The Myth of the Two-Liter Nasal Cannula

Abstract

Background:

Healthcare professionals often document that patients on 2-L nasal cannula (NC) are receiving 28% FIO₂. This is simply misleading. NC are low-flow devices with variable oxygen delivery depending on anatomy and breathing pattern, as the physiology of nasal oxygen therapy is based oxygen filling of the oropharynx during expiration and delivery of oxygen to the lower airways during inspiration. Our objective for this study is to quantify the actual FIO₂ delivered by the NC considering the role of minute ventilation (VE) and inspiratory flow (PIF) on oxygen concentration.

Methods:

An anatomic model which comprised of a mannequin connected to a Drager ventilator via a Michigan Lung created spontaneous ventilation. We utilized an oxygen analyzer in the trachea to measure the oxygen level delivered by the 2-L NC at different VE in L and PIF in L/min. A combination of tidal volumes (V_T) in mL and respiratory rates (RR) in breaths/min were used to generate VE. Our protocol examined VE 2 L (V_T 500 mL, RR 4 breaths/min) at PIF of 40 L/min and 80 L/min, VE 5 L (V_T 500 mL, RR 10) at PIF 40 L/min and 80 L/min, VE 10 L (V_T 500 mL, RR 20) at PIF 40 L/min and 80 L/min, and VE 20 L (V_T 800 mL, RR 25) at PIF 60 L/min and 100 L/min, to simulate the clinical scenarios of opiate overdose, normal ventilation, and central hyperventilation, respectively. Readings were allowed to equilibrate for 3-5 min. Measurements were then recorded five times at ten second intervals. Continuous variables were expressed as means with standard deviation (SD) and minimum-maximum values.

Results:

The minimum-maximum FIO₂ generated by the 2 L NC were 27.7% - 62.5%. The mean (SD) oxygen levels were: VE 2 L at IFR 40 L/min and 80 L/min 61.1% ( ± 1.45%) and 61.46% ( ± 0.91%) vs. VE 5 L at IFR 40 L/min and 80 L/min 55.48% ( ± 1.03%) and 53.08% ( ± 1.13%) vs. VE 10 L at 40 L/min and 80 L/min 38.16% ( ± 0.10%) and 36.28% ( ± 0.26%) vs. VE 20 L at 60 L/min and 80 L/min 29.14% ( ± 0.10%) and 27.74% ( ± 0.05%).

Conclusions:

Our study demonstrated that the 2 L NC can deliver variable FIO₂ between 28% to over 60%. Our study also illustrated that VE (V_T x RR) played a significant role in the FIO₂ delivered by the 2 L NC, with the highest FIO₂ provided at the lowest VE studied, while PIF had only minimal impact. Various clinical conditions such as opiate overdose and central hyperventilation have a huge impact on the actual FIO₂ delivered.