Background

A fundamental role of both surgeon and anaesthetist is to seek to identify patients who may be at increased risk of mortality or serious morbidity following surgical intervention.

Perioperative cardiovascular events are a major source of adverse outcomes. The incidence of death following major non-cardiac surgery in the United Kingdom is 0.5–1.5% (approximately 25 000 deaths per year). A further 2–3.5% of patients suffer major cardiac complications. A major focus of ongoing research is concerned with how to identify this high-risk subgroup of patients and what interventions may minimize the risk.

 To quantify known disease, and to identify subclinical disease, aiming to intervene and optimize where possible

 To facilitate informed patient consent: a better appreciation of risk allows patients and clinicians to discuss the risk–benefit ratios of alternative procedures and/or conservative treatment

 To assist in appropriate allocation of critical care or high-dependency beds

 To assist decision-making in respect of both anaesthesia and surgery: for example, in deciding between open or laparoscopic surgery, or whether to use regional techniques as an adjunct or alternative to general anaesthesia.

Cardiac risk indices

The time-honoured approach to patient assessment is based upon history, examination and investigations. In the past, several scoring systems have emerged based on this principle.

The first widely used cardiac risk index was that proposed by Goldman et al in 1977.¹ Nine independent criteria were identified as indicators of increased risk (Box 2.1). The Goldman Index has been revised by subsequent workers, notably Detsky² and Lee.³

In 2007, the American College of Cardiology (ACC) and American Heart Association (AHA)⁴ sought to stratify apparent cardiac risk factors into three categories – those that require further investigation, and others that may or may not actually impose increased risk (Box 2.2).

A step-by-step approach to risk assessment

Subsequent guidelines propose a stepwise approach to the evaluation of a potential high-risk surgical patient. The aim is to assist in creating an individualized cardiac risk assessment, and to suggest appropriate interventions before surgery in terms of optimization. The process is summarized in Box 2.3 and expanded upon in the sections that follow.

Assessing the risk of the surgical procedure

The risk of serious cardiac complications following surgery depends not only on the presence of risk factors, such as those described above, but also varies according to the type of surgery performed. Surgery induces a physiological stress response, with sympatho-humoral activation, increased myocardial oxygen demands and hyper-coagulability. With regard to cardiac risk, surgical interventions fall into one of three categories: low, intermediate or high-risk, according to the risk of myocardial infarction (MI) and cardiac death within 30 days of surgery (Table 2.1).

Tests of functional capacity including cardiopulmonary exercise testing

A potential consequence of the physiological response to major surgery is an imbalance between oxygen supply and demand: hence the interest in measuring a patient’s exercise capacity as an index of global cardiorespiratory reserve. Tests of individual components of exercise capability (e.g. exercise electrocardiography (ECG), pulmonary function tests) have shown poor correlation as predictors of postoperative problems.

A careful history may, of course, give some indication of a patient’s exercise tolerance, but may not be accurate. Efforts to make this more objective have included structured questionnaires, such as the Duke Activity Status Index, which grades exercise tolerance according to the ability to perform tasks ranging from washing and dressing through to strenuous activities such as tennis.

In the shuttle walk test, the patient is observed walking back and forth between two fixed points, usually 10 m apart, against a timed bleep which is made progressively shorter as the test continues. The completed distance within the allowed time is taken as a measure of exercise ability and has shown reasonable correlation with postoperative mortality and morbidity after major surgery.

Cardiopulmonary exercise (CPEX) testing is increasingly regarded as a gold-standard for preoperative exercise testing, yielding considerable data on oxygen uptake and utilization. CPEX testing is cheap and relatively non-invasive, and aims to determine the patient’s anaerobic threshold. Since it evaluates both the cardiovascular and respiratory systems, it is ideal for investigation of the patient with exertional breathlessness. The patient exercises on a bicycle ergometer, with measurement of gas exchange at the mouth together with ECG monitoring. CPEX detects the change from aerobic to partial anaerobic metabolism (Fig. 2.1): at the anaerobic threshold (AT), production of CO₂ relative to consumption of O₂ increases. An AT of less than 11 ml/min/kg has been associated with a higher perioperative cardiovascular mortality.

Other cardiac investigations

Pharmacological strategies to reduce risk

Pharmacological interventions to reduce perioperative risk have been the focus of much interest and research. A number of classes of drug have been investigated.

Myocardial revascularization

Patients with unstable angina who require non-cardiac surgery are high-risk. The mainstays of management are antiplatelet anticoagulant therapy and beta-blockade, proceeding to prompt revascularization. Most patients will undergo a percutaneous coronary intervention (PCI), often with bare-metal stents (see below) if the proposed surgery is urgent.

The evidence differs, however, in respect of surgical patients with stable ischaemic heart disease. Coronary artery bypass grafting (CABG) improves prognosis and relieves symptoms in patients with significant left main-stem disease and/or significant triple vessel disease, especially when there is poor left ventricular function, and in patients with other categories of lesion PCI is now a valuable alternative. Nonetheless, evidence is lacking that prophylactic revascularization reduces perioperative mortality in stable cardiac patients undergoing non-cardiac surgery.

Management of antiplatelet therapy

An increasing number of patients now present for non-cardiac surgery having previously undergone myocardial revascularization. Most will be receiving single or dual antiplatelet therapy.

Two sorts of stent are commonly employed: bare-metal stents have generally been superseded by drug-eluting stents which carry a reduced risk of re-stenosis but a higher risk of stent thrombosis. Drug eluting stents require continuous dual antiplatelet therapy (aspirin + clopidogrel) for at least 12 months after implantation. It is now generally accepted that elective surgery should not take place within 12 months of drug-eluting stent implantation. After 12 months, surgery can proceed, but with at least continuation of aspirin therapy. It is no longer acceptable simply to discontinue all antiplatelet therapy in all patients, and discussion between surgeon, anaesthetist and cardiologist is to be recommended. The recommendations in respect of the timing of non-cardiac surgery after PCI are summarized in Figure 2.2.

Rationale for oxygen therapy

Mild-to-moderate hypoxaemia during the postoperative period is extremely common and may contribute to poor outcome in a variety of areas (Box 2.4).

Certain patient groups are at particular risk from hypoxaemia: these include patients at extremes of age, pregnant women, obese patients, smokers and those with pre-existing cardiorespiratory disease.

Factors contributing to postoperative hypoxaemia

From first principles, adequate tissue oxygenation depends on:

Anaesthesia and surgery may disrupt each of these processes. The main factors that contribute to postoperative hypoxaemia are conveniently classified anatomically from respiratory drive onwards, and are summarized in Box 2.5.

Assessment and detection of hypoxaemia

Mild-to-moderate hypoxaemia is difficult to detect by purely clinical methods. More profound cases may result in:

A high index of clinical suspicion is required, and pulse oximetry should be routinely available, with a low threshold for arterial blood gas analysis to confirm a hypoxic state.

Oxygen therapy devices

Increasing the inspired concentration of oxygen provides a higher gradient for diffusion of oxygen from the alveolar gas into the pulmonary capillary blood. Two sorts of device are available – variable and fixed performance (Fig. 2.3).

Recognition and management of respiratory failure

Respiratory failure is defined as a failure of oxygenation of arterial blood to achieve a partial pressure of oxygen (PaO₂) of 8kPa breathing room air at sea level. Two types are described: in type 1, ventilation is preserved (PaCO₂ < 6.5 kPa). In type 2 respiratory failure, there is a failure of both oxygenation and ventilation (PaCO₂ > 6.5 kPa).

The common causes in surgical patients are as listed in Box 2.5 and the clinical manifestations are as described above. In terms of investigations, these should include arterial blood gas analysis and an urgent chest X-ray (CXR). It is important to note that arterial gases do not require to be taken on air for a diagnosis to be made – this is dangerous, and may provoke severe desaturation.

The initial management of the hypoxaemic patient is high-flow oxygen therapy: as explained above, the scenario of the patient with chronic CO₂ retention losing his or her hypoxic drive in response to oxygen therapy is not common. If there is any doubt, high-flow oxygen should be given pending a respiratory opinion.

If, despite oxygen therapy, the oxygen saturation cannot be maintained above 92% (or the PaO₂ above 9 kPa), then further respiratory support may be required. In essence, this may be one of three types: