Standardization of Methods for Sampling the Distal Airspace in Mechanically Ventilated Patients using Heat Moisture Exchange Filter Fluid.

Abstract

Non-invasive 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 filters (HME) 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 4 different filter types by loading increasing volumes of saline and measuring volume of fluid recovered. We collected filters after 1, 2 and 4 hours of dwell time for measurement of fluid volume and total protein from 13 subjects. After identifying 4 hours as the optimal dwell time, we measured total protein and IgM in HME fluid from 42 subjects with ARDS and 9 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 (IQR 1.2-2.7) after 4 hours. Total protein was higher in the 42 subjects with ARDS compared to 9 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 to hydrostatic edema. These studies establish a standardized HME fluid collection protocol and confirm that HME fluid analysis is a novel non-invasive tool for study of the distal airspace in ARDS.