Standardization of methods for sampling the distal airspace in mechanically ventilated patients using heat moisture exchange filter fluid

Abstract

Noninvasive sampling of the distal airspace in patients with acute respiratory distress syndrome (ARDS) has long eluded clinical and translational researchers. We recently reported that fluid collected from heat moisture exchange (HME) filters closely mirrors fluid directly aspirated from the distal airspace. In the current study, we sought to determine fluid yield from different HME types, optimal HME circuit dwell time, and reliability of HME fluid in reflecting the distal airspace. We studied fluid yield from four different filter types by loading increasing volumes of saline and measuring volumes of fluid recovered. We collected filters after 1, 2, and 4 h of dwell time for measurement of fluid volume and total protein from 13 subjects. After identifying 4 h as the optimal dwell time, we measured total protein and IgM in HME fluid from 42 subjects with ARDS and nine with hydrostatic pulmonary edema (HYDRO). We found that the fluid yield varies greatly by filter type. With timed sample collection, fluid recovery increased with increasing circuit dwell time with a median volume of 2.0 mL [interquartile range (IQR) 1.2-2.7] after 4 h. Total protein was higher in the 42 subjects with ARDS compared with nine with HYDRO [median 708 µg/mL (IQR 244-2017) vs. 364 µg/mL (IQR 136-578), P = 0.047], confirming that total protein concentration in HME is higher in ARDS compared with hydrostatic edema. These studies establish a standardized HME fluid collection protocol and confirm that HME fluid analysis is a novel noninvasive tool for the study of the distal airspace in ARDS.

Keywords: ARDS; acute respiratory distress syndrome; biomarkers; exhaled breath; pulmonary edema.

Figure 1.

Fluid recovery of pre-loaded volume by filter type. 50-mL conical tubes were modified using four screws to firmly hold the HME filter in place prior to placing the tube in the centrifuge (A and B). Increasing volumes of saline were added to four different HME filters (three adult and one neonatal). Filters were placed in customized tubes and fluid was recovered by centrifugation. C: percent recovery was calculated by volume recovered/volume loaded. HME, heat moisture exchange.

Figure 2.

Timed HMEF sample collection. HME filters were collected after 1, 2, and 4 h of circuit dwell time from 13 mechanically ventilated patients with ARDS, HYDRO, mixed, or another cause of respiratory failure. HMEF volume was measured at each time point and recovery increased as dwell time increased (A). Protein concentrations were measured in the same samples and did not change significantly as HMEF dwell time increased (B). ARDS, acute respiratory distress syndrome; HME, heat moisture exchange.

Figure 3.

HMEF comparison between ARDS and HYDRO. HMEF was collected after 4 h of HME circuit dwell time within the first 2 days of endotracheal intubation from ARDS (n = 42) and HYDRO (n = 9) patients. There was no difference in total volume of fluid collected (A), but protein concentration was significantly higher in fluid from ARDS (n = 35) patients versus HYDRO (n = 9) (B) and there was a trend toward higher IgM in patients with ARDS (n = 35) compared with HYDRO (n = 5) patients (C). ARDS, acute respiratory distress syndrome; HME, heat moisture exchange.