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Mechanisms of dyspnea in obesity remain unclear. This study was undertaken to determine the relationships between dyspnea and pulmonary function including inspiratory muscle endurance (IME) in morbidly obese patients before bariatric surgery.Fifty-five patients with a mean±s.d. body mass index (BMI) of 49.4±7.0 kg/m2 were included. Dyspnea was evaluated by the Baseline Dyspnea Index (BDI; 0-12, 0 = maximal dyspnea). Pulmonary function tests included a plethysmography, maximal inspiratory pressure (PImax) and IME was assessed by the incremental threshold loading test, determining the maximal pressure sustained for 2 min (Plim2) and Plim2/PImax ratio. Patients were classified according to their BMI in two groups: BMI ≤49 (n = 27) and >49 kg/m2 (n = 28).Breathlessness was higher in the BMI >49 kg/m2 group compared to the BMI ≤49 kg/m2 group (BDI score at 6.9±2.2 in the BMI >49 kg/m2 group vs 8.9±2.5 in the BMI ≤49 kg/m2 group, P<0.01). Patients with BMI >49 kg/m2 had significantly higher PaCO2 level and significantly lower vital capacity, inspiratory capacity and PImax values compared with the BMI ≤49 kg/m2 group. Correlations between BDI and lung function were moderate: forced expiratory volume in 1 s (FEV1)% pred: Rho = 0.27; P = 0.05; vital capacity % pred: Rho = 0.40; P = 0.004; and Plim2/PImax: Rho = 0.40; P = 0.003. Higher correlations with dyspnea were found in the BMI ≤49 kg/m2 group: FEV1% pred: Rho = 0.38; P = 0.05; and Plim2/PImax: Rho = 0.49; P = 0.01.Inspiratory muscle performance is moderately reduced in morbid obesity. Dyspnea in these patients remains moderately related to lung function and inspiratory muscle performance. However, inspiratory muscles performance correlates more significantly with dyspnea in patients with a BMI ≤49 kg/m2.