Abstract

Peri-operative pulmonary complications are a significant cause of morbidity and mortality in the surgical patient population, especially among those with pre-existing pulmonary pathology. This article discusses the causes of peri-operative pulmonary complications, the identification of high-risk patients and interventions to optimise their pulmonary function pre-operatively to improve patient outcome.

Keywords: anaesthesia, peri-operative pulmonary complications, respiratory disorders, risk factors

Introduction

Despite advances in anaesthesia practice and surgical care, peri-operative pulmonary complications remain a significant cause of morbidity, mortality, and prolonged hospital stay. Pulmonary complications are most commonly seen post-operatively. They are at least as common as or more common than cardiac complications in 17 of 25 studies of post-operative complications.¹ Patients with pre-existing pulmonary pathology are at increased risk. It is thus important to identify these patients pre-operatively and implement appropriate measures to optimise their pulmonary function, with the aim of preventing or reducing peri-operative morbidity.

Adverse Effects of Anaesthesia and Surgery

Peri-operative pulmonary complications include pneumonia, respiratory failure (usually defined as the need for mechanical ventilatory support), bronchospasm, atelectasis, hypoxaemia and exacerbation of underlying chronic lung disease.

Anaesthesia

Anaesthesia causes significant anatomical and physiological disruption to the respiratory system.² There is impairment of central respiratory regulation and tone of respiratory muscles. Anaesthetics and many other drugs used in the peri-operative period affect the central regulation of breathing, changing the distribution and timing of neural drive to respiratory muscles, which include the intercostal muscles and diaphragm. At high doses, some of these drugs may cause a global depression of muscular activity and reduce the efficiency of respiratory effort.

Deformation of chest wall also decreases the functional residual capacity, and produces atelectasis in dependent lung regions. Chest wall distortion and atelectasis also occur when the respiratory muscles are inactive during mechanical ventilation and persist even with positive end-expiratory pressure. Reflex stimulation during airway manipulation and release of inflammatory mediators by some anaesthetic agents can produce bronchospasm, resulting in hypoxia, hyperinflation and air trapping with risk of barotrauma and gas exchange abnormalities. Dry anaesthetic gases and prolonged mechanical ventilation may impair normal mucociliary mechanism. This causes retention of secretions which can lead to atelectasis from mucus-plugs and pneumonia from superimposed infection. Acute airway obstruction during spontaneous breathing can result in negative pressure pulmonary oedema. There is also a risk of aspiration pneumonitis with general anaesthesia when airway reflexes are obtunded.

These factors mandate vigilant clinical observation, coupled with continuous monitoring (pulse oximetry, heart rate, blood pressure, cardiac rhythm, airway pressure, end-tidal carbon dioxide tension, and blood gas analysis in indicated cases) of an anaesthetised patient peri-operatively.

Surgery

Surgical incisions cause physical disruption of respiratory muscles, such as intercostals and abdominal muscles, and impair their mechanical efficiency. Post-operative pain, especially from thoracic and upper abdominal incisions, causes voluntary limitation of respiratory effort.

Stimulation of viscera intra-operatively, such as esophageal dilatation and traction on gallbladder, markedly decreases phrenic neuronal output and reduces the force of diaphragmatic contraction during inspiration. All these factors contribute to a decrease in functional residual capacity and vital capacity, resulting in lung atelectasis.

Risk Assessment

Risk factors are multifactorial and complex (Table 1). Lawrence, Goldman and Chariston risk indices allow risk assessment by implementing a calculus that assigns relative values to each of a number of variables. These variables are broadly classified as patient and procedure factors.^2-8

Table 1. Risk factors for development of perioperative pulmonary complications.2-5

Patient Factors

General Health Status

The widely used American Society of Anaesthesiologists (ASA) classification, developed to evaluate the risk of overall peri-operative mortality, has been found to be strongly predictive of post-operative pulmonary complications.³ Poor exercise tolerance reflected as symptom-limited stair climbing also appears to identify those at risk.⁴

Smoking

Smoking of 40 or more cigarette pack-year significantly increases the risk of peri-operative pulmonary complications even among those without chronic lung disease.⁵ It causes chronic deterioration in pulmonary function after surgery in patients with chronic lung disease.^2,6 The relative risk of pulmonary complications among smokers as compared to non-smokers ranges from 1.4 to 4.3.⁷ It takes 8 weeks of abstinence for this risk to decline. However, even a short period of abstinence (12h) before anaesthesia will allow time for clearance of nicotine, a coronary vasoconstrictor, and a fall in the levels of carboxyhaemoglobin which results in improved oxygen carrying capacity of blood.

Chronic Pulmonary Disorders

The common disorders include chronic obstructive pulmonary disease and asthma.

Chronic Obstructive Pulmonary Disease

Complications arise from airflow obstruction, mucus hypersecretion and repeated infections. Incidence of pulmonary complications varies according to the severity of lung disease, with relative risk between 2.7 to 4.7.⁷

Asthma

Asthma is a result of hypersensitive airways with oedema, inflammation and narrowing due to smooth muscle spasm, which is characteristically reversible. Fortunately, bronchospasm which can occur during anaesthesia, is rarely associated with serious morbidity.⁶

Obstructive Sleep Apnoea (OSA)

Patients with sleep apnoea are at increased risk of severe hypoxaemia, hypercapnia and deterioration of sleep disordered breathing in the post-operative period.⁹ These patients are usually obese males who invariable have a history of snoring. OSA is also associated with hypertension, arrhythmias, congestive heart failure, coronary heart disease and stroke. This group of patients may have excessive pharyngeal tissues which pose a challenge during tracheal intubation and extubation. Regional anaesthesia for post-operative analgesia is preferred as the use of post-operative opioids for analgesia may be associated with increased risk of pharyngeal collapse.¹⁰

Obesity

Opinions vary with respect to obesity as a risk factor. Obese patients are at increased risk of difficult intubation. They may have delayed gastric emptying and/or gastroesophageal reflux which predisposes them to aspiration during anaesthesia. Obesity decreases expiratory reserve volume and functional residual capacity of the lungs, causing peri-operative basal atelectasis leading to hypoxia. Morbid obesity causes restrictive lung disease and decreased thoracic compliance, which lead to hypoventilation. Obesity also predisposes patients to obstructive sleep apnoea with post-operative airway compromise. Most studies, however, have not found any significant association between obesity and increased risk of pulmonary disorders.^7,11-13

Age

A recent prospective study recognised age 65 years or older as a strong clinical predictor of the risk of pulmonary complications.⁵ However, many other studies which reported an increased risk of pulmonary complications among the elderly, have not been controlled for coexisting conditions. When data are stratified according to ASA status, pulmonary complications are more strongly related to coexisting conditions than to chronological age.^3,7,11,14

Pulmonary Function Tests

There is no consensus on the routine use of pre-operative pulmonary function testing in the risk assessment for peri-operative pulmonary complications.^2-4,7,15-16 The position paper of American College of Physicians recommended spirometry for patients with history of smoking or dyspnoea undergoing coronary bypass or upper abdominal surgery, patients with unexplained dyspnoea or pulmonary symptoms undergoing head and neck, orthopaedic or lower abdominal surgery and all patients who are undergoing lung resection.¹⁵ An increased risk of pulmonary complications is associated with Forced Expiratory Volume in one second (FEV1) or Forced Vital Capacity (FVC) of less than 70% predicted, or the ratio of FEV1/FVC of less than 65%. Even then, patients at very high risk as defined by spirometry can still undergo surgery at an acceptable risk of pulmonary complications.^2,7 As such, pulmonary function testing should be viewed as a management tool to optimise pre-operative pulmonary function and not as a means to assess risk. Besides, clinical findings are consistently found to be more predictive of peri-operative pulmonary complications than spirometric results in the few studies that evaluated both factors.^4,16 Self-reported or directly observed exercise capacity has also been shown to be predictive of risk for major post-operative pulmonary complications. Defining 4 flights of stairs as good exercise capacity resulted in sensitivity of 71% and a specificity of 77%.⁴

A recent prospective study did not find an elevated PaCO2 to be a risk factor among surgical candidates, hence clinicians should not use arterial blood gas analyses to identify patients for whom the risk of surgery is prohibitive.¹⁷

Procedure-related Factors

Surgical Site

This is the single most important predictor for peri-operative pulmonary complications. The risk increases as the incision approaches the diaphragm: upper abdominal and thoracic surgery carries the greatest risk, ranging from 10 to 40%.^2,7,11-13

Degree of Surgical Trauma (Laparoscopic vs Open Surgery)

The risk is lower in laparoscopic cholecystectomy (0.3 to 0.4%) compared to open cholecystectomy (13 to 33%).^7,11-13

Anaesthetic Techniques and Post-operative Analgesia

Most studies report a lower risk of pulmonary complications with regional anaesthesia compared to general anaesthesia.^18-21 Regional anaesthesia avoids the associated risks of anaesthetic agents, airway manipulation and mechanical or assisted ventilation which disturbs lung mechanics. Two large multi-centre trials reported significant reduction in respiratory failure with regional anaesthesia.^22,23 A recent meta-analysis also supports the utility of epidural analgesia for decreasing post-operative pulmonary complications and morbidity.²⁴

A prospective study also showed that there is a higher risk of pulmonary complications among patients who receive the long-acting neuromuscular relaxant pancuronium than those receiving the shorter-acting atracurium or vecuronium.²⁵ Post-operative residual neuromuscular blockade with the use of pancuronium may result in frequent and prolonged post-operative hypoventilation. Thus, pancuronium should be used with caution in patients at risk of pulmonary complications.

Duration of Surgery

Surgery lasting more than 3 hours is associated with a higher risk of peri-operative pulmonary complications.¹²

Emergency Surgery

Patients with respiratory conditions for emergency surgery tend to be more sick, and have significantly less time for optimisation of their coexisting disorders. Also, they may not be adequately fasted, predisposing them to aspiration.

Optimisation of Pulmonary Status

Asthma

It is important to determine the frequency of asthma exacerbations, their severity including the need for ventilatory support in intensive care unit, and drug history (beta-2 adrenergic agonists, steroids, anticholinergics). Asthma can be classified into mild episodic, intermittent asthma, mild, moderate or persistent asthma. Inhalational steroids and systemic steroids are prescribed to patients with more persistent symptoms.^14,26,27 Trends in peak expiratory flow rate (PEFR) are useful in evaluating the control of asthma.²⁶

The risk of bronchospasm in the peri-operative period is low in stable asthmatic patients and is usually not associated with serious morbidity.^6,7 Patients at risk of peri-operative pulmonary complications include those with current or recent asthmatic exacerbation, persistent asthmatic symptoms, PEFR <80% predicted or personal best or a history of tracheal intubation for asthma exacerbation.^6,14,26 Airway hyperreactivity persists for weeks after an acute exacerbation. Before surgery, patients should ideally be free of wheeze, with a PEFR greater than 80% of the predicted or personal best value. Elective surgery should be deferred for those with acute asthma exacerbation.^14,28

High risk patients may be started on steroids (prednisolone 40mg or IV hydrocortisone 100mg q8h) 1 to 2 days pre-operatively because the beneficial effect of steroids on airway reactivity occurs over a period of hours. Some physicians advocate a week long course of steroids.^6,7,14 Patients who have been on systemic steroids for more than 3 weeks in the preceding 6 months should be assumed to have hypothalamic-pituitary-adrenal suppression and should receive stress-dose steoroid coverage peri-operatively. Avoid known allergens. Aspirin or non-steroidal anti-inflammatory drugs should be used with caution.

In summary, prevention of bronchospasm intra-operatively includes (1) Preinduction administration of inhaled beta 2 adrenergic agonists and muscurinic antagonists which can effectively attenuate airway reflex responses to intubation. (2) Choice of regional anaesthesia versus general anaesthesia. During general anaesthesia, airway manipulation should be kept to a minimum and only attempted when an adequate plane of anaesthesia has been achieved. Current inhalational agents in use have similar bronchodilator properties but the use of non-irritant agents such as sevoflurane and halothane are potentially beneficial in asthmatic patients with hypersensitive airways to noxious stimuli. Agents that release histamine such as morphine and atracurium should be avoided or used judiciously.¹⁴ Although the appropriate use of regional anaesthesia may seem logical as it avoids tracheal intubation and minimize risk of bronchospasm, there is no clear evidence that these techniques result in fewer respiratory conditions than after general anaesthesia.¹⁴ (3) Treatment of bronchospasm. Intraoperative bronchospasm can be detected clinically (stridor, poor air entry on auscultation) and through recognition of high airway pressure or altered capnography tracing. It can be relieved by nebulised beta-2 adrenergic agents delivered via the endotracheal tube. All volatile anaesthetic gases are bronchodilators with no clinically significant differences between them with respect to their efficacy for treating bronchospasm.^6,14,27 (4) Ventilation. Mechanical ventilation may be required for major or long procedures. In cases with severe airway obstruction, slow ventilator rate and long expiratory time allow for the slow expiration. Residual neuromuscular blockade should be fully reversed.¹⁴

Finally, bear in mind that not all wheezing is related to asthma. Other causes of wheezing include pulmonary oedema, pneumothorax, drug reactions, aspiration and endobronchial intubation.

Chronic Obstructive Lung Disease (COLD)

According to the Global Initiative for Chronic Obstructive Lung Disease, COLD can be classified from mild (Stage I) to very severe (Stage IV) depending on spirometric parameters of Forced expiratory volume in one second (FEV1)/Forced vital capacity (FVC), FEV1 predicted, and respiratory symptoms.²⁹ Patients with severe COLD are more likely to have a major post-operative complication. Best predictors of post-operative ventilation were arterial oxygen tension and dyspnoea at rest.³⁰

Despite the increased risk, there is no prohibitive level of pulmonary function for which surgery is contraindicated.^30,31 The management goal for this group of patients includes (1) Control of acute exacerbation. Patients with acute infectious exacerbation of COLD (increased sputum production, change in colour of sputum, fever, worsening cough) should be treated with antibiotics.^14,29 Patients should cease smoking. Elective surgery should be deferred if an acute exacerbation is present. (2) Continue medications. Mild COLD can usually be controlled with inhaled beta-adrenergic agonists.²⁹ Inhaled anti-cholinergics can be added for synergistic effect for symptomatic relief. Not all patients with COLD will respond to corticosteroid therapy. Many physicians feel that a short pre-operative course of systemic corticosteroid is reasonable for patients who continue to have symptoms despite bronchodilator therapy and who are not at their best-personal baseline level as determined by symptoms, chest findings and spirometry.¹⁴ (3) Physiotherapy and incentive spirometry. Post-operative maneouvres to increase mean lung volumes include intermittent positive pressure breathing, deep breathing exercises, incentive spirometry and chest physiotherapy. Critical review and meta-analysis have shown that all these measures are equally efficacious in reducing the frequency of peri-operative pulmonary complications after upper abdominal surgery.³² Currently, incentive spirometry is frequently used as it is simple, inexpensive and provides objective goals for monitoring the patient’s performance. (4) Oxygen therapy. Patients with severe COLD have chronic hypoxaemia which can cause pulmonary hypertension and cor pulmonale. Continous oxygen therapy should be started as this improves pulmonary hypertension and cardiac function.¹⁴ (5) Adequate post-operative analgesia. Regional analgesic techniques can improve post-operative pulmonary function by reducing pain and reflex inhibition of respiratory muscles. This helps to increase lung expansion and prevent post-operative pulmonary atelectasis and pneumonia. Techniques such as segmental epidural blockade with local anaesthetics can increase tidal volume, vital capacity and improve diaphragmatic activity after thoracic and upper abdominal surgery.^33,34

Obstructive Sleep Apnoea

Patients who may have sleep apnoea should have a formal polysomnographic sleep study preoperatively to confirm the diagnosis and assess the severity. Patients who require nasal continuous positive airway pressure (CPAP) therapy should receive it pre-operatively. CPAP can improve cardiac function, reduce pulmonary artery pressures, and reduce systolic hypertension, all of which may reduce complications.⁹

The intra-operative and post-operative use of sedatives and narcotics should be minimised.^11,35,36 Careful monitoring in the post-operative period, including elective admission to intensive care unit should be considered for worsening sleep apnoea, development of airway obstruction, or carbon dioxide retention. In the initial nights after surgery, sleep is fragmented with decreased REM sleep. In succeeding nights, REM sleep is increased, resulting in increased risk of hypoventilation and hypoxia.³⁵ Patients with sleep apnoea often benefit from regional anaesthesia rather than general anaesthesia.

Coryza (Common Cold)

Most patients with minor upper respiratory tract infections without fever, myalgia or productive cough can proceed for elective surgery. However, surgery should be postponed if common cold is present in patients with other underlying respiratory disease or in those having major abdominal or thoracic surgery.¹⁴

Respiratory Tract Infections

Patients with fever and productive cough should be treated before undergoing elective surgery as there is an increased risk of post-operative pulmonary complications. When these patients present for emergency surgery, a course of antibiotics should be administered.¹⁴

Conclusion

Patients with respiratory disorders are predisposed to the development of peri-operative pulmonary complications resulting in significant morbidity and mortality. This risk can be effectively minimised through the timely recognition of pre-existing respiratory disorders, optimisation of the patient’s conditions pre-operatively, judicious choice of anaesthetic techniques and vigilant post-operative monitoring. A multidisciplinary team approach involving anaesthetists, surgeons, pulmonologists, physiotherapists and recovery room personnel is crucial.

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