Respiratory complications of surgery

Up to 15% of patients suffer from respiratory complications associated with general anaesthesia and major operations. The most common of these are atelectasis and pneumonia.

Pre-existing lung disease greatly increases the risk of complications. Severely ill patients, including those with acute pancreatitis, and burns or trauma victims, are susceptible to the development or ARDS.

Effects of anaesthesia and surgery on respiratory function
Anaesthesia and surgery predispose to postoperative complications by altering lung function and compromising normal defence mechanisms as follows:

• Lung tidal volume may be reduced by as much as 50%, depending on the incision site. Thoracic, upper abdominal and lower abdominal incisions (in decreasing order of effect) reduce lung volume.
  
  
• Lung expansion is reduced by the supine posture during and after operation, pain, abdominal distension, abdominal constriction by bandages and the effects of sedative drugs.
  
  
• Ventilation rate usually increases and there is loss of normal periodic hyperinflation.
  
  
• Diminished ventilation and pulmonary perfusion result in reduced gaseous exchange.
  
  
• Airway defences are compromised by loss of the cough reflex and diminished ciliary activity, which both lead to accumulation of secretions.
  
  
• Problems associated with laparoscopic abdominal surgery - the pneumoperitoneum and head-down position lead to diaphragmatic splinting, a reduction in functional residual capacity and changes in intrathoracic blood volume. These lead to atelectasis, pulmonary shunting and hypoxaemia.

Atelectasis

Atelectasis or alveolar collapse occurs when airways become obstructed and air is absorbed from the air spaces distal to the obstruction.

Bronchial secretions are the main cause of this obstruction. Predisposing factors include shallow ventilation, loss of periodic hyperinflation, inhibition of coughing and pooling of mucus. All of these are particular problems after thoracic or abdominal surgery.

The resulting V/Q mismatch produces a degree of R to L shunting of the blood and tends to cause a fall in PaO2.

If the obstructed airways are small, there is only minor segmental collapse, in which case localising signs are minimal and x-ray appearance is unremarkable. Despite this, the overall extent of collapse may be large and cause significant hypoxia.

Obstruction of a major airway causes collapse and consolidation of a whole lobe, resulting in the typical clinical signs of dullness to percussion and reduced breath sounds or bronchial breathing.

CXR will show the lobe to be contracted and opacified with mediastinal shift and compensatory expansion of other lobes.

Most cases of atelectasis are relatively mild and pass undiagnosed, although the patient may be slow to recover from operation.

The patient may be cyanosed, resulting from mild hypoxaemia, and have a mild tachypnoea, tachycardia and low-grade pyrexia, which all resolve spontaneously within a few days. Sputum culture (if any is produced) is usually negative but infection may complicate severe cases.

Prevention and Rx:

1. Preop and postop physiotherapy. This includes deep breathing exercises, regular adjustments of posture and vigorous coughing. During physiotherapt, wounds should be supported by the patient's hand.
2. Analgesia, e.g. infiltration of the wound with local anaesthetic or epidural analgesia, facilitates physiotherapy and mobility.
3. Nebulised bronchodilators e.g. salbutamol may assist the patient to cough up secretions.
4. Severe cases of diffuse atelectasis may require endotracheal intubation and positive-pressure ventilation.
5. Lobar or whole lung collapse requires intensive physiotherapy and sometimes flexible bronchoscopy to aspirate occluding mucus plugs.

Pneumonia

Bronchopneumonia is the usual form of chest infection seen in surgical patients. It occurs secondarily to chronic lung disease, chronic smoking or following atelectasis or aspiration of gastric contents.

Haemophilus and Strep. pyogenes are the common infecting organisms but coliforms may be responsible in elderly, debilitated or seriously ill patients.

Pseudomonas bronchopneumonia occurs in patients on ventilators or with bronchiectasis.

Infection is manifest by pyrexia, tachypnoea, tachycardia, increased leucocyte count.

Mucopurulent sputum is thick, copious and green.

Antibiotics, usually amoxicillin or co-trimoxazole, are given on a best-guess basis until sputum culture and sensitivities are available.

Physiotherapy and encouragement to cough are equally important for recovery.

Incomplete - Missing aspiration pneumonitis, aspiration pneumonia, ARDS. INfo from Burkitt pg 177.