The inspired sinewave technique: A novel method to measure lung volume and ventilatory heterogeneity

Abstract

The Inspired Sinewave Technique (IST) is a novel method which can provide simple, non-invasive cardiopulmonary measurements. Over successive tidal-breaths the concentration of a tracer gas (i.e. nitrous oxide, N₂O) is sinusoidally modulated in inspired air. Using a single-compartment tidal-ventilation lung model, the resulting amplitude/phase of the expired sinewave allows estimation of end-expired lung volume (ELV), pulmonary blood flow and three indices for ventilatory heterogeneity (VH; ELV₁₈₀/FRC_pleth, ELV₁₈₀/FRC_pred and ELV₆₀/ELV₁₈₀).

This investigation aimed to determine: the repeatability and agreement of ELV with FRC_pleth, and, as normal ageing results in well-established changes in pulmonary structure and function, whether the IST estimates of ELV and VH are age dependent.

48 healthy never-smoker participants (20–86 years) underwent traditional pulmonary function testing: (e.g. spirometry, body plethysmography) and the IST test which consisted of 4 minutes of quiet breathing through a facemask while inspired N₂O concentrations are oscillated in a sinewave pattern with a fixed mean (4%) and amplitude (3%) and a period of either 180 seconds or 60 seconds.

ELV₁₈₀/FRC_pleth and ELV₁₈₀/FRC_pred were age dependent (average decreases of 0.58% and 0.48% per year) suggesting an increase in VH with advancing age. ELV showed a mean bias of −1.09L vs. FRC_pleth, but when normalised for the effects of age this bias reduced to −0.35L. The IST test has potential to provide clinically useful information necessitating further study (e.g. for mechanically ventilated or obstructive lung disease patients), but these findings suggest that the increases in VH with healthy ageing must be accounted for in clinical investigations.

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