Management of the High-Risk Surgical Patient

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Management of the High-Risk Surgical Patient

A  high-risk surgical procedure can be considered as one in which there is an accepted postoperative mortality rate of more than 1%. This includes cardiothoracic surgery, vascular surgery and major intra-abdominal cancer surgery, either as elective or emergency procedures. There are a number of factors that can put a patient at risk from such procedures. These can be divided into two broad categories: first, the technical hazards of the surgical procedure itself, e.g. the construction of a gastrointestinal tract anastomosis and the potential for it to break down; and second, the presence of significant co-morbidities in the patient before surgery, usually of a cardiorespiratory nature, which are severe enough to cause impaired preoperative functional status. In most patients, poor outcome from major surgery arises from a combination of these factors, in which the patient with impaired physiological reserve is unable to cope with the physiological demands of the surgery, leading to multi-organ dysfunction syndrome (MODS), multi-organ failure (MOF), and death in the worst cases.

Cardiothoracic anaesthesia and emergency anaesthesia are considered elsewhere (Chs 33, 34 and 37), and this chapter will concentrate on the patient undergoing scheduled major non-cardiac surgery. However, the principles of treatment, particularly fluid management, generally apply also to the patient undergoing an emergency procedure.

WHAT MAKES AN OPERATION HIGH-RISK?

Major surgery generates a systemic inflammatory response which is driven by the release of pro-inflammatory cytokines such as tumour necrosis factor (TNF) and interleukin-6 (IL-6). The magnitude of the inflammatory response, as judged by the levels of pro-inflammatory cytokines in the circulation, is associated directly with postoperative outcome, with higher concentrations of circulating IL-6 associated with an increased incidence of postoperative complications.

Pro-inflammatory responses are particularly marked in surgery involving the gastrointestinal tract, major vascular surgery and cardiac surgery. Other factors which increase the inflammatory response include the need for major blood transfusion, emergency surgery and the presence of decreased tissue perfusion, particularly in the gastrointestinal tract.

The effect of the inflammatory responses is a postoperative increase in oxygen requirements of up to 50% above basal levels. This substantial increase in oxygen demand is met normally by increases in cardiac output and tissue oxygen extraction. Most patients can meet the increased oxygen demand by increasing cardiac output and usually recover well after surgery. However, there is a group who may not have the physiological reserve to increase cardiac output to the required level and these patients are at higher risk of complications after surgery.

In addition to the systemic inflammatory response, major surgery generates a neuroendocrine stress response. Although this stress response may be attenuated to a degree, it is difficult to modify an established systemic inflammatory response and treatment strategies for the high-risk patient have relied on identifying patients early and optimizing various aspects of patient care in order to reduce risk and improve outcome.

IDENTIFYING THE HIGH-RISK SURGICAL PATIENT

Large-scale audits of surgical deaths in the UK have found that patients at risk are usually elderly, and 60–70% have established cardiorespiratory disease. When cardiac output monitoring is used in patients undergoing major surgery, it has been found that patients are more likely to die if they are unable to increase their cardiac output spontaneously in response to the physiological demands of the procedure. Poor outcome after major surgery is also associated with other related physiological factors which are all markers of impaired tissue perfusion, either globally or more specifically.

Because these abnormalities may manifest themselves only during surgery or in the postoperative period, the challenge for the anaesthetist is to identify high-risk patients before surgery, wherever possible.

RISK PREDICTION SCORING SYSTEMS

Shoemaker’s Criteria

In the 1980s, Shoemaker noted that a decreased oxygen delivery (DO2) was associated with poor outcome, and described a list of clinical risk factors associated with decreased survival, known as the Shoemaker criteria.

These criteria have been used to select patients for therapeutic trials, and are a useful guide to highlight at-risk patients, but do not provide an individual-specific risk assessment.

POSSUM Score

The Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM score) was developed specifically to provide predicted risk scores for complications and death after surgery.

The score requires physiological data from the patient’s condition prior to surgery, and intraoperative data from the surgical procedure itself.

POSSUM score physiological variables:

POSSUM score surgical variables:

The POSSUM system assigns different scores to degrees of abnormalities demonstrated by the variables. The total scores for the physiological and operative components are entered into an equation which gives predicted percentage values for the risks of mortality and morbidity (a specified range of complications). The original POSSUM system was devised 20 years ago in a general surgical population and the values of predicted mortality and morbidity reflect the current standards of care at that time. Subsequent research has produced more procedure-specific and location-specific versions of the POSSUM score but the overall structure of the system remains unchanged. The requirement for intraoperative data severely limits the real-time use of POSSUM as a tool for predicting the risk of an individual patient prior to surgery, but it has been shown to be a useful tool for retrospective assessment of the risk of comparative patient groups for audit and research purposes.

Revised Cardiac Risk Index

The Revised Cardiac Risk Index (RCRI) is a more system-specific score, designed to predict the risk of a patient developing a cardiac-related complication following non-cardiac surgery. Six variables are identified as independent predictors.

The risk of cardiac events ranges from 0.4% without any factors to 11% if three or more factors are present. Although simple to use, the RCRI predicts only specific cardiac morbidity. In addition, the score was derived during the 1990s, and subsequent developments in treatment for secondary prevention of ischaemic heart disease are likely to have led to a reduction in the incidence of postoperative events, and to an over-prediction of risk.

LABORATORY INVESTIGATIONS FOR RISK ASSESSMENT

Assessment of Functional Capacity

The physiological response to major surgery is a dynamic situation and assessment of the patient’s preoperative functional capacity has been recognized as a useful test with some predictive value. A patient who has limitation of cardiorespiratory reserve on exercise may be less able to elevate cardiac output in response to postoperative demands, and therefore may be considered at greater risk of complications.

Traditionally it has been taught that patients are at higher risk of complications after surgery if they are unable to perform exercise to a level that equates to 4 metabolic equivalents (METs), where MET is the energy expenditure at rest of a 40-year-old, 70 kg male. The degree of exercise equating to 4 METs would be climbing two flights of stairs. Questionnaires have been devised which allow practitioners to estimate a patient’s level of fitness, but these inevitably rely on the patient giving an accurate and honest appraisal of their levels of activity, and are therefore subject to bias.

Simple Exercise Testing

Objective testing of exercise capacity can provide a reasonable estimate of risk. This can vary from simple stair-climbing to more formal tests such as the shuttle walk test, in which the subject walks between two cones placed 10 m apart until unable to keep pace with a timed beep. The results of the test are expressed in metres walked, and the further the subject walks, the better the outcome after surgery is likely to be.

Although simple walking or climbing tests are useful in giving an overall impression of a patient’s reserve, they have significant limitations; for example, some elderly patients have significant mobility problems of the lower limb due to osteoarthritis and may find walking difficult. The main value of these simple tests lies in their negative predictive value, which means that fit patients can be identified who are very unlikely to have complications after surgery. However, these tests give little useful information about the underlying causes of impairment of functional capacity in patients who do not perform well in the test. This is an important limitation, because some patients perform poorly due to an underlying disease state, most commonly cardiac impairment, whilst others perform poorly simply through being out of condition due to lack of physical activity. Recognition of the cause of decreased functional capacity may allow effective preoperative interventions to improve the patient’s performance and reduce the risk of surgery. To evaluate patients in this way, more sophisticated testing is required.

Cardiopulmonary Exercise Testing (CPET)

During CPET, a patient undergoes exercise of increasing intensity which requires an increase in metabolic activity in the exercising muscles, which in turn demands increases in ventilation and cardiac output. Limitations in either respiratory or cardiac function (or both) result in decreased oxygen uptake by the exercising muscles.

CPET is performed with the patient pedalling on a static bicycle with a flywheel to which increasing resistance is applied. During the test, oxygen uptake and carbon dioxide production are measured using a metabolic monitoring cart. A 12-lead ECG is recorded simultaneously. Cardiorespiratory performance during exercise is usually defined by the measurement of oxygen uptake by the tissues (oxygen consumption, VO2), either as the maximum oxygen uptake measurable (VO2max), or as the oxygen uptake at the onset of lactate production, commonly known as the anaerobic threshold (AT). Other useful parameters obtained include measures of ventilatory efficiency and the ability to diagnose myocardial dysfunction from heart failure or ischaemic heart disease.

Anaerobic Threshold

AT occurs usually at 50–60% of VO2max, and is independent of patient motivation. If the AT is the prime objective of the CPET, the test can be stopped after it has been reached, and this may be advantageous in the frail or elderly surgical patient. This variant of CPET is known as a submaximal test, as the intention is not to test the patient to the maximum effort.

AT is identified as the point at which there is onset of lactate production through the activation of anaerobic pathways. The lactate produced is buffered by bicarbonate to produce water and carbon dioxide. The net effect is an increase in the slope of the graph of carbon dioxide production relative to oxygen uptake (Fig. 23.1)

Patients whose AT occurs at oxygen uptake values less than 11 mL kg− 1 min− 1 are at 6–7 times increased risk of mortality after surgery compared with those who have a higher oxygen uptake at AT.

Ventilatory Efficiency

This is estimated from the ventilatory equivalent of carbon dioxide, which is the ratio of minute ventilation to carbon dioxide production, VE/VCO2. A normal value is around 25–30, and increases in the ratio reflect impairment of V/Q mismatch, either from respiratory causes or from impaired cardiac function. In patients with heart failure, a value for VE/VCO2 greater than 34 is associated with a poor prognosis, particularly when combined with an AT less than 11 mL kg− 1 min− 1. The same holds true for surgical patients, because the implication is that this combination reflects a patient with more severe underlying cardiac dysfunction. These patients are often asymptomatic at rest, but the mortality rate after surgery in this group of patients is elevated, reflecting abnormal cardiac function in response to the stress of surgery.

In a typical UK population of elderly surgical patients, approximately 30% have reduced AT combined with reduced ventilatory efficiency, and are therefore in a high-risk group for surgical intervention. This group of patients should be optimized medically before surgery if possible, and managed on high dependency care after surgery.

Identification of Myocardial Dysfunction

Two parameters derived from gas exchange data can be used to identify patients with underlying myocardial dysfunction.

image Oxygen uptake per heart beat (VO2/HR, also known as the ‘oxygen pulse’). This should increase steadily during exercise as a reflection of increases in underlying stroke volume initially, followed by increases in oxygen extraction. Flattening of the slope of the graph during exercise may reflect underlying myocardial wall motion abnormalities.

image Oxygen uptake to work rate relationship (VO2/WR) (Fig. 23.2). For each 1 W increase in work rate, oxygen uptake should increase by 10 mL min− 1.

A value of VO2/WR significantly less than 10 mL min− 1 W− 1 may indicate underlying heart failure. A sudden change of the VO2/WR relationship during exercise may indicate the onset of myocardial ischaemia with consequent myocardial wall motion abnormalities.

Changes in either oxygen pulse or the VO2/WR relationship occur usually 2–3 min before changes in the ST segment of the exercise ECG are observed. A patient who develops these abnormalities may benefit from referral for cardiological assessment if symptomatic and if other variables such as AT are significantly impaired, or from cardiac protection from cardioselective β-blockade if asymptomatic (see below).

PREOPERATIVE INTERVENTIONS TO REDUCE RISK

β-Blockade

The purpose of cardioselective β-blockade is to reduce the risk of myocardial ischaemia after surgery through the prevention of tachycardia and the consequent decrease in myocardial oxygen demand and improvement in myocardial perfusion time. β-Blockers are now also used widely in the treatment of heart failure as well as the prevention of ischaemic events, but in heart failure, it is necessary to establish β-blockade over a period of time for full effect, starting with a low dose.

In practice, the introduction of β-blockers has not always shown clinical benefit. The use of bisoprolol (a cardioselective β-blocker) in patients with demonstrable wall motion defects on stress echocardiography significantly reduces the risks of death and myocardial infarction after surgery. However, it may be necessary to start treatment with bisoprolol several weeks before surgery. Metoprolol started immediately before surgery results in a small but significant reduction in cardiac events, but at the expense of increased hypotension, an increase in cerebrovascular events and an increase in overall mortality.

Retrospective studies have shown that the protective effect of β-blockade is greater in patients who have an increased number of RCRI cardiac risk factors, and there is even a suggestion that β-blockade in patients with no cardiac risk factors may be harmful.

Many patients are already established on long-term β-blockade, and it is important that β-blockade is continued around the time of surgery, as sudden cessation is definitely associated with a worse outcome. Parenteral preparations of atenolol and metoprolol are available for the patient who is ‘nil by mouth’ and there is some suggestion that the longer-acting atenolol may be more beneficial.

Coronary Revascularization

A cardiological evaluation is required for patients with unstable angina, or severe exercise limitation accompanied by ischaemia. These patients are at risk of adverse cardiac events irrespective of their surgical disease, but in practice constitute a small proportion of surgical patients.

There is a much larger group of patients who are asymptomatic, but have cardiac risk factors, and in whom coronary lesions can be identified on angiography. Prophylactic revascularization of such lesions before non-cardiac surgery has not been shown to have any overall benefit when the morbidity of the revascularization is taken into account.

Surgery may be further complicated in patients who have undergone percutaneous coronary intervention (PCI) with stenting because they need to take dual anti-platelet therapy for a minimum of 6 weeks in the case of bare metal stents, and 12 months for drug-eluting stents. In patients who are known to require surgery after their PCI, a bare metal stent should be used to minimize the time on dual anti-platelet therapy.

Correction of Anaemia

Most healthy patients tolerate a haemoglobin concentration of 8 g dL− 1, but the high-risk patient is more likely to be elderly and to have ischaemic heart disease, and may therefore require a higher haemoglobin concentration.

The patient may already be anaemic due to the nature of the surgical disease, particularly tumours of the right colon, stomach and urogenital tract, and anaemia is an independent risk factor for the development of complications after colonic surgery.

In patients with limited reserve, the anaemia should be reversed, if not completely corrected, in the first instance with either oral or parenteral iron administration. In more severe cases, the clinician needs to decide whether a preoperative transfusion is justified, although with improving parenteral iron therapy, the need for transfusion should be reduced.

IDENTIFYING PATIENTS IN NEED OF POSTOPERATIVE CRITICAL CARE

High-dependency care unit (HDU) and intensive care unit (ICU) beds are expensive resources in all hospitals, and demand usually outstrips supply. Some patients are admitted automatically to an HDU bed after major surgery such as open aortic aneurysm repair or upper gastrointestinal cancer surgery. However, after other more common surgical procedures, such as colorectal cancer surgery, weight reduction surgery or complex orthopaedic surgery, it is often unclear which patients are more likely to benefit from the extra monitoring and additional supportive measures which the HDU can provide.

A reduced value of anaerobic threshold (AT), measured from cardiopulmonary exercise testing (CPET), has been used to define groups at higher risk of complications after surgery. A protocol for triage of patients to ICU, HDU or to ward care after surgery based on CPET results was described by Older in 1999. Mortality rates were very low among patients who were allocated to ward care on the basis of good AT values, indicating that CPET was accurate in identifying patients who did not need HDU care, irrespective of clinical history or age. This model of care is now becoming established in hospitals with access to preoperative CPET. A typical protocol is shown in Figure 23.3.

The usefulness of this approach is emphasized by the results from a recent large audit of surgical outcomes in a UK tertiary centre, which showed that clinical judgement alone does not identify high- or low- risk surgical patients with the same degree of accuracy.

PERIOPERATIVE MANAGEMENT OF THE HIGH-RISK PATIENT

Ideally, the high-risk patient presents for surgery having been identified as being at increased risk, and having had medical co-morbidities optimized as much as possible at the preoperative assessment clinic stage.

This section concentrates on the anaesthetic management of the patient, with emphasis on haemodynamic monitoring and optimization of the circulation.

Choice of Anaesthetic Technique

General anaesthesia is usually necessary for patients who undergo major body cavity surgery, whereas a regional anaesthetic technique may be considered advantageous for peripheral surgery in a high-risk patient, although there is little evidence that the risk of developing complications is significantly reduced by use of regional anaesthesia.

In many patients who undergo body cavity surgery, general anaesthesia is supplemented by a neuraxial block technique, usually using a thoracic epidural catheter inserted before induction. The main purpose is to provide a high quality of pain relief after surgery, but a carefully managed epidural also reduces the need for systemic opioid analgesia during surgery. However, an epidural block is likely to produce a sympathetic block which causes peripheral vasodilatation and hypotension, and this can lead to decreased tissue perfusion, which may be significant in the compromised patient.

Haemodynamic Monitoring, Fluid Therapy, and Optimization of Oxygen Delivery

Oxygen Delivery

Many studies have shown that patients are more likely to develop complications after surgery if they show signs of impaired tissue perfusion and oxygen delivery during the perioperative period.

Oxygen delivery (DO2) is dependent upon cardiac output and the oxygen content of arterial blood.

Additionally, there is a small but clinically insignificant amount of dissolved oxygen.

Assuming that the haemoglobin concentration is satisfactory (> 8 g dL− 1), the variable most likely to alter during anaesthesia is the cardiac output.

Cardiac output (CO) depends on the stroke volume (SV, volume of blood ejected during systole) and the heart rate (HR).

Cardiac output is usually expressed as an indexed value, where the absolute value of cardiac output (L min− 1) is divided by the body surface area, which allows closer comparison between individuals of different sizes.

Stroke Volume

Stroke volume is dependent on cardiac preload, afterload and cardiac contractility, factors which can all change during anaesthesia and surgery.

In clinical practice, the most important of these variables is the volume of preload, and its effect on stroke volume. The Frank–Starling law states that increasing venous return to the left ventricle increases left ventricular end-diastolic pressure and volume, resulting in an increase in stroke volume. Increasing the preload increases the active tension developed by the muscle fibre and increases the velocity of fibre shortening, assuming that afterload and inotropic states remain constant (if the afterload is changed or inotropic activity changes, the shape of the Frank–Starling curve alters).

Figure 23.4 shows the effect of boluses of fluid on stroke volume at different points on the Frank–Starling curve. On the lower portion of the curve, when preload is low, a fluid bolus is likely to produce an increase in stroke volume of greater than 10%. As preload increases, the increases in stroke volume reduce, until a rise of less than 10% is obtained, indicating that the plateau of the Frank–Starling curve has been reached, and that further fluid boluses are not required.

The high-risk surgical patient benefits from measures which aim to optimize oxygen delivery through the careful administration of fluid, guided by careful monitoring of circulatory flow and preload.

The techniques used to measure stroke volume or preload, and the protocols used for fluid therapy, vary, but essentially fall into one of three categories:

All of these approaches have been shown to improve outcome through reductions in mortality and complication rates.

Preoperative Optimization of Oxygen Delivery

Early studies of outcome after high-risk surgery used the observation from Shoemaker in the 1980s that survival after surgery was associated with a DO2 index of greater than 600 mL− 1 min− 1 m− 2. Patients were admitted to critical care beds before surgery and a pulmonary artery catheter was inserted to measure cardiac index. A DO2 of 600 mL− 1 min− 1 m− 2 was targeted using fluid boluses initially, and an infusion of an inotropic drug was started if the target oxygen delivery was not achieved with fluid alone.

In trials in which the patients were at very high risk, as shown by mortality in the control group, this approach produced significant reductions in mortality. However, further studies of patients who were at lower risk failed to show benefit. In addition, the use of the pulmonary artery catheter to measure cardiac output has largely fallen out of favour due to its invasive nature and the need for expert interpretation. New techniques for measurement of cardiac output have now been developed which are less invasive than the pulmonary artery catheter, and easier to use in the general surgical patient.

Intraoperative Stroke Volume Optimization

The development of the oesophageal Doppler device has allowed direct measurement of the stroke volume. Placed in the oesophagus after induction of anaesthesia, the Doppler probe is focused on the optimal waveform in the descending portion of the thoracic aorta.

The measured parameters of the Doppler waveform (Fig. 23.5) include:

A value for corrected flow time of less than 330 ms indicates hypovolaemia, and stroke volume usually increases if a bolus of fluid is given. Flow time increases if systemic vascular resistance decreases, which is usually the case in the anaesthetized patient; administration of vasoconstrictors increases systemic vascular resistance and decreases flow time. Because of these confounding influences on flow time, most practitioners use the derived estimate of stroke volume to guide fluid therapy.

Figure 23.6 shows a typical protocol for Doppler-guided fluid administration in which boluses of fluid are given until no further increases in stroke volume occur.

Doppler-guided fluid strategies have been shown to reduce complication rates and hospital length of stay in cardiac, general and orthopaedic surgery. Doppler-guided stroke volume optimization is associated with improved perfusion of the gut mucosa, and with reduced interleukin-6 concentrations after surgery. These studies suggest that correcting occult hypovolaemia at an early stage during surgery has an important beneficial role through improvement of gut perfusion with consequent reduction in the magnitude of the inflammatory response.

The oesophageal Doppler has some limitations. For consistent measurements, the patient needs to be either anaesthetized or deeply sedated. The technique is contraindicated in patients with oesophageal pathology, and the values are inaccurate during clamping of the aorta.

Preload Responsiveness to Guide Fluid Therapy

Stroke volume can be measured through analysis of the arterial pulse waveform (pulse contour analysis). This requires insertion of an arterial line, but this should be considered routine monitoring in the high-risk patient. A patient who has a sustained increase in stroke volume after a fluid challenge can be described as being preload responsive. As an alternative to direct observation of stroke volume responses to a fluid challenge, preload responsiveness can be estimated by assessing changes in the arterial pressure or plethysmographic waveform during mechanical ventilation. To date, several preload responsiveness variables have been described:

SPV, PPV and SVV all require insertion of an arterial line, whereas PVI is derived from analysis of the pulse oximeter waveform and is therefore non-invasive.

The ability of these variables to determine preload responsiveness is based on observation of the cyclical changes which occur in stroke volume in response to mechanical ventilation (Fig. 23.7).

Respiratory variations in stroke volume are the main determinant of the respiratory change in pulse pressure, as long as arterial compliance remains the same. In hypovolaemia, SVV and PPV are increased as the under-filled right atrium and vena cavae are more compliant, and hence collapsible during inspiration, and the heart is generally more sensitive to changes in preload because of the position on the steeper portion of the Frank–Starling curve. Consequently, variations in stroke volume, pulse pressure and, to a lesser extent, systolic pressure increase in hypovolaemic conditions. A pulse pressure or stroke volume variation greater than 12–13% is highly sensitive and specific for hypovolaemia, and a patient with this degree of variability normally responds to a fluid challenge with an increase in stroke volume.

Most studies of preload responsiveness have been conducted in patients undergoing cardiac surgery or in the ICU. Some surgical outcome studies have been undertaken and show that a fluid administration protocol based on stroke volume or pulse pressure variation can lead to a reduction in postoperative complications, although the evidence base is smaller compared with that for stroke volume optimization guided by Doppler, which has been available for longer.

PVI is obtained using recent technology in which the variation of the amplitude of the plethysmogram waveform during the respiratory cycle is analysed. It has been shown that values of PVI in excess of 14% are associated with hypovolaemia in stable cardiac surgical patients.

Preload responsiveness monitoring variables share the same significant limitations in that they require a stable heart rhythm, and are only validated in patients whose lungs are ventilated mechanically. It is unlikely that one technique is superior to the other, as long as the principles of measuring the response to a fluid challenge are adhered to, with the aim of ensuring that the patient has an optimal preload, and is on the plateau portion of the Frank–Starling curve.

Choice of Fluid

Both colloid and crystalloid solutions are used regularly for fluid therapy in the high-risk surgical patient. Although some anaesthetists express strong views in favour or against one or other of these types of fluid, it is likely that a combination of both fluids, used for different purposes, is the optimal solution.

Crystalloids should be used for provision of maintenance fluid until the patient is able to tolerate oral fluid. This should be given at a rate of at least 1.5 mL kg− 1 h− 1, although larger doses are often used. Crystalloids are also the vehicle for electrolyte replacement therapy. Surgical patients need 1–2 mmol kg− 1 of sodium daily and 1 mmol kg− 1 of potassium. Solutions which contain dextrose only, or a mixture of dextrose and hypotonic saline (0.18%), lead to hyponatraemia and hypokalaemia, and should be avoided.

Colloid solutions should be reserved for correction of hypovolaemia and optimization of tissue perfusion, given as fluid challenges guided by flow-based measurements such as stroke volume or the preload responsiveness variables described above. Two types of colloid have been used in anaesthetic practice, the gelatins and starches. Gelatins are generally cheaper and provide a plasma volume expansion effect for 2–3 h. Starch-based colloids have now been withdrawn in the UK following after results from large randomised clinical trials and a meta-analysis reported an increased risk of renal dysfunction and mortality in critically ill or septic patients who received hydroxyethyl starch compared with crystalloids.

Infusion of large volumes of 0.9% sodium chloride (‘normal saline’) has been associated with the development of a hyperchloraemic metabolic acidosis. This acidosis is usually mild, and the clinical consequences are unclear. The most significant problem is probably the potential for erroneous interpretation of metabolic acidosis, and administration of unnecessary treatment in an otherwise normal patient. There may even be some benefit to a mild acidosis as it induces a leftward shift in the oxygen dissociation curve leading to increased oxygen delivery to the tissues.

It remains clinically unproven whether physiologically balanced crystalloids, such as Ringer’s lactate, or colloids suspended in physiologically balanced solutions, have a clear clinical advantage over those containing saline.

Fluid Restriction Regimens

There is some evidence that giving patients unrestricted amounts of crystalloid solutions around the time of surgery can lead to increased complications. Although crystalloid fluids have some immediate volume expansion effects when given as a fast bolus, these effects are short-lasting because crystalloid solutions (Ringer’s lactate or 0.9% saline) are largely dispersed throughout the extracellular fluid compartment, which is 14 L in the average adult. Of 1 L of crystalloid solution administered, approximately only 25–30% remains within the circulation, with the remainder adding to the interstitial fluid volume.

The net effect is to increase the likelihood of tissue oedema, which, in the lungs, can cause impairment of gas exchange and, in the gut, can lead to ileus and increased gut permeability, with a delay in return of normal gut function.

Some studies of regimens in which crystalloid use has been restricted have shown improved outcome, but the wide range of regimens used, the lack of the use of goal-directed fluid therapy and the results of other studies showing no benefit, has led to uncertainty over the benefits of true ‘restrictive’ fluid regimens.

In summary, the important factor is how carefully intravenous fluids are administered. Crystalloids should not be given in large volumes, but used for maintenance support, and the use of colloid should be restricted to optimization of circulating volume and replacement of circulatory losses until the point at which blood component therapy is required, which is usually when the haemoglobin concentration decreases to below 8 g dL− 1.

POSTOPERATIVE MANAGEMENT

Successful management strategies for the high-risk surgical patient are based on preventing complications where possible through appropriate cardiovascular support as described above, and early recognition and treatment of situations in which surgical complications or unforeseen medical problems cause deteriorating organ function.

The level of monitoring required depends on the risk status of the patient, hopefully identified prior to surgery, so that the appropriate arrangements can be made in advance. Major elective surgery in a high-risk patient should proceed only if it has been confirmed that the appropriate level of postoperative care is available.

A patient previously identified as high-risk should be managed in a critical care bed after surgery, usually a High Dependency Unit (HDU, level 2 care) bed, unless mechanical ventilation or renal support is necessary, in which case an Intensive Care Unit (ICU, level 3 care) bed is required.

It may be possible to manage patients who undergo major surgery but who are not considered as being at high risk in the Post-Anaesthesia Care Unit (PACU) for a few hours after surgery before returning to the surgical ward. Some surgical wards have an ‘enhanced care’ area (level 1 care), in which patients can have an additional level of continuous monitoring, usually ECG, pulse oximetry and non-invasive blood pressure, as well as additional nursing support to assist mobilization and physiotherapy.

Monitoring on the General Ward

General ward monitoring is normally restricted to standard clinical observations, although some wards may be able to monitor central venous pressure (CVP) as well. The same principles of monitoring apply to both general ward patients and critical care patients in that the clinician is using the monitoring to detect organ dysfunction and decreased tissue perfusion at the earliest possible stage. The variables shown in Table 23.1 should be monitored routinely. These variables are usually combined into a scoring system known as a ‘patient at risk’ (PAR) score, or ‘early warning system’ (EWS) score. A particular PAR or EWS score value, or change in value, is pre-defined as a trigger point for nursing staff to summon medical assistance so that a potentially deteriorating patient can be identified early and appropriate treatment can be given, including transfer to critical care if indicated.

Monitoring in the Critical Care Unit

Critical care provides an environment in which the invasive monitoring used in the operating room can continue to be used safely after surgery. This applies particularly to the use of arterial cannulae, which should never be used on the general ward because an unobserved disconnection can lead to profound blood loss. Information gained from invasive monitoring in the postoperative critical care unit can be used to detect organ dysfunction at an early stage.

Arterial Cannula

image Adequacy of oxygenation and ventilatory function through measurement of PaO2 and PaCO2.

image Assessment of pH and identification of causes of abnormalities, e.g. whether an acidosis is metabolic, respiratory or hyperchloraemic in origin.

image Assessment of tissue perfusion using lactate and bicarbonate, with an elevated lactate, decreased bicarbonate (‘base deficit’) and decreased pH indicating that tissue perfusion is inadequate, and that treatment with fluids and possibly inotropic drugs may be required.

image Cardiac output monitoring using a pulse contour analysis device. Using this technology, stroke volume and cardiac output can be estimated in the awake patient without the need for pulmonary artery catheterization. Fluid challenges can be given to increase stroke volume, and subsequent improvements in tissue perfusion can be assessed using base deficit, lactate or central venous oxygen saturation as described above.

INOTROPIC SUPPORT FOR THE HIGH-RISK SURGICAL PATIENT

Protocols which target an oxygen delivery value have usually incorporated the use of an inotropic agent to increase cardiac contractility if the oxygen delivery target has not been achieved with fluid loading alone. Adrenaline (α1-, β1– and β2-agonist), dobutamine (β1-agonist) and dopexamine (β2-, DA1– agonist) have all been used in this context.

There has been particular interest in the use of dopexamine in this group of patients, as it has splanchnic vasodilator and anti-inflammatory properties in addition to a mild inotropic activity. Early work suggested that dopexamine was associated with a reduction in complications after surgery, but more recent studies have shown that when fluid is given in a goal-directed manner, targeting either stroke volume or stroke volume variation, the routine use of dopexamine does not confer an additional significant clinical advantage.

Some patients may require inotropic or vasoconstrictor support if, despite adequate fluid loading, they still have signs of inadequate tissue perfusion (large base deficit, increased lactate concentration, decreased central venous oxygen saturation, etc.). In these situations, cardiac output monitoring should ideally be used to help guide treatment.

FURTHER READING

Canet, J., Galart, L., Gomar, C., et al. Prediction of postoperative pulmonary complications in a population-based surgical cohort. Anesthesiology. 2010;113:1338–1350.

Copeland, G.P., Jones, M.W. POSSUM: a scoring system for surgical audit. Br. J. Surg. 1991;78:355–360.

Davies, S.J., Wilson, R.J.T. Preoperative optimisation of the high-risk surgical patient. Br. J. Anaesth. 2004;93:121–128.

Davies, S.J., Wilson, R.J.T. Rationalising the use of surgical critical care: the role of cardiopulmonary exercise testing. In: Vincent J.L., ed. Yearbook of Intensive Care and Emergency Medicine. Berlin: Springer-Verlag, 2009.

Grocott, M.P.W., Mythen, M.G., Gan, T.J. Perioperative fluid management and clinical outcomes in adults. Anesth. Analg. 2005;100:1093–1106.

Lee, T.H., Marcantonio, E.R., Mangione, C.M., et al. Derivation and validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation. 1999;100:1043–1049.

Loftus I., ed. Care of the critically ill surgical patient, third ed., London: Hodder Arnold, 2010.

McConachie I., ed. Anaesthesia for the high-risk patient, second ed., Cambridge: Cambridge University Press, 2009.

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