Overestimation of thoracic gas volume during the airway resistance maneuver. A potential error in the diagnosis of air trapping

There are no data published about the agreement between the measurement of thoracic gas volume (TGV) during the airway resistance (TGV-Raw) and the conventional technique described by Dubois. The aim of this study was to establish the agreement between both methods to measure TGV. We studied eighty consecutive subjects. Only sixty-six performed acceptable plethysmography maneuvers. The patients were measured with a constant volume plethysmograph (Medical Graphics 1085 DL). TGV was performed in the same patient with two techniques: 1) during the airway resistance (Raw) measurement (TGV-Raw) and 2) during quiet breathing at the end of expiration (TGV). The panting frequency was 1 to 2 Hz with both maneuvers. The differences between both techniques were expressed in percentage (deltaTGV %) and absolute values (deltaTGV). The TGV-Raw of the whole group was higher than TGV (3.69 +/- 1.08 l vs 3.28 +/- 1.05 l, p < 0.001). Similarly, the subgroups of patients had a greater TGV-Raw than TGV (Normal: 3.44 +/- 0.77 l vs 2.98 +/- 0.72 l , p < 0.001; Obstructive: 4.08 +/- 1.19 l vs 3.71 +/- 1.15 l, p < 0.001; Restrictive: 2.62 +/- 0.49 l vs 2.25 +/- 0.51 l, p < 0.01). There was a considerable lack of agreement between the TGV-Raw and TGV, with discrepancies of up to +0.95 l or +34%. The deltaTGV % was similar between the patients' subgroups and between the subjects with different degree of airflow obstruction (Normal: 16.5 +/- 10%, Obstructive: 10.8 +/- 9.4%, Restrictive: 18 +/- 14.3%, p NS; mild obstruction: 10.7 +/- 11%, moderate obstruction: 12.3 +/- 5.7, severe obstruction: 10.1+/- 6.6, p NS). In conclusion, TGV-Raw was larger than TGV. This was because the patients generally panted at a volume above FRC when performing the TGV-Raw maneuver. TGV-Raw should not be used to estimate FRC because FRC would be overestimated and the diagnosis of air trapping may be erroneous.