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

Abstract

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, N 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; ELV /FRC , ELV /FRC and ELV /ELV ). This investigation aimed to determine the repeatability and agreement of ELV with FRC 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 N 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 ELV /FRC and ELV /FRC were age dependent (average decreases of 0.58 and 0.48% year ), suggesting an increase in VH with advancing age. The ELV showed a mean bias of -1.09 litres versus FRC , 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.

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

“…IST measurements of absolute ELV are below those measured by SF 6 and CT by 12e13% and do not fall within 5% of the reference method recommended by the European Respiratory Society/American Thoracic Society consensus statement. 33 These results are consistent with a previous study of IST, in which Bruce and colleagues 17 showed that IST underestimated functional residual capacity (FRC) by 32% (1080 ml) compared with body plethysmography in humans, where the mean FRC was 3.4 L. The underestimation was smaller in our study, with a mean bias of e12 to e13% (e129 to e161 ml), where the overall mean ELV was 809 ml. IST measures the volume of ventilated regions of the lung, and it is expected that IST will underestimate lung volume compared with CT, which estimates the entire thoracic gas volume.…”

“…IST uses commercially available components commonly used in anaesthetic/intensive care practice, and has proven efficacy in spontaneously ventilating healthy volunteers. 17,18 To date, IST's ability to measure ELV in mechanically ventilated lungs has not been tested.…”

“…The repeatability of IST measurements of ELV in the same subject under the same conditions was within the reported international acceptable limits of coefficient of variation (<10%) 34 and consistent with studies of IST in other circumstances. 17 IST has good repeatability and minimal bias compared with CT (mean bias¼e1 ml) when measuring changes in lung volume. This provides support for the use of IST to monitor trends in lung volume, where the change (or lack of) in volume after a change in ventilator setting is potentially of most interest to the bedside clinician.…”

Noninvasive cardiac output monitoring in a porcine model using the inspired sinewave technique: a proof-of-concept study

“…Details of the IST and inspired sinewave device (ISD) have been detailed elsewhere, 15 but in brief: at the start of each inhalation a small volume of N 2 O is injected into inspired gas by a mass flow controller (Alicat Scientific, Inc., Tucson, AZ, USA). Airflow is measured using an ultrasonic flowmeter (VenThor 22/2A; Thor Laboratories, Budapest, Hungary), and N 2 O concentration using a rapid mainstream infrared sensor (Square One Technology, Boulder, CO, USA).…”

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Fig 1A typical example of recorded data, with inspired N 2 O concentration oscillating sinusoidally around a fixed mean (4%), with a predetermined amplitude (3%) and period (180 s). Green line is the expired N 2 O concentration; the blue and red crosses are the N 2 O concentrations in inspired gas and end-tidal gas, respectively; the blue and red lines are the inspired and expired N 2 O sinewaves, respectively (reproduced from Bruce and colleagues 15 ).

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