Partial flow/volume curves and airway resistances in obese subjects: new tools for investigating flow limitation?

The number of obese patients referred to our laboratory for pulmonary function tests is increasing: sleep breathing disorders, unexplained effort dyspnea, evaluations before surgical procedures are the main problems we are expected to cope with. In some patients with normal FEV1 and FEV1/VC values, we have observed an overlap between the tidal volume expiratory flow and the maximal forced expiratory flow, a pattern that suggests expiratory flow limitation (EFL) [1]. EFL has been reported in obese patients without bronchial obstruction [2] and it is not surprising that some of our obese patients may be flow limited at rest. Partial sub-maximal flow/volume curves (PSCs) have been reported as a tool for detecting expiratory flow limitation in patient with airway obstruction [3]. In COPD we described that when PSCs suggest the occurrence of EFL, a loop in the expiratory phase of quiet breathing airway resistance (Raw-qb) is frequently seen. We hypothised that a loop in the expiratory portion of Raw-qb could be a marker of EFL [4]. With the aim of seeing if the same issue was true for obese patients, we studied 45 consecutive obese subjects with no history of asthma, no bronchial hyperreactivity to methacoline challenge, and no acute or chronic obstructive lung disease. Thirteen patients had no graphic overlap between maximal flow-volume curves and tidal volume; the others had tidal volume expiratory flows impinging maximal forced expiratory flows in all manoeuvers, suggesting the likelihood of EFL. The Raw-qb were obtained twice, when a patient was breathing at tidal volume, trying to minimise factors that could artificially create a loop (phase delay in flow measurement, artefacts due to gas temperature changes, air losses from the mouthpiece and cheek movements, plethysmographic losses). After at least eight stable tidal breaths, patients were invited to sligthly increase their respiratory rate to 18-24 breath/minute paced by technician: during this phase Raw qb were collected. PSCs, obtained according to the description by Pellegrino et alii, are extensively described elsewhere [3, 4]. The manoeuvres, were repeated 2-4 times. All measurements were obtained at the same session with patients seated, using the Vmax 20 Sensor Medics. The data was analysed using Systat statistical software, rel. 11.05 for Windows (SPSS Inc. Chicago). The results were expressed as means ± standard deviation and as frequency percentages. The differences in variables between groups were evaluated by Student’s unpaired two tailed ttest and ANOVA. The level of statistical significance for each test was set at <0.05. The association between variables was examined using bivariate correlations and Spearman’s coefficient calculation.