Positive-pressure ventilation

Positive-pressure ventilation assists inhalation by raising pressure at the airway opening, driving gas into the lungs when that pressure exceeds alveolar pressure. It differs mechanically from spontaneous expansion of the thorax or negative-pressure ventilation, which lowers pressure around the lungs to produce the relevant distending difference. A ventilator's mask or tube supplies the applied airway connection.^[ref]

The direction of flow still depends on a pressure difference, not on whether either pressure is above atmospheric. For a simple resistive path,with positive $Q$ directed inward and $R$ the airway resistance. In an original example, pressures 10 and 8 cm H${}_2$O and $R=4$ cm H${}_2$O·s/L give inward flow 0.5 L/s. Pressures 8 and 10 cm H${}_2$O give outward flow 0.5 L/s. Both pairs are positive relative to atmosphere.

This explains why positive end-expiratory pressure can coexist with exhalation. The applied pressure can remain above atmospheric while elastic recoil produces a still higher alveolar pressure and drives gas outward. At equal opening and alveolar pressures, the same ideal resistive model gives zero flow.

The resulting tidal volume also depends on respiratory-system compliance, airway resistance, duration and any active respiratory muscle effort. The no-flow interval used to interpret plateau airway pressure removes one resistive contribution only under the required measurement conditions.^[ref] Positive pressure is therefore a mode of applying mechanical assistance, not a guarantee of a particular inward flow, volume or gas-exchange result.