Anaesthesia-related techniques

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Anaesthesia-related techniques

TECHNIQUES TO ASSESS PERIOPERATIVE RISK

Assess

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

Box 2.1   Goldman Risk Index Criteria

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).

Box 2.2   ACC/AHA Guidelines (2007)

Active cardiac conditions requiring further evaluation:

Clinical risk factors that may affect outcome:

Factors not proven to increase perioperative risk

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.

Box 2.3   A step-by-step approach to cardiac risk assessment

(after AHA/ACC 2009)

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).

Table 2.1

Risk of MI/cardiac death within 30 days of surgery

Low risk (<1%) Intermediate risk (1–5%) High risk (>5%)
Breast Abdominal Aortic and major vascular surgery
Dental Carotid Peripheral vascular surgery
Endocrine Endovascular aneurysm repair
Eye Head and neck
Gynaecology Neurosurgery
Plastic/reconstructive Major orthopaedic
Minor orthopaedic Renal transplant
Minor urology Major urology

Action

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

Assessment of resting left ventricular function

Trans-thoracic echocardiography and radionuclide angiography can be used to measure resting left ventricular (LV) function. Although an association has been demonstrated between poor LV ejection fraction (<40%) and an increased risk of adverse perioperative cardiac events, the predictive value of such tests is increased if dynamic images are taken under stress.

Aftercare

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.

β-blockers

Part of the physiological stress response to surgery is a catecholamine surge with increased heart rate and myocardial oxygen consumption. In surgical patients with known ischaemic heart disease, Mangano et al5 reported a reduced 2 year mortality after 7 days’ perioperative β-blockade.1 These findings were swiftly incorporated into new guidelines recommending use of β-blockade in patients with overt ischaemic heart disease or with risk factors. Subsequent studies produced more equivocal results and a more cautious approach followed, recommending use of β-blockers in high-risk patients rather than in all patients at risk.

Then came the POISE (PeriOperative Ischaemia Study Evaluation) study,6 which measured 30-day mortality and morbidity after oral metoprolol. There was a significant reduction in the number of cardiac events, but the overall mortality rate actually increased, with a significant excess of strokes – possibly because of the excess of patients suffering from hypotension and bradycardia amongst those treated.

More recent work again suggests that high-risk patients benefit from β-blockade – and certainly that withdrawal of established therapy is dangerous.

Close monitoring of blood pressure and heart rate intra- and postoperatively is, however, essential.

Other drugs

Angiotensin converting enzyme inhibitors (ACEI) are of proven benefit in reducing disease progression in patients with cardiac failure and it is postulated they may improve postoperative outcomes. They may, however, interact with anaesthesia to cause significant hypotension – hence common practice is to discontinue ACEI therapy 24 hours preoperatively, especially when prescribed for hypertension. In patients with stable chronic heart failure, it may be preferable to continue ACEI throughout the perioperative period, with appropriately close haemodynamic monitoring.

Statins are widely used in patients with cardiovascular disease because of their lipid-lowering effect. They also have plaque-stabilizing properties and have been postulated to reduce the incidence of perioperative myocardial infarction. Several studies have confirmed benefit, and it is recommended that statins be started preoperatively in high-risk surgical patients, and be continued throughout the perioperative period.

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.

REFERENCES

1. Goldman L, Caldera DL, Nussbaum SR, et al. Multifactorial index of cardiac risk in noncardiac surgical procedures. N Engl J Med 1977;297(16):845–50.

2. Detsky AS. Cardiac assessment for patients undergoing non cardiac surgery: a multifactorial clinical risk index. Arch Intern Med 1996;146(11):2131–4.

3. Lee TH. Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation 1999;100:1043–9.

4. Fleisher LA. ACC / AHA 2007 Guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery. J Am Coll Cardiol 2007;50(17):e159–242.

5. Mangano DT, Layug EL, Wallace A, et al. Effect of atenolol on mortality and cardiovascular morbidity after noncardiac surgery. Multicenter Study of Perioperative Ischaemia Research Group. N Engl J Med 1996;335(23):1713–20.

6. POISE Study Group. Effects of extended-release metoprolol succinate in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled trial. Lancet 2008;371:1839–47.

OXYGEN THERAPY

Appraise

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).

Box 2.4   Potential adverse consequences of hypoxaemia

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.

Box 2.5   Factors contributing to postoperative hypoxaemia

Assess

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.

Action

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:

PERIPHERAL VENOUS ACCESS

Appraise

Peripheral venous access is used for fluid and intravenous drug administration. When selecting an appropriate cannula (Fig. 2.4), it is important to remember that flow rates increase in proportion to the fourth power of the radius (Poiseuille’s law). Hence volume resuscitation requires a large-bore (14 G or 16 G), short cannula. Smaller diameter devices are suitable for maintenance fluids and/or drug administration.

Action

Having selected a cannula of suitable size, according to the indication, an appropriate site should be chosen. This will usually be the upper limb and, most conveniently, the dorsum of the hand or the radial border of the forearm (cephalic vein). It is preferable to avoid insertion sites over the wrist and elbow joints.

Venous access in the lower limb is generally avoided and carries a greater risk of thrombosis.

A meticulous aseptic technique should be employed, preparing the skin with 2% chlorhexidine. It should be routine practice to use 1% or 2% lidocaine (via a 25 G needle) to reduce discomfort when inserting cannulae of sizes larger than 20 G.

A few tips for successful peripheral venous cannulation are given in Box 2.6.

Box 2.6   Tips for successful peripheral venous cannulation

Try not to be rushed: allowing time for adequate venous distension from a proximal tourniquet will increase the chances of success.

Try to immobilize the vein distally with the free hand.

Keep the angle of insertion low (essentially parallel to the vein), making puncture of the posterior wall of the vein less likely.

Remember that the needle extends several millimeters beyond the tip of the cannula: rather than ‘freezing’ when a flashback is obtained, instead aim to confidently insert the needle/cannula assembly in one movement such that both are within the lumen: the needle can then be withdrawn slightly and the device advanced easily along the vein.

CENTRAL VENOUS ACCESS

Appraise

Central venous lines are usually multichannel devices comprising three to five lumens (ranging from 20 G to 14 G size). Indications for central venous access are summarized in Box 2.7.

Box 2.7   Indications for central venous access

Central lines allow simultaneous infusions of multiple drugs and fluids, in addition to measurement of central venous pressure. Single-lumen catheters are sometimes used for administration of total parenteral nutrition, since the risk of catheter-related sepsis is less compared to multi-lumen devices.

Traditional central venous lines are unsuitable for large volume resuscitation since flow rates decrease with increasing length. Special rapid infusor central lines are available if potential volume requirements exceed what can be provided through large-bore peripheral cannulae. There are essentially three possible approaches: internal jugular, subclavian and femoral.

Prepare

A central venous line insertion is by no means a risk-free procedure, and it is therefore important to take certain precautions. Any pre-existing coagulopathy should be corrected, especially if using a subclavian approach, where direct pressure cannot be applied. When preparing for an internal jugular or subclavian approach, it is important to exclude abnormalities on the contra-lateral side (such as pneumothorax or haemothorax) – otherwise a complicated line insertion may result in bilateral pathology and a risk of significant cardiorespiratory compromise.

Informed consent should be obtained, and full monitoring applied (3-lead ECG, pulse oximetry and non-invasive blood pressure monitoring).

Central venous lines remain the commonest source of hospital-acquired bloodstream infections, and an aseptic insertion technique (gown, gloves, mask, sterile drapes and 2% chlorhexidine skin preparation) is mandatory. A ready-made sterile pack containing gown, drapes, syringes, etc., is helpful.

Local skin infiltration with 1% or 2% lidocaine is required in the awake patient – who must be able to lie flat throughout the duration of the procedure.

To promote venous distension and to reduce the risk of air embolism, the patient may be placed in a head-up (femoral) or head-down (subclavian/internal jugular) tilt.

INSERTION TECHNIQUE

All multi-lumen lines are inserted using a Seldinger technique (needle, guidewire, dilator, line). The skin incision should be kept superficial but sufficiently generous to allow easy passage of the dilator – it is imperative not to use undue force.

Ultrasound guidance (Fig. 2.5) is strongly recommended for internal jugular lines, and has been demonstrated to reduce the incidence of complications. The surface landmark for the vein lies over a triangle formed from the two heads of sternomastoid (medial and lateral) and the clavicle (inferior). In the absence of ultrasound, the needle should be advanced at an angle of about 300 towards the ipsilateral nipple. A high approach reduces the risk of pneumothorax but increases the risk of arterial puncture – the converse is true of a low approach.

A subclavian approach tends to be more comfortable for the patient but carries a greater risk of pneumothorax and haemothorax. Ultrasound visualization is less reliable than with the internal jugular route and an inadvertent arterial puncture is not amenable to direct pressure. The needle entry point should be about one finger’s breadth inferior to the clavicle, at the junction of the outer and middle thirds, aiming towards the suprasternal notch (i.e. perpendicular to the sagittal plane of the body). It is important to begin sufficiently distant from the clavicle as to be able to pass under it with the needle almost horizontal – in so doing, the risk of pneumothorax will be reduced.

A CXR should be obtained after internal jugular or subclavian cannulation: the tip of the line should reside above the pericardial reflection, within the superior vena cava (not the right atrium): this corresponds to the level of the carina on the CXR.

A femoral line insertion is again facilitated by ultrasound guidance, and may be preferred in the presence of coagulopathy.

COMPLICATIONS OF CENTRAL VENOUS ACCESS

The most important complications are immediate damage to adjacent structures (manifesting as haemothorax, pneumothorax, etc.) and later-onset infection (Box 2.8).

Box 2.8   Complications of central venous line insertion

Strategies to reduce the risk of complications are listed below:

GENERAL ANAESTHESIA TECHNIQUES

Appraise

Within the operating theatre, the primary responsibility of the anaesthetist is to ensure patient safety during surgical procedures. Self-evidently, this involves measures to relieve pain and discomfort, but fundamentally, anaesthesia aims to minimize the physiological disturbance from surgery and to support vital functions – respiratory, cardiovascular, metabolic and so on, whilst providing suitable operating conditions for the surgeon.

Anaesthesia may be general, regional or local, and sometimes a combination of these: a careful preoperative visit will inform the decision-making process.

There is certainly evidence, for example, that combining epidural anaesthesia with general anaesthesia reduces respiratory complications after major abdominal surgery and promotes faster recovery of gut function, though no overall mortality benefit has been demonstrated.

In medically unstable or high-risk patients, evidence supports a period of ‘pre-optimization’, usually in an intensive care or high-dependency environment, with correction of fluid deficits prior to anaesthetic induction.

Action

Airway management

The unconscious patient requires support of the airway, which will otherwise become obstructed. To achieve airway patency requires ‘head tilt-chin lift’ and ‘jaw thrust’ procedures. The airway may then be maintained through a variety of techniques:

Endotracheal intubation

Endotracheal intubation provides the ‘definitive’ airway and is required if there is a significant risk of aspiration, or if positive-pressure ventilation requires high inflation pressures (for example, in the obese patient). Intubation is usually performed via the oral route, but nasal intubation may be indicated for oro-facial procedures or to allow longer-term ventilation, especially in children.

A key aspect of the preoperative assessment is an evaluation of the likely difficulty or otherwise of endotracheal intubation (and, even more importantly, of mask ventilation). Certain clinical features predict possible difficulty (Box 2.9).

Box 2.9   Clinical predictors of difficult intubation

Ventilation during anaesthesia

Following anaesthesia induction and after establishing a stable airway, ventilation must be continued throughout surgery: either a spontaneous or a controlled ventilation technique may be used.

Spontaneous ventilation is typically employed in slim patients requiring relatively minor and superficial procedures of short duration, and the airway is usually maintained with an LMA.

Controlled breathing, usually with neuromuscular blockade, is generally used in longer, more major procedures, in obese patients and in those with poor respiratory reserve. It is also required when there is a surgical need for full muscle relaxation (e.g. intra-abdominal surgery) or when tight control of the arterial PaCO2 is indicated (e.g. to control intracranial pressure). In appropriate situations, an LMA may be employed during controlled ventilation: for instance, when the anaesthetist wishes to avoid a hypertensive response to intubation in a relatively short procedure and when the patient’s anatomy is favourable.

Principles of monitoring

Indications for invasive monitoring

In complex cases, an enhanced level of monitoring may be indicated. An arterial line allows continuous, beat-to-beat recording of the arterial blood pressure (Box 2.11). Further advanced cardiovascular monitoring may include measurement of cardiac filling pressures (most commonly the central venous pressure), cardiac output and mixed venous oxygen saturation.

Box 2.11   Indications for invasive arterial pressure monitoring

Measurement of central venous pressure (CVP) gives an estimate of the cardiac pre-load, but becomes unreliable in the presence of impaired cardiac function. Nonetheless, observation of the trend in CVP and its response to fluid challenges provides useful information in respect of volume status both in the operating theatre and in intensive care.

Measurement of cardiac output may be extremely useful in both anaesthesia and intensive care. A variety of techniques are available: the pulmonary artery flotation catheter provides much haemodynamic information, but it is an invasive procedure and some controversy persists as to whether it confers a survival benefit.

More recently, less invasive techniques have emerged based on arterial pulse contour analysis, for example the PiCCO (using thermodilution) and the LiDCO (uses lithium dilution).

The oesophageal Doppler has been used successfully to guide fluid therapy, with demonstrable improvements in outcome amongst surgical patients. Trans-oesophageal echocardiography is a semi-invasive procedure requiring a high degree of operator skill, but gives information regarding filling status and contractility in addition to revealing structural abnormalities such as valve lesions or pericardial collections.

There is increasing interest in the use of central and mixed venous oxygen saturation (SvO2) to guide perioperative interventions. Venous oxygen saturation reflects the relationship between global oxygen delivery (reflecting cardiac output + arterial oxygen content) and consumption. A high SvO2 may reflect increased O2 delivery (e.g. inotrope therapy) or reduced O2 utilization (e.g. sedation/hypothermia/sepsis). Conversely, a low SvO2 reflects increased tissue O2 extraction (e.g. anaemia/hypoxia) or reduced O2 delivery (e.g. hypovolaemia/pulmonary embolism/heart failure).

Aftercare

The recovery room

The anaesthetist is responsible for the safe transfer of patients to the recovery room for continued care and 1:1 nursing following a detailed handover of all pertinent information including the procedure performed, analgesic/fluid therapy and nil by mouth status. Tracheal extubation requires the presence of the anaesthetist, and it is usually safer for this to take place in theatre before transfer. LMAs are more usually removed in recovery. Monitoring of oxygen saturation and NIBP are essential, as are facilities for more intensive monitoring if required. The recovery bed space should contain all essential airway equipment.

With guidance from the anaesthetic and surgical teams, the recovery nursing staff will monitor the patient’s vital signs and operative site, and attend to common postoperative problems such as pain and nausea. A further detailed handover should take place between the recovery and ward nursing staff.

Levels of postoperative care

An ever-increasing number of patients undergo day case surgery or are discharged from hospital within 24 hours. For those who require inpatient management postoperatively, several levels of care are defined (Box 2.12).

Box 2.12   Levels of postoperative care

Level 0

For patients whose needs can be met through normal ward care in an acute hospital

Level 1

For patients at risk of deteriorating (or after recent transfer from a higher level of care) whose needs can be met on an acute ward with advice/support from ITU team

Level 2

For patients requiring more intensive observation monitoring (e.g. invasive BP) and/or single organ support (not mechanical ventilation)

Level 3

For patients requiring mechanical ventilation and/or support of other organs

Analgesic techniques

A robust strategy for managing postoperative pain is essential since untreated pain has a variety of adverse consequences (Box 2.13).

Box 2.13   Adverse effects of postoperative pain

Psychological

Anxiety/distress/sleep disturbance/loss of confidence in healthcare team

Socio-economic

Prolonged hospital stay/delayed rehabilitation and return to work.

The pharmacological management of acute pain includes:

Pre-emptive analgesia (i.e. before the skin incision) may have an impact on postoperative pain.

Most acute hospitals run an acute pain service, under the guidance of a consultant anaesthetist and usually run by a clinical nurse specialist. The team predominantly looks after patients with epidurals and patient-controlled analgesia (PCA) devices, but also offers staff education and training, and provides advice in difficult situations.

LOCAL ANAESTHESIA TECHNIQUES

Local anaesthesia is widely used, as a sole technique or as an adjunct to general anaesthesia. Local anaesthetic agents are potentially dangerous, and a knowledge of safe doses and of the management of suspected toxicity is paramount. These subjects are discussed, together with examples of a few blocks in common use.

Appraise

Operative procedures are frequently undertaken under local anaesthesia (LA), both in and out of theatre. LA techniques are well suited to minor procedures, and cause less systemic upset than general anaesthesia.

LA agents may be administered in a variety of ways according to the required area of analgesia:

LOCAL ANAESTHETIC AGENTS

Various LA agents are available, and are classified into two groups – esters and amides – according to the structure of their carbonyl linkage group. The agents in most common clinical use (lidocaine, bupivacaine and prilocaine) are all amides.

LAs block sodium channels to cause a reversible interruption of nerve impulse conduction. Most are weak bases and will exist in both ionized and unionized forms according to the pH of the tissue fluid. LAs are relatively ineffective in an acid pH (e.g. inflamed or infected tissues), in which the ionized (non-lipid soluble) form predominates.

Addition of a vasoconstrictor (e.g. adrenaline (epinephrine)) prolongs the duration of action of LAs. Epinephrine is added to LA in concentrations ranging from 1:80 000 to 1:300 000. The commonest strength is a 1:200 000 (5 μg per ml) concentration of adrenaline (epinephrine) (Box 2.14).

Box 2.14   Preparation of LA solutions

Adrenaline (epinephrine) may cause tachycardia and hypertension, and should be used with caution in patients with cardiovascular disease. The use of adrenaline (epinephrine) is absolutely contraindicated in areas supplied by end arteries (e.g. digits, penis).

Important features of the different LA agents are summarized in Table 2.2.

It is always sensible to calculate the maximum safe dose for the individual patient: for example, the maximum safe dose of lidocaine is 3 mg/kg without adrenaline (epinephrine) and 7 mg/kg with adrenaline (epinephrine). In a 70 kg adult, therefore, the maximum safe dose of plain lidocaine is 210 mg. This equates to 21 ml of a 1% solution (10 mg/ml). If larger volumes are required, the concentration should be reduced, or adrenaline (epinephrine) added.

LOCAL ANAESTHETIC TOXICITY

All local anaesthetics may exert toxic effects if administered in excess of the safe maximal dose. Systemic absorption is influenced by the site of injection (more rapid in vascular tissues, e.g. intercostal blocks) and by the addition of adrenaline (epinephrine) (slows absorption). Inadvertent intravascular injection may cause rapid cardiovascular and central nervous system collapse.

Strategies to reduce the risk and/or impact of LA toxicity include:

LA toxicity typically presents with clinical features relating to the central nervous and cardiovascular systems:

INTRAVENOUS REGIONAL ANAESTHESIA

Intravenous regional anaesthesia (IVRA) was first described for forearm anaesthesia (Bier’s block), but can also be used on the lower limb and for sympathetic blocks in chronic pain states.

A dilute solution of LA is injected intravenously into an exsanguinated limb kept isolated by a tourniquet cuff from the rest of the circulation.

The block is technically simple (Box 2.15) yet potentially dangerous: escape of LA into the systemic circulation may cause severe toxicity. Prilocaine 0.5% (without adrenaline (epinephrine)) is thought to be the safest agent (maximum 6 mg/kg or up to 300 mg).

Box 2.15   Technique for intravenous regional anaesthesia

The most important potential complication is systemic LA toxicity from cuff failure. The tourniquet may produce pressure-related damage. The technique is not suitable in the grossly obese, in hypertensive patients (systolic BP > 200 mmHg) or in those with peripheral vascular disease.

CENTRAL NEURO-AXIAL BLOCKS

Spinal or subarachnoid block and epidural blocks are the major neuro-axial techniques.

SPINAL ANAESTHESIA

The introduction of LA solutions into the cerebrospinal fluid (CSF) produces spinal anaesthesia. The LA does not have to cross tissue barriers and the central attachments of the ventral and dorsal nerve roots are un-myelinated, which allows for a rapid uptake of the LA drug. There is a rapid onset of effect (within a few minutes with lidocaine but up to 20 minutes for bupivacaine) and the dose of drug required is small (2 to 4 ml). Lidocaine (5%) or heavy bupivacaine (0.5%) are commonly used. This is a ‘one-shot’ technique and the duration of action should be adequate to perform the intended surgery. Offset may be as rapid as 30–40 minutes following lidocaine and 90–120 minutes following bupivacaine, although the addition of adrenaline (epinephrine) will prolong the duration of the block. Spinal anaesthetics are useful for urological and gynaecological procedures, lower limb surgery and also obstetric procedures.

EPIDURAL ANAESTHESIA

This can be used as a sole anaesthetic for procedures involving the lower limbs, perineum, pelvis and lower abdomen. It is possible to perform upper abdominal and even thoracic procedures under epidural anaesthesia alone, but the height of the block required, with its attendant side-effects, makes it difficult to avoid patient discomfort and risk. The advantage of epidural over spinal anaesthesia is the ability to maintain continuous anaesthesia after placement of an epidural catheter, thus making it suitable for procedures of a longer duration. This feature also enables the use of the technique into the postoperative period for analgesia, using lower concentrations of local anaesthetic drugs or in combination with different agents, usually opiates.

Technique

The tip of a hollow bored needle with a bevelled end (Tuohy needle) is introduced into the epidural space, after it has passed through the ligamentum flavum. The epidural space is really only a potential space, as the dura and ligamentum flavum are usually closely adjacent. The epidural space contains adipose tissue, lymphatics and the epidural veins. The space has to be carefully identified as the bevel of the needle passes through the ligamentum flavum as the dura will be penetrated shortly after if the needle is advanced any further. The most common method used is pressure applied to a syringe attached to the Tuohy needle, and a sudden loss of resistance is felt as soon as the epidural space is entered. Saline or sometimes air is used in the syringe. The block is usually performed with the patient awake and in the sitting position or sometimes the lateral decubitus position.

The quality and extent of the block is determined by the volume as well as the total dose of the drug. The spread of the block may be more extensive in pregnancy as the volume of the space is reduced by venous engorgement.

INDICATIONS FOR EPIDURAL ANAESTHESIA/ANALGESIA

1. Hip and knee surgery: Internal fixation of a fractured hip is associated with less blood loss when central neuro-axial blocks are used. The incidence of deep vein thrombosis is reduced in patients undergoing total hip and knee replacement under an epidural technique.

2. Vascular reconstruction of the lower limbs and endovascular arterial reconstructions: Epidural anaesthesia improves distal blood flow and can be used as the sole anaesthetic technique. Patients undergoing lower limb amputation may have a reduced incidence of phantom limb pain if neuro-axial blockade is established before surgery.

3. Postoperative pain relief following abdominal and thoracic surgery: Low concentration bupivacaine (0.125%), often in combination with an opioid such as fentanyl or preservative-free morphine provides effective pain relief. It also minimizes the effects of surgery on cardiopulmonary reserve, such as diaphragmatic splinting and the inability to cough effectively. This is especially important in patients with compromised respiratory function, e.g. chronic obstructive airways disease, morbid obesity and the elderly. Adequate analgesia allows better cooperation with chest physiotherapy. Epidural analgesia also facilitates earlier mobilization and reduces deep vein thrombosis.

EFFECTS ON ORGAN SYSTEMS

Cardiovascular: Sympathetic blockade (sympathetic outflow T1–L2) results in vasodilatation of resistance and capacitance vessels, causing relative hypovolaemia and tachycardia, with a resulting fall in blood pressure. This is managed with fluid loading and/or a vasoconstrictor. If the block is as high as T2 the sympathetic supply to the heart (T2–T5) is also interrupted, leading to bradycardia.

Respiratory: Usually unaffected, unless the blockade is high enough to affect the intercostal muscle nerve supply (thoracic nerve roots) leading to reliance on diaphragmatic breathing alone.

Gastrointestinal: Blockade of the sympathetic outflow to the GI tract leads to a predominance of parasympathetic (vagus and sacral parasympathetic) tone, with active peristalsis and relaxed sphincters and a small contracted gut which can enhance surgical access. Urinary retention is a common problem with epidural anaesthesia.

CONTRAINDICATIONS

EPIDURALS AND ANTICOAGULANT THERAPY

The incidence of epidural haematoma is unknown but it has increased since the use of low-molecular-weight heparin (LMWH) therapy for thromboembolic prophylaxis. Over 80% of epidural haematomas are related to haemostatic abnormalities or procedural difficulties with catheter insertion.

MANAGEMENT OF SURGICAL PATIENTS RECEIVING LONG-TERM ANTICOAGULANT OR ANTIPLATELET THERAPY

Increasing numbers of patients are receiving anticoagulant or anti-platelet therapy. When such patients require surgery, a balance of risks must be considered:

ANTICOAGULANT THERAPY

There are a number of indications for long-term anticoagulant therapy, including the presence of atrial fibrillation, a prosthetic heart valve or a history of arterial or venous thromboembolism.

Patients treated with a vitamin K antagonist (VKA) may require interruption of anticoagulation prior to surgery. Frequently, either LMWH or unfractionated heparin (UFH) is used to bridge the gap in therapy since these agents have a relatively rapid onset and offset of action compared to warfarin.

Patients on oral anticoagulants undergoing elective surgery

The key issues are:

The difficulties presented by these issues are reflected in the current wide variation in practice regarding bridging therapy for perioperative anticoagulation.

Procedures which do not require warfarin interruption: Patients on warfarin may undergo minor procedures such as dental extraction without discontinuing their treatment, provided their INR is in the therapeutic range and they receive tranexamic acid mouthwashes.1

For minor dermatological and ophthalmological (e.g. cataract extraction) procedures, it is also recommended that patients do not stop their VKA therapy.

Stratification of thromboembolism risk

image Atrial fibrillation: approximately 50% of all patients receiving warfarin therapy have atrial fibrillation (AF) which is, therefore, the most common clinical condition requiring a decision about bridging therapy. The average risk of perioperative stroke in patients with AF who do not receive antithrombotic therapy is 4.5%. The risk can be further stratified based on a ‘CHADS’ score (1 point each for congestive cardiac failure, hypertension, age > 75 years and diabetes, and 2 points for history of stroke or transient ischaemic attack). The American College of Physicians recommends low dose LMWH or no bridging for a score of 0–2, and bridging with therapeutic LMWH or UFH for CHADS scores of 4 and above. Intermediate levels of risk can be managed with higher prophylactic doses of LMWH.

image Mechanical heart valves: the risk of thromboembolism is such that bridging therapy is essential. The risk varies according to the type of valve and also its position (mitral > aortic). If the patient’s target INR is 3, then bridging therapy with therapeutic/full-dose LMWH is required. If the target INR is 2.5, then low-dose LMWH bridging is sufficient. Whenever surgery is planned, the risk of procedure-related bleeding must be balanced against the possible risk of thromboembolic events.

image Venous thromboembolic disease (VTE): therapeutic dose bridging is recommended for high-risk patients. These include patients who have suffered an episode of VTE within the previous 3 months, or those with known thrombophilia (such as deficiency of Protein S, Protein C or antithrombin III, or the presence of antiphospholipid antibodies). Moderate-risk patients (e.g. those with VTE within 3–12 months or with Factor V Leiden mutation) also require full-dose bridging therapy. Low-risk patients require either no bridging or prophylactic dose LMWH only.

Bleeding risk with bridging therapy: The risk of surgery when a patient is on full-dose bridging therapy varies markedly with the type of surgery. The risk of major bleeding is low for minor surgery such as inguinal hernia repair, but for major surgery, including knee and hip replacement, the risk of major bleeding is significantly greater. LMWH bridging therapy should be stopped 24 hours before surgery and therapeutic doses resumed 24–48 hours postoperatively. Low-dose LMWH may be considered as an alternative option during resumption of anticoagulant bridging after major surgery. LMWHs are very dependent on adequate renal function for their elimination, and reduced doses may be required in the presence of renal impairment or in the very elderly. In general, monitoring of LMWH activity is not required, but factor Xa levels can be measured where necessary.

A scheme for management of perioperative bridging therapy according to the risk of thromboembolic events is presented in Table 2.3.

New oral anticoagulant drugs

Warfarin has a variable dose–response, a narrow therapeutic index and numerous drug and dietary interactions, and requires frequent monitoring.

Recently, new oral anticoagulant drugs have been developed for the prevention and treatment of thromboembolic disease and also for the prevention of stroke in patients with atrial fibrillation. These drugs are given once a day, have a wider therapeutic index and do not require monitoring:

Emergency surgery in patients on anticoagulant therapy

For patients on warfarin therapy requiring urgent surgery or if there is life-threatening bleeding (e.g. intracranial) give prothrombin concentrate concentrate (PCC) 20–50 units/kg and 5 mg of vitamin K intravenously. PCC contain factors II, VII, IX and X and produces rapid and effective reversal. Fresh frozen plasma is no longer recommended as a means of reversing warfarin therapy. UFH can be readily reversed with protamine (50 mg), but protamine is far less effective at reversing the anticoagulant effects of LMWH. There is no reversal agent for the new oral anticoagulant drugs – however, recombinant VIIa (NovoSeven™) 90 μg/kg, has been suggested as a possible agent in these circumstances. It is advisable to seek guidance from a haematologist.

ANTIPLATELET THERAPY

The two most commonly used antiplatelet drugs are aspirin, which irreversibly inhibits platelet cyclo-oxygenase-1 (COX-1), and clopidogrel, which binds irreversibly to the platelet ADP P2Y12 receptor. Dual therapy is known to provide more effective platelet inhibition, as the effects are synergistic and is routine therapy in patients who have received drug eluting stents (DES). These drugs inherently increase bleeding risk but discontinuing them will in many patients lead to an increased risk of thrombosis.

Stratifying the bleeding risk: Because there is considerable inter individual variability in response to both aspirin and, especially, clopidogrel therapy, some patients may be at greater risk than others for adverse bleeding outcomes. It is now becoming apparent that the degree of platelet inhibition in patients treated with the same antiplatelet regime is highly variable and up to 30% of patients may show no demonstrable platelet inhibition on standard therapy. This has implications not only for the risks of recurrent ischaemic events in ‘hypo-responders’, but at the other end of the spectrum for bleeding risks in ‘hyper-responders’. In terms of antiplatelet therapy there is undoubtedly an optimal therapeutic window, but there are many challenges left to define the best method for monitoring platelet function and to identify ‘cut-off’ values where the risk of ischaemic events or bleeding becomes a significant risk. There is accumulating evidence that bleeding risk increases as the degree of irreversible platelet inhibition increases. Prasugrel is a third generation thienopyridine that achieves 4–5 times more potent ADP P2Y12 receptor blockade than clopidogrel. It significantly reduced ischaemic events in the TRITON – TIMI trial but the occurrence of major bleeding was also significantly increased. Prasugrel is increasingly used in patients with coronary stents who have had a poor response to conventional therapy. Point of care platelet function monitoring as a means of assessing the efficacy of these drugs is still under evaluation, but the most promising techniques in terms of assessing bleeding risk are platelet mapping™, which is a modification of the thromboealstographic technique and the Multiplate® analyser.

Perioperative management: It is currently recommended that patients on aspirin as primary prevention should continue therapy up until the day of surgery and those on clopidogrel should discontinue treatment at least 5 days prior to surgery. However, there are serious thrombotic risks associated with the discontinuation of these agents when they are used for secondary prevention of vascular disease or after coronary revascularization. It is generally agreed that aspirin should never be discontinued before surgery unless the risk of bleeding is thought unacceptable, e.g. intracranial surgery. Clopidogrel alone appears to increase the bleeding risk more than for aspirin alone. Dual therapy increases the relative risk of bleeding by 50% and the absolute risk by 1%. This risk remains increased in patients who stopped clopidogrel less than 5 days before surgery.

The difficulty is how to manage patients who have to stay on their antiplatelet therapy because they have coronary stents (see also management of antiplatelet therapy in previous section) or who currently are on treatment because they are presenting as an emergency or have been asked to stop their therapy prior to elective surgery and have forgotten to do so. A multidisciplinary approach to this problem is essential, with discussion between the patient’s cardiologist, surgeon and anaesthetist. Prior to surgery, patients on dual therapy should always continue their aspirin until the day of surgery. If it is felt prudent to discontinue clopidogrel, bridging therapy with UFH or LMWH will be necessary.

In patients on antiplatelet therapy requiring urgent surgery, platelet concentrates should be ordered and available for transfusion if required. Without assessment of the degree of platelet inhibition, the increased risk of perioperative bleeding is undefined and prophylactic transfusion is generally unjustified. Patients who are taking Prasugrel will almost certainly have significant platelet inhibition and would probably benefit from platelet transfusion prior to surgical procedures with a high risk of bleeding.

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