Physics in Anaesthesia, second edition by Middleton Ben;Phillips Justin;Stacey Simon;

Author:Middleton, Ben;Phillips, Justin;Stacey, Simon;
Language: eng
Format: epub
Publisher: Scion Publishing

14.10 Non-invasive ventilation

Non-invasive ventilation is a method of ventilating patients without intubation: a tight-fitting mask is used instead. The modes and principles of non-invasive ventilation available are almost identical to the invasive modes already discussed.

For patients with obstructive sleep apnoea, airway obstruction rather than oxygenation is the issue. Their condition is improved by the application of positive pressure (CPAP) at night. As a supplementary oxygen supply is often not required, the positive pressure is created by an air compressor. For comfort, a nasal mask is usually used and the CPAP keeps the oropharynx patent in spite of reduced pharyngeal muscle tone found during sleep.

The management of a hypercapnoeic patient may involve non-invasive ‘BiPAP’ to help control their carbon dioxide. This mode of ventilation, strictly speaking, should be called bilevel positive airway pressure (BPAP). Non-invasive BPAP ventilation is conceptually exactly the same as invasive pressure support ventilation. However, on many machines designed to deliver just non-invasive ventilation the term PEEP has been replaced with EPAP (expiratory positive airway pressure) and pressure support with IPAP (inspiratory positive airway pressure). Although the terms differ, the effect is the same, the goal being to deliver a larger tidal volume thereby improving minute ventilation and reducing carbon dioxide.

High flow nasal oxygen therapy

High flow nasal oxygen (HFNO) therapy delivers warmed, humidified oxygen at high flow rates via a tight-fitting nasal cannula: the fraction of inspired oxygen can be adjusted by manipulating the flow of oxygen entering the driving gas. The high flow rates both reduce anatomical dead space and generate low levels of positive pressure (CPAP) in the upper airways.

Oxygen consumption of respiratory support devices

Clinicians should be aware of the consumption of oxygen for the various respiratory support devices. Up to 100 L·min−1 of oxygen can be required to drive wall CPAP systems. HFNO systems and some NIV ventilators can also use over 50 L·min−1 of oxygen. Hospital oxygen delivery systems can become overwhelmed and even fail if large numbers of patients are treated with wall CPAP and HFNO systems.

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