Benefits of heated humidification for ventilated COVID‑19 patients
7. COVID‑19 patients, who are critically ill with severe respiratory disease, need high levels of humidity to assist with secretion management, promote efficient ventilation and gas exchange, and to ensure optimal mucociliary function
The upper airway naturally heats and humidifies inhaled air to 37 °C and 100% Relative Humidity (44 mg/L Absolute Humidity).11,12 Invasively ventilating a patient with lower levels of heat and humidity can have the following adverse effects:
- Mucociliary transport system dysfunction11,13,14
- Airway drying15
- ETT blockages3,16
- Thick, difficult to suction secretions17,18
- Increased rates of Ventilator Acquired Pneumonia (VAP)19
Heated humidifiers aim to deliver optimum levels of heat and humidity to patients (37 °C, 44 mg/L). HMEs achieve a maximum humidity level of 32‑33 mg/L, with many producing less than 30 mg/L.20 Using an HME provides patients with significantly lower levels of humidity than a heated humidifier, and studies show that delivering just 10% less humidity for 15 minutes can have a significant impact on mucociliary function.21
8. Heated humidifiers provide humidification without increasing instrumental dead space, an essential requirement for effective lung-protective ventilation
COVID‑19 patients require lung-protective ventilation strategies. When compared to HMEs, heated humidification may enable patients to be ventilated with reduced tidal volume (VT), reducing the partial pressure of carbon dioxide (PaCO2) and plateau pressures (Pplat), resulting in increased alveolar ventilation and gas exchange. Heated humidifiers are recommended for lung-protective ventilation,22 as reduced tidal volume delivery requires minimal instrumental dead space, which cannot be achieved with HMEs.
Lung-protective ventilation is a combination of ventilation settings and associated procedures which can have a direct impact on mortality.23–27 A key aspect of lung-protective ventilation is minimizing instrumental dead space, which can have a substantial impact on work of breathing, gas exchange, and alveolar ventilation.24,28–33 Several clinical guidelines recommend lung-protective ventilation for invasively ventilated COVID‑19 patients, or those meeting the criteria for ARDS.10,34
- The use of a heated humidifier does not add any instrumental dead space, whereas an HME can add up to 100 mL of dead space. Several studies have demonstrated that dead space reduction using a heated humidifier can have a significant impact on gas exchange along with a decrease in PaCO2 which is proportional to the reduction in dead space.29–33 Prat et al.31 showed that using a heated humidifier compared to an HME resulted in a PaCO2 decrease (80 to 63 mmHg) without changing any other settings.
- Moran et al.30 showed that using a heated humidifier compared to an HME resulted in the ability to decrease tidal volume (VT) by 81 mL, peak pressure (Ppeak) by 7 cmH2O and plateau pressure (Pplat) by 4 cmH2O.
9. Heated humidifiers can support more effective weaning in difficult-to-wean patients compared to HMEs
COVID‑19 patients will likely be difficult to wean off mechanical ventilation due to the nature of the disease and likely development of Acute Respiratory Distress Syndrome (ARDS). Heated humidification reduces dead space and resistance to flow for optimized weaning when compared to HMEs.28
Girault et al.28 compared the use of HMEs and heated humidifiers in difficult-to-wean patients. They found that using HMEs required pressure support to be increased by 8 cmH2O in the HME group compared to the heated humidifier group. The study recommended not using HMEs in this patient group.
10. Heated humidification has no contraindications, but it is important to understand the contraindications to HME use that may present in some COVID-19 patients
There are no contraindications to the physiological conditioning of inspired gas during mechanical ventilation. However, clinicians should consider the device used to heat and humidify respiratory gases. While there are no documented contraindications for heated humidifiers, HMEs are contraindicated in some circumstances.
These include, but are not limited to:
- Patients with thick, copious secretions16,22,35 or those requiring long term ventilation, as humidification of inspired gas may be insufficient to prevent airway dysfunction and subsequent HME occlusion.
- Patients with large mask leaks, as the exhaled volume is insufficient to replenish heat and moisture for subsequent inspiration.
- Patients with body temperature <32 °C or with expired tidal volume <70% of the delivered volume, as reductions to exhaled heat and moisture impair HME efficiency.16,22
- Patients with low tidal volumes, acute respiratory failure36,37, chronic respiratory failure28, ARDS31 or high minute volumes (>10 L/min)5,38, as HMEs add dead space to the circuit22,29–31,36 which reduces alveolar ventilation and can substantially elevate PaCO2,30,37 resistance to flow, work of breathing, and thus ventilatory requirements.29,35,39,40
The American Association for Respiratory Care also recommends the use of heated humidifiers over HMEs in noninvasive ventilation, as this may improve patient comfort and tolerance.22 The additional dead space and resistance added by the HME system may negate the effects of noninvasive positive pressure, elevating work of breathing and ventilatory requirements.37
Note: The simultaneous use of heated humidification and an HME should not be used as per a patient safety alert (PSA) notification issued in 2015 by the NHS and MHRA41. For further information please refer to the PSA: Risk of using different airway humidification devices simultaneously.