Background and goal of study: Effective lung volume (ELV) can be calculated continuously using a capnodynamic method utilizing a breathing pattern with intermittent cyclic prolongation of inspiratory pauses and correlates well with functional residual capacity (FRC) in healthy lungs in a porcine model. (1) The aim of this study was to evaluate and validate ELV as a volume correlated to FRC in healthy volunteers.

Materials and methods: 10 healthy volunteers were passively ventilated in an upright position using the capnodynamic methods breathing pattern without PEEP. ELV was measured in at least three consecutive minutes where breaths were showing a good fit (low error) to a mathematically predicted ELV volume. The mean ELV were then compared to FRC measured on the same study subjects using body box plethysmography (FRCbp), considered to be the golden standard in lung volumetrics. In four out of ten study subjects we also measured FRC by a nitrogen wash out method for measuring FRC in spontaneous breathing subjects (Exhalyzer), (FRCex).

Results:  Three of the study subjects were excluded since they were not able to let themselves be passively ventilated by the capnodynamic breathing pattern.
ELV showed a FRC (median (range)) of 3350 ml (2700-4500 ml). FRCbp in the same objects were 3700 ml (2600-4400 ml). FRCex was slightly higher 3950 ml (2900-5000 ml). Blant-Altman statistics showed high agreement between ELV and FRC with a bias (95% limits of agreement of) of 21 (-754 to 796 ml) ml and a percentage error of 22 %.
 
Conclusions: In healthy subjects in the up-right position ELV and FRC showed a high agreement. This indicates that ELV could be used as a continuous online measurement of FRC during mechanical ventilation. Further, ongoing, studies will evaluate the performance of ELV on anaesthetized patients.

Reference: 1. Capnodynamic assessment of effective lung volume during cardiac output manipulations in a porcine model. Hällsjö Sander C, Lönnqvist PA, Hallbäck M, Sipmann FS, Wallin M,  Oldner A, Björne H. J Clin monit Comput 2015 Sept: 1-9


10 / Validation of a capnodynamic method for measuring Effective Lung Volume in healthy volunteers