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. 2015 Apr 3:407–435.e18. doi: 10.1016/B978-1-4557-3383-5.00025-7

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Copyright © 2016 Elsevier Inc. All rights reserved.

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eFigure 25-4 — Physical principles underlying pressure-volume plethysmography.

A, The theoretical “true” instantaneous volume event. During this event, plethysmographic pressure increases rapidly and then decays exponentially (B). If the plethysmographic pneumotachygraph is linear, the flow signal has a shape similar to that of the pressure transducer (C). This flow signal is integrated to obtain volume (D), which reaches the same level as the true volume event, but the shape does not conform to the “true” event. The difference is a result of the compression of a large volume of gas in the plethysmograph and is directly proportional to the plethysmograph pressure. Therefore, by adding a portion of the plethysmograph pressure to the integrated plethysmograph flow (E), the true volume event is reconstructed accurately: ΔV = Pbox + . Thus the true volume is obtained by adding the plethysmographic compression volume (Pbox) and the displacement volume (). More precisely, (1) box pressure (Pbox) is multiplied by a constant (Kbox), a factor to correct pressure to volume that is proportional to the gas volume in the box (total box volume − patient volume); and (2) Pbox is also divided by the box flowmeter resistance (Rbox) to yield box flow (), and integrated to obtain volume (Vbox). These two signals are added together to yield the change in lung volume: ΔV = PboxKbox + Pbox/Rbox .

Inline graphic — Physical principles underlying pressure-volume plethysmography.

A, The theoretical “true” instantaneous volume event. During this event, plethysmographic pressure increases rapidly and then decays exponentially (B). If the plethysmographic pneumotachygraph is linear, the flow signal has a shape similar to that of the pressure transducer (C). This flow signal is integrated to obtain volume (D), which reaches the same level as the true volume event, but the shape does not conform to the “true” event. The difference is a result of the compression of a large volume of gas in the plethysmograph and is directly proportional to the plethysmograph pressure. Therefore, by adding a portion of the plethysmograph pressure to the integrated plethysmograph flow (E), the true volume event is reconstructed accurately: ΔV = Pbox + . Thus the true volume is obtained by adding the plethysmographic compression volume (Pbox) and the displacement volume (). More precisely, (1) box pressure (Pbox) is multiplied by a constant (Kbox), a factor to correct pressure to volume that is proportional to the gas volume in the box (total box volume − patient volume); and (2) Pbox is also divided by the box flowmeter resistance (Rbox) to yield box flow (), and integrated to obtain volume (Vbox). These two signals are added together to yield the change in lung volume: ΔV = PboxKbox + Pbox/Rbox .