Contribution of rostral fluid shift to intrathoracic airway narrowing in asthma

Abstract

In asthma, supine posture and sleep increase intrathoracic airway narrowing. When humans are supine, because of gravity fluid moves out of the legs and accumulates in the thorax. We hypothesized that fluid shifting out of the legs into the thorax contributes to the intrathoracic airway narrowing in asthma. Healthy and asthmatic subjects sat for 30 min and then lay supine for 30 min. To simulate overnight fluid shift, supine subjects were randomized to receive increased fluid shift out of the legs with lower body positive pressure (LBPP, 10-30 min) or none (control) and crossed over. With forced oscillation at 5 Hz, respiratory resistance (R5) and reactance (X5, reflecting respiratory stiffness) and with bioelectrical impedance, leg and thoracic fluid volumes (LFV, TFV) were measured while subjects were seated and supine (0 min, 30 min). In 17 healthy subjects (age: 51.8 ± 10.9 yr, FEV1/FVC z score: -0.4 ± 1.1), changes in R5 and X5 were similar in both study arms (P > 0.05). In 15 asthmatic subjects (58.5 ± 9.8 yr, -2.1 ± 1.3), R5 and X5 increased in both arms (ΔR5: 0.6 ± 0.9 vs. 1.4 ± 0.8 cmH2O·l-1·s-1, ΔX5: 0.3 ± 0.7 vs. 1.1 ± 0.9 cmH2O·l-1·s-1). 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 asthmatic subjects, 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-1·s-1, P = 0.32). We conclude that in asthmatic subjects fluid shifting from the legs to the thorax while supine contributed to increases in the respiratory resistance and stiffness.NEW & NOTEWORTHY In supine asthmatic subjects, application of positive pressure to the lower body caused appreciable increases in respiratory system resistance and stiffness. Moreover, these changes in respiratory mechanics correlated positively with increase in thoracic fluid volume. These findings suggest that fluid shifts from the lower body to the thorax may contribute to overnight intrathoracic airway narrowing and worsening of asthma symptoms.

Keywords: asthma mechanisms; fluid shift; lung physiology; respiratory mechanics; thoracic fluid.

Copyright © 2017 the American Physiological Society.

Figures

Fig. 1.

Changes in the leg and thoracic fluid volumes (ΔLFV and ΔTFV, respectively) from 0 to 30 min supine during the control and lower body positive pressure (LBPP) study arms. Data are represented as means ± SD. Black and gray bars represent data from the healthy and asthmatic subjects, respectively. In both groups, LFV decreased and TFV increased in both study arms. However, the changes were significantly larger in the LBPP arm than in the control arm. Except for the increases in TFV in the control arm (P = 0.01), the changes in the fluid volumes were similar between the healthy and asthmatic subjects (NS, not significant).

Fig. 2.

Changes in the respiratory system resistance at 5 Hz (R5) in healthy subjects (black) and asthmatic subjects (gray) from 0 to 30 min in the control and lower body positive pressure (LBPP) study arms. Each line represents an individual subject. Bars on either side of data lines represent group mean value with SE.

Fig. 3.

Changes in the respiratory system reactance at 5 Hz (X5) in healthy subjects (black) and asthmatic subjects (gray) from 0 to 30 min in the control and lower body positive pressure (LBPP) study arms. Each line represents an individual subject. Bars on either side of data lines represent group mean value with SE. Note that X5 is negative, but to avoid confusion, in the text we discuss the changes in the magnitude of X5.

Fig. 4.

Average changes in the respiratory system reactance at 5 Hz (ΔX5) with posture change (moving from seated to supine position) and rostral fluid shift in lower body positive pressure (LBPP) study arm in the asthmatic subjects. Error bars indicate SD.

Fig. 5.

Correlation between increases in the respiratory system resistance at 5 Hz, R5 (normalized to R5 values at supine 0 min) and thoracic fluid volume (TFV) in the asthmatic subjects (n = 14) during the lower body positive pressure (LBPP) study arm (r = 0.73, P = 0.002).