A study of the humidification performance of Heat and Moisture
Exchanging Filters in Semi-Closed Circle Anaesthesia
Karen Kendall
Karen Wilkins
Scientific & Laboratory Services
Pall Europe
______________________________________________________________________________
Introduction
Humidification of dry medical gases has long been recognized as important for
preventing damage to the respiratory tract and for decreasing the extent of post
operative hypothermia [1], Humidification of inspiratory gases has also been shown to be
necessary in semi-closed anaesthesia, despite the humidity levels otherwise generated
in such circuits [2]. Heat and moisture exchangers (HME’s) have been used to provide
this humidification because they are more economical and convenient to use than hot
water bath humidifiers. Heat and moisture exchanging filters (HMEF’s) have the added
advantage of reducing circuit and ventilator contamination [3].
Several studies have been performed to evaluate the heat and moisture exchanging
properties of HMEF’s [3 - 8]. They have mostly involved the use of laboratory models
(artificial patients) designed to simulate lung function by producing saturated water at or
near body temperature. Several methods have been used to monitor humidity levels
including use of hygrometer probes, collection of expired water vapour and gravimetric
techniques which determine water loss from the articial patient [9].
The current ISO Standard for heat and moisture evaluation, ISO 9360 [10], is a
gravimetric method. It also recognizes that the articial patient water vapour output is of
critical importance and thus specifies that the test apparatus must produce fully
saturated water vapour at 34°C. Whilst welcoming the aims and principles of ISO 9360,
at least two eminent working groups in this area have listed various practical problems
encountered when trying to run the test apparatus [9, 11]. Such problems have mainly
focused on the design of the artificial patient, points which have been addressed in the
test apparatus used in this study.
The test methodology given in ISO 9360 evaluates HME performance in an open circuit,
dry gas situation typical of intensive care environments. It does not address the issue of
HME performance in other circuit configurations. In this study, HME performance was
assessed using a test apparatus designed to simulate semi-closed circle anaesthesia
systems whilst maintaining the essential principles of ISO 9360.
Materials
Ventilators
Two ventilators were utilized; a Blease 8100S anaesthesia ventilator obtained from
Blease Medical Equipment Limited, Chesham and a Servo 900C obtained from
Siemens plc, Sunbury on Thames.
Artificial Patient
The artificial patient was constructed using a Bennett Cascade Mk II water bath
humidifier obtained from Puritan Bennett UK Limited, Hounslow; standard breathing
system flutter valves, connectors and tubing; and a one way valve obtained from
Abacus Valve Manufacturers Limited, Alton. These were assembled as Figure 1.
Apparatus
The following pieces of equipment were obtained from the manufacturers listed.
Circle absorber and Durasorb (self indicating soda lime) – M & IE, Dentsply, Exeter;
Relative humidity / temperature probe – Solomat, Devon;
Total airflow meter – British Gas Sothern, Portsmouth;
Datex CO2 monitor – Datex Instrumentation Corp., Helsinki, Finland.
Test Devices
The following devices were tested.
Device 1 – Pall BB25 pleated hydrophobic membrane filter (lot 101210)
Device 2 – composite device consisting of electret filter media with hygroscopic element
Device 3 – composite device consisting of electret filter media with hygroscopic element
Methods
Evaluation of Artificial Patient Performance
The performance of the artificial patient was evaluated at the start of each experimental
run to confirm that it produced exhaled air fully saturated with water vapour at 34 °C ie,
100 % relative humidity (RH).
The artificial patient was assembled as in Figure 1. New flutter valves were used for
each experimental run due to their short life at elevated temperatures. The artificial
patient was filled to the ‘maximum’ mark with deionised water. It was connected to a
standard non-recirculating circuit as Figure 2 using a Siemens Servo 900C ventilator set
at 6 l/min and 12 breaths/min.
The ventilator dry gas supply had a humidity not exceeding 0.88 mg/l and a temperature
of 23 ± 2 °C. The room ambient temperature was 23 ± 2 °C. The airflow patient exhaled
temperature was set at 34 ± 1 °C as measured at the T piece.
During a 30 minute stabilisation period (necessary to allow the artificial patient exhaled
temperature to reach 34 °C) the circuit was leak tested using the inspiration pause hold
function on the ventilator. Testing was only continued if no leaks were apparent.
At the end of the stabilization period, the ventilator was switched off and the weight of
the artificial patient and the total air flow meter reading recorded. The circuit was
reconnected and run for one hour before the artificial patient weight and total airflow
meter reading were recorded again. The water loss (mg/l) from the artificial patient was
calculated by dividing the difference in weight of the artificial patient (mg) by the total
airflow (l).
Testing was only continued if the water loss from the artificial patient was 37.6 ± 3.0
mg/l.
Evaluation of HME Performance in Semi-Closed Circle Anaesthesia
The pre-warmed, pre-weighed artificial patient was connected to an anaesthesia circuit
as Figure 3.
The ventilator tubing, water traps, airflow meter and absorber canister had been dried in
a laminar flow cabinet for at least 12 hours prior to use to eliminate any moisture
residues.
Fresh Durasorb was added to the dry absorber as per the manufacturer’s instructions
(The Durasorb was discarded at the end of each experimental run regardless of whether
it was totally or partially exhausted).
The humidity and temperature of the room air was measured and recorded. This was
compared to the initial humidity reading from the circuit to confirm the lack of moisture
residues. If moisture residues were indicated, the equipment was re-dried in the laminar
flow cabinet for a further 12 hours.
The Blease 8100S ventilator was set at 6 l/min and 12 breaths/min. The total airflow
meter reading was recorded. Fresh gas was introduced at the absorber at either 1 or 3
l/min. Carbon dioxide was introduced into the expiratory limb of the artificial patient at a
level sufficient to produce a peak exhaled breath reading of 5 % as measured via the
monitoring port of the HMEF or via a T piece positioned in place of the HMEF (control).
At 1, 2, 3 and 4 hours the ventilator was switched off and the artificial patient
disconnected from the circuit at the Y piece. To prevent leakage from the circuit the Y
piece was capped.
The artificial patient weight, HMEF and catheter mount weight, total airflow meter
reading and circuit humidity and temperature were recorded. The water loss (mg/l) from
the artificial patient was calculated by dividing the difference in weight of the artificial
patient (mg) by the total air flow (l). This was repeated for each of the devices and also
without any device in place at both 1 and 3 l/min fresh gas flow.
Results
The data for water loss from the artificial patient when evaluating the three devices are
shown in Tables 1 - 2.
At 1 l/min fresh gas flow, the mean water loss for hour 1 was 8.38, 8.00 and 8.12 mg/l
for device 1, device 2 and device 3 respectively. This value fell steadily over the four
hour test period until during hour 4, the mean water loss was measured as 6.64, 8.79
and 5.26 mg/l for device 1, device 2 and device 3 respectively. No significant difference
was observed between the three test devices during each hour period (p < 0.05) using
conventional analysis of variance [12]. With no HMEF in the circuit, the mean water loss
was 15.44 and 16.08 mg/l for hour 1 and hour 4 respectively.
At 3 l/min fresh gas flow, the mean water loss for hour 1 was 9.26, 8.47 and 8.56 mg/l
for device 1, device 2 and device 3 respectively. This again fell steadily until at hour 4,
the mean water loss was measured as 7.34, 7.90 and 6.87 mg/l for the device 1, device
2 and device 3 respectively. No significant difference was observed between the three
test devices during each hour period (p < 0.05). With no HMEF in the circuit, the mean
water loss was measured as 17.06 and 15.53 mg/l for hour 1 and hour 4 respectively.
The data obtained for circuit humidity measurements are shown in Tables 3 - 4. At both
1 and 3 l/min, the circuit humidity was shown to increase over time and reached 100 %
RH (at 23 ± 2 °C) by hour 2. This supports the findings from the water loss experiments
which demonstrate that all three HMEF’s performed similarly.
Discussion
The current ISO Standard for heat and moisture evaluation, ISO 9360, specifies that the
test apparatus (artificial patient) must produce fully saturated water vapour at 34 °C.
This was confirmed in this study at the beginning of each experimental run. The water
loss from the artificial patient was demonstrated to be 37.6 ± 3.0 mg/l (The predicted
water loss is 37.6 mg/l for air fully saturated with water vapour at 34 °C but it must be
recognized that the unavoidable dead space within the patient model will have some
influence on water loss results obtained). Confirmation of the consistency of artificial
patient performance also ensured that any differences in subsequent test results
obtained were due to changes in test parameters and not due to artificial patient
variability.
The semi-closed circle anaesthesia test circuit incorporated a canister containing soda
lime. The mechanism of action of soda lime is to produce water as a product of carbon
dioxide absorption. This results in an increasing circuit humidity level with time. Water
loss from the artificial patient will vary depending on the humidity of the inspired air.
Thus, if the inspired air is dry, a large amount of moisture is required from the artificial
patient to humidify the air and produce exhaled gas at 100 % RH.
Conversely, if the inspiratory air is already partially humidified, less moisture is required
from the patient and the measured water loss will be less. It was therefore essential in
the experimental protocol followed to standardize the humidity level in the semi-closed
anaesthesia circuit at the beginning of each test run. This was achieved by the use of
fresh soda lime for each run and ensuring that there were no moisture residues in the
circuit prior the start of each test.
Results from this study demonstrate that in a semi-closed circle anaesthesia system,
there is no significant difference in the water loss from an artificial patient when using
device 1, device 2 or device 3, ie, there is no difference in measured HME performance.
Although HMEF’s have been shown to perform to varying degrees of efficiency in a nonrecirculating circuit [3], this was not found to be the case in this study. Therefore, with
individual manufacturers’ products providing equivalent heat and moisture exchanging
ability, it is important to consider the other characteristics when choosing a HMEF.
The danger of cross contamination through anaesthesia and ventilator circuits is well
known (due to a number of case reports) and the importance of taking measures to
avoid such cross-contamination is generally accepted [13].
Recently the Australian New South Wales Health Department issued a Public Health
bulletin [14] detailing the likely patient-to-patient transmission of hepatitis C in a private
hospital. It was proposed that patient A had coughed at some stage of an anaesthetic
procedure and that respiratory secretions were consequently introduced into the reusable part of the anaesthetic circuitry. This acted as a reservoir for the virus, which was
able to be transmitted to four subsequent patients (patients B, C, D and E).
HMEF’s do not perform equally with regard to filtration capabilities. In various studies
only Pall pleated hydrophobic membrane HMEF’s have consistently been shown to
prevent the passage of both aqueous and airborne suspensions of bacteria and viruses
[15 - 17]
.
In conclusion, the HME performance of three HMEF devices was found to be similar in a
semi-closed anaesthesia circuit. Other characteristics, such as filtration capability, have
been shown to vary considerably between devices and should be carefully considered
when choosing a HMEF for use in anaesthesia.
TABLE 1: EVALUATION OF HME PERFORMANCE IN SEMI-CLOSED CIRCLE
ANAESTHESIA SYSTEM AT 1 l/min FRESH GAS FLOW
Device
Device 1
Device 2
Device 3
No Device
ARTIFICIAL PATIENT WATER LOSS (mg/l)
HOUR 1
Hour 2
Hour 3
Hour 4
6.29
6.38
6.31
8.89
8.29
6.82
8.14
9.09
6.24
6.23
7.83
7.78
5.73
5.75
7.39
7.75
χ = 6.64
χ = 6.29
χ = 7.42
χ = 8.38
8.04
10.21
6.45
8.36
7.76
6.46
8.77
7.71
14.66
5.69
9.94
7.15
7.17
6.99
7.44
7.96
6.32
6.90
5.51
8.84
χ = 8.79
χ = 7.25
χ = 7.62
χ = 8.00
4.16
7.06
7.32
8.73
5.85
6.28
7.77
7.78
6.07
4.22
6.17
9.66
4.95
5.08
5.59
6.32
χ = 5.26
χ = 5.66
χ = 6.71
χ = 8.12
16.65
16.32
15.34
16.08
14.23
17.02
16.44
16.08
χ = 15.44
χ = 16.67
χ = 15.89
χ = 16.08
TABLE 2: EVALUATION OF HME PERFORMANCE IN SEMI-CLOSED CIRCLE
ANAESTHESIA SYSTEM AT 3 l/min FRESH GAS FLOW
Device
Device 1
Device 2
Device 3
No Device
ARTIFICIAL PATIENT WATER LOSS (mg/l)
HOUR 1
Hour 2
Hour 3
Hour 4
5.44
7.24
7.89
9.34
11.10
7.72
9.50
9.29
5.31
4.85
8.64
9.53
7.52
7.37
8.21
8.89
χ = 7.34
χ = 6.79
χ = 8.56
χ = 9.26
5.72
5.61
7.78
8.53
5.96
6.79
6.05
9.95
9.07
7.91
8.43
10.87
4.50
5.79
6.99
χ = 7.90
χ = 6.20
χ = 6.54
χ = 8.47
8.52
8.49
7.60
8,78
6.35
5.69
7.80
5.86
5.54
6.94
9.63
6.76
6.44
9.58
8.04
χ = 6.87
χ = 6.54
χ = 8.04
χ = 8.56
15.10
15.12
17.36
18.23
15.96
15.70
15.00
16.09
χ = 15.53
χ = 15.41
χ = 16.18
χ = 17.06
TABLE 3: RELATIVE HUMIDITY (%) MEASURED IN SEMI-CLOSED CIRCLE
ANAESTHESIA CIRCUIT – 1 l/min FRESH GAS FLOW*
RELATIVE HUMIDITY (%) MEASURED IN CIRCUIT AT TIME
0 HOUR
1 Hour
2 Hour
3 Hour
4 HOUR
Device 1
25
86
100
100
100
33
100
100
100
100
39
95
100
100
100
45
99
100
100
100
Device 2
43
100
100
100
100
49
100
100
100
100
20
87
100
100
100
36
77
100
100
100
Device 3
27
73
100
100
100
36
68
100
100
100
46
100
100
100
100
40
100
100
100
100
*Note: Circuit temperature = 23 ± 2 °C
Device
TABLE 4: RELATIVE HUMIDITY (%) MEASURED IN SEMI-CLOSED CIRCLE
ANAESTHESIA CIRCUIT – 3 l/min FRESH GAS FLOW*
RELATIVE HUMIDITY (%) MEASURED IN CIRCUIT AT TIME
0 HOUR
1 Hour
2 Hour
3 Hour
4 HOUR
Device 1
42
100
100
100
100
47
100
100
100
100
34
100
100
100
100
24
100
100
100
100
Device 2
37
100
100
100
100
39
73
100
100
100
42
74
100
100
100
29
51
100
100
100
Device 3
43
84
100
100
100
43
73
100
100
100
40
62
100
100
100
33
69
96
100
100
*Note: Circuit temperature = 23 ± 2 °C
Device
References
1.
Bengtson, J.P., Bengtson, A., Svengvist, O. (1989). The circle system as a
humidifier. British Journal of Anaesthesia 63, 453 – 457.
2.
Tontschev, G., Luder, M., Benson, Ch. (1979). Humidity and temperature of
breathing gas in semi-closed and closed anaesthesia circle systems. Anaesthesia
U Reanniat H2, 78 – 86.
3.
Shelly, M., Bethune, D. W., Latimer, R. D. (1986). A comparison of five heat and
moisture exchangers. Anaesthesia 41, 527 – 532.
4.
Walker, A. K. Y. and Bethune, D. W. (1976). A comparative study of condenser
humidifiers. Anaesthesia 31, 1086 – 1093.
5.
Mebius, C. (1983). A comparative evaluation of disposable humidifiers. Acta
Anaesthesiology Scandinavia 27, 403 – 409.
6.
Weeks, D. B. (1986). A laboratory evaluation of recently available heat-andmoisture exchangers. Anaesthesiology Review 13, 33 – 36.
7.
Kugimiya, T., Phuc, T. G. Numata, K. (1989). Laboratory evaluation of heat and
moisture exchangers. Journal of Anaesthesia 3, 80 – 85.
8.
Eckerbom, B. and Lindholm, C. E. (1990). Laboratory evaluation of heat and
moisture exchangers. Assessment of the draft international standard (ISO/DIS
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9.
Bethune, D. W. (1991). Evaluation of test methods used to determine moisture
conservation. Italian Hospitals Anaesthesia / Intensive Care Association Update
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and Bacterial / Viral Protection. Are 330 15 June 1991
10. ISO 9360 1992(E) 1st edition. Anaesthetic and respiratory equipment – heat and
moisture exchangers for use in humidifying respired gases in humans.
11. Heat and moisture exchanger (HME). Pall BB22-15F (1991). Medical Devices
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14. Chant, K., Kociuba, K., Munro, R., Crone, S., Kerridge, R., Quinn, J., Wyland, M.,
Miller, G., Turner, I., Brown, J., Baird, L., Locarni, S., Bowden, S., Kenrick, K. G.,
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16. Lee, M. G., Ford, J. L., Hunt, P. B., Ireland, D. S., Swanson, P. W. (1992). Bacterial
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as a barrier to hepatitis C transmission within breathing systems. Centre for Applied
Microbiology and Research.
Ventilator tubing
Low resistance
check valve
Flutter valve
Independent
temperature
probe
2 litre rubber lung
Bennett Cascade
humidifier
Water level kept to
maximum mark
Tubing between dotted lines insulated to prevent rain out
FIGURE 1: ARTIFICIAL PATIENT
Temperature
control box
Total flow meter
Ventilator
Artificial
patient
FIGURE 2: STANDARD NON-RECIRCULATING CIRCUIT
Humidity and
Temperature Probe
CO2 Analyser
Water Trap
Ventilator
Total flow
meter
Soda Lime
Canister
Test
device
Water Trap
Artificial
patient
CO2
Temperature
Probe
FIGURE 3: CIRCLE ANAESTHESIA TEST RIG
Gas to Drive
Ventilator
Fresh Gas Flow