= Positive-pressure ventilation
\b[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.<What Is a Ventilator? by NHLBI>[Machine action and mask/tube interfaces]{ref}
The direction of flow still depends on a pressure difference, not on whether either pressure is above atmospheric. For a simple resistive path,
$$
Q=\frac{p_{\rm opening}-p_{\rm alv}}R,
$$
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.<Measured values for Ppeak and Pplateau by Hamilton Medical>[Definition of airway pressures]{ref} Positive pressure is therefore a mode of applying mechanical assistance, not a guarantee of a particular inward flow, volume or gas-exchange result.
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