Bedside monitoring of lung volume available for gas exchange.

Background: 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 (PaO) decline during a breath-holding manoeuvre could be used to estimate lung volume in a single-compartment physiological model of the respiratory system.

Methods: 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 PaO rate of decline (V) and whole-lung computed tomography scan (V) during apnoea at different positive end-expiratory and end-inspiratory pressures.

Results: A total of 146 volume measurements was completed (range 134 to 1869 mL). A linear correlation between V and V was found both in control (slope = 0.9, R = 0.88) and in saline-lavaged pigs (slope = 0.64, R = 0.70). The bias from Bland-Altman analysis for the agreement between the V and V 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.

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