Positive Airway Pressure Ventilation

Overview:

Positive airway pressure (PAP) helps keep airways that might otherwise collapse open, whether it be the upper (e.g. in obstructive sleep apnoea) or lower airways (e.g. COPD or ARDS).

Continuous PAP (CPAP)

In obstructive sleep apnoea, the obstruction is due to collapse in the oropharynx and a little continuous positive airway pressure (CPAP) into the airways can prevent this from occurring. During a sleep study, CPAP pressure can be titrated up until apnoea ceases. CPAP is also useful in diseases where smaller airways in the lung are liable to collapse and helps in the same way, by providing positive pressure within the airway to oppose whatever forces are acting to encourage collapse. While these are effective interventions, patients don’t always find wearing a mask and the sensation of positive pressure comfortable.

Bilevel PAP

Bilevel PAP devices not only prevent airway closure, they assist breathing.. These devices can be set to have a waveform on top of CPAP, so airway closure is prevented and breathing is aided by more positive pressure peaks (Inspiratory PAP; IPAP) during inspiration.

Figure 1: Comparison of normal ventilation and the effect of CPAP and Bilevel PAP. CPAP provides a simple, continuous positive pressure on which normal, otherwise unassisted ventilation is superimposed. Bilevel PAP takes this a step further, by providing CPAP (set as Expiratory PAP; EPAP) with intermittent bursts of increased pressure (set as Inspiratory PAP; IPAP) to assist inspiration.

We can apply our model of CO₂ washout to show how bilevel PAP helps patients with COPD, who have lost elastin fibres responsible for keeping small airways open (radial traction) and in whom obstruction is caused by collapse of these airways on expiration. The increased pressure within these airways acts as a “pneumatic splint”, preventing collapse, improving flow in and out of the alveoli in which CO₂ (or dirt in the bottom of a vase in this model) is accumulating:

This splinting effect can also be used to recruit alveoli that would otherwise collapse when the lungs retain fluid, such as in pneumonia.