Contribution of Rostral Fluid Shift to Intrathoracic Airway Narrowing in Asthma

Introduction: In asthma, supine posture and sleep increase intrathoracic airway narrowing. When supine, due to gravity, fluid moves out of the legs and accumulates in the thorax. We hypothesized that fluid shift out of the legs into the thorax contributes to the intrathoracic airway narrowing in asthma. Methods: Healthy and asthmatic subjects sat for 30min and then lay supine for 30min. To simulate overnight fluid shift, supine subjects were randomized to receive increased fluid shift out of the legs using lower body positive pressure (LBPP, 10 to 30min) or none (control), and crossed over. Using forced oscillation at 5Hz, respiratory resistance (R5) and reactance (X5, reflecting respiratory stiffness), and using bioelectrical impedance, leg and thoracic fluid volumes (LFV, TFV) were measured while seated and supine (0min, 30min). Results: In 17 healthy subjects (age:51.8±10.9 years, FEV1/FVC Z-score: -0.4±1.1), changes in R5 and X5 were similar in both study arms (p>0.05). In 15 asthmatics (58.5±9.8 years, -2.1±1.3), R5 and X5 increased in both arms (R5:0.6±0.9 vs. 1.4±0.8 cmH₂O/L/s, X5:0.3±0.7 vs. 1.1±0.9 cmH₂O/L/s). The increases in R5 and X5 were 2.3 and 3.7 times larger with LBPP than control, however (p=0.008, p=0.006). The main predictor of increases in R5 with LBPP was increases in TFV (r=0.73, p=0.002). In asthmatics, the magnitude of increases in X5 with LBPP was comparable to that with posture change from sitting to supine (1.1±0.9 vs. 1.4±0.9 cmH₂O/L/s, p=0.32). Conclusion: In asthmatics, fluid shift from the legs to the thorax while supine contributed to increases in the respiratory resistance and stiffness.

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