Comparison of 2 exchangers of heat and humidity in anesthesia (Hygrobac vs Ultipor BB50)

Comparison between Hygrobac and Ultipor BB50, two commercially available heat and moisture exchangers, shows that the former is more efficient: temperature (T degrees C) and water content (absolute humidity, AH) of the inspired gas is greater with Hygrobac than Ultipor BB50. However, both HMEs work at the low limit of the normally accepted range of T degrees C and AH for intubated patients.     

Contamination control in long-term ventilation

Twenty-eight patients who required periods of mechanical ventilation for up to 22 days in the intensive therapy unit were studied to evaluate the clinical use of the Pall Ultipor Breathing System Filter (BB50T) as a heat- and moisture-exchanging bacterial filter. Results in this group of patients showed that there was no longer any need to sterilise breathing systems or decontaminate ventilators if these filters were used. They also performed satisfactorily as a heat and moisture exchanger in patients in need of long-term ventilation, and their use appears to offer substantial advantages as regards cost, ease of use and patient safety.     

Effects of a heat and moisture exchanger on carbon dioxide equilibrium during mechanical ventilation with the Bain circuit

The introduction of a heat and moisture exchanger (HME) into the anaesthetic circuit may cause a rise in carbon dioxide (CO2) tension through an increase in dead space. We studied the effects of the Ultipor Pall BB50 filter included 'in series' in the Bain circuit on CO2 equilibrium. Arterial carbon dioxide tension (PaCO2) was measured in 81 patients scheduled for elective surgery before and after the insertion of the filter. Results showed that: females were always more hyperventilated than males when fresh gas flow was set at 70 ml kg-1 ideal body weight; the inclusion of the filter increased the PaCO2 in the group as a whole (the difference was statistically, but not clinically, significant); PaCO2 increased after the application of the filter only in females; the effects of the filter were completely independent of the patient's age. It is concluded that the use of the Ultipor Pall BB50 filter is a safe procedure during mechanical ventilation with the Bain breathing system and there is no need to modify ventilation.     

BACTERIAL RETENTION PROPERTIES OF HEAT AND MOISTURE EXCHANGE FILTERS

We have examined the properties of six heat and moisture exchangefilters (HMEF) to ascertain their resistance to liquid flowand their ability to retain a challenge bacterium, Pseudomonasdiminuta, from aqueous and nebulized suspensions. Only one HMEF,the Pall Ultipor was able to withstand a significantly greaterpressure of liquid than that found in clinical practice. However,when breached, the HMEF were unable to prevent transmissionof micro-organisms from aqueous suspension. Only the Darex Hydrobacfilter failed to meet the manufacturer's claim for filter efficiencyfor nebulized bacteria, mainly because the filter housing failedunder test. When the reduction in bacterial cells after passageof the nebulized Pseudomonas diminuta through the HMEFs wasanalysed statistically, the data showed that the HMEF producedby Pall (Ultipor) and Intersurgical (Filter therm) were superiorto those produced by DAP, Mediplan (Hygrobac), Intertech (HME225–2835–800) and Gibeck (Humid-vent). (Br. J. Anaesth.1992;69:522–525)     

Retention of airborne latex particles by a bacterial and viral filter used in anaesthesia apparatus

We have developed a series of laboratory tests to evaluate the efficiency of a heat and moisture exchanger filter (Pall BB25) in retaining latex particles in order to protect allergic patients during anaesthesia. Latex particles were nebulised with cornstarch as a support and collected for assay in a flask, with or without the filter integrated into the experimental circuit. With the Pall BB25 filter in the circuit, no natural latex proteins were detected by measurement of either total protein or antigenic latex proteins. The Pall BB25 filter may represent a useful means of preventing inhalation of latex particles during anaesthesia in susceptible patients.     

Performance evaluation of six heat and moisture exchangers according to the Draft International Standard (ISO/DIS 9360)

Six commonly available heat and moisture exchangers (HMEs) were tested according to a draft ISO standard for evaluating these devices (ISO = The International Organization for Standardization). The devices tested were: Pall Ultipor Filter, Mallinckrodt Inline, Siemens Servo 152, Engstrom Edith, Triplus Icor, and Portex Humid Vent 1. The trial period was 24 h and three different ventilator settings were used. For each device the following were determined: humidity-conserving ability, heat-conserving ability, resistance to air flow, internal volume, and gas leakage. The water loss (mg/1) at a common ventilator setting (101/min, 20 · min^-1) was as follows: Pall 10.8, Inline 7.5, Servo 9.0, Edith 6.6, Icor 6.2, and for Humid Vent 13.9, as compared to a control value (= no HME) of 24.8. The temperature differences (°C) between exhaled and inhaled gas at the patient port of the HME were: Pall 2.39, Inline 1.31, Servo 1.21, Edith 1.40, Icor 1.12, and for Humid Vent 2.80 as compared to a control value of 5.34. Ventilator settings with higher tidal volumes generally resulted in decreased efficiency. Resistance to air flow was less than 3 hPa · 1^-1 · s^-1 for all devices tested. The internal volumes ranged from 11 to 87 ml. The gas leakage was zero for all devices. Based on our findings the HMEs could be divided into three groups: 1) Icor, Servo, Inline, Edith: very good performance, 2) Pall: good performance for tidal volumes up to about 0.7 1, and 3) Humid Vent 1: acceptable performance for tidal volumes up to 0.5 1.     

Laboratory evaluation of heat-and-moisture exchangers

We conducted a laboratory study on six commercially available heat and moisture exchangers in order to determine and compare their water retaining efficiency and their contribution to airway resistance.The Gambro-Engström Edith Flex device was the most desirable of the six devices we evaluated in terms of its water retaining efficiency. The NMI Pneumoist 1 and the Siemens Servo Humidifier 153 units had good water retaining capacity but their higher airflow resistance need close monitoring, especially after prolonged clinical use. The Pall HME 15–22 and the Portex Humid-Vent 1 devices were also efficient in water retaining capacity. The Pall also demonstrated low airflow resistance and the minimum increase in airflow resistance after water immersion. The pathogen filtering capacity of the Pall should also be considered an additional advantage, especially in infected patients. The Terumo Breathaid device performed worst of all six devices, but it was still better than no HME at all.(Kugimiya T, Phuc TG, Numata K: Laboratory evaluation of heat-and-moisture exchangers. J Anesth 3: 80–85, 1989)     

An evaluation of six disposable heat and moisture exchangers

Six disposable heat and moisture exchangers were tested on patients undergoing anaesthesia requiring mechanical ventilation. Inspiratory humidity and temperature were monitored to find the steady-state values reached with each device together with the times taken for these to be achieved. The exchangers were tested in a non-rebreathing T-piece circuit and in a conventional circle system with a fresh gas flow of 6 l/min: the Siemens 150 provided 28 and 32 mg of water/litre of inspired gas (at about 30 degrees C) in 10 and 5 min respectively, but is rather heavy and bulky. The Portex Humidvent provided 25 and 30 mg/l, and although taking longer to reach steady state (27 and 15 min respectively) is small, light and cheap. The Siemens 151 provided 25 and 27 mg/l in 18 and 10 min respectively, but is heavier than the Portex exchanger. The performances of these three devices were not significantly different from each other in either study (P less than 0.05). For the T-piece system the Pall and Engstrom exchangers were the next most efficient. The Pall device provided 18 and 23 mg/l (in 18 and 8 min respectively) and the Engstrom provided 20 and 23 mg/l (in 19 and 10 min respectively). In the circle system, there were no significant differences between the performances of the Portex, Siemens 151, Pall and Engstrom exchangers. The Pall is also a very effective bacterial filter and has been found to be satisfactory in the intensive care setting. The Terumo appeared to perform no better than a circle system with catheter mount (13 mg/l at 27 degrees C). It would seem that more complex humidification equipment is not necessary during anaesthesia if an efficient heat and moisture exchanger is used.     

A Comparative Evaluation of Disposable Humidifiers

The performance of four hygroscopic condenser humidifiers (HCH) and two conventional heat and moisture exchangers (HME), all commercially available, have been evaluated in laboratory tests. A clinical study was also made in order to confirm the test results. It was found that the new generation of heat and moisture exchangers, which have hygroscopic properties, are superior to conventional HME units. When dry inspired gases are used, the HCH units, but not the HME units, deliver a moisture level that is comparable to what is produced in the upper trachea during normal breathing the nose. Our clinical experience as well as the extensive body of literature on humidification requirements support the conclusion that in most cases they can be recommended to be used for extended time periods even with dry gases. The humidification efficiency decreases with increasing tidal volume for all units. However, the best units can be used up to tidal volumes of 1000-1500 ml. The dead space of these units varies from 90 ml to 100 ml and the resistances are only about 50 Pa at 0.5 l/s. The weight is in the range 18-40 g. One of the best HCH devices also has distinct antimicrobial properties. The others are found at least not to promote organism growth. This feature added to others such as safety and simplicity in use make an HCH device an attractive alternative to conventional humidification techniques.     

A comparison of the filtration properties of heat and moisture exchangers

Heat and Moisture Exchangers are used increasingly as filters to prevent the contamination of breathing apparatus and to limit cross-infection. A series of laboratory tests has been developed to evaluate the microbial filtration and air flow resistance properties of these devices. The tests were designed to simulate the clinical situation and therefore evaluated devices in both dry and wet conditions. The devices tested (Engstrom Edith, Pall BB50T, Dar Hygrobac, Intersurgical Filtatherm and Intersurgical Filtaguard) were each representative of a particular type of construction. The simple hygroscopic device (Engstrom Edith) showed poor airborne and liquid-borne filtration efficiency, but its resistance to air flow remained low in all conditions. The composite devices (Dar Hygrobac, Intersurgical Filtatherm and Intersurgical Filtaguard), which all possessed relatively large pores, performed well in terms of dry airborne filtration efficiency, but showed substantial increases in air flow resistance and poor filtration efficiency in the presence of liquid. The pleated membrane filter (Pall BB50T), which possessed small pores, showed good airborne filtration efficiency and prevented the passage of liquid. The latter property enabled this device to prevent the passage of liquid-borne contamination and to maintain a low resistance to air flow in wet conditions. It would appear that in terms of contamination control and air flow resistance the pleated membrane filter provides a wider margin of safety than either the hygroscopic or composite devices.     

Some observations on blood microfilters.

Under laboratory conditions 3 types of filter designed to prevent the passage of microemboli into the circulation during the transfusion of stored blood were compared with the standard Baxter BR10 filter, designed only to hold back macroemboli, in respect of resistance to blood flow and damage to the cellular elements of the blood. In both respects the Pall Ultipor microfilter proved more efficient than the Swank IL200 microfilter and the Bentley Polyfilter (PF-127) and, except with 3-week-old blood, was comparable to the Baxter filter.     

Measurement of heat and moisture exchanger efficiency

Deciding between a passive heat and moisture exchanger or active humidification depends upon the level of humidification that either will deliver. Published international standards dictate that active humidifiers should deliver a minimum humidity of 33 mg.l^?1; however, no such requirement exists, for heat and moisture exchangers. Anaesthetists instead have to rely on information provided by manufacturers, which may not allow comparison of different devices and their clinical effectiveness. I suggest that measurement of humidification efficiency, being the percentage moisture returned and determined by measuring the temperature of the respired gases, should be mandated, and report a modification of the standard method that will allow this to be easily measured. In this study, different types of heat and moisture exchangers for adults, children and patients with a tracheostomy were tested. Adult and paediatric models lost between 6.5 mg.l^?1 and 8.5 mg.l^?1 moisture (corresponding to an efficiency of around 80%); however, the models designed for patients with a tracheostomy lost between 16 mg.l^?1 and 18 mg.l^?1 (60% efficiency). I propose that all heat and moisture exchangers should be tested in this manner and percentage efficiency reported to allow an informed choice between different types and models.     

Heat and moisture exchange devices: are they doing what they are supposed to do?

Heat and moisture exchangers (HMEs) are used to provide humidification and warming of the inspiratory gases during general anesthesia. The performance specifications provided by manufacturers of HMEs are based on in vitro measurements of moisture output using the International Standards Organization (ISO) 9360 method. We studied the in vivo performance of three different HMEs with similar ISO specifications in a randomized crossover fashion in patients under general anesthesia. The effect of each HME on temperature, convective heat loss, evaporative heat loss, total heat loss, relative humidity, and absolute humidity of inspiratory gases was determined. Although all HMEs in general improved baseline variables, we found significant differences in performance for the different HMEs. In only one type did the moisture output correspond with ISO specifications. We conclude that the in vivo performance of HMEs may not correspond with manufacturer's specifications. IMPLICATIONS: There is considerable variability in the in vivo performance of heat and moisture exchangers that have similar manufacturer specifications. These specifications, based on the International Standards Organization 9360 standard, which is measured in vitro, cannot be used to predict clinical performance.     

Gradual Reduction of Endotracheal Tube Diameter during Mechanical Ventilation via Different Humidification Devices

Background: Limited data suggest that increased resistance to flow within endotracheal tubes (ETT) may occur in patients whose lungs are mechanically ventilated for more than 48 h, especially when airway humidification is inadequate. This could lead to sudden ETT obstruction or induce excessive loading during spontaneous breathing.

Methods: Twenty-three such patients were randomly assigned to three types of airway humidifier based on three different working principles: a Fisher Paykell hot water system (n = 7), a Pall BB2215 heat and moisture exchanger (HME) hydrophobic filter (n = 8), and a Dar Hygrobac 35254111 HME hygroscopic filter (n = 8). The decrease in internal pressure along the ETT and the flow rate were measured in each patient every 2 days. An "effective inner diameter" was derived from these measurements and allowed the inner ETT configuration to be monitored.

Results: On the first day of intubation, the mean diameter was similar in the three groups, and was slightly smaller than the in vitro diameter (mean +/- SD: 7.6 +/- 0.6 mm for Fisher-Paykell, 7.7 +/- 0.4 for Pall, and 7.5 +/- 0.4 for Dar). The mean diameter tended to decrease from day to day. At the end of the study, the overall reduction in mean diameter was significantly greater with the hydrophobic HME (Pall) than with the two other systems (Pall: -6.5 +/- 4% vs. -2.5 +/- 2.5% for Dar and -1.5 +/- 3% for Fisher-Paykell; P < 0.01 with analysis of variance). The same was true of the mean reduction in effective inner ETT diameter expressed per day of ventilation (-1.6 +/- 1.5% per day for Pall vs. -0.5 +/- 0.4% for Dar and -0.2 +/- 0.4% for Fisher-Paykell; P < 0.01). In four patients, the ETT became obstructed and emergency repeated tracheal intubation was required. The Pall HME and the Fisher-Paykell system were being used in three and one patient, respectively. Before obstruction, the reduction in ETT diameter was significantly greater for these four patients than for the remaining 23 patients (7.8 +/- 1.4% vs. 3.1 +/- 4.1%; P < 0.01).     

Gas conditioning in artificial respiration

The authors review the main systems used in the clinical setting to condition inspiratory gases during mechanical ventilation. More in details, the functional principles of hot water humidifiers and heat and moisture exchangers are described.     

The benefit of using a heat and moisture exchanger during short operations in young children

We studied the efficiency of a heat and moisture exchanging filter (HMEF; Pall BB25) as a means of compensating for the heat and moisture loss during anaesthesia in young children using cold and dry gas supplied from open circuits. Forty ASA I children (mean age: 48 months±20; mean weight: 16±3.5 kg) were randomized into two groups: Group I without HMEF/Group II with HMEF. The two groups did not show any significant differences for morphometric data or ventilation parameters. Relative humidity and temperature measurements in anaesthetic gases were taken using a combined temperature/humidity probe introduced into the circuit. Absolute humidity in the circuit was calculated from these measurements. In Group II, a significant increase (P<0.001) in absolute humidity was demonstrated (Group I: 12 mg H₂O·l^?1vs Group II: 22 mg H₂O·l^?1). This increase appeared immediately after introduction of the HMEF in the circuit and remained constant throughout the duration of the operation. Thus, the use of the device is recommended for young children, even for operations of short duration.     

Humidification of inspired gases during mechanical ventilation

Humidification of inspired gas is mandatory for all mechanically ventilated patients to prevent secretion retention, tracheal tube blockage and adverse changes occurring to the respiratory tract epithelium. However, the debate over "ideal" humidification continues. Several devices are available that include active and passive heat and moisture exchangers and hot water humidifiers Each have their advantages and disadvantages in mechanically ventilated patients. This review explores each device in turn and defines their role in clinical practice.     

Comparison of filtering efficiency of four new in-line blood transfusion filters.

Efficient removal of debris from stored human blood prior to transfusion has become increasingly important. The debris, consisting largely of microaggregates of platelets and fibrin, is not effectively removed by passage through a standard transfusion filter. This study evaluated the performance of four of the currently available small pore in-line blood transfusion filters. Filters tested included the Bentley PF-127, the Pall Ultipor SQ-40, the Swank In-Line IL-200 and the Fenwal Microaggregate Blood Filter. A standard blood administration filter was also tested, the McGraw V-2950. The rate of blood flow through the filters was recorded using single and multiple units of blood. The screen filtration pressure and debris weight of the filtered blood were studied to compare effectiveness of filtration. The Swank filter was effective in debris removal and maintained good flow rates. The Bentley and Fenwall filters removed debris nearly as well, but had reduction of flow rates after smaller infusions. The Pall filter maintained high flow rates but did not remove debris as effectively, particularly with pressure infusion. The standard 170 mu pore blood transfusion filter does not remove microaggregates.     

Humidification in intensive care

The normal physiological function of the upper respiratory tract is to filter and humidify inspired air. In intensive care units the upper respiratory tract is frequently bypassed. The importance of humidifying and warming the dry, cold, piped gas is well documented. The results of lack of adequate humidification include endotracheal tube obstruction, impairment of the mucociliary elevator and altered pulmonary function. Optimal levels of humidification are as yet undefined and useful clinical markers of adequate humidification are not available. As a result there is a bewildering array of humidification devices available at present, the most recent of which are heat and moisture exchangers with or without specific filtration properties. This article reviews available data on these humidification devices, and recommends an approach to their appropriate use, based on the probable physiological needs of individual patients.     

Failure of a heat and moisture exchanger as a cause of disconnection during anaesthesia

Two cases of ventilator tubing mishaps, resulting from defective heat and moisture exchangers, are described. The report emphasises the need for preoperative inspection of the anaesthetic machine and associated equipment as well as the importance of a disconnect alarm device.