The following are side effects of mechanical ventilation:

• Ventilator-induced lung injury (VILI):
  • This term encompasses barotrauma (injury due to excessive pressure) and volutrauma (injury due to excessive volume) – in combination, these produce alveolar strain (defined as the ratio between the amount of gas volume delivered compared with the amount of aerated lung receiving it).
  • Crucially, damage is also caused by atelectrauma (injury due to repeated opening and closing of alveoli) and biotrauma (distal organ dysfunction caused by the release of inflammatory mediators because of VILI).
  • Preventing VILI is difficult, and the best way to do so is not established. In ARDS, avoid plateau pressures > 30 cm H₂O, use tidal volumes of no more than 6 ml/kg of ideal body weight, select PEEP to prevent end-expiratory alveolar closure without increasing alveolar strain (see further reading), and perhaps use intermittent recruitment maneuvers.
  • These techniques, in combination with permissive hypercapnia and permissive hypoxia, are together termed Lung Protective Ventilation.
• Haemodynamic instability:
  • Increasing intrathoracic pressure with positive pressure ventilation has haemodynamic consequences.
  • In general, it causes a reduction in cardiac preload (venous return) with a corresponding decrease in cardiac output and organ perfusion. This may be masked by vasoconstriction, which will protect blood pressure.
  • The effect of an increase in intrathoracic pressure in an individual patient will depend upon:
    • Volume status: Hypovolaemic patients are very sensitive to this effect.
    • Lung compliance: Patients with reduced compliance can tolerate higher intrathoracic pressures. Less of the airway pressure is transmitted to the blood vessels.
    • The precise effect in an individual patient is difficult to predict.
• Ventilator-associated pneumonia (VAP):
  • VAP is an infection of the lungs that develops after 48 hours of ventilation.
  • The presence of an endotracheal tube bypassing natural defenses against infection, drugs causing gastric stasis and incompetence of gastrointestinal sphincters, patient positioning, and the general hypoimmunity seen in many critically ill patients, all predispose to the development of VAP.
  • Recently, ventilator bundles combined with the Scottish Patient Safety methodology have dramatically reduced the prevalence of this complication in Scottish ICUs.