The inspired sine-wave technique: A novel method to measure lung volume and ventilatory heterogeneity

What is the central question of this study? We present a new non-invasive medical technology, the inspired sine-wave technique, which involves inhalation of sinusoidally fluctuating concentrations of a tracer gas. The technique requires only passive patient cooperation and can monitor different cardiorespiratory variables, such as end-expired lung volume, ventilatory heterogeneity and pulmonary blood flow. What is the main finding and its importance? In this article, we demonstrate that the measurements of end-expired lung volume are repeatable and accurate, in comparison to whole-body plethysmography, and the technique is sensitive to the changes in ventilatory heterogeneity associated with advancing age. As such, it has the potential to provide clinically valuable information.

The inspired sine-wave technique (IST) is a new method that can provide simple, non-invasive cardiopulmonary measurements. Over successive tidal breaths, the concentration of a tracer gas (i.e. nitrous oxide, N₂ O) is sinusoidally modulated in inspired air. Using a single-compartment tidal-ventilation lung model, the resulting amplitude/phase of the expired sine wave 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. Forty-eight 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 min of quiet breathing through a face mask while inspired N₂ O concentrations were oscillated in a sine-wave pattern with a fixed mean (4%) and amplitude (3%) and a period of either 180 or 60 s. The ELV₁₈₀ /FRC_pleth and ELV₁₈₀ /FRC_pred were age dependent (average decreases of 0.58 and 0.48% year^-1 ), suggesting an increase in VH with advancing age. The ELV showed a mean bias of -1.09 litres versus FRC_pleth , but when normalized for the effects of age this bias reduced to -0.35 litres. 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 taken into account in clinical investigations.