The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

HEAT AND MOISTURE EXCHANGER (HME) EFFICIENCY FACTORS AND DEADSPACE VOLUMES.

Jeanette Lucato MS, Alexander Adams RRT, MPH, Rogerio Souza MD , Carlos Carvalho MD, John Marini MD. Clinical Hospital, Sao Paulo, Brazil/Regions Hospital, St.Paul, MN.

Background: As a substitute for heated humidifier use in mechanical ventilator circuits, HMEs have important advantages such as cost savings and fewer condensate management problems. While HMEs add to the circuit-deadspace burden and they can become a resistive element, HMEs have been widely adopted for short-term use - one to several days. After extended use in some patients, however, a humidity deficit from HME use can result in thick secretions that are potentially occlusive. For the HMEs currently available, we evaluated their humidification efficiency under a range of settings that varied tidal volume, frequency, flow rate and FIO2.

Methods:
A test system was assembled included a heated chamber (plastic container heated to 37°C. by hair dryer), a preserved swine lung (NASCO), hygrometer (Vaisala), standard ventilator circuitry and mechanical ventilator (P-B 840). Humidity levels were measured during control conditions (no HME) and with 8 interposed HMEs as ventilator settings were varied: VT = 200, 500, 1000 ml, f = 5, 10, 20/min, Flow = 30, 60, 90 L/min, FIO2 = 0.21, 0.50, 1.0. We also conducted a separate analysis of the effect of deadspace volume on CO2 retention in a test lung (TTL) system.

Results:
Relative humidity under control conditions was lower (21-79%) than during HME use (58-86%). The Hygrobac S model tended to maintain higher intracircuit humidity levels than other units. Increasing tidal volume had a significant effect by lowering maintained humidity in all HME models (left figure). Changing the frequency, flow rate or FIO2 had little effect on humidity levels. In our deadspace analysis study, CO2 was retained in a direct relationship to the HME deadspace volume (right figure - VD in parentheses).





Conclusions:
Our findings suggest that HME humidification efficiency can be markedly affected by delivered tidal volume and that greater HME - VD increases CO2 retention.

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