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(2)) 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(2) rate of decline (V(PaO2)) and whole-lung computed tomography scan (V(CT)) 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(CT) and V(PaO2) was found both in control (slope = 0.9, R(2) = 0.88) and in saline-lavaged pigs (slope = 0.64, R(2) = 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. CONCLUSION: Bedside measurement of PaO(2) rate of decline during apnoea is a potential approach for estimation of lung volume changes associated with different levels of airway pressure.