Non-invasive ventilatory support

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Objectives The objectives of this chapter are to understand: • The mechanisms of action of non-invasive ventilation. • The clinical applications for non-invasive ventilation in the acute setting. • The role of continuous positive airway pressure (CPAP) versus non- invasive positive pressure ventilation (NIPPV). • The limitations and complications of non-invasive ventilation. • The practical application of non-invasive ventilation. Introduction Non-invasive ventilation (NIV) is the umbrella term used to describe the provision of ventilatory support through the patient's upper airway using a mask or similar device. NIV is principally indicated in patients with chronic obstructive pulmonary disease (COPD) with respiratory distress and hypercapnia, and in acute cardiogenic pulmonary oedema. Evidence indicates a reduction in intubation rates and mortality, particularly in patients with COPD. NIV may also be used in patients who are not considered suitable for intubation. A ceiling of treatment, and whether escalation to intubation is indicated, must be defined at the outset. Do not use NIV as a substitute for intubation and invasive ventilation if the latter is more appropriate. Early consultation with the intensive care team is imperative in both cases. Modes of non-invasive ventilation The terminology used to describe different types of NIV is complex. Outside the intensive care unit the two main types of NIV used in the UK are: • Continuous positive airway pressure (CPAP). During CPAP a constant positive pressure is applied throughout the respiratory cycle. • Non-invasive positive pressure ventilation (NIPPV). NIPPV provides wo levels of pressure support during the ventilator cycle – one during expiration and a higher one during inspiration. Mechanisms of action Continuous positive airway pressure CPAP is typically used to correct hypoxaemia in type 1 respiratory failure (Pa O2 <8 kPa with a normal or low Pa CO2). It has several mechanisms of action: • Increase in functional residual capacity (FRC) – the volume of gas remaining in the lungs at the end of a normal expiration. A low FRC causes atelectasis and lung collapse, leading to ventilation/perfusion mismatch, reduced pulmonary compliance and increased airway resistance. This increases the work of breathing. Restoration of the FRC towards normal improves oxygenation and reduces work of breathing. © College of Emergency Medicine, London, 2015.