Bedside monitoring of lung volume available for gas exchange

Bedside measurement of lung volume may provide guidance in the personalised setting of respiratory support, especially in patients with the acute respiratory distress syndrome at risk of ventilator-induced lung injury. We propose here a novel operator-independent technique, enabled by a fibre optic oxygen sensor, to quantify the lung volume available for gas exchange. We hypothesised that the continuous measurement of arterial partial pressure of oxygen (PaO₂) decline during a breath-holding manoeuvre could be used to estimate lung volume in a single-compartment physiological model of the respiratory system.

Thirteen pigs with a saline lavage lung injury model and six control pigs were studied under general anaesthesia during mechanical ventilation. Lung volumes were measured by simultaneous PaO₂ rate of decline (V_PaO2) and whole-lung computed tomography scan (V_CT) during apnoea at different positive end-expiratory and end-inspiratory pressures.

A total of 146 volume measurements was completed (range 134 to 1869 mL). A linear correlation between V_CT and V_PaO2 was found both in control (slope = 0.9, R² = 0.88) and in saline-lavaged pigs (slope = 0.64, R² = 0.70). The bias from Bland-Altman analysis for the agreement between the V_CT and V_PaO2 was - 84 mL (limits of agreement ± 301 mL) in control and + 2 mL (LoA ± 406 mL) in saline-lavaged pigs. The concordance for changes in lung volume, quantified with polar plot analysis, was - 4º (LoA ± 19°) in control and - 9° (LoA ± 33°) in saline-lavaged pigs.

Bedside measurement of PaO₂ rate of decline during apnoea is a potential approach for estimation of lung volume changes associated with different levels of airway pressure.