Evaluation of phase correction and low gas density to improve thoracic gas volume measurement

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Abstract
Two methods of decreasing the error on plethysmographic determinations of thoracic gas volume (TGV) related to cheeks movements during panting maneuvers were investigated: lowering gas density in the airways with an 80% He-20% O2 mixture and computing TGV from the in-phase component of the plethysmographic signal (TGVr). The methods were tested by measuring how TGV estimates varied when panting frequency was raised from 0.8-2.5 Hz during the same occlusion. The measurements were performed in 6 normal subjects and 12 patients with chronic bronchitis with and without cheeks support and when the airway was connected to an external device simulating an increased cheeks compliance. A small negative frequency dependence of TGV (.DELTA.TGV/.DELTA.f = -1.2 .+-. 0.8%/Hz with cheeks support), most probably unrelated to upper airway walls, was found in normal subjects. .DELTA.TGV/.DELTA.f was positive and algebraically larger in patients than in normals, reaching 2.2 .+-. 3.4%/Hz without cheeks support and 11.8 .+-. 8.0%/Hz with the additional cheeks. The latter value was only 20% smaller when computed on the basis of TGVr, demonstrating the limited usefulness of the phase-based correction. In contrast, breathing He-O2 decreased .DELTA.TGV/.DELTA.f to .apprx. 50% of its air value (P < 0.01) and appears as an effective way to diminished the error in obstructive patients.