= Negative-pressure ventilation
\b[Negative-pressure ventilation] assists <inhalation> by reducing the <pressure> outside the chest relative to that at the airway opening. A chamber encloses the torso while the mouth and nose communicate with gas outside the chamber. Lower external pressure allows chest expansion and makes alveolar pressure transiently lower than the airway opening, drawing gas into the <lungs>.
The original 1929 apparatus report by <Philip Drinker> and <Louis A. Shaw> describes a sheet-metal tank, a sealing neck collar and the head supported outside the tank. Alternating tank pressure moved the chest while the head remained exposed to atmospheric pressure.<An apparatus for the prolonged administration of artificial respiration: I. A design for adults and children by Philip Drinker and Louis A. Shaw>[Pages 230–231, Figures 1–2]{ref} This is a concrete distinction between applying a pressure change to the body surface and applying it through an airway interface.
An original static example uses <transpulmonary pressure> $P_L=P_{\rm alv}-P_{\rm pl}$. With an open airway and no flow, suppose alveolar pressure is zero on an atmospheric gauge. Changing <pleural pressure> from $-6$ to $-10$ cm H$_2$O raises $P_L$ from 6 to 10 cm H$_2$O. For stipulated <lung compliance> $0.05$ L/cm H$_2$O, the linear model predicts
$$
\Delta V=C_L\Delta P_L=0.05(4)=0.20\ \mathrm L.
$$
During the actual filling transient, pressure differences also overcome <airway resistance>. The static calculation does not predict the instantaneous flow waveform.
Tank pressure is a boundary condition, not a direct measurement of <pleural pressure>. Chest-wall mechanics and body geometry affect pressure transmission. Consequently, an equal numerical pressure at a positive-pressure airway interface and a negative-pressure body chamber need not produce identical volume changes or circulatory effects. The useful unifying quantity for lung distension is the pressure difference across the lung, together with its mechanical response.
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