31
Anaesthesia for Vascular, Endocrine and Plastic Surgery
MAJOR VASCULAR SURGERY
General Considerations
Most of these are considered independent risk factors for perioperative cardiac complications after major surgery (see Ch 18).
The broad aims of preoperative evaluation before vascular surgery are to:
assist risk assessment, permit further investigation if appropriate and allow optimization of coexisting medical conditions
evaluate and discuss the risks with the patient and surgical team
establish the best surgical options (e.g. non-invasive or endovascular surgery) for an individual
plan the anaesthetic technique, perioperative monitoring and postoperative care, and allow required facilities (e.g. ICU) to be organized.
Vascular surgery is associated with a high morbidity and mortality, resulting mostly from cardiac complications (myocardial infarction, arrhythmias and cardiac failure) (see Chs 18 and 43). It is therefore vital that cardiac function is assessed preoperatively and that the risks of surgery are evaluated and discussed with the patient. Although the outcome of subsequent vascular surgery is improved in those who have previously undergone coronary revascularization by coronary artery bypass grafting (CABG), this is associated with additional risks. Percutaneous coronary interventions (insertion of bare metal or drug-eluting intracoronary stents (ICS)) are being used increasingly as an alternative to CABG in suitable patients. There is a risk of stent thrombosis after ICS so patients receive dual antiplatelet therapy (aspirin and clopidogrel) for 6 weeks after bare metal and 1 year after drug-eluting stent insertion. The risks of perioperative cardiac events or stent thrombosis are very high if surgery is performed during the period when dual antiplatelet drugs are needed because of increased haemorrhage if antiplatelet therapy is continued, or acute thrombotic events if it is interrupted. The most recent guidelines suggest that elective surgery should be postponed for at least 4–6 weeks after bare metal stent insertion (and ideally delayed for 3 months) and for 1 year after drug-eluting stent insertion, unless surgery is more urgent. It should be remembered that coronary revascularization should be performed only if indicated because of the severity of coronary disease and is not justified simply to improve outcome from subsequent vascular surgery.
Preoperative Medical Therapy in Vascular Surgical Patients
Minimum investigations before major vascular surgery should include ECG, chest X-ray, full blood count and serum urea and electrolyte concentrations, but more invasive or specialized tests may be required (see Ch 18), including cardiopulmonary exercise testing where available. Some assessment of exercise tolerance should be made in all patients because it is a useful indicator of functional cardiac status, although many vascular patients are limited by intermittent claudication or old age and may have a sedentary lifestyle. In this case, a patient with severe coronary artery disease may have no symptoms of angina and a normal resting ECG. In some patients (e.g. those with limited functional capacity or life expectancy because of severe intractable coexistent medical conditions), the risks of elective vascular surgery may outweigh the overall potential benefits and invasive surgery may not be appropriate.
Abdominal Aortic Aneurysm
Elective Open AAA Repair
Several important considerations apply to patients undergoing aortic surgery (Table 31.1). The following are required:
TABLE 31.1
Major Anaesthetic Considerations for Patients Undergoing Aortic Surgery
High incidence of coexisting cardiovascular and respiratory disease
Cardiovascular instability during induction of anaesthesia, aortic cross-clamping and declamping
Large blood loss and fluid shifts during and after surgery
Prolonged major surgery in high-risk patients
Marked heat and evaporative fluid losses from exposed bowel
Potential postoperative impairment of respiratory, cardiac, renal and gastrointestinal function
two large (14-gauge) cannulae for infusion of warmed fluids
arterial catheter for intra-arterial pressure monitoring and blood sampling for acid-base and blood gas analysis
multilumen central venous catheter for drug administration and determination of right atrial pressure
continuous ECG monitoring for ischaemia (CM5 position) preferably with ST-segment analysis
oesophageal or nasopharyngeal temperature probe
Three specific stimuli may give rise to cardiovascular instability during surgery:
Tracheal intubation. Laryngoscopy and tracheal intubation may be accompanied by marked increases in arterial pressure and heart rate which may precipitate myocardial ischaemia in susceptible individuals. This response may be attenuated by the i.v. administration of a β-blocker (e.g. esmolol 1.5 mg kg–1) or a rapidly-acting opioid (e.g. alfentanil 10 μg kg–1) before intubation.
Cross-clamping of the aorta. Clamping of the aorta causes a sudden increase in aortic impedance to forward flow and hence left ventricular afterload. This increases cardiac work and may result in myocardial ischaemia, arrhythmias and left ventricular failure. Arterial pressure proximal to the clamp increases acutely even though left ventricular ejection fraction and cardiac output are reduced. The effects on preload are variable. The degree of cardiovascular disturbance is greater when the clamp is applied more proximally (greater at the supracoeliac > suprarenal > infrarenal levels). A vasodilator, e.g. glyceryl trinitrate (GTN), is often infused just before clamping (and continued up to clamp release) to obviate these problems. Deepening of volatile anaesthesia or an additional dose of an opioid may also be used at aortic clamping. While the aorta is clamped, blood flow distal to the clamp decreases, and distal organ perfusion is largely dependent on the collateral circulation. The lower limbs, pelvic and abdominal viscera suffer variable degrees of ischaemia during which inflammatory mediators are released from white blood cells, platelets and capillary endothelium. These mediators include oxygen free radicals, neutrophil proteases, platelet activating factor, cyclo-oxygenase products and cytokines.
Aortic declamping. Declamping of the aorta causes sudden decreases in aortic impedance, systemic vascular resistance and venous return with reperfusion of the bowel, pelvis and lower limbs and redistribution of blood. Inflammatory mediators are swept into the systemic circulation causing vasodilatation, metabolic acidosis, increased capillary permeability and sequestration of blood cells in the lungs. This is a critical period of anaesthesia and surgery because hypotension after aortic declamping may be severe and refractory unless circulating volume has been well maintained. If relative hypervolaemia is produced during the period of clamping by infusion of fluids to produce a CVP of greater than 12–14 mmHg (and perhaps administration of GTN until shortly before clamp release), declamping hypotension is less of a problem and metabolic acidosis may be diminished. Declamping hypotension usually resolves within a few minutes but vasopressors or positive inotropes are often required; these can be given before clamp release in anticipation. Good communication with the surgeon and slow or sequential clamp release helps the anaesthetist to manage aortic declamping. Renal blood flow decreases even when an infrarenal cross-clamp is used, and steps to maintain renal function are often required. The single most important measure is the maintenance of extracellular fluid volume (i.e. CVP > 12–14 mmHg). Prophylactic mannitol 0.5–1.0 g kg–1 i.v. or furosemide may also be administered, although the evidence for their efficacy is conflicting.
The Postoperative Period: Postoperatively, the patient should be transferred to a high-dependency or intensive care unit. The decision to continue artificial ventilation or to extubate the trachea after surgery depends on the patient’s previous medical condition and physiological stability during and at the end of surgery. Artificial ventilation should be continued until body temperature and acid-base status are normalized, cardiovascular stability restored and effective analgesia provided. Patients have a high incidence of postoperative cardiovascular and respiratory complications; renal dysfunction and ileus are also common. Close monitoring is required for several days.