The Inspired Sinewave Technique: A Comparison Study With Body Plethysmography in Healthy Volunteers

Abstract

The inspired sinewave technique is a noninvasive method to measure airway dead space, functional residual capacity, pulmonary blood flow, and lung inhomogeneity simultaneously. The purpose of this paper was to assess the repeatability and accuracy of the current device prototype in measuring functional residual capacity, and also participant comfort when using such a device. To assess within-session repeatability, six sinewave measurements were taken over two-hour period in 17 healthy volunteers. To assess day-to-day repeatability, measurements were taken over 16 days in 3 volunteers. To assess accuracy, sinewave measurements were compared to body plethysmography in 44 healthy volunteers. Finally, 18 volunteers who experienced the inspired sinewave device, body plethysmography and spirometry were asked to rate the comfort of each technique on a scale of 1-10. The repeatability coefficients for dead space, functional residual capacity, and blood flow were 48.7 ml, 0.48L, and 2.4L/min respectively. Bland-Altman analyses showed a mean BIAS(SD) of -0.68(0.42)L for functional residual capacity when compared with body plethysmography. 14 out of 18 volunteers rated the inspired sinewave device as their preferred technique. The repeatability and accuracy of functional residual capacity measurements were found to be as good as other techniques in the literature. The high level of comfort and the non-requirement of patient effort meant that, if further refined, the inspired sinewave technique could be an attractive solution for difficult patient groups such as very young children, elderly, and ventilated patients.

Keywords: Lung function test; cardiac output; functional residual capacity; lung inhomogeneity; medical device.

FIGURE 1.

The Inspired Sinewave Device. (1): nasal mask with seal; (2): bacterial and moisture filter; (3): N2O infrared sensor; (4): flow sensor; (5): gas mixing chamber; (6): articulated arm.

FIGURE 2.

An example of inspired and expired sinewaves of N₂O in an inspired sinewave technique test. The green line is N₂O concentration measured by the infrared mainstream sensor. The blue dots are the mean inspired concentration, whereas the red crosses are the end expired concentration. The blue and red sinewaves are the inspired and expired sinewaves respectively.

FIGURE 3.

One-compartment “balloon on a straw” lung model. is the inspired concentration, is the expired concentration, is the airway deadspace, is the lung/alveolar volume, and is the alveolar concentration.

FIGURE 4.

(a-c) Repeated sinewave measurements of , FRC, for 17 participants over 2 hours. Black dots are individual measurements. Blue diamonds are the average.

FIGURE 5.

(a-c) Day-to-day repeated sinewave measurements for 3 participants over 16 days.

FIGURE 6.

Bland Altman plot comparing FRC measured by the inspired sinewave technique and by body plethysmography. Teal line represents the bias, and red lines represent the limits of agreement. Legends: x-axis is (FRCISD + FRCpleth)/2; and y-axis is FRCISD – FRCpleth.

FIGURE 7.

Assessment of comfort between the ISD, body plethysmography and spirometry.

FIGURE 8.

When lung inhomogeneity presented, even in healthy volunteers, the FRC estimated from the ISD would depend on the inspired sinewave’s frequency. The x-axis is the inspired sinewave period. The y-axis is the ratio between FRC estimated by the ISD and FRC estimated by body plethysmography. The lower mean ratio of the asymptomatic asthma group suggested that, indeed, it has higher lung inhomogeneity than the healthy volunteer group.