Effects of anaesthesia & prediction of complications

Question 1

Effects ofsurgery

Answer 1

• Upper abdominal operations are associated with pulmonary complications in 20–40% of the general surgical population.
• Incidence with lower abdominal surgery is2–5%.
• Upper abdominal or thoracic surgery is associated with profound reductions in lung volume; vital capacity (VC) is reduced by 50–60%, and functional residual capacity (FRC) reduced by about30%.
• Diaphragmatic dysfunction plays an important role in these changes, but pain and splinting are also factors.
• Reductions in lung volumes are not seen with surgery on the extremities.

Question 2

Effects ofanaesthesia

Answer 2

• On induction of anaesthesia, FRC decreases by 15–20% (~450mL); the diaphragm relaxes and moves cranially, and the rib cage moves inward.
• FRC may be reduced by 50% of the awake supine value in morbidly obese patients. Positive end-expiratory pressure (PEEP) may reduce these effects. FRC is relatively maintained during ketamine anaesthesia.
• Under anaesthesia, the closing capacity (the lung volume at which airway closure begins) encroaches upon the FRC, and airway closure occurs, contributing to the risk of atelectasis, pneumonia, and ventilation/ perfusion (V/Q) mismatching. this happens more readily in smokers, the elderly, and those with underlying lung disease.
• Chest computed tomography (Ct) shows atelectasis in the dependent zones of the lungs in >80% of anaesthetized subjects. Microatelectasis results in areas of the lungs that are perfused but not ventilated, leading to impaired gas exchange and consequent post-operative hypoxaemia.
• Intubation halves the dead space by circumventing the upper airway.
• the ventilatory response to hypercapnia is blunted, and the acute responses to hypoxia and acidaemia almost abolished by anaesthetic vapours at concentrations as low as 0.1 minimum alveolar concentration(MAC).
• Inhibition of cough and impairment of mucociliary clearance of respiratory secretions contribute to the risk of post-operative infection.
• Most of these adverse changes are more marked in patients with lung disease but usually improve within a few hours post-operatively. After major surgery, they may last severaldays.

Question 3

Predicting post-operative pulmonary complications

Answer 3

• Post-operative pulmonary complications include atelectasis, infection, prolonged mechanical ventilation, respiratory failure, exacerbation of underlying chronic lung disease, and bronchospasm.1
• Post-operative pulmonary complications are as prevalent as cardiac complications and contribute similarly to morbidity and mortality.
• Preoperative identification of patients with pre-existing respiratory dysfunction reduces post-operative complications.
• Even for patients with severe pulmonary disease, surgery that does not involve the abdominal or chest cavities is inherently of low risk for serious perioperative pulmonary complications.
• Large and rigorous studies to identify risk factors for pulmonary complications are lacking (in contrast to those identifying cardiacrisk).

Question 4

Factors shown topredict perioperative pulmonary complications

Answer 4

Patient factors 
•  Increasing age (>50yr). 
•  Chronic obstructive lung disease. 
•  Smoking within 8wk of  surgery. 
•  Obstructive sleep apnoea (OSA).2 •  Pulmonary hypertension.3 
•  ASA grade 2 or greater. 
•  CCF. 
•  Functional dependence. 
•  Serum albumin <30g/dL. 

Procedure-related factors
• Prolonged surgery.
• Residual neuromuscular blockade (nMB).
• Upper abdominal and thoracic surgery.
• neurosurgery, head and neck surgery.
• Vascular surgery, especially aortic aneurysm repair.
• Emergency surgery.