How to avoid an epidemic of endotracheal tube occlusion.

We are writing this Comment out of concern over the alarming number of publications describing endotracheal tube occlusions in patients with COVID-19 receiving mechanical ventilation.1, 2, 3 This potentially fatal complication had become rare, but high rates of occlusions (up to 72%) have been described in patients with COVID-19. This increase in prevalence has prompted questionable recommendations, such as the systematic change of endotracheal tubes every week in this patient group, or the routine use of tube cleaning devices.

Although relative luminal tube narrowing can occur frequently, the rates of tube occlusions in patients without COVID-19 on mechanical ventilation are typically reported to be below 0·5%.3, 6 Several factors might favour occlusions in patients with COVID-19, including prolonged duration of intubation (mean duration 2–3 weeks), reduction of suctioning opportunities caused by isolation of patients and desire to reduce caregiver exposure, patient's dehydration, or specific properties of the bronchial mucus.2, 3 Surprisingly, the role of under-humidification has almost never been discussed in articles that have reported endotracheal tube occlusions. However, there is ample evidence showing that under-humidification is the main risk factor for this complication.8, 9

During the surge of COVID-19 cases, difficulties in procuring respiratory therapy consumables have explained suboptimal choices in heat and moisture exchangers (HMEs), with preference for filtration capability over humidification quality, combined with imprecise specifications from manufacturers.

Under-humidification can occur with heated humidifiers or HMEs through different mechanisms. First, with heated wire humidifiers, high ambient temperatures (>28°C) or high gas temperature from the ventilator (in some turbines) increase the inlet chamber temperature, leading to a decrease in heater plate temperature, which reduces inspiratory humidity.1, 10 Furthermore, inadequate decisions have been made throughout the pandemic that would have been previously considered inconceivable, such as turning off humidifiers to reduce the risk of potentially infectious aerosols. Second, HMEs are less dependent on external factors than are heated wire humidifiers; however, their humidification performances are highly heterogeneous. Nevertheless, the choice of a HME is crucial as only a few provide sufficient humidity, even among those that claim optimum humidification properties.

Under-humidification of inspiratory gases can cause bronchial inflammation, epithelial injury, and a change in mucus properties, leading to a reduction in endotracheal tube diameter, increased tube resistance, or occlusion of the endotracheal tube in a matter of hours. This clinical scenario might require an emergent change of the endotracheal tube (a procedure particularly at risk of aerosol production), cardiac arrest, or even death. Sub-occlusions result in increased resistance of the endotracheal tube, which increases respiratory drive and work of breathing, potentially aggravating lung injury or delaying weaning.

The minimum level of inspiratory humidity recommended is 30 mg H2O/L (International Organization for Standardization [ISO] 9360) for HME and 33 mg H2O/L for heated humidifiers (ISO 8185). Using the psychrometric method, we showed that the risk of endotracheal tube occlusion was increased when the level of humidity delivered was below 28 mg H2O/L.

The ISO standard that identifies the safe limits relies on the gravimetric method for the measurement of the humidity delivered. Unfortunately, this method of measurement is accurate only if it is performed rigorously in specialised laboratories, given that the sources of error are numerous (eg, water loss weight measurements, ambient conditions, leaks in the system, or minute ventilation measurements of the gases in variable conditions of temperature and humidity). Almost all of the HMEs or filters involved in cases of endotracheal tube occlusion during the COVID-19 pandemic had passed the ISO standard. In a bench study, we evaluated three of these HMEs involved in tube occlusions: DYNJAAHME1B (Medline, Northfield, IL, USA), FH603008 (Sunmed, Grand Rapids, MI, USA), and Inter-Therm (Intersurgical, Wokingham, UK). We measured the level of humidity delivered by these devices with a psychrometric method, using the same bench that has been previously described. We measured an unacceptably low humidity output (mean 25·8 mg H2O/L [SD 0·9]), despite the fact that manufacturers reported a reassuring humidity of 32·8 mg H2O/L (1·6) on the basis of the ISO 9360 standard (figure ). Such differences between measured and claimed performances have been previously reported. This finding seriously questions the validity of these standards and how these measurements are controlled to discriminate between efficient and non-efficient HMEs, which should never be used in critically ill patients on mechanical ventilation. Additionally, we measured the level of humidity delivered when the heated humidifiers were turned off. In this situation, the mean absolute humidity of inspiratory gases was 7·8 mg H2O/L (0·2), which is extremely low.

Figure

Absolute humidity of devices according to manufacturers’ values and psychrometric measurements

Medline, Sunmed, Inter-Therm, and MR 850 devices were involved in endotracheal tube occlusions in patients with COVID-19; HygrobacS was not. A humidity of 28 mg H2O/L (red line) was considered to be a safe limit, with psychrometric and gravimetric measurements performed in independent and specialised laboratories.9, 15 Blue lines represent the recommended minimum humidity for heat and moisture exchangers (ISO 9360) and heated humidifiers (ISO 8185). ISO=International Organization for Standardization. *Measurements were performed according to ISO standards, with tidal volumes of 500 mL (HygrobacS) or 250 mL (Inter-Therm), or unreported conditions (Medline and Sunmed); data were collected from the manufacturers’ websites or measurements of hygrometry with heated humidifiers and heat and moisture exchangers in our laboratory in June, 2021.

The need to humidify gases has been recognised since the genesis of mechanical ventilation. The main basic principles of humidification, which were described 50 years ago, still need to be followed. Humidification devices that deliver at least 28 mg H2O/L, with an accurate humidity method of measure and measurements that are obtained independently from the manufacturer, should be provided to patients on mechanical ventilation.9, 15 It is time to protect patients with adequate humidification devices and adequate standards that discriminate humidification devices to avoid preventable mortality and morbidity.

FL declares financial contributions from Fisher & Paykel to develop a smartphone app (VentilO) to help the implementation of lung protective ventilation, and to conduct bench humidification evaluations, and honoraria for a symposium on dead space and high flow. LB declares honoraria for a symposium on high flow from Fisher & Paykel. All other authors declare no competing interests. We thank Eduardo Mireles-Cabodevila, Scott Neuhofs, and Samuel P Wiles (Cleveland clinic, Cleveland, OH, USA), who provided the HMEs used at their centre to conduct bench tests. We also thank Tsuneaki Kenzaka (Kobe, Japan) and Rohan K Panchamia (New York, NY, USA) for their help to determine the reference of the HMEs used at their centre.