Introduction

The decision to wean from ventilatory support must balance two opposing concerns: the increased risk of ventilator-associated pneumonia and sepsis with prolonged ventilation versus the increase in morbidity and mortality with premature extubation. The general principle of this chapter is that liberation from the ventilator should commence as soon as possible after the conditions warranting intubation have begun to resolve. The following are considered:

1. • The physiological effects of weaning from mechanical ventilation.

2. • Identifying patients ready to commence weaning.

3. • Methods of weaning.

4. • Identifying patients ready for extubation or decannulation of tracheostomy.

5. • Failed extubation and prolonged weaning. Emerging techniques.

The cardiovascular pathophysiology of weaning

The withdrawal of mechanical ventilation has the potential to cause cardiovascular instability. This is especially true in cardiac surgery patients.

Increased oxygen demand

The increased respiratory muscle activity of spontaneous ventilation results in increased oxygen demand. This necessitates a rise in cardiac work and myocardial oxygen demand. This may lead to myocardial ischaemia in patients with untreated coronary artery disease, incomplete revascularization or recovering myocardium. Patients with impaired left ventricular (LV) function may be unable to raise their cardiac output sufficiently to meet these demands. They will maintain oxygen supply by either increasing oxygen extraction (noted by falling Svo2) or by blood flow redistribution, typically away from the splanchnic region. This leads to a fall in gastric pH and a risk of mucosal ischaemia.