PURPOSE: It is known that increased vagal tone contributes to asthma pathophysiology and that horizontal body position is associated with increased vagal tone. In this study we aimed at evaluating and comparing changes in expiratory flow rates and airway resistance, occurring when shifting from the upright (UP) to the horizontal position (HP),in both asthmatic and normal subjects.
METHODS: We studied 50 patients with stable asthma (20 male, 30 female) aged 37.4±11.1 yrs. and 40 normal volunteers (14 male, 26 female) aged 36.1 ± 10.3 yrs., by recording their maximal flow-volume loops both in UP and HP, using an electronic spirometer. All measurements were made at about 8.00 am and with a randomly selected sequence of positions.We compared the positional changes of both groups in Forced Expiratory Volume in 1 second (FEV1), mean mid-expiratory flow rate (MMFR), flow rates at 50%and 75% of vital capacity (FEF50 and FEF75 respectively) and the mathematically derived value of flow resistance (Raw). For statistical analysis we used t-test for paired observations.
RESULTS: Both groups showed a reduction of flow rates and increase in flow resistance on changing from UP to HP. These changes were respectively as follows: Normals (mean±SD):FEV1= 4.2±1.0 vs. 4.0±1.4 l, MMFR= 4.5±1.2 vs. 4.3±1.1 l/s, FEF50=6.3±1.8 vs.5.8±1.7 l/s, FEF75 = 2.1±0.7 vs.1. 9±0.5 l/s and Raw =1.6±0.6 vs. 1.7±0.7cmH20/l/s. Asthmatics: FEV1= 2.4±0.9 vs. 2.1±0.8 l, MMFR= 2.1±1.3 vs. 1.4±1.4 l/s (p<0,01), FEF50=3.6±1.1 vs. 2.6±1.0 l/s,(p<0.01), Raw= 2.8±0.8 vs.3.3±1.0 cmH2O/l/s p<0.01).
CONCLUSION: Asthmatics exhibit a more prominent and statistically sigificant fall in flow rates and an increase in airway resistance at HP compared to normal subjects.
CLINICAL IMPLICATIONS: This effect may be a component of the worsening of asthmatic symptoms deterioration at night when patients resume the horizontal position.
DISCLOSURE: Stylianos Michaelides, No Financial Disclosure Information; No Product/Research Disclosure Information