Consequences of Pausing Heated Humidification During Invasive Ventilation

Abstract

BACKGROUND:

During invasive mechanical ventilation, where medical gases are very dry and the upper airway is bypassed, appropriate gas conditioning and humidification are mandatory at all times. Results of in vitro studies suggest that dry gases may improve lung deposition during nebulization, but this has not been confirmed through in vivo studies. The objective of this study was to measure gas humidity under multiple conditions to better describe gas hygrometry when heated humidifiers are turned off.

METHODS:

We measured, on a bench, the hygrometry of different gases at steady state: medical gases, at the Y-piece without humidifier, with the humidifier switched off, and with humidifier switched on. We measured gas humidity every 10–60 s during dynamic conditions after switching off the heated humidifier and after switching on the heated humidifier. Hygrometry was measured by using the psychrometric method with at least 3 measurements for each tested condition.

RESULTS:

We performed 287 psychrometric measurements in different situations. The mean ± SD gas absolute humidity at steady state during different conditions were the following: 1.6 ± 0.2 mg H₂O/L for the medical gases, 4.5 ± 0.9 mg H₂O/L at the Y-piece without humidifier, 9.1 ± 0.3 mg H₂O/L at the Y-piece with heated humidifier turned off, and 34.2 ± 2.2 mg H₂O/L at the Y-piece with the heated humidifier turned on. During the dynamic evaluation, after turning off the humidifier, humidity was < 30 mg H₂O/L after a few minutes, attained 15 mg H₂O/L after 15 min, and was below 10 mg H₂O/L after 1 h but never reached the level of dry medical gases. After turning on the heated humidifier, the gas hygrometry reached 30 mg H₂O/L after 5 min.

CONCLUSIONS:

When heated humidifiers are turned off, gas humidity levels are very low but not as low as medical gases. The clinical impact of repeated shutdowns is unknown. As recommended, heated humidifiers should never be turned off during nebulization.

Introduction

Proper gas humidification is mandatory at all times during invasive mechanical ventilation.¹ Indeed, gases may arrive at the distal opening of the endotracheal tube with insufficient humidity for several reasons, including the use of (i) dry medical gases (1-3 mg H₂O/L), especially when delivered by an ICU ventilator by using a mixture of medical oxygen and medical air; and (ii) bypassing the upper respiratory tract, which is usually responsible for heating and humidifying gases. However, in recent years, it has been suggested that heated humidifiers can be turned off, particularly when nebulizing medications. In this situation, several in vitro studies have reported potential benefits of turning off heated humidifiers during aerosolization to increase lung deposition,^2-5 but these benefits have been questioned in in vivo studies.^6,7

In the studies that compared lung deposition with dry or humidified gases, the “dry gas” conditions are heterogeneous. In several studies, gases were delivered directly through standard circuits without humidification, with turbine ventilators² or ICU ventilators.^3,4 In other studies, the heated humidifier was turned off.^6,7 The gas humidity in dry and humid conditions were rarely measured in those studies, except in the research by Fink et al.^4,8 The objectives of our study were to quantify the different humidity levels of the “dry gases” used in in vitro studies that evaluated lung deposition during aerosol therapy, to evaluate the time necessary to obtain dry gases when the humidifiers are turned off and the time necessary to return with humidified gas after turning on the heated wire humidifier.

QUICK LOOK.

Current Knowledge

Results in laboratory studies suggest that dry gases improve lung deposition during nebulization; however, the dry condition has not been well defined during in vitro studies. The consequences of insufficient temperature and humidity on the tracheobronchial epithelium and ciliary function following endotracheal intubation are well known.

What This Paper Contributes to Our Knowledge

The gas humidity at steady state was ∼2 mg H₂O/L for the medical gases, 5 mg H₂O/L at the Y-piece without humidifier, 9 mg H₂O/L with the heated humidifier turned off (cold humidification) and 35 mg H₂O/L with heated humidifier turned on (heated humidification). After turning the humidifier off, humidity was < 30 mg H₂O/L after a few minutes, attained 15 mg H₂O/L after 15 min, and was below 10 mg H₂O/L after 1 h. The study echoes the recent recommendation not to turn off heated humidifiers when nebulizing medications during invasive ventilation. The approximate duration of nebulization (∼5 min for a bronchodilator and 30 min for antibiotics) is indicated, but the humidity during nebulization is not indicated because it may not be measured accurately.

Methods

We conducted a bench study in the O₂-H₂O-CO₂ laboratory at the Institut Universitaire de Cardiologie et de Pneumologie de Québec. The conditions of the experiments were the following: the ambient air of the laboratory was kept constant at 25 ± 0.5°C; the ventilator used was an ICU ventilator Dräger V500 (Dräger Medical, Lübeck, Germany) connected to a test lung (Test lung 190 – 1 L, Maquet Critical Care, Solna, Sweden). The ventilator was set to provide a constant minute ventilation with the following settings: tidal volume ( ${\text{V}}_{\text{T}}$), 500 mL; breathing frequency, 20 breaths/min; PEEP, 5 cm H₂O; and ${\text{F}}_{{\text{IO}}_{\text{2}}}$, 0.21. A heated humidifier was used (MR850, Fisher & Paykel Healthcare, Auckland, New Zealand), software 8.0, set at 37/40 (37°C at the humidifier and 40°C at the Y-piece).

We conducted 3 to 5 hygrometric measurements, by using a Yokogawa GP10 psychrometer (Yokogawa Test & Measurement, Tokyo, Japan), for each tested condition by using a previously described method.^9,10 Additional explanations on the psychrometric method are available (Supplementary Fig. S1 [see the supplementary materials at http://www.rcjournal.com]). We measured gas hygrometry (absolute and relative humidity) under different conditions at steady state:

• medical air and oxygen, at the outlet of the wall after decompression, at a constant flow of 30 L/min

• gases at the ventilator outlet

• gases at the Y-piece without a heated humidifier in the circuit

• gases at the Y-piece with a heated humidifier turned off (cold humidification)

• gases at the Y-piece with a heated humidifier turned on (heated humidification)

We also measured gas hygrometry in dynamic situations after turning the heated humidifier on and off. Hygrometry was measured every 10 s at the beginning of each testing condition, then every 60–120 s after 10 min. When the humidifier was turned on, we also collected the heater plate temperature from the device submenu as previously described¹¹ as well as the displayed temperature on the main screen. In addition, we performed two measurements of gas humidity with the heated humidifier turned off during 24 h (cold humidification).

Results

A total of 543 data points were measured or recorded (absolute humidity of different gases at steady state and during dynamic conditions, heater plate temperature, and temperature displayed on the humidifier’s main screen), including 287 hygrometric measurements, mainly obtained during dynamic measurements. The hygrometry of the medical gases that come from the wall, at the ventilator outlet, without humidifier, and with the humidifier turned off or on are displayed in Figure 1. The dynamic gas hygrometry when turning the heated humidifier off or on are shown in Figure 2. The decrease in gas humidity was relatively slow after the humidifier is turned off, whereas the increase in humidity was much faster when the humidifier is turned back on.

Fig. 1.

The mean ± SD absolute humidity of different gases at steady state are represented: medical gases (air or oxygen), at the outlet of an ICU ventilator, at the Y-piece without the humidification device in the circuit, at the Y-piece with the humidifier switched off (cold humidification) or with the heated humidifier switched on (heated humidification).

Fig. 2.

Absolute humidity of the gas delivered to the Y-piece after switching off and after switching on the heated humidifier. Representation of different experiments are shown on the graph. Two minutes after switching off the humidifier, the humidity was < 30 mg H₂O/L, it took 20 min to reach 15 mg H₂O/L, and after 1 h, the absolute humidity was < 10 mg H₂O/L. After turning the humidifier on, the humidity was > 30 mg H₂O/L after only 5 min.

After the heated humidifier was turned off, the absolute humidity decreased at 10 mg H₂O/L after 30 min. but never dropped < 5 mg H₂O/L, even after several hours. After turning the humidifier off for 24 h, the gas humidity with cold humidification was ∼7.5 mg H₂O/L. The heater plate temperature increased after turning the heated humidifier on, followed by the absolute humidity of the gas and gas temperature (Fig. S2 [see the supplementary materials at http://www.rcjournal.com]). The psychrometric measurements when turning the humidifier on and off, and during the nebulization are provided in the online supplement (Figs. S3 to S5 [see the supplementary materials at http://www.rcjournal.com]).

Discussion

In this bench study, we described the gas humidity at steady state during different conditions, from < 5 mg H₂O/L for the medical gases to almost 35 mg H₂O/L with active humidification. After turning the humidifier off, humidity was < 30 mg H₂O/L after a few minutes, dropped to 15 mg H₂O/L after 15 min, and was < 10 mg H₂O/L after 1 h but never reached the level of dry medical gases. After turning the heated humidifier on, gas hygrometry reached 30 mg H₂O/L after 5 min. We assessed gas humidity levels when heated humidifiers are turned off and on to estimate the gas humidity during nebulization with dry gases. To achieve dry gases (∼15 mg H₂O/L), the humidifier must be turned off for a minimum of 15 min with the heated humidifier tested in this study. When considering the duration of the nebulizations (from 5 min for β₂ agonists to 30 min for antibiotics) and the time required to return to acceptable humidity, each nebulization procedure could lead to low gas humidity for 30 min to 1 h if heated humidifiers must be turned off before nebulization. This may lead to complications related to under-humidification.¹²

However, it is possible if heated humidifiers are switch off intermittently, that the mucosal function, which is affected rapidly by dry gases, is restored when delivered gases are rehumidified.¹³ There is no evidence that repeated, brief, discontinuation of heated humidity affects bronchial function. However, because nebulization can be delivered several times a day, there is the risk of not turning the device back on each time. In addition, hyperventilation with dry gases can cause bronchoconstriction and have effects similar to those of methacholine in triggering bronchial hyperreactivity.¹⁴ It would, therefore, be paradoxical to use dry gases when inhaled medications are administered to manage bronchoconstriction in mechanically ventilated patients. However, it may seem wise to increase lung deposition by using dry gases to deliver nebulized antibiotics to reduce the costs of this treatment,¹⁵ but the benefits of dry gases in the clinical setting are unclear.^6,7

The beneficial effects of dry gases on lung deposition with “dry gases” were found in laboratory studies,^2,3,8 whereas in vivo studies did not find similar benefits.^6,7 In the study by Dugernier et al,⁶ humidity levels in dry conditions were likely similar to what we found after turning the heated humidifier off (∼10 - 15 mg H₂O/L). Dugernier et al did not find statistically significant differences in lung deposition when the humidifiers were on versus off.⁶ In Fink et al study,⁸ aerosols deposition was increased when dry gases were used during nebulization. The “dry conditions” in this studies were likely much drier (<5 mg H₂O/L), and this may, in part, explain the different findings between the studies that compared effects of dry and humidified conditions. It seems that the balance between the benefits and risks of dry gases during invasive mechanical ventilation argues in favor of not using dry gases. It is now clearly recommended not to turn off heated humidifiers when nebulizing antibiotics or other medications.⁵

This issue of pausing heated humidifiers could be relevant for other situations, for example, when patients are transported to the radiology department for different procedures or in cases of transport to other hospitals.¹⁶ In certain cases, when the ${\text{V}}_{\text{T}}$ is very low and breathing frequency is high, such as in cases of severe ARDS, it could be dangerous to use a heat-and-moisture exchanger instead of the heated humidifier because of the dead space, which leads to a risk of increased respiratory acidosis.¹⁷ However, just turning off the humidifier during transport is not an option due to the rapid decrease in humidity as shown in the present study. The question of integrating a battery into heated humidifiers to continue to humidify gases during transport could then arise.

When the heated humidifier is turned off for a prolonged period, the absolute humidity was consistently < 10 mg H₂O/L (cold humidification) in the present study. As a reference, it must be kept in mind that, with a poor performing heat-and-moisture exchanger providing between 20 and 25 mg H₂O/L, the rate of endotracheal tube occlusions can be high.⁹ The hydrophobic heat-and-moisture exchanger Pall BB2215 (Pall corporation, Port Washington, NY) was evaluated in 5 studies, with endotracheal tube occlusions occurring in 37 of 294 subjects (12.6%).^18-22 This heat-and-moisture exchanger delivered ∼22 mg H₂O/L with the psychrometric method (similar to the method used in the present study) in a bench study⁹ and in a clinical study.²³ Thus, humidifiers should not be turned off in patients on invasive mechanical ventilation for prolonged periods because it leads to levels of humidity < 10 mg H₂O/L. During the pandemic at least one group proposed turning off heated humidifiers to reduce the risk to caregivers.²⁴ This is probably an anecdotal practice at best, but, as expected, it led to “frequent endotracheal tube occlusions” despite utilization of frequent saline solution instillations, which is not a recommended practice, due to its frequent harmful effects.²⁵ Bacterial aerosols have been described with bubble-through humidifiers,²⁶ but there is no rational for this complication to occur with pass-over humidifiers.

This study has limitations. First, we did not conduct measurements in patients but the data were obtained on a bench model with controlled conditions, with the ambient temperature in the laboratory kept at 25°C. It is likely that gas humidity delivered with various ambient temperature would be different.^10,11 However, we think gas humidities with and without heated humidification are the most relevant data. We are confident that, in patients, similar data would be obtained. Clinical consequences of dry gases cannot be directly derived from this bench study. However, a relation was previously shown between hygrometric data obtained from a bench study and clinical complications.^9,27 Another limitation is that gas humidity measured during nebulization is probably not reliable,²⁸ and we could not evaluate the level of humidity in this situation. Indeed, the psychrometric method is intended to measure water content in a gaseous state while the water is in a liquid state in droplets during nebulization.²⁹ It is likely that ${\text{V}}_{\text{T}}$ may slightly influence the humidity delivered for very low ${\text{V}}_{\text{T}}$ .³⁰ In this study, we used an average ${\text{V}}_{\text{T}}$ for patients without ARDS (ie, 500 mL, which corresponds to 7–9 mL/kg of predicted body weight for average heights in men and women).^31-33

CONCLUSIONS

This study showed very low levels of gas humidity when the humidifier was turned off, even for short periods of time and this situation should be avoided. Preventive antibiotic nebulization to prevent ventilator-acquired pneumonia may be used more frequently after a recent randomized controlled study demonstrated benefits of this therapy.¹⁵ However, the present study shows that very low humidity is delivered when humidifiers are turned off, and, as recently recommended, these devices should be kept on, even when delivering nebulization.⁵ It must be reiterated that proper gas humidification should be provided at all times to any patient receiving invasive mechanical ventilation.^1,5