Influence of hyperpnea on airway surface fluid volume and osmolarity in normal humans

The effects of hyperpnea on exhaled nitric oxide synthesis in normal subjects

STUDY OBJECTIVES

To determine if the concentration of nitric oxide (NO) in the lungs increases with hyperpnea by contrasting calculated production (ie, the product of the fractional expired NO concentration [FeNO] and minute ventilation [Ve]) [Vno] with the amount of NO in equilibrium with the conducting airways (eNOair) and the amount of NO diffusing from the alveoli (eNOalv).

DESIGN

Observational study.

SETTING

University teaching hospital.

PARTICIPANTS

Normal subjects.

INTERVENTIONS

Measurements were made in 16 healthy people during and after 4 min of tidal breathing (10 L/min) and isocapnic hyperventilation of 60 L/min.

MEASUREMENTS AND RESULTS

FeNO was measured by collecting the exhaled air during the last minute of each trial and passing it through a chemiluminescence analyzer. The expired NO levels in the plateau phases of slow (30 mL/s) and fast (200 mL/s) single-breath exhalations were also obtained before and after hyperventilation. The Vno (mean +/- SEM) increased from 89.8 +/- 12.3 to 329.1 +/- 36.2 nL/min as Ve rose (p < 0.001). However, neither the quantities of eNOair nor eNOalv changed with hyperventilation (eNOair range before to after, 34.9 +/- 7.7 to 30.9 +/- 6.4 parts per billion [ppb], p = 0.96; eNOalv range before to after, 7.3 +/- 1.5 to 6.5 +/- 1.1 ppb, p = 0.97).

CONCLUSIONS

These data demonstrate that the amount of NO in equilibrium with the airway walls and alveoli are not altered by hyperpnea. Rather, the apparent augmentation in Vno in such circumstances appears to be an arithmetic artifact.

Desiccation and hypertonicity of the airway surface fluid and thermally induced asthma

To determine whether drying and hypertonicity of the airway surface fluid (ASF) are involved in thermally induced asthma, nine subjects performed isocapnic hyperventilation (HV) (minute ventilation 62.2 +/- 8.3 l/min) of frigid air (-8.9 +/- 3.3 degrees C) while periciliary fluid was collected endoscopically from the trachea. Osmolality was measured by freezing-point depression. The baseline 1-s forced expiratory volume was 73 +/- 4% of predicted and fell 26.4% 10 min postchallenge (P > 0.0001). The volume of ASF collected was 11.0 +/- 2.2 microl at rest and remained constant during and after HV as the airways narrowed (HV 10.6 +/- 1.9, recovery 6.5 +/- 1.7 microl; P = 0.18). The osmolality also remained stable throughout (rest 336 +/- 16, HV 339 +/- 16, and recovery 352 +/- 19 mosmol/kgH(2)O, P = 0.76). These data demonstrate that airway desiccation and hypertonicity of the ASF do not develop during hyperpnea in asthma; therefore, other mechanisms must cause exercise- and hyperventilation-induced airflow limitation.