Validation of a Novel Compact System for the Measurement of Lung Volumes

Abstract

Background: Current techniques for measuring absolute lung volumes rely on bulky and expensive equipment and are complicated to use for the operator and the patient. A novel method for measurement of absolute lung volumes, the MiniBox method, is presented.

Research question: Across a population of patients and healthy participants, do values for total lung capacity (TLC) determined by the novel compact device (MiniBox, PulmOne Advanced Medical Devices, Ltd.) compare favorably with measurements determined by traditional whole body plethysmography?

Study design and methods: A total of 266 participants (130 men) and respiratory patients were recruited from five global centers (three in Europe and two in the United States). The study population comprised individuals with obstructive (n = 197) and restrictive (n = 33) disorders as well as healthy participants (n = 36). TLC measured by conventional plethysmography (TLC_Pleth) was compared with TLC measured by the MiniBox (TLC_MB).

Results: TLC values ranged between 2.7 and 10.9 L. The normalized root mean square difference (NSD) between TLC_Pleth and TLC_MB was 7.0% in healthy participants. In obstructed patients, the NSD was 7.9% in mild obstruction and 9.1% in severe obstruction. In restricted patients, the NSD was 7.8% in mild restriction and 13.9% in moderate and severe restriction. No significant differences were found between TLC values obtained by the two measurement techniques. Also no significant differences were found in results obtained among the five centers.

Interpretation: TLC as measured by the novel MiniBox system is not significantly different from TLC measured by conventional whole body plethysmography, thus validating the MiniBox method as a reliable method to measure absolute lung volumes.

Keywords: novel measurement; physiology; plethysmograph; pulmonary function test; total lung capacity.

Figure 1

Bar graph showing the NSD for TLC data when body plethysmography was compared with those obtained using He, N2, CT scanning, and the MB systems. Also shown are the number of participants (N) and the plethysmography devices in each study. Of note, the NSDs were calculated for these published studies using the identical definition that was applied in the present study. He = helium dilution; MB = MiniBox; N2 = nitrogen washout; NSD = normalized root mean square difference; TLC = total lung capacity.

Figure 2

Illustrations of MiniBox device. A, Patient breathing through mouthpiece of the MiniBox, with the spirometer and the tablet computer (both detachable) attached to the device. B, Schematic of the device setup and airflow during the inhalation phase. When the valve is closed, air flows from the rigid container to the lungs, causing a pressure drop in the container.

Figure 3

Diagram showing the physical basis of the MiniBox method. A rigid container of volume V_C is connected to a container of volume V_L, which expands at the rate F₀. Here V_L mimics the lungs and V_C mimics the volume of the MiniBox device. The rarefication of gas in the expanding system induces a flow of gas F_C from the rigid container to the expanding container.

Figure 4

Graphs showing breathing maneuvers and events of the MiniBox. A, Minibox events (circles) are triggered toward the end of the inhalation phase of normal breathing. B, Airflow during a typical MiniBox event. The flow levels F₀ and F_C are calculated by averaging (over intervals of approximately 20 ms) the flow just before the valve begins to close and right after the valve fully closes. ERV = expiratory reserve volume; FRC = functional residual capacity; IC = inspiratory capacity; RV = residual volume; SVC = slow vital capacity; TLC = total lung capacity; VC = vital capacity.

Figure 5

Comparison of TLC_MB and TLC_Pleth for all participants in the study. A, Bland-Altman plot in which the mean discrepancy and deviations of 1.96 SD from it are shown in solid and dashed lines, respectively. The NSD and the mean discrepancy are shown. B, Identity plot showing TLC_Pleth vs TLC_MB. Solid line is the identity line. Dashed lines show 95% CIs about a linear Deming regression. The R², P value, and slope (1.024 ± 0.003) of the linear Deming regression also are shown. NSD = normalized root mean square difference; TLC_MB = total lung capacity measured by the MinBox; TLC_Pleth = total lung capacity measured by plethysmography.

Figure 6

Comparison of TLC_MB and TLC_Pleth for the participants with obstructive dysfunction in the study. A, Bland-Altman plot in which the mean discrepancy and deviations of 1.96 SD from it are shown in solid and dashed lines, respectively. The NSD and the mean discrepancy are shown. B, Identity plot showing TLC_Pleth vs TLC_MB. Solid line is the identity line. Dashed lines show 95% CIs about a linear Deming regression. The R², P value, and slope (1.009 ± 0.003) of the linear Deming regression also are shown. NSD = normalized root mean square difference; TLC_MB = total lung capacity measured by the MinBox; TLC_Pleth = total lung capacity measured by plethysmography.

Figure 7

Comparison of TLC_MB and TLC_Pleth for the participants with restrictive dysfunction in the study. A, Bland-Altman plot in which the mean discrepancy and deviations of 1.96 SD from it are shown in solid and dashed lines, respectively. The NSD and the mean discrepancy are shown. B, Identity plot showing TLC_Pleth vs TLC_MB. Solid line is the identity line. Dashed lines show 95% CIs about a linear Deming regression. The R², P value, and slope (1.004 ± 0.031) of the linear Deming regression also are shown. NSD = normalized root mean square difference; TLC_MB = total lung capacity measured by the MinBox; TLC_Pleth = total lung capacity measured by plethysmography.