A novel method to quantify breathing effort from respiratory mechanics and esophageal pressure.

Abstract

Breathing effort is important to quantify to understand mechanisms underlying central and obstructive sleep apnea, respiratory-related arousals, and the timing and effectiveness of invasive or non-invasive mechanically assisted ventilation. Current quantitative methods to evaluate breathing effort rely on inspiratory esophageal or epiglottic pressure swings or changes in diaphragm electromyographic (EMG) activity, where units are problematic to interpret and compare between individuals and to measured ventilation. This paper derives a novel method to quantify breathing effort in units directly comparable to measured ventilation by applying respiratory mechanics first principles to convert continuous transpulmonary pressure measurements into "attempted" airflow expected to have arisen without upper airway obstruction. The method was evaluated using data from eleven subjects undergoing overnight polysomnography, including 6 obese patients with severe obstructive sleep apnea (OSA), including one who also had frequent central events, and 5 healthy-weight controls. Classic respiratory mechanics showed excellent fits of airflow and volume to transpulmonary pressures during wake periods of stable unobstructed breathing (mean ± SD r² = 0.94 ± 0.03), with significantly higher respiratory system resistance in patients compared to healthy controls (11.2 ± 3.3 vs 7.1 ± 1.9 cmH2O·l-1·sec, P=0.032). Subsequent estimates of attempted airflow from transpulmonary pressure changes clearly highlighted periods of acute and prolonged upper airway obstruction, including within the first few breaths following sleep onset in patients. This novel technique provides unique quantitative insights into the complex and dynamically changing inter-relationships between breathing effort and achieved airflow during periods of obstructed breathing in sleep.