Patient and Device Interface Variables Affect Relative and Absolute Humidity Delivery

Abstract

Background:

Humidity is important when bypassing the upper airway (ETT and tracheostomy tube) or when high gas flows are used. When humidity is not appropriately considered, it can affect the structure and function of the respiratory epithelium. This can cause increased irritation to the airways, as well as the inability to mobilize secretions resulting in mucus plugging leading to increased work of breathing.

Methods:

Multiple minute volume combinations were simulated using a mechanical ventilator and a dual chamber lung simulator. The lung simulator was partitioned to simulate a muscle compartment connected to the ventilator and a lung compartment used to simulate patient minute volume. The lung compartment was fitted with one-way valves to measure inspiratory humidity delivery and not be influenced by humidity in an expiratory reservoir. The three device configurations studied were nasal cannula, with and without a bubbler, trach collar and direct trach adaptor, and high flow nasal cannula. The effect of multiple device flows and FIO₂ settings were assessed. Varying minute volumes were simulated by using different combinations of frequency and tidal volume.

Results:

Relative humidity (RH) comparison of nasal cannula without (RH range 4.6-22.0%) and with (RH range 58.0-77.5%) a bubbler demonstrates an added RH and absolute humidity (AH) benefit with an addition of a bubbler at all flows and minute volume combinations (Figure 1). Large volume aerosol delivery demonstrated an overall significant difference in the AH delivery when trach collar (AH range 11.16-14.08 mg/L) was compared to a direct trach adapter (AH range 14.11-15.50 mg/L) (P < .001) at all FIO₂ and minute volume combinations (Figure 2). With a constant heater temperature, high-flow nasal cannula (HFNC) demonstrated a higher mean AH at 60 L/min (31.25 mg/L) when compared to 30 L/min (28.67 mg/L).

Conclusions:

The addition of a bubbler to nasal cannula at flow settings ranging 1-6 L/min has a RH and AH benefit. The benefit increases as the flow increases. A direct trach adapter had a statistically significant increase in AH delivery when compared to a trach collar at similar settings. Clinical significance is unknown. HFNC AH delivery remained relatively constant when minute volume combinations were assessed. However, there was an increased AH delivery at 60 L/min when compared to 30 L/min.