Rapid breath analysis for acute respiratory distress syndrome diagnostics using a portable two-dimensional gas chromatography device

Abstract

Acute respiratory distress syndrome (ARDS) is the most severe form of acute lung injury, responsible for high mortality and long-term morbidity. As a dynamic syndrome with multiple etiologies, its timely diagnosis is difficult as is tracking the course of the syndrome. Therefore, there is a significant need for early, rapid detection and diagnosis as well as clinical trajectory monitoring of ARDS. Here, we report our work on using human breath to differentiate ARDS and non-ARDS causes of respiratory failure. A fully automated portable 2-dimensional gas chromatography device with high peak capacity (> 200 at the resolution of 1), high sensitivity (sub-ppb), and rapid analysis capability (~ 30 min) was designed and made in-house for on-site analysis of patients' breath. A total of 85 breath samples from 48 ARDS patients and controls were collected. Ninety-seven elution peaks were separated and detected in 13 min. An algorithm based on machine learning, principal component analysis (PCA), and linear discriminant analysis (LDA) was developed. As compared to the adjudications done by physicians based on the Berlin criteria, our device and algorithm achieved an overall accuracy of 87.1% with 94.1% positive predictive value and 82.4% negative predictive value. The high overall accuracy and high positive predicative value suggest that the breath analysis method can accurately diagnose ARDS. The ability to continuously and non-invasively monitor exhaled breath for early diagnosis, disease trajectory tracking, and outcome prediction monitoring of ARDS may have a significant impact on changing practice and improving patient outcomes. Graphical abstract.

Keywords: 2D GC; Acute respiratory distress syndrome gas chromatography; Breath analysis; Machine learning.

Fig. 1

Conceptual illustration of using a portable GC device to analyze breath from a patient on a mechanical ventilator

Fig. 2

Detailed description of the experimental setup. The portable GC was connected to the output of a ventilator via a 2 m long polytetrafluoroethylene (PTFE) tubing (0.64 cm i.d.). The portable GC weighed less than 5 kg. The patient breath was drawn into and captured by the thermal desorption tube in the GC device at a flow rate of 70 mL/min for 5 minutes. The total assay time was 33 minutes, which included 5 minutes of sample collection time, 5 minutes of desorption/transfer time, 13 minutes of separation time, and 10 minutes of cleaning time. For details of GC operation, see the ESM

Fig. 3

Layout of the portable 1×2-channel 2D GC device. It consisted of three detachable modules: sampling module, 1^st-dimensional separation module, and 2^nd-dimensional separation module. The 1^st-dimensional module consisted of a micro-thermal injector (μTI), a non-polar column, and a micro-photoionization detector (μPID). The 2^nd-dimensional module had two identical channels, each of which consisted of a μTI, a polar column, and a μPID. During operation, breath was collected via the sampling tube and captured by the thermal desorption tube. Then the analytes were transferred to μTI 1 and injected into the ¹D column. The elution from the ¹D column was detected by μPID 1. If 2^nd-dimensional separation was needed, the 2^nd-dimensional module was attached to the outlet of the ¹D column and the elution from the ¹D column was sliced and sent alternately to one of the two ¹D columns via a micro-Deans switch. The portable GC can operate as 1D GC alone when the 2^nd-dimensional module is disabled or detached, or as comprehensive 2D GC. Comprehensive 2D GC operation required additional 20 seconds compared to 1D GC operation alone, which was negligible considering the overall assay time of ~30 minutes. For details of GC operation and 2D GC chromatogram construction, see the ESM

Fig. 4

(a) and (b) Representative 1D chromatogram and 2D chromatogram for an ARDS patient, respectively. (c) and (d) Representative 1D chromatogram and 2D chromatogram for a non-ARDS (control) patient, respectively. In the zoomed-in 2D chromatogram for the control patient, four co-eluted ¹D peaks are separated into eight peaks in the 2D chromatogram. Other zoomed-in portions of (b) and (d) can be found in ESM Fig. S1

Fig. 5

All 97 peaks found collectively in 85 breath samples from 48 patients plotted in a 2D chromatogram, among which 18 pairs (36 peaks) are co-eluted and approximately another 30 peaks are partially co-eluted (with doublets or triplets and separation of adjacent peaks is less than 2σ) from the ¹D column. Each dot represents the center of a peak in the contour plot (see, for example, Fig. 4, for a peak contour plot). Note that not all 97 peaks appear in a 2D chromatogram for a particular patient

Fig. 6

Evolution of the 2D chromatogram of an ARDS patient (Patient #11) during 3 days of monitoring

Fig. 7

PCA plot of all recruited patients. X-axis (PC 1) is the 1^st principal component and Y-axis (PC 2) is the 2^nd principal component. The red and black symbols denote respectively the ARDS and non-ARDS patients adjudicated by physicians using the Berlin criteria. The patient numbers are given by the symbol. For example, “11.1” and “11.3” denote Patient #11, Day 1 and Day 3 results, respectively. The bottom/top zone below/above the boundary line represents respectively the ARDS/non-ARDS region using the breath analysis method. The corresponding Q-residuals for this PCA model are shown in ESM Fig. S5

Fig. 8

The trajectory on PCA plot for patient #11, #27, #36, and #47. #11 and #27 are the upgrade case (initially listed as potential ARDS on the first day) and #36 and #47 are recovery cases (extubated and discharged from ICU 24–48 hours after the last test). The bottom/top zone below/above the boundary line represents respectively the ARDS/non-ARDS region using the breath analysis method Note: If the breath test results do not match the clinical adjudication, we consider the test as “misclassification” when calculating the overall classification accuracy, even for the cases like #36.3 and #47.4 that suggest that the breath analysis was able to predict the trajectory of ARDS (i.e., earlier diagnosis).