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 (ELV_IST) and compared with gold-standard measures (sulphur hexafluoride wash in/washout [ELV_SF6] and computed tomography (CT) [ELV_CT]). Measured volumes were used to calculate changes in ELV (ΔELV) between PEEP levels for each method (ΔELV_IST, ΔELV_SF6, and ΔELV_CT).

Results

The coefficient of variation was <5% for repeated ELV_IST measurements (n=13 pigs). There was a strong linear relationship between ELV_IST and ELV_SF6 in uninjured lungs (r²=0.97), and with both ELV_SF6 and ELV_CT in the ARDS model (r²=0.87 and 0.92, respectively). ELV_IST had a mean bias of –12 to 13% (95% limits=±17 – 25%) compared with ELV_SF6 and ELV_CT. ΔELV_IST was concordant with ΔELV_SF6 and ΔELV_CT in 98–100% of measurements, and had a mean bias of –73 to –77 ml (95% limits=±128 – 186 ml) compared with ΔELV_SF6 and –1 ml (95% limits ±333 ml) compared with ΔELV_CT.

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.