even with a nasogastric tube in situ.
Clear oral antacids (e.g. sodium citrate) may be used to raise the pH of gastric contents immediately before induction. However, this also increases gastric volume. Particulate antacids should not be used, as they may be very damaging to the airway if aspirated. The preoperative administration of H2-receptor antagonists consistently raises gastric pH and may reduce the chance of chemical pulmonary injury occurring in the event of inhalation. Although this is standard practice in obstetric anaesthesia, few anaesthetists employ these measures for emergency general surgery. The regimens available are described in Chapter 35.