Low tidal volume ventilation facilitates spontaneous increase in bronchoconstriction and air trapping that can be resolved by deep inspiration and bronchodilator.

Abstract

Prolonged absence of deep inspiration (DI) increases airway resistance. The underlying mechanism is not entirely clear. We hypothesize that DI prohibition allows basal airway smooth muscle (ASM) tone to narrow and close airways over time, resulting in elevation of airway and lung resistance, as well as air trapping. We further hypothesize that DI or pharmacological bronchodilators can prevent or alleviate the resistance increase and air trapping. Physiological respiration was simulated in ex vivo sheep lungs. Lung resistance, elastance, and volume were measured using small tidal volume (120 mL), ventilation frequencies of 0.25 and 2 Hz, and transpulmonary pressure of 7.5 cmH₂O in the presence and absence of DI and bronchodilators. A DI maneuver, involving rapid inflation to total lung capacity followed by deflation to zero transpulmonary pressure, was used to resolve air trapping. Lung resistance and elastance were recorded pre- and post-DI. The experiments were also conducted in the presence of the bronchodilator salbutamol to assess the role of ASM. Ventilation without DI increased lung resistance and elastance, as well as air trapping. DI effectively resolved air trapping, restoring resistance and elastance to their initial values. Salbutamol also alleviated the increase in lung resistance, elastance, and air trapping. DI prevented air trapping and reduced lung resistance and elastance in ex vivo sheep lungs during tidal ventilation, playing a similar role as a pharmacological bronchodilator.NEW & NOTEWORTHY We showed that air trapping is a consistent feature in ex vivo sheep lungs possessing spontaneous bronchoconstriction, when the lungs are ventilated with small tidal volume without intermittent deep inspirations. We further demonstrated that in the presence of salbutamol, air trapping does not occur. This explains the importance of deep inspirations in normal breathing and indicates that airway smooth muscle tone could result in air trapping in the absence of deep inspiration.