= Pleural-pressure calibration and compensating component errors
\b[Pleural-pressure calibration and compensating component errors] arise when an internal-pressure estimate partitions total respiratory stiffness into lung and chest-wall contributions. A gain or offset error can shift those inferred components in opposite directions while their sum still agrees perfectly with the measured total.
An original local model treats the <volume> increment $\Delta V$ as known. Let true pleural-pressure change be $\Delta P_{\rm pl}$ and the measured surrogate be
$$
\Delta P_s=k\,\Delta P_{\rm pl}+d.
$$
Here $k$ is gain and $d$ is an offset change between the two <volume> states. With wall elastance $E_W=\Delta P_{\rm pl}/\Delta V$ and known total elastance $E_T$, inference gives
$$
\widehat E_W=kE_W+\frac d{\Delta V},\qquad
\widehat E_L=E_T-\widehat E_W.
$$
Thus the component errors are equal and opposite. A constant absolute <pressure> offset cancels when taking increments, but a changing offset or gain error remains.
For stipulated $\Delta V=0.5$ L, total <pressure> increment 5 cm water and true pleural increment 2 cm water, the true component elastances are $E_W=4$ and $E_L=6\,\mathrm{cm\,H_2O\,L^{-1}}$. A surrogate gain $k=1.5$ with $d=0$ instead gives 6 and 4. Checking only their sum cannot identify the <measurement calibration> error. The GUM requires uncertainty and correlations from shared <measurement calibration> inputs to remain in the measurement model.<Guide to the expression of uncertainty in measurement by JCGM>[§5.2.2, correlated inputs; §5.2.4, shared reference effects]{ref}
The <esophageal balloon occlusion test> supplies an experimental constraint. In the 1982 primary study, <Ahmet Baydur> and colleagues measured changes in esophageal and mouth <pressure> during inspiratory efforts against an occluded airway in ten subjects and several postures. Pressure-change ratios were near unity in sitting and lateral positions; some supine discrepancies could be reduced by repositioning the balloon.<A Simple Method for Assessing the Validity of the Esophageal Balloon Technique by Ahmet Baydur and others>[Abstract, ten-subject occlusion and posture comparison]{ref}
The underlying mechanical argument is that an ideal occlusion effort with negligible <volume> change produces negligible change in elastic <transpulmonary pressure>. Mouth and <pleural pressure> changes should then track each other. Comparing the recorded changes tests <pressure> transmission under those conditions. It does not by itself identify an absolute <pressure> zero or establish that one position represents every regional <pleural pressure> during all breathing states. The measurement, its validation maneuver and the inferred elastic partition therefore remain distinct pieces of evidence.
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