Drug management of postoperative pain

Many different methods of pain relief are available, using several different routes of administration. Opiates and derivatives (such as morphine, pethidine, fentanyl) make up a large proportion of these drugs and morphine arguably remains the benchmark drug (Barrat 1997). Table 12.1 gives a summary of common routes of administration of drugs. In Table 12.2 a list of the common drugs used for postoperative pain is shown, together with their potential side effects.

Table 12.1 Common routes of drug administration for postoperative analgesia

Table 12.2 Information about commonly used analgesic drugs

Opioids are drugs that bind with specific opioid receptors. They mimic endogenous peptide transmitters involved in pain modulation and act principally within the central nervous system. Non-steroidal antiinflammatory drugs (NSAIDs), such as indomethacin, are also used in management of acute postoperative pain and can act as opioid sparing agents. They decrease production of prostaglandins that sensitize nociceptor nerve endings to inflammatory mediators and also have an antipyretic effect (reduce fever). Local anaesthetic agents such as bupivacaine, block the initiation and propagation of action potentials by blocking sodium channels. They are used to produce nerve root blocks and to depress action potentials in sensory neurons, thereby reducing pain (NHMRC 2005).

The use of multimodality analgesia rather than single analgesic administration is more common in the new millennium as is the existence of hospital pain management teams led by anaesthetists and nurses.

Opiates have significant respiratory depression effects (Sabanathan et al 1999) and they are only partially effective in relieving pain. Richardson and Sabanathan (1997) report that since opioids may only relieve pain transmitted by C fibres, where opioid receptors exist, the sharp pain transmitted by A fibres still exists. More recently, use of local anaesthetics (bupivacaine, ropivacaine) by the epidural route has gained more favour in postoperative pain management, often used in combination with opioids. The introduction of these multimodal methods has also seen an increase in the use of NSAIDs such as indometacin. These are used as opioid sparing agents, but have also been shown to improve analgesia by a reduction in inflammation and stress inhibition (Richardson & Sabanathan 1997). There is level 1 evidence that oral administration of NSAIDs is as effective as intravenous (National Health and Medical Research Council 2005). Oral paracetamol or narcotic drugs may also be added to these regimens. Multimodal or balanced analgesia has been developed in response to the commonly associated side effects of monotherapy – nausea, vomiting, paralytic ileus and respiratory depression in the case of opioids and urinary retention, motor block and hypotension in the case of local anaesthetic agents. Balanced analgesia was also developed to allow early postoperative ambulation and enteral feeding, which minimize the respiratory and gut complications associated with use of opioids, leading to earlier patient discharge from hospital. The drugs are used in smaller doses than if used separately and provide effective pain relief as a result of their synergistic actions (Peeters-Asdourian & Gupta 1999).

The most common methods for pain control following abdominal surgery are patient-controlled analgesia (PCA) with intravenous opioids (Fig. 12.1) and continuous epidural analgesia delivering a combination of local anaesthetic and opioids (Fig. 12.2). In a recent systematic review it was concluded that continuous epidural analgesia is superior to PCA in relieving postoperative pain for up to 72 hours in patients undergoing intra-abdominal surgery. Similarly a large review of analgesia reports that all techniques of epidural analgesia provide better postoperative pain relief compared with parenteral opioid administration (NHMRC 2005). However, PCA remains a common method of analgesic delivery, as patients often prefer it (Werawatganon & Charuluxanun 2005). Continuous infusion may be associated with increased risk of respiratory depression compared with bolus IV administration and evidence for improved pain relief with continuous administration is lacking (NHMRC 2005).

Figure 12.1 A microprocessor pump (IMED) for delivery of patient-controlled analgesia.

Figure 12.2 Spinal and epidural anaesthesia. (A) Position of needle in subarachnoid space for spinal anaesthesia. (B) Position of needle in epidural space for epidural anaesthesia/analgesia.

Non-pharmacological methods for managing postoperative pain are generally perceived to be adjuncts to pharmacological methods, but there is growing evidence for the value of their contribution (National Health and Medical Research Council 2005). Education including providing procedural information about treatment (such as provided by physiotherapists preoperatively), combined with sensory information describing the sensory experiences a patient may expect and information regarding coping strategies, may be effective in reducing negative affect, pain medication use and improving clinical recovery after surgery (National Health and Medical Research Council 2005). Preoperative education may encourage a more positive attitude toward pain relief, although there is no evidence that preoperative education about pain has any effect on postoperative pain after cardiac surgery (National Health and Medical Research Council 2005). Implementation of an acute pain management service may also improve pain relief.

Measurement of pain

Pain is difficult to measure as it is a purely individual and sensory experience (Dodson 1985). Postoperative pain is acute and initiated by tissue injury during surgery, but reduces with time and the natural healing process. The measurement of pain is often necessary to assess the results of an intervention or to measure intensity of pain, such as postoperative pain. Regular measurement of pain leads to improved acute pain management. Most measures of pain are based upon self-report but can provide sensitive and consistent results if performed properly (National Health and Medical Research Council 2005).

Several different instruments may be used to measure pain, these include:

numerical rating scales

verbal descriptor scales (VDS)

pain questionnaires, such as the McGill

the visual analogue scale (VAS).

Except for the McGill pain questionnaire, these methods are unidimensional; that is, they only measure intensity of pain in absolute terms or changes in pain intensity. Verbal scales may use words that have different meanings for different people, such as mild, moderate or severe pain. These categorical scales are quick and simple but less sensitive than numerical rating scales such as the visual analogue scale (VAS) (National Health and Medical Research Council 2005). The McGill questionnaire uses 20 groups of two to six words and the patient is asked which word in each group best describes their pain. While this offers more valid information than VDS, it is very time consuming and it not used extensively for the measurement of acute postoperative pain. Verbal rating scales are commonly used in clinical practice and use of the VAS is the most commonly used method. Visual analogue scales usually consist of a straight line, 10 cm long, the extremes of which are taken to represent the limits of the subjective experience being measured. In the case of pain measurement, one end of the line may be defined as ‘no pain’ and the other as ‘severe pain’ or ‘worst possible pain’. The subject is asked to place a mark on the line corresponding to the severity of their pain. The distance from the mark to the end of the scale is taken to represent pain severity. The most common way to use a VAS in the study of postoperative pain is to ask the patient to score the pain they are experiencing at the time of completion of the VAS. Visual analogue scales may also be used to obtain a pain score that reflects pain or pain relief over the preceding 24 hours. Commonly, physiotherapists ask patients to rate their pain on activity in the postoperative period to provide more meaningful information. The VAS has been shown to be a linear scale for patients with postoperative pain of mild to moderate intensity. Therefore results are equally distributed across the scale so that the difference between each number on the scale is equal. It is reported that values greater than 70 mm are indicative of severe pain while values between 45 and 74 mm represent moderate pain and those between 5 and 44 mm mild pain (National Health and Medical Research Council 2005).

Box 12.1 shows the physiotherapy key points with regard to analgesia.

Box 12.1 Physiotherapy key points with regard to analgesia

Always assess the adequacy of pain relief before treatment.

Always note if the patient has pinpoint pupils and is drowsy.

Always check vital signs, particularly respiratory rate and blood pressure, as hypotension is a common side effect of pain management. Most important in position changes.

If a patient has had a spinal block or epidural alwaysassess motor and sensory function of the lower limbs, especially before upright mobilization.

Ask the patient if they need to use their PCA or PCEA before a physiotherapy treatment session or ask about a bolus dose of analgesia before treatment.

Always liaise with medical and nursing staff before treating the patient and know the local guidelines for mobilizing patients with an epidural in situ.

PCA, patient-controlled analgesia; PCEA, patient-controlled epidural analgesia

Lung volumes

The characteristic abnormality of respiratory mechanics following major surgery is a restrictive ventilatory defect manifest by changes in vital capacity (VC) and functional residual capacity (FRC) (Wahba 1991). The VC can reduce to 40% of preoperative values, while the FRC may gradually reduce to be 70% of preoperative value at 24 hours postoperatively. These changes may persist for 5–10 days following surgery (Craig 1981). The timing of greatest reduction in FRC, while varying between studies, is generally on the first or second postoperative day. In morbidly and even mildly obese patients there is a significant reduction in FRC compared with patients within the ideal weight range (Jenkins & Moxham 1991). Although most other lung volumes also reduce following major surgery, it is thought that the reductions in FRC represent the most clinically important changes because of the functional consequences. The alteration in lung volumes after lower abdominal and laparoscopic surgery is less pronounced.

Functional residual capacity and closing capacity

FRC may be affected by a number of factors. It is linearly related to height and is 10% less in females for the same body height (Nunn 1993). The FRC is affected by gravity and therefore body position. In supine the abdominal contents push the diaphragm cephalad, reducing intrathoracic volume and FRC. In normal subjects FRC is reduced by approximately 500–1000 ml upon adopting the supine position (Macnaughton 1994). It is highest in standing and reduces with recumbency (Fig. 12.3). A change in body position from bed to sitting in a chair increased the FRC by 17% in 10 patients following upper abdominal surgery.

Figure 12.3 Functional residual capacity in different body positions.

(Adapted from Nunn 1993 p 55 with permission from Butterworth-Heinemann, Oxford)

The relationship of FRC with the closing capacity of the lungs explains the functional significance of perioperative reductions in FRC. Closing capacity (CC) is defined as the lung volume at which dependent airways begin to close, or cease to ventilate (Macnaughton 1994). The small airways (less than 1.0 mm diameter) in the periphery of the lung are not supported by cartilage and are therefore influenced by transmitted pleural pressures. Normally the transpulmonary pressure or distending pressure is less than atmospheric, producing a positive pressure, which distends the lungs. Breathing at lower lung volumes produces a higher pressure in gravity-dependent lung regions. This produces a negative distending pressure and causes small, unsupported airways to narrow or close, resulting in reduced ventilation. The relationship between FRC and CC is an important determinant of dependent airway closure. If the CC exceeds the FRC, as it does during tidal breathing in the upright position in people over 70 years of age, then dependent lung regions underventilate, resulting in / mismatch and hypoxaemia. The relationship between CC and FRC is responsible for the reduction in arterial oxygen tension (PaO₂) with increasing age. Closing capacity increases with age due to loss of elastic lung tissue. It also increases in chronic lung disease and with cigarette smoking, due also to changes in lung elasticity. In combination with these increases in CC, any factor which at the same time reduces FRC will significantly affect the relationship between the two volumes, such that dependent airway closure occurs during normal tidal breathing (Macnaughton 1994) (Fig. 12.4).

Figure 12.4 The relationship between functional residual capacity (FRC) and closing capacity.

(Adapted from Nunn 1993 p 56 with permission from Butterworth-Heinemann, Oxford)

Anaesthesia, surgery and recumbency reduce FRC. The reduction in FRC together with possible increases in CC in some subjects may account for the regional changes in ventilation that occur perioperatively and lead to reduced compliance, altered ventilation and perfusion (/), arterial hypoxaemia and atelectasis from absorption of trapped gas behind closed airways. Postoperative hypoxaemia is inevitable, but usually subclinical, and like FRC, is usually lowest on the first and second days after surgery. Therefore supplemental oxygen is routinely given postoperatively (Craig 1981, Fairshter & Williams 1987).

The mechanisms for the perioperative reductions in FRC and VC in upper abdominal surgery (UAS) are thought to be: mechanical disruption of the thorax and abdomen, absence of spontaneous sighs, shallow breathing, pain and inhibition of diaphragmatic function (Wahba 1991). Abdominal distension, presence of a nasogastric tube and the use of analgesics to control postoperative pain may impact on ventilatory function.

Mucociliary clearance

Mucociliary clearance is a major function of the airway epithelium. This important function depends both on the physicochemical properties of the airway mucus and on the activity of the cilia (Kim 1997). Anaesthesia, intubation, mechanical ventilation reduced lung volumes and reduced cough effectiveness perioperatively present a significant insult to the mucociliary escalator. A summary of the perioperative contributions to altered mucociliary clearance is given in Box 12.2.

Box 12.2 Factors promoting postoperative mucociliary dysfunction

Anaesthetic agents

Endotracheal intubation

Pain medication

Higher inspired oxygen concentrations and airway humidification

Atelectasis

Reduced lung volumes

Reduced cough efficiency

Increased viscosity of secretions

(Adapted from Denehy and van de Leur 2004)

Respiratory muscle function

Diaphragmatic excursion has been shown in research literature to be reduced following abdominal and thoracic surgery and postoperative pain may contribute to diaphragm dysfunction. This is associated with reduced VC, changed pattern of ventilation to predominately rib cage movement rather than abdominal movement and postoperative hypoxaemia. The reduced function may last up to 1 week following surgery. Reflex inhibition of the phrenic nerve may occur after UAS, but research findings are inconsistent. The breathing pattern observed after abdominal surgery may act as a protective mechanism by splinting the abdomen, allowing faster healing of abdominal incisions and reducing the risk of peritoneal infection (Ford et al 1993).

Postoperative pulmonary complications

One of the aims of physiotherapy in the perioperative period is to counteract the adverse pulmonary changes produced as a result of the surgical process (Stiller et al 1994). The factors described above; dependent atelectasis, hypoxaemia and altered V, although occurring in most patients, can lead to clinically significant PPC in some patients after surgery. Two basic theories have been proposed to explain the pathogenesis of PPC following major surgery. Firstly, regional hypoventilation and blockage of airways by mucus and, secondly, absorption of alveolar air distal to a mucus plug in the proximal airways, may lead to eventual collapse unless fresh air enters through collateral channels (Marini 1984). Regional hypoventilation results from reductions in FRC and the altered relationship between FRC and CC together with diaphragmatic dysfunction postoperatively as discussed above. The precise sequence and relative contributions of each of the two mechanisms for developing PPC is unclear. It is possible that they vary among patients and that both alveolar hypoventilation and secretion plugging coexist to contribute to postoperative lung changes (Denehy & Browning 2007). Other risk factors that may predispose to increased risk of mucus plugging may be a history of cigarette smoking, weak cough, prolonged intubation, presence of a nasogastric tube and prolonged postoperative atelectasis (Smith & Ellis 2000).

The definition of PPC can include atelectasis or pneumonia (atelectasis and collapse are terms that are often used interchangeably). Significant PPCs have been defined as complications that alter the patient’s clinical course (O’Donohue 1992). PPC may be defined by using radiological and bacteriological criteria, clinical signs and symptoms, or a combination of these (Pasquina et al 2006). The definition of PPC impacts on the incidence obtained. Using only radiological evidence of atelectasis for example, gives a higher incidence of complications than using a combination definition. To date, no valid definition has been established and as a result the definitions used are variable.

The clinical signs and symptoms of PPC may include the following:

arterial desaturation (measured using a pulse oximeter), often defined as <90% on two consecutive days

radiological evidence of atelectasis or pneumonia (routine chest X-rays are not common after surgery)

raised oral temperature (febrile is >37.5°C), often defined as >38°C on more than one consecutive day as a raised temperature on the first day after surgery is a common finding resulting from the surgical insult

production of yellow or green sputum (where different to preoperative)

abnormal lung auscultation (given that the majority of patients have some dependent atelectasis, reduced breath sounds are commonly found in the first 2 days after surgery)

bacterial growth on sputum microbiology

otherwise unexplained raised white cell count >11 × 10/l)

prescription of an antibiotic specific for lung infection (many patients are given routine antibiotics immediately postoperatively depending on type of surgery).

Many of these signs and symptoms occur in patients after major surgery and a combination of three or four, occurring together, could be considered as clinically significant PPC.

The incidence of postoperative atelectasis is reported to be 70% following UAS but the incidence of clinically significant complications is reported to range from 5% to 20% (Denehy et al 2001). There is a lower reported incidence following cardiac surgery of around 5%–7% (Brasher et al 2003, Pasquina et al 2003). The incidence following thoracic surgery varies but is reported to be 8–32% (Gosselink et al 2000). A higher incidence of 16% (Law & Wong 2006) to 30% (Gosselink et al 2000) is reported in patients following oesophageal surgery.

Risk factors for postoperative pulmonary complications

Advances in operative technique and postoperative patient management have led to surgery of increasing complexity being performed routinely in patient populations with more severe comorbidities. Estimation of surgical risk is therefore important for all health professionals involved in the management of surgical patients. Surgical risk is the probability of morbidity and mortality secondary to the presence of pre-, intra- and postoperative risk factors. This discussion will be limited to the development of PPC that most affects physiotherapists. Assessment of risk of developing PPC is important for the physiotherapist as it allows prioritized respiratory care for high-risk subjects and more appropriate use of often scarce resources in physiotherapy staffing. Surgical complications such as wound breakdown, bleeding, renal failure and other respiratory problems such as development of pulmonary embolus (PE) will not be discussed in detail. However, it is important to consider a PE in differential diagnosis of a respiratory complication such as pneumonia. The symptoms of both may be quite similar and include pleuritic chest pain, moderate to severe hypoxaemia, breathlessness and fever. Anticoagulation with heparin and then warfarin is indicated for PE. The identification of any pain in the calf on assessment is also important as it may indicate a deep vein thrombosis (DVT), although clinical diagnosis of DVT is unreliable with 50% of patients with DVT on venography showing symptoms.

Several patient characteristics are associated with an increased risk of developing complications. There is a large volume of literature published on this topic. In a systematic review (Fisher et al 2002), 40 variables were reported as possible risk factors for patients having non-thoracic surgery. There have been attempts to find a group of risk factors (model) that predict most complications in a particular patient population; several different models currently exist and none provide perfect prediction. The most common patient, operation and postoperative factors considered to increase risks of developing a PPC are described below. In physiotherapy research, a weighted model was developed to predict the risk of patients having abdominal surgery developing PPC (Scholes et al 2006). Five main risk factors (all these risk factors occurring together in one patient) were identified in this model, which predicted 82% of patients who developed a PPC in a population of 268 patients having upper abdominal surgery. Patients predicted as high risk were eight times more likely than those predicted to be at low risk of developing a PPC. The risk factors identified were:

duration of anaesthesia >180 minutes

type of surgery performed (upper abdominal)

presence of preoperative respiratory problems, e.g. COPD

current smoking (within last 8 weeks)

reduced level of preoperative activity (measured using a questionnaire).

In addition to the risk factor model above, a systematic review of non-cardiopulmonary surgery (Smetana et al 2006) reports good evidence for the following risk factors to increase incidence of PPC:

advanced age

American Society of Anestheologists (ASA) classification of comorbidity of class 3–5

functional dependence

respiratory and cardiac disease

serum albumin <3 g/dl.

Procedure-related risk factors that were supported by good evidence were thoracic, abdominal, emergency and prolonged surgery. There is fair evidence for significant intraoperative blood loss, oesophageal surgery and abnormal chest radiograph. However, in this systematic review, which presents the highest level of evidence, there was good evidence that the following were not important risk factors: obesity, asthma, hip and gynaecological surgery (lower abdominal surgery). These results challenge some traditional views, especially that of obesity being considered a risk factor. The results from this systematic review support the risk factors identified in the model by Scholes et al (2006). In patients having oesophageal surgery, age, operation duration and location of tumour in the proximal oesophagus were identified in one study as risk factors for PPC in 421 patients (Law et al 2004).

The ASA score (1–5) divides patients into five groups and collectively rates patient risk from anaesthesia. It was developed as a standardized way for anaesthetists to convey information about the patients’ overall health status and to allow outcomes to be stratified by a global assessment of their severity of illness. In practice, the ASA score may be the only overall documentation of preoperative condition that is used widely. Generally, the attending anaesthetist ascribes a score to each patient upon preoperative assessment.

The classification of physical status recommended by the House of Delegates of the American Society of Anesthesiologists (1963) is:

Box 12.3 shows key points in the surgical process.

Box 12.3 Key points in the surgical process

Functional residual capacity (FRC) is reduced perioperatively as a result of anaesthesia, surgery and recumbency.

FRC is increased in sitting and standing positions.

The altered relationship between FRC and closing capacity is an important determinant of dependent airway collapse.

Ventilation/perfusion (/) mismatch and arterial hypoxaemia commonly occur after major surgery, although increases in CO₂ are rare unless patients are narcotized.

Mucociliary clearance and cough effectiveness are reduced after surgery.

Changes in pulmonary function after surgery generally resolve spontaneously with time from surgery.

Clinically relevant postoperative pulmonary complication (PPC) may develop in a subset of patients having major surgery.

A combination of clinical signs and symptoms is used to diagnose a PPC.

Assessment of risk factors for developing PPC is important for the physiotherapist as it allows prioritized respiratory care for high-risk patients.

Carcinoma of the lung

Cigarette smoking is the single most common predisposing factor for lung carcinoma, but other factors such as environmental or occupational exposure to hydrocarbons or asbestos are also implicated. There are several different pathological types of lung carcinoma; these are divided into non-small cell and small cell carcinoma. The non-small cell types are squamous cell, adenocarcinoma, large cell and adenosquamous carcinoma. Of these, squamous cell and adenocarcinoma are most common, making up approximately 80% of all lung cancers (Smith 2006). Small cell carcinomas are the most malignant and make up about 10% of presentations of lung cancers. Extrathoracic spread is common at the time of presentation.

Thoracic clinical signs and symptoms of lung cancer include: cough, haemoptysis, chest pain, hoarse voice (if recurrent laryngeal nerve involved), shortness of breath, arm pain and weakness. There may be other symptoms depending on the extent and site of metastases at presentation, such as bone, liver or central nervous system involvement. Physical findings may include loss of weight, fatigue, finger clubbing, pleural effusion or lung collapse. Several investigations may be undertaken to determine the cell type, extent and location of the carcinoma; these include chest radiograph, pulmonary function tests, computed tomography (CT) scan, needle biopsy of tumour under CT control, bronchoscopy, mediastinoscopy, positron emission tomography (PET) scan, ventilation/perfusion scan, and sputum cytology.

Stay updated, free articles. Join our Telegram channel

Join