Mechanical ventilation is used during surgery or respiratory failure to optimise gas exchange until the end of surgery or while waiting for the improvement of the underlying respiratory disease.

Modern mechanical ventilation involves positive pressure insufflation of gas into the lungs rather than negative pressure generated by the respiratory muscles, and is therefore harmful by default. However, short spells of mechanical ventilation during anaesthesia and surgery seem to be well tolerated by the majority of patients without major side effects.

Positive pressure ventilation of the lungs causes a number of undesirable side effects which can lead to lung injury, even in healthy lungs. These can sometimes lead to multiorgan dysfunction or failure (Figure 23.1).

For this reason the least harmful mode of mechanical ventilation until the end of surgery or respiratory failure recovery is likely to yield most patient benefit.

Mechanical ventilation can be non-invasive (through face masks or hoods), or invasive (through tracheal or bronchial tubes, or tracheostomy).

Both modes of mechanical ventilation employ the same principles, but non-invasive ventilation:

• Does not require sedation

• Can be done at home

• Does not impair the mucociliary apparatus

Continuous positive airways pressure (CPAP) is not per se a mode of mechanical ventilation, but often used as such. It involves administration of positive pressure throughout the respiratory cycle (inspirium and expirium) but the patient has to generate negative pressure to inflate the lungs. This mode is least invasive, and efficient for:

• Improving pulmonary oedema (by increasing the intra-alveolar pressure during expirium and reducing the work of breathing)

• Improving oxygenation

• Improving lung collapse and atelectasis