Invasive Mechanical Ventilation
An algorithm for heating and humidifying inspired gases during invasive mechanical ventilation is proposed in Figure 51-7.
Algorithm for heating and humidifying inspired gases during invasive mechanical ventilation. ARDS, acute respiratory distress syndrome; HME, heat and moisture exchange.
Which Device for Which Patients
There are very few and rare contraindications to the use of HMEs. Because they act passively, the amount of heat and moisture they deliver to the inspired gases depends on the amount of heat and moisture they retain during expiration. Therefore, patients with profound hypothermia or important bronchopleural fistulas should preferably be ventilated with a heated humidifier. In addition, a heated humidifier may be preferable in patients ventilated for acute asthma or acute respiratory distress syndrome, in whom a drastic decrease in tidal volume induces extreme respiratory acidosis (to avoid increased dead space with HMEs).39,40 A heated humidifier can be used until the patient’s respiratory condition improves sufficiently to enable the use of an HME. Apart from these specific situations, HMEs should be considered first to heat and humidify inspired gases of all ventilated patients. They are as effective as heated humidifiers, and are much cheaper and much easier to use. Importantly, medical as well as surgical patients can benefit from them.
Despite the former practice of restricting their use to patients without a history of respiratory disease and only for the first 5 days of mechanical ventilation,74,76 several studies have clearly shown that they may be used in any ventilated patient, even in patients with COPD and for any length of mechanical ventilation. Because patients with COPD represent the most important subgroup of ventilated patients in the United States,77 and because their duration of ventilation is longer than that of other patients,77 adequate humidification is probably more critical in such patients than in patients ventilated for shorter periods. Numerous studies show that long-term mechanical ventilation can be safely conducted in such patients with HMEs.19,55,56,75,78–80 Three studies have specifically compared humidity output of HMEs in patients with and without COPD and consistently found that the measured values for absolute humidity were very similar (Fig. 51-8).19,75,80
Comparison of the absolute humidity delivered by heat and moisture exchangers in patients with and without chronic obstructive pulmonary disease (COPD). Values are similar, indicating that these devices are suitable for patients with COPD undergoing mechanical ventilation.
Frequency of Heat and Moisture Exchange Replacement
There is now considerable evidence that HMEs may be used for longer than the 24 hours recommended by manufacturers.18,19,56,60,61,73,75,79–82 Compelling results stem from rigorous clinical evaluation56,60 or extensive bedside measurement of humidity delivery.18,19,61,75,79–81Figure 51-9 shows that humidity delivery of a combined HME (Hygrobac, DAR, Mirandola, Italy) used for 7 days without change was remarkably stable throughout this period. Progressive clogging of the device with tracheal secretions (that seriously increase resistance to airflow) could have been a drawback. Repeated measurements of the resistance of the HME over 7 days indicate that this phenomenon did not occur (Fig. 51-9).75
Absolute humidity (left Y axis) and resistance (right Y axis) measured over time in combined heat and moisture exchangers used for 7 days without change. Note the perfect stability of the humidity output and the lack of increase in resistance over time. (Adapted, with permission, from Ricard et al.53)
Keeping the HME vertically above the tracheal tube (see Fig. 51-3) (and having nurses and doctors repeatedly check the position of the HME) prevents secretions from refluxing from the tracheal tube and obstructing the HME.19,75 Use of HMEs may be extended in both medical patients (including those with COPD19,75,79,80) and surgical patients.18,61,73,82 This practice is now widely accepted,83 and recommendations have been made to change HMEs only once a week.64 In our own published75 and unpublished experience,75 we have been changing HMEs only once a week since 1997, and have not encountered endotracheal tube occlusions.
Noninvasive Mechanical Ventilation
The question of heat and humidification requirements during noninvasive ventilation has gone unanswered until very recently. The 2001 International Consensus Conference on Noninvasive Ventilation made no recommendation as to whether a humidifying device should be used or not.84 This absence stems from the uncertainties regarding the level of humidification required during noninvasive ventilation (given the fact that the upper airways are not bypassed) and that can be achieved during noninvasive ventilation with HMEs and heated humidifiers.
This aspect has received attention in a recent study that compared the water content of gas delivered obtained during noninvasive ventilation with either an HME or a heated humidifier, and the comfort or discomfort experienced by healthy subjects.85 In the absence of humidification, water content was very low when an ICU ventilator was used (5 mg H2O/L), but equivalent to ambient air hygrometry with a turbine ventilator at minimal fractional inspired oxygen concentration (FIO2) (13 mg H2O/L). HME and heated humidifier had comparable performances (25 to 30 mg H2O/L) although HME’s effectiveness was reduced with leaks (15 mg H2O/L). During continuous positive airway pressure, dry gases (5 mg H2O/L) were less tolerated than humidified gases. Gases humidified at 15 or 30 mg H2O/L were equally tolerated. These data indicate that in favorable conditions, HME and heated humidifiers provide equivalent comfort to the patient through similar water content to the inspired gases.
This study contrasts with some physiologic studies of the influence of the humidifying device on respiratory variables. In a randomized crossover study in nine patients receiving noninvasive ventilation for moderate to severe acute hypercapnic respiratory failure, a significant increase in work of breathing was found with HMEs in comparison with heated humidifiers), although this did not significantly impact the partial pressure of arterial carbon dioxide (PaCO2).44 The dead space of HMEs is obviously responsible for these observations and a recent study showed that use of an HME with a very small dead space does not alter respiratory variables, compared with heated humidifiers.86
The question that arises is whether the choice of a given humidifying device during noninvasive ventilation affects outcome, especially the intubation rate. To address this issue, a large multicenter randomized trial tested the hypothesis that the use of HMEs would increase intubation rates.87 During 13 months, 247 patients were randomized to noninvasive ventilation either with an HME or a heated humidifier. The intubation rate (failure of noninvasive ventilation) was higher with a heated humidifier than with an HME, although the difference did not reach statistical significance (37.6% vs. 30.6%; p = 0.31). Mortality tended to be higher in the heated humidifier group (21.5% vs. 14.1%; p = 0.18). PaCO2 tended to be lower in the heated humidifier group (66 vs. 72 mm Hg; p = 0.08).87 These preliminary results suggest that the observed increases in work of breathing, minute ventilation, and PaCO2 in carefully conducted physiologic studies (but on a small number of selected patients) may only play a marginal role in the clinical setting. Taken together, it can now be stated that noninvasive ventilation can be safely and efficiently performed with HMEs, provided attention is given to minimize leaks and that a small internal volume HME is chosen.
Adjustments at the Bedside and Troubleshooting
Assessing humidification at the bedside is desirable, if not essential, for at least two reasons: (a) a life-threatening tracheal tube occlusion24 may occur without any precursory clinical signs of insufficient humidification; and (b) devices may not deliver, in some instances, the heat and humidity that is expected of them, because they are either malfunctioning14 or performance in the clinical setting does not attain that stated by the manufacturer.14,80
Such assessment raises two questions: (a) Is the device delivering enough heat and humidity (according to the standards)? (b) Is the amount of heat and moisture delivered appropriate for a given patient?
Assessing Heat and Moisture Exchangers and Heated Humidifiers at the Bedside
A simple means of evaluating the humidity delivered by a given device is to assess the amount of condensation seen in the flexible tubing that connects either the Y-piece (when using a heated humidifier) or the HME to the endotracheal tube.14,17,88 Indeed, the amount of condensation seen in the flexible tubing is positively correlated with the absolute humidity measured at the bedside.14,17 If the tubing remains constantly dry over time, or only very few droplets of water are seen, then absolute humidity delivered by a device is probably below 25 mg H2O/L, and the patient is at risk of endotracheal tube occlusion. When numerous droplets are seen in the tubing or if it is dripping wet, the device is probably delivering sufficient absolute humidity to prevent endotracheal tube occlusion.17
Assessing Adequacy of Inspired Gas Conditioning
Although it is conceivable that some patients have special humidification needs, the prevailing literature indicates that heated humidifiers and HMEs equally meet humidification requirements in the vast majority of ventilated patients. Therefore, monitoring the characteristics of secretions (thick and tenacious, or watery and abundant) as a guide to insufficient or excessive humidification may be of limited value, because changes in mucus characteristics may be entirely caused by the patient’s condition (respiratory status, fluid balance, and so on), and not a consequence of the humidifying device. It has been shown that air humidification with either an HME or a heated humidifier has similar effects on mucus rheologic properties, contact angle, and transportability by cilia in patients undergoing mechanical ventilation.89