Volume/pressure curve of total respiratory system in paralysed patients: artefacts and correction factors

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Abstract
The volume/pressure (V/P) curve of the total respiratory system in paralysed patients is drawn assuming that volume changes of the respiratory system (ΔV resp) equals volume displacement of the measuring apparatus (ΔV syr), usually a supersyringe. However, in 93 VP curves we found that O2 removed from the lung-syringe system during the procedure (proportional to the time) largely exceedes the CO2 added to the lung-syringe system (ΔV gas). This results in a net loss of volume from the system (ΔV resp<ΔV-syr). Deflation compliance, hysteresis area and ratio are significantly affected by this phenomenon. Inflation compliance is less influenced by ΔV gas, partially compensated by the intrapulmonary gas expansion due to the temperature changes. We conclude that the parameters computed on the deflation limb of V/P curve are misleading if proper correction of the volume scale is not introduced.