Continuous positive airway pressure

Continuous positive airway pressure (CPAP) is the addition of positive pressure to the expiratory side of the breathing circuit of a spontaneously ventilating patient who may or may not be intubated. This sets the baseline upper airway pressure above atmospheric pressure, prevents alveolar collapse, and may recruit already collapsed alveoli. It is usually administered in 2.5cmH2O increments to a maximum of 10cmH2O. It is applied either via a tight-fitting face mask (face CPAP), nasal mask (nasal CPAP), a whole head helmet, or the expiratory limb of a T-piece breathing circuit. A high-flow (i.e. above peak inspiratory flow) inspired air–oxygen supply or a large reservoir bag in the inspiratory circuit is necessary to keep the valve open. CPAP improves oxygenation and may reduce the work of breathing by reducing the alveolar-to-mouth pressure gradient in patients with high levels of intrinsic PEEP. Transient periods of high CPAP (e.g. 40cmH2O for 40s) may be a useful manoeuvre for recruiting collapsed alveoli and improving oxygenation in ARDS.

Indications

• Hypoxaemia requiring high respiratory rate, effort, and FIO2.

• Left heart failure to improve hypoxaemia and cardiac output.

• Weaning modality.

• Reduces work of breathing in patients with high PEEPi (e.g. asthma, chronic airfl ow limitation). NB. Use with caution, monitor closely.

Management

1 Measure blood gases, monitor haemodynamic variables and respiratory rate.

2 Prepare T-piece circuit with a 5cmH2O CPAP valve on the expiratory limb. Connect inspiratory limb to flow generator/large volume

reservoir bag. Adjust air–oxygen mix to obtain desired FIO2 (measured by O2 analyser in circuit). Use a heat-moisture exchanger to humidify the inhaled gas. If not intubated, consider nasal, face, or helmet CPAP. Attach mask to the patient’s face by an appropriate harness, attach a T-piece to the mask, and ensure no air leak is present. If using a nasal

mask, encourage patient to keep their mouth closed.

3 Repeat measurements after 15–20min.

4 Consider further changes in CPAP (by 2.5cmH2O increments).

5 Consider: (i) fluid challenge (or vasoactive drugs) if circulatory compromise, and (ii) nasogastric tube if gastricatony present.

Complications

• With mask CPAP, the risk of aspiration increases as gastric dilatation may occur from swallowed air. Insert a nasogastric tube, especially if consciousness is impaired or gastric motility is reduced.

• Reduced cardiac output due to reduced venous return (raised intrathoracic pressure). May need additional fl uid or even inotropes.

• Overinflation leading to air trapping and high PaCO2. Caution is urged in patients with chronic airflow limitation or asthma.

• May reduce venous return and increase intracranial pressure.

• Occasional poor patient compliance with tight-fitting face mask due to feelings of claustrophobia and discomfort on bridge of nose.

• Inspissated secretions due to high flow, dry gas.