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MOUTH PRESSURE MAY OVERESTIMATE LUNG VOLUME IN PATIENTS WITH EXPIRATORY FLOW LIMITATION

Baylor College of Medicine, Houston, TX

PURPOSE: Plethysmographic determination of lung volumes using oral pressure (TLC) is spuriously high in patients with expiratory flow limitations during high frequency panting. This artifact is reportedly avoided by using esophageal pressure (TLC_e) or by panting less than 1 HZ. In this stud we evaluated the factors that may contribute to overestimation of TLC using oral pressure.

METHODS: We analyzed lung mechanic data of 150 subjects retrospectively. An esophageal balloon was placed at 45 cm from nostrils. Each subject sat in a volume displacement body box. The subjects were asked to pant against a close valve at a rate of less 1 HZ. TLC and RV were estimated using both oral and esophageal pressures. DTLC was determined as (TLC- TLC_e)/TLC. In addition, inspiratory and expiratory lung resistance and forced expiratory maneuver were measured. Airflow limitation was defined as FEV1/FVC less than 70 percent.

RESULTS: Subjects with airflow limitation had significantly higher DTLC compared to ones without airflow limitation (3% SD 8% versus –1% SD 14%; p < 0.05). On average TLC was 330 ml (SD 790 ml) higher than TLC_e in subjects with flow limitation. In subjects with no flow limitation, TLC was 27 ml (SD 898 ml) higher than TLC_e. Inspiratory and expiratory resistance, RV/TLC positively predicted DTLC while FEV1, PEF, and FVC negatively predicted DTLC. In multiple regression analysis DTLC correlated with inspiratory resistance, RV/TLC and FEV1.

CONCLUSION: In patients with airflow limitation, using oral pressure may overestimate TLC. The TLC overestimate directly correlates with indices of airflow limitation.

CLINICAL IMPLICATIONS: The spuriously high TLC measured using oral pressure may confound comparison of lung volume (TLC and RV) in response to an intervention that may change the degree of airflow limitation. Special attention should be given to frequency of panting to avoid spuriously this effect.

DISCLOSURE: Hanan Abdel-Monem, None.