Reduced Baseline Airway Caliber Relates to Larger Airway Sensitivity to Rostral Fluid Shift in Asthma

Abstract

Background: We have previously shown that when asthmatics go supine, fluid shifts out of the legs, accumulates in the thorax, and exacerbates lower airway narrowing. In the retrospective analysis of our previous work presented here, we test the hypothesis that the sensitivity of this process relates inversely to baseline caliber of the lower airways.Methods: Eighteen healthy (six women) and sixteen asthmatic subjects (nine women) sat for 30 min, and then lay supine for 30 min. While supine, lower body positive pressure (LBPP, 40 mm Hg) was applied to displace fluid from the legs similar in amount to the overnight fluid shift. Respiratory resistance and reactance at 5 Hz (R5 and X5) and leg and thoracic fluid volumes (LFV and TFV) were measured at the beginning and end of the supine period.Results: With LBPP, healthy, and asthmatic subjects had similar changes in the LFV and TFV (p = 0.3 and 0.1, respectively). Sensitivity to fluid shift, defined by ΔR5/ΔTFV, was larger in the asthmatics than in the healthy subjects (p = 0.0001), and correlated with baseline R5 in the supine position in the asthmatics (p = 0.7, p = 0.003). No such association was observed in the healthy subjects (p = 0.6). In the asthmatics, women showed a greater reduction in X5 than men with LBPP (p = 0.009).Conclusions: Smaller baseline airway caliber, as assessed by larger R5, was associated with increased sensitivity to fluid shift in the supine position. We conclude that asthmatics with narrower small airways such as obese asthma patients, women with asthma and those with severe asthma may be more sensitive to the effects fluid shift while supine as during sleep.

Effects of physical exercise training on nocturnal symptoms in asthma: Systematic review

“…Although, the included studies did not evaluate overnight changes in lung function, it is unlikely that exercise training will cause overnight changes if it did not change daytime lung function. Another factor that may be associated with the nocturnal worsening of asthma is the overnight rostral fluid shift from the legs to the thorax which can increase fluid accumulation in the lower airway wall, narrow airway lumen, and worsen asthma symptoms [ 25 , 26 ]. Previous studies in patients with sleep apnea have shown that physical training during the day can reduce overnight fluid shift out of the legs and improve sleep apnea severity [ 89 ] and snoring [ 90 ].…”

“…During sleep, recumbent posture causes a reduction in the lung volumes, respiratory muscle tone, and lung compliance [ 22 – 24 ]. Moreover, recumbent posture also causes a shift of fluid from the legs to the thorax (rostral fluid shift) that is associated with airway narrowing in asthma [ 25 , 26 ]. In addition, during sleep there is an overnight decrease in plasma cortisol and an increase in airway inflammation, which in turn increases cholinergic tone [ 16 , 17 ], and enhances airway hyper-responsiveness.…”

Thoracic fluid accumulation and asthma symptoms: A new contributor mechanism

“…Recent evidences show that rostral fluid shift from the legs to the thorax during supine posture 19 20 and fluid overloading 21 are potential contributors to poor asthma control and nocturnal worsening of asthma symptoms. Fluid accumulation in the thorax may cause a cascade of events that exacerbate airway narrowing.…”

“… 19 20 In order to control for the confounding effects of sleep on airway resistance, we performed a daytime study in healthy controls and patients with asthma. 19 20 We simulated nocturnal rostral fluid shift by applying lower body positive pressure with inflatable trousers. After 30 minutes, lower body positive pressure moved similar amount of fluid out of the legs to that which happens overnight.…”

“…We found that despite similar amount of fluid moving to the thorax in healthy controls and asthmatics with lower body positive pressure: (1) only in asthmatics, fluid accumulation in the thorax increased lower airway narrowing 19 ; (2) changes in lower airway resistance were directly related to the amount of fluid moving to the thorax 19 ; and (3) the effects of fluid shift were larger in women than in men. 20 …”