Abstract
BACKGROUND: At high ambient temperatures in ICU rooms, the humidification performances of heated-wire humidifiers are significantly reduced, with delivered gas humidity well below 30 mg H2O/L, which leads to an increased risk of endotracheal occlusions, subocclusions, or mucociliary dysfunction. The objective of the study was to evaluate the humidity delivered at the Y-piece with new-generation heated-wire humidifiers with advanced algorithm (FP950 [Fisher & Paykel Healthcare, Auckland, New Zealand] and VHB20 [Vincent Medical, Inspired, Hong Kong]) while varying ambient temperatures.
METHODS: We measured, on the bench, the hygrometry of inspiratory gases delivered by a new generation of heated-wire humidifiers (i) FP950, (ii) VHB20 and a previous generation of heated-wire humidifiers, (iii) MR850 (Fisher & Paykel) with the usual settings (37°C at the chamber/40°C at the Y-piece), (iv) MR850 with no temperature gradient (40°C/40°C), and (v) MR850 with the automatic compensation algorithm activated. Hygrometry was measured with the psychrometric method after 1 h of stability while varying the room temperature from 20 to 30°C.
RESULTS: Two hundred ninety-four hygrometric bench measurements were performed at steady state for the different tested conditions. With the new heated-wire humidifiers (FP950 and VHB20), gas humidity delivered remained > 30 mg H2O/L in all tested conditions, even at high ambient temperatures (>25°C). With previous generations of heated-wire humidifiers (MR850), at high ambient temperature, humidity delivered was adequate in only 26% (11/42) of the measurements when the usual settings were used (37°C/40°C) and 30% (11/37) with automatic compensation. When no temperature gradient was set (40°C/40°C), humidity delivered was > 30 mg H2O/L in 91% (30/33) of the measurements at a high ambient temperature. With an ambient temperature < 25°C, almost all devices and settings provided adequate humidity.
CONCLUSIONS: The new FP950 and VHB20 heated-wire humidifiers by using advanced algorithms demonstrated stable performance while varying the ambient temperature by 20–30°C, better than the previous generation of heated humidifiers when ambient temperatures were high.
Introduction
Proper humidification of the gases delivered to the patients during mechanical ventilation is mandatory1,-,3 and was recognized as such in the first publication of mechanical ventilation in 1953.4 Heated humidifiers as well as heat-and-moisture exchangers (HME) are used for this purpose.1,3 It has been shown that high ambient temperature and high outlet ventilator temperature are responsible for heated-wire humidifiers’ dysfunction, with absolute humidity delivered well below recommended values, which leads to an increased risk of endotracheal tube occlusion and sub-occlusions.5 When the temperature of the gas entering the humidification chamber is high, the temperature of the heating plate will decrease to maintain the humidification chamber outlet temperature at 37°C according to the working principles.5 The humidity delivered at the Y-piece is closely related to the heater plate temperature, as previously shown,6 consequently, every situation leading to high temperature in the humidification chamber (high ambient or ventilator temperature, sun shining directly on the humidifier will reduce heater plate temperature and humidity delivered). During the COVID-19 pandemic, high rates of endotracheal tube occlusions have been described with poor performing HMEs and even with heated-wire humidifiers when not used in optimal conditions.7,-,12 Endotracheal tube occlusions were the second cause of unexpected cardiac arrests in mechanically ventilated patients with COVID-19 and are frequently associated with unfavorable outcome.13 This complication is related to under-humidification of the gases delivered to patients.14,15 Gas humidity > 30 mg H2O/L with HME (International Organization for Standardization [ISO] 9360) and > 33 mg H2O/L with heated-wire humidifiers (International Organization for Standardization [ISO] 8185:1997)3 are recommended for patients on mechanical ventilation. A humidity level of 33 mg H2O/L is not attained with many HMEs15 nor with heated-wire humidifiers when the ambient temperature is high.5
The performances of heated-wire humidifiers are partially improved with specific settings (increased chamber temperature to 40°C or activation of the compensation algorithm),5 but these settings have limitations. The aim of the study was to evaluate new-generation heated wire humidifiers (FP950 [Fisher & Paykel Healthcare, Auckland, New Zealand] and VHB20 [Vincent Medical, Inspired, Hong Kong]) that add parameters in their algorithm, which include sensors for ambient air temperature, inlet chamber temperature, and relative humidity, with the objective to maintain a stable humidity delivered to the patients who are intubated whatever the ambient temperature and other conditions.
QUICK LOOK
Current Knowledge
Heated-wire humidifiers efficiently warm and humidify gases delivered to mechanically ventilated patients when optimal conditions are met. When the ambient temperature or outlet temperature of the ventilator is high, the heater plate temperature decreases accordingly and humidity delivered to patients may not be sufficient, which leads to increased risk for endotracheal tube occlusion or subocclusion. New heated humidifiers with an advanced algorithm have been developed to address this limitation.
What This Paper Contributes to Our Knowledge
New heated-wire humidifiers with advanced algorithms provided adequate humidification at ambient temperatures from 20 to 30°C, while previous-generation humidifiers had diminished performance when the ambient temperature was > 25°C. We report that high ambient temperatures were not rare, even in an ICU room equipped with air conditioning in Québec, a city with a northern latitude.
Methods
We conducted a bench study in our laboratory at the Research Center of the Institut Universitaire de Cardiologie et de Pneumologie de Québec. We measured the hygrometry of inspiratory gases delivered by new generation of heated wire humidifiers (i) FP950 (ii) VHB20 with recommended settings (36°C at the chamber/39°C at the Y-piece) and set at 37°C/40°C and previous generation of heated-wire humidifiers, (iii) MR850 (Fisher & Paykel) with the usual settings (37°C at the chamber/40°C at the Y-piece), (iv) MR850 with no temperature gradient (40°C/40°C), and (v) MR850 with an automatic compensation algorithm activated.
The new humidifiers have an advanced algorithm, including the ambient temperature (FP950) and the relative humidity measurements (VHB20). However, we do not have other details on these algorithms. The double heated-wire circuits used in this study were the following: Ventilator Dual Heated Circuit kit – reference 950A91J (Fisher & Paykel) for the FP950, dual limb heated circuit, Inspired, – reference 51005683 (Vincent Medical, Inspired, Hong Kong) for the VHB20.
We measured the relative and absolute humidity of the inspiratory gases with the psychrometric method, as previously described.5,15 An ICU V500 ventilator (Dräger Medical, Lübeck, Germany) connected to a test lung (Test lung 190 – 1 L, Maquet Critical Care, Solna, Sweden) was set to provide a constant minute ventilation (10 L/min with the tidal volume set at 400 mL and breathing frequency set at 25 breaths/min). The laboratory ambient temperature was changed via the thermostat 2 to 3 times each study day, with incremental increases of 2 to 3°C to achieve target temperatures between 20 and 30°C. The ambient temperature was kept stable for at least 2 h before the humidity measurements with the tested humidifiers were performed. There was no predefined ambient temperature target program. However, our goal was to achieve comparable average ambient temperatures and an equivalent number of “high” and “low” ambient temperatures with each device evaluated. Therefore, the latest measurements were carried out to achieve comparable ambient temperatures with all tested devices.
The “humidification laboratory” was designed with the biomedical department to maintain the ambient temperature stable. The ambient temperatures were collected with high precisions thermometers (Duotemp, Fisher & Paykel) and we recorded the heater plate temperatures from the submenus of the MR850 heated humidifier and the FP950. Several temperatures (ambient, ventilator outlet, humidification chamber inlet and outlet, Y-piece, heater plate) were recorded as previously described.5 The different temperatures were recorded at the same time as the hygrometric measurements. Hygrometric measurements were performed after 2 h of steady state by using the psychrometric method.5,15,16 We previously determined that, after activating the heated humidifiers, stability was achieved within 30 min (data not shown), therefore, a 2-h steady state was likely sufficient. The psychrometer used was the Yokogawa µR10000 psychrometer (Yokogawa Test & Measurement, Tokyo, Japan), the same model that was used in the previous experiments. In another series of measurements, we recorded ambient temperatures every 30 min over several months in ICUs in 2 Québec City hospitals equipped with air conditioning (see the supplementary materials at http://www.rcjournal.com).
Statistical Analysis
Data were expressed by using mean ± SD to summarize the data. We compared absolute humidity (mg H2O/L) delivered at the Y-piece among the different heated humidifiers (MR850 with different settings, FP950 and VHB20 at different settings). We did these comparisons for all conditions, for ambient temperatures ≤ 25°C and ambient temperature > 25°C. The graphic representation of the relationship between the heater plate temperature and absolute humidity suggests defining a statistical model by using a spline curve to fit the data as the smoothest way. To compare different heated humidifiers over different ambient temperatures, a linear model was defined. A fixed factor was associated with the comparison of the 6 heated humidifiers, and the smoothing trend analysis for ambient temperature was defined as a fixed spline effect that varied by humidifiers. Another statistical approach was to split the ambient temperature data into 2 categories: ≤25°C and >25°C.
Rather than analyzing the ambient temperature as a continuous variable, it was proposed to analyze this variable as a 2-level fixed factor. A 2-way analysis of variance with an interaction term between the 2 fixed factors was defined to compare the different heated humidifiers at the 2 ambient temperature categories. The normality assumption was verified with the Shapiro-Wilk tests by using studentized residuals from the statistical model. The graphic representation of marginal linear predictors with studentized residuals suggests the homogeneity of the variances. Statistical significance was present with a 2-tailed P < .05. Analyses were performed by using SAS version 9.4 (SAS Institute, Cary, North Carolina).
Results
A total of 294 hygrometric measurements were performed at steady state for the different tested conditions; 59 to 64% of the measurements were performed at high ambient temperatures (>25°C) with the different heated humidifiers (Table 1). When the ambient temperatures were < 25°C, almost all the measurements were > 30 mg H2O/L with previous and new-generation heated humidifiers. With the new heated-wire humidifiers (FP950 and VHB20), the mean humidity delivered remained stable at > 30 mg H2O/L of delivered absolute humidity, even with ambient temperatures > 25°C. With previous generations of heated-wire humidifiers (MR850) at a high ambient temperature, the mean ± SD humidity delivered was adequate, that is, 36.0 ± 3.5 mg H2O/L, only when no temperature gradient between the humidification chamber and the Y-piece was set (40°C/40°C). With the MR850, when the ambient temperature was high, the mean ± SD absolute humidity delivered was 26.0 ± 4.8 and 29.3 ± 2.5 mg H2O/L with the usual settings 37°C/40°C and automatic compensation, respectively (Table 1 and Fig. 1).
The absolute humidity at different ambient temperatures (>25°C and <25°C) delivered by the previous generation (MR850 at different settings: 37/40, 40/40 and with compensation activated) and new-generation heated humidifiers (FP950 and VHB20). The minimum recommended absolute humidity (30 mg H2O/L) is represented by a gray line. *P < .001 for comparisons of absolute humidity values when ambient temperatures were above or below 25°C (2-way analysis of variance [ANOVA] comparisons). Differences of absolute humidity between the new and old generation of humidifiers are statistically significant, with P < .001 (multiple comparisons).
The Absolute Humidity Delivered at the Y-piece for Previous Generation of Heated-Wire Humidifiers (MR850 at different settings: 37/40, 40/40 and with automatic compensation activated) and New Generation Humidifiers for Ambient Temperatures Above or Below 25°C*
When comparing the new heated humidifiers, both performed well at > 30 mg H2O/L, whatever the ambient temperature from 20 to 30°C, with slightly higher humidity with the VHB20 compared with FP950 (Fig. 2). Previous generations of heated-wire humidifiers (MR850) were markedly influenced by the ambient temperature, with progressive reduction of the humidity delivered from 20 to 30°C, especially when > 25°C, with the usual settings (37°C/40°C) and with automatic compensation (Fig. 3). With these settings, low humidity was frequent and, in extreme conditions (near 30°C of the ambient temperature), the absolute humidity at the Y-piece was as low as 20 mg H2O/L. However, when no gradient was set (40°C/40°C) with the MR850, the occurrence of under-humidification was not frequent and only happened with very high ambient temperature, near 30°C (Fig. 3). The heater plate temperature was related to the absolute humidity delivered with different settings of the MR850 (Fig. 4). The ambient temperature was measured continuously in 2 ICUs of 2 different hospitals in Québec during 17 months and 11 months. Ambient temperatures > 24°C were present > 20% of the time in one ICU room, and a temperature > 25°C was found almost 10% of the time in the same ICU room. In other rooms, these temperatures were not rare events, and high temperatures did not seem seasonal (see the supplementary materials at http://www.rcjournal.com).
Humidification performances of new heated-wire humidifiers with varying ambient temperatures from 20 to 30°C. Each point represents a measurement of absolute humidity with the psychrometric method measured at the Y-piece at steady state for different ambient air temperatures. The gray horizontal line represents the minimum humidity expected with heated humidifiers (30 mg H2O/L). Whatever ambient temperatures from 20 to 30°C, all measurements of humidity were > 30 mg H2O/L with these humidifiers. Spline curves for absolute humidity delivered at different ambient temperatures for VHB20 and FP950 are represented. *Two-way analysis of variance (ANOVA) comparison for spline relationship.
Humidification performances of previous-generation heated-wire humidifiers with varying ambient temperatures from 20 to 30°C. The gray horizontal line represents the minimum humidity expected with heated humidifiers (30 mg H2O/L). The gray dotted line represents the limit of high ambient temperature (arbitrarily defined as 25°C). Each point represents a measurement of absolute humidity with the psychrometric method measured at the Y-piece at steady state for different ambient air temperatures. When the ambient temperature was < 25°C, most humidity measured was above 30 mg H2O/L whatever the MR850 setting. When the ambient temperature was > 25°C, absolute humidity < 30 mg H2O/L was frequent with the usual MR850 settings (37/40: 74% of measurements) as well as with advanced settings (40/40: 9%; Automated correction mode: 70%). Spline curves for different settings of the MR850 and ambient temperature are represented. *P values represent the interaction effect between settings and the ambient temperature.
Relationship between heater plate temperature and humidity delivered with MR850 and different settings (usual settings 37/40, upper panel) and advanced settings (automatic compensation and 40/40, lower panels). Heater plate temperature monitoring provides information on humidity delivered with previous-generation heated humidifiers (data obtained with MR850). When this temperature was > 62°C, 100% of the absolute humidities measured were > 30 mg H2O/L, as previously described. Coefficient of determination (R2) for the relation between absolute humidity delivered and heater plate temperature is displayed.
Discussion
This bench study evaluated the impact of the ambient temperature on previous and new-generation heated humidifiers. The study showed that new-generation heated-wire humidifiers, when using advanced algorithms, were stable and delivered the proper humidification, >30 mg H2O/L, even with a high ambient temperature. Previous-generation heated-wire humidifiers delivered low humidified gases when the ambient temperature was > 25°C, except when specific settings were used (40°C/40°C). To our knowledge, this is the first study that evaluated new heated-wire humidifiers specifically designed to address their known limitation of reduced humidification performance with high ambient temperatures.5
The previous-generation heated-wire humidifiers, represented in this study by the MR850, which is likely one of the most-used heated humidifiers worldwide, was known for years to have reduced humidification performances in specific conditions, leading to high inlet chamber temperature.5 High ventilator temperatures and a high ambient temperature (or even the sun directly on the humidifier) lead to low or very low humidity output. This may explain that, among the high rate of endotracheal tube occlusion during the recent pandemic, several cases were related to heated humidifiers.9,12 Even in ICUs with air conditioning in Québec, we found that an ambient temperature > 25°C was not an uncommon event and is probably related to ICU room expositions. Both ICUs were build in the last ten years and the air conditioning system had recently been updated and revised during the COVID-19 pandemic.12 It is likely that, in hospitals without an air-conditioning system or in other latitudes, the ambient temperatures in the ICUs are even higher.
The minimum absolute humidity to be delivered in patients on mechanical ventilation is usually define as > 28–30 mg H2O/L, depending of the reference method used.7,15,17 Below these levels, there are increasing risks of occlusions,14,18,-,20 a late marker of under-humidification or of endotracheal tube subocclusions21,-,23 or mucociliary dysfunction.24 The main risk factors of endotracheal tube occlusion are the low humidity delivered by the humidification devices1,15 or the duration of mechanical ventilation.9,21 There is abundant literature that describes the impact of under-humidification on airway mucosal and mucociliary transport dysfunction and bronchial inflammation.24 Epithelial dysfunction modifies the properties of the mucus, which leads to intraluminal depositions of viscous mucus, reduction of endotracheal diameter and increased tube resistances in a few hours,21-23 or endotracheal tube occlusions after several days.15 While previous-generation heated humidifiers deliver humidity > 30 or even 35 mg H2O/L when optimal conditions are met, they deliver gas with low or very levels of humidity (<20 mg H2O/L) when temperatures at the humidification chamber inlet are high.5
To cope with these limits, the 2 heated humidifiers tested in the present study were developed with an advanced algorithm. The FP950 incorporates additional sensors, including the ambient temperature, and this study demonstrated stability of the humidification performances even when the ambient temperature is high. The VHB20 uses an algorithm that considers the relative humidity that is measured at the distal part of the inspiratory circuit. Whereas advanced algorithms are developed to overcome the limitations of previous generations of heated humidification, cautious clinical and laboratory evaluations of the new-generation devices must be undertaken to detect unexpected limitations of these technologies. It must be emphasized that, to date, there are no data that demonstrate better clinical outcomes with heated humidifiers compared with performing HMEs, even in large randomized controlled trials, that compared these 2 humidification strategies.25,26
Our study has some limitations because the evaluation was a bench study and additional clinical evaluation will be required to evaluate other aspects, such as user interface, simplicity to use the devices, the presence of condensation in the circuits, robustness, and cost-effectiveness of these technologies. With regard to this last parameter, it is likely that these new devices will come with increased costs, but, if the proper humidity delivered reduces the duration of mechanical ventilation or severe complications, for example, endotracheal tube occlusions, then these additional costs are acceptable. In addition, the impact of minute ventilation with new heated humidifiers was not evaluated in this study.
Conclusions
The new heated-wire humidifiers FP950 and VHB20 demonstrated stable performances while varying ambient temperatures from 20 to 30°C, performing better than the previous generation of heated humidifiers when ambient temperatures were high. The bench evaluation showed good performances in terms of humidification but clinical evaluations are required to assess the practical utilization and potential issues related to the circuits. There is currently no clinical experience with very high humidity delivered (>40 mg H2O/L). Analysis of these data demonstrates the progressive improvement of the technology by the industry in response to clinical concerns.5
Comprehensive recommendations for the selection of a humidification device should consider the clinical condition, with the choice of HMEs as first-line strategy in many situations and heated humidifier to reduce dead space during protective mechanical ventilation with low tidal volumes (≤6 mL/kg predicted body weight as recommended3,27) and in the case of HME contraindications such as the presence of hypothermia or bloody secretions.1 In addition, the choice of a heated humidifier should also include considering the parameter of the variations of ambient temperature in the ICU rooms and the risk of high ambient temperature. If there is a greater risk of high ambient temperatures, then previous generations of heated-wire humidifiers should be used cautiously, with dedicated settings (low or no gradient between the humidifier and the Y-piece) and with a specific monitoring.6 With a new generation of heated humidifiers, the risk of under-humidification seems very low in tested conditions and excessive humidity has been described for much higher humidity levels.28
Clinical recommendations, for example, turning off humidifiers, are not acceptable.29 Similarly, it is not acceptable that mechanically ventilated patients who are critically ill experience potentially fatal complications related to under-humidification when both good performing HMEs and heated-wire humidifiers are now available, with the only restriction to use these devices properly.
ACKNOWLEDGMENTS
We thank Serge Simard MSc, for the statistical analysis and Jean-Claude Lefebvre for the participation to the ambient temperature measurements at the Hospital Hôtel Dieu de Québec.
Footnotes
- Correspondence: François Lellouche MD, Centre de Recherche de l’Institut Universitaire de Cardiologie et de Pneumologie de Québec, 2725 Chemin Sainte-Foy, G1V4G5, Québec, QC, Canada. E-mail: francois.lellouche{at}criucpq.ulaval.ca
Supplementary material related to this paper is available at http://www.rcjournal.com.
Dr Lellouche discloses relationships with Fisher & Paykel Healthcare and Vincent Medical. The other authors have disclosed no conflicts of interest.
Devices and consumables were provided by Fisher & Paykel Healthcare and Vincent Medical who had no other involvement in the study.
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