1Department of Anaesthesia and Intensive Care and 2Sheffield University Department of Anaesthesia, Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, UK. 3Department of Anaesthesia and Intensive Care, Doncaster Royal Infirmary, Armthorpe Road, Doncaster DN2 5LT, UK*Corresponding author. Present address: Department of Anaesthesia and Intensive Care, Northern General Hospital, Herries Road, Sheffield S5 7AU, UK
Presented in part at the European Society of Intensive Care Medicine Annual Congress, Rome, Italy, October 2000.
Accepted for publication: March 19, 2001
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Abstract |
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Br J Anaesth 2001; 87: 28991
Keywords: equipment, heat and moisture exchangers; equipment, bacterial filters; complications
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Introduction |
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We have examined the performance of three HMEFs under wet conditions in an attempt to determine whether any particular design features might predispose to such complications. The test procedure described assumes that the coexistence of the following factors could cause blockage: (1) a device would have to retain secretions, (2) the retained secretions should not be apparent on inspection of the device, and (3) they would increase air flow resistance across the device.
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Methods and results |
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Retention testing
Increments of 5 ml 0.9% saline were instilled into the patient end of the device, which was gently shaken with the patient end occluded and then inverted to determine if spillage of saline occurred. Further increments were added in this manner and the maximum volume instilled that did not result in spillage was defined as the retention volume.
Concealment testing
Increments of 5 ml 0.9% saline were instilled into the patient end of the device, which was gently shaken with the patient end occluded and then inspected for the appearance of free saline within the patient side of the device housing. Further increments were added and the maximum volume instilled that did not result in the appearance of free saline was defined as the concealment volume.
Dead space testing
The dead space of the device on the patient side was measured by adding 1 ml saline increments to the patient end of vertically mounted devices up to the point of spillage.
Five devices of each type were used for each of the above test sequences and mean (SD) values calculated. Retention and concealment volumes were expressed as a percentage of the device dead space on the patient side (Table 1).
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Inspiratory flow resistance was measured by connecting the air flow to the non-patient end of the vertically mounted device (patient end uppermost). After baseline testing the device was removed from the test rig and 5 ml of 0.9% saline instilled into the patient end. The device was gently shaken with the patient end occluded, reinserted into the test rig and the measurement repeated. Testing continued with the addition of further aliquots of saline until the air flow through the device resulted in ejection of saline from the patient end. Expiratory flow resistance was similarly measured but with the air flow connected to the patient end of the device and with testing continuing until the resistance presented by the filter exceeded the pressure generating capabilities of the BiPAP S/T-D30 (30 cm H2O). Ten devices of each type were tested for each air flow direction and mean (SD) values calculated (Table 2).
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Comment |
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Blockage is a recognized complication of HMEF usage and most manufacturers warn users of the potential for accumulated secretions to cause occlusion. The cellulose exchanger of the composite HMEFs in this study retained and concealed large volumes of saline (Table 1). This produced a tampon effect associated with bi-directional air flow resistances well in excess of the international standard of a 5 cm H2O pressure drop across the device at 60 l min1 air flow (Table 2).5 The tampon effect was most pronounced in the DAR Hygrobac-S, which had the greatest retention volume when expressed as a percentage of the patient sided device dead space (74%). In the clinical setting such secretion accumulation might be detectable if blood stained or purulent but contamination with clear secretions would not be obvious.
The pleat only HMEF showed a significant increase in expiratory air flow resistance (Table 2) but only after the saline was seen (Table 1). If hanging down and collecting secretions, a pleat only HMEF could allow inspiration, but prevent expiration.6 However, the filter would have to be held in a dependent position for sputum retention to occur and the patient's attendants would have to fail to notice that the filter housing was full of secretions. Our data suggest that secretions would be seen before expiratory air flow resistance increased significantly.
The superior moisture output of composite HMEFs results in a lower incidence of tracheal tube blockage in comparison to pleat only devices.1 However our data imply that composite designs may have greater potential to block or cause excessive work of breathing from occult accumulation of patient secretions. In the UK, the Medical Devices Agency performs regular evaluations of HMEFs, however, these are performed in accordance with the International Standard for Anaesthetic and Respiratory Equipment test procedure5 and do not include a liquid challenge. Testing under wet conditions would provide clinicians with useful information when assessing the safety profile of individual devices and in particular would allow comparison between different designs of composite HMEFs with regard to the risk of device blockage.
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References |
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2 Johnson PA, Raper RF, Fisher MMcD. The impact of heat and moisture exchanging humidifiers on work of breathing. Anaesth Intensive Care 1995; 23: 697701[ISI][Medline]
3 Prasad KK, Chen L. Complications related to the use of a heat and moisture exchanger. Anesthesiology 1990; 72: 958[ISI][Medline]
4 Hedley RM, Allt-Graham J. A comparison of the filtration properties of heat and moisture exchangers. Anaesthesia 1992; 47: 41420[ISI][Medline]
5 International Organisation for Standards. Anaesthetic and Respiratory EquipmentHeat and Moisture Exchangers for Use in Humidifying Respired Gases in Humans. Geneva: International Organisation for Standardisation Technical Committee, 1992, International Standard ISO 9360,2
6 Wilkes AR. Resistance to gas flow in heat and moisture exchangers. Anaesthesia 1992; 47: 1095[Medline]