Influence of Mouth Pressure on Measurement of Diffusing Capacity in the Clinical Pulmonary Function Laboratory

Current American Thoracic Society/European Respiratory Society guidelines recommend that patients hold their breath with minimum effort at total lung capacity during measurement of the single-breath diffusing capacity of the lung for carbon monoxide (D_LCO) to avoid excessively positive or negative mouth pressures. We asked to what extent do these pressures range during single-breath D_LCO testing and whether mouth pressures are associated with single-breath D_LCO.

We analyzed mouth pressures measured during clinical single-breath D_LCO testing in an academic pulmonary function laboratory over a 3-month period. We compared mouth pressures with single-breath D_LCO and determined the influence of obesity, restriction, and emphysema on mouth pressures. We used multiple linear regression to evaluate whether mouth pressure was an independent determinant of single-breath D_LCO.

We analyzed data from 336 subjects who presented with a variety of diseases, the most common of which were unexplained dyspnea, interstitial lung disease, sarcoidosis, and emphysema. The median mouth pressure was 4.5 cm H₂O, with a range of -13 to 31 cm H₂O. The single-breath D_LCO did not correlate with mouth pressure (P = .08). There was no difference in mouth pressures between individuals with and without obesity, with and without restriction, and with and without interstitial lung disease. Mouth pressure was lower among the subjects with emphysema. There was no difference in single-breath D_LCO % predicted between individuals who were obese and individuals who were not obese. Multiple linear regression demonstrated that only age and FEV₁, but not mouth pressure, were independently associated with single-breath D_LCO % predicted (ß-coefficient: age, -0.35 [P = .003]; FEV₁%, 0.26 [P = .004]; adjusted R², 0.16).

Mouth pressures varied widely during single-breath D_LCO measurement but were not associated with the measurement of single-breath D_LCO in the clinical setting of pulmonary function testing. Overall, these findings indicate that pulmonary function technologists need not discard efforts made during measurement of single-breath D_LCO if only mild changes in mouth pressure occur.