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.