Validating the inspired sinewave technique to measure the volume of the 'baby lung' in a porcine lung-injury model.

ABSTRACT

BACKGROUND: Bedside lung volume measurement could personalise ventilation and reduce driving pressure in patients with acute respiratory distress syndrome (ARDS). We investigated a modified gas-dilution method, the inspired sinewave technique (IST), to measure the effective lung volume (ELV) in pigs with uninjured lungs and in an ARDS model. METHODS: Anaesthetised mechanically ventilated pigs were studied before and after surfactant depletion by saline lavage. Changes in PEEP were used to change ELV. Paired measurements of absolute ELV were taken with IST (ELVIST) and compared with gold-standard measures (sulphur hexafluoride wash in/washout [ELVSF6] and computed tomography (CT) [ELVCT]). Measured volumes were used to calculate changes in ELV (ΔELV) between PEEP levels for each method (ΔELVIST, ΔELVSF6, and ΔELVCT). RESULTS: The coefficient of variation was <5% for repeated ELVIST measurements (n=13 pigs). There was a strong linear relationship between ELVIST and ELVSF6 in uninjured lungs (r2=0.97), and with both ELVSF6 and ELVCT in the ARDS model (r2=0.87 and 0.92, respectively). ELVIST had a mean bias of -12 to 13% (95% limits=±17 - 25%) compared with ELVSF6 and ELVCT. ΔELVIST was concordant with ΔELVSF6 and ΔELVCT in 98-100% of measurements, and had a mean bias of -73 to -77 ml (95% limits=±128 - 186 ml) compared with ΔELVSF6 and -1 ml (95% limits ±333 ml) compared with ΔELVCT. CONCLUSIONS: IST provides a repeatable measure of absolute ELV and shows minimal bias when tracking PEEP-induced changes in lung volume compared with CT in a saline-lavage model of ARDS.