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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.

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 (DELV) between PEEP levels for each method (DELV IST , DELV SF6 , and DELV CT). Results: The coefficient of variation was <5% for repeated ELV IST measurements (n¼13 pigs). There was a strong linear relationship between ELV IST and ELV SF6 in uninjured lungs (r 2 ¼0.97), and with both ELV SF6 and ELV CT in the ARDS model (r 2 ¼0.87 and 0.92, respectively). ELV IST had a mean bias of e12 to 13% (95% limits¼±17 e 25%) compared with ELV SF6 and ELV CT. DELV IST was concordant with DELV SF6 and DELV CT in 98e100% of measurements, and had a mean bias of e73 to e77 ml (95% limits¼±128 e 186 ml) compared with DELV SF6 and e1 ml (95% limits ±333 ml) compared with DELV 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.

Background Cardiac output monitoring can support the management of high-risk surgical patients, but the pulmonary artery catheterisation required by the current ‘gold standard’—bolus thermodilution —has the potential to cause life-threatening complications. We present a novel noninvasive and fully automated method that uses the inspired sinewave technique to continuously monitor cardiac output . Methods Over successive breaths the inspired nitrous oxide (N 2 O) concentration was forced to oscillate sinusoidally with a fixed mean (4%), amplitude (3%), and period (60 s). was determined in a single-compartment tidal ventilation lung model that used the resulting amplitude/phase of the expired N 2 O sinewave. The agreement and trending ability of were compared with during pharmacologically induced haemodynamic changes, before and after repeated lung lavages, in eight anaesthetised pigs. Results Before lung lavage, changes in and from baseline had a mean bias of –0.52 L min −1 (95% confidence interval [CI], –0.41 to –0.63). The concordance between and was 92.5% as assessed by four-quadrant analysis, and polar plot analysis revealed a mean angular bias of 5.98° (95% CI, –24.4°–36.3°). After lung lavage, concordance was slightly reduced (89.4%), and the mean angular bias widened to 21.8° (–4.2°, 47.6°). Impaired trending ability correlated with shunt fraction ( r =0.79, P <0.05). Conclusions The inspired sinewave technique provides continuous and noninvasive monitoring of cardiac output, with a ‘marginal–good’ trending ability compared with cardiac output based on thermodilution. However, the trending ability can be reduced with increasing shunt fraction, such as in acute lung injury.