Venturi mask
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The venturi mask, also known as an air-entrainment mask, is a medical device to deliver a known oxygen concentration to patients on controlled oxygen therapy.[1][2] The mask was invented by Moran Campbell at McMaster University Medical School as a replacement for intermittent oxygen treatment, a practice he described as "bringing a drowning man to the surface – occasionally".
Mechanism
Venturi masks are considered high-flow oxygen therapy devices. This is because venturi masks are able to provide total inspiratory flow at a specified FiO2 to patients therapy. The kits usually include multiple jets, which are usually color-coded, in order to set the desired FiO2.
Other brands of masks have a rotating attachment that controls the air entrainment window, affecting the concentration of oxygen. This system is often used with air-entrainment nebulizers to provide humidification and oxygen therapy.
The mechanism of action is usually incorrectly quoted as depending on the venturi effect. Despite there being no evidence for this, many textbooks and journal articles cite this as the mechanism. However, a fixed performance oxygen delivery system, despite often being called a venturi mask works on the principle of jet mixing.[3][4]
Flow problems
Air entrainment masks, although considered high flow systems, are not always able to guarantee the total flow with oxygen percentages above 35% in patients with high inspiratory flow demands. The problem with air entrainment systems is that as the FiO2 is increased, the air to oxygen ratio decreases. For example, for 30% the ratio is eight parts air to one part oxygen.
For 40% the ratio decreases to 3:1. Since the jets in venturi masks generally limit oxygen flow to 12 to 15 liters per minute, the total flow decreases as the ratio decreases.
At an oxygen flow rate of 12 liters per minute and a 30% FiO2 setting, the total flow would be 108 L/min. At a 40% FiO2 setting, the total flow would decrease to 48 L/min.
References
- ^ Use of a reservoir nasal cannula in hospitalized patients with refractory hypoxemia; Sheehan, JC, O'Donohue, WJ; Chest. 1996; 110:s1.
- ^ Bateman NT, Leach RM (1998). "ABC of oxygen. Acute oxygen therapy". BMJ. 317 (7161): 798–801. doi:10.1136/bmj.317.7161.798. PMC 1113909. PMID 9740573.
- ^ Kittredge P (1983). "Neither Venturi nor Bernoulli". Lancet. 1 (8317): 182. doi:10.1016/s0140-6736(83)92779-4.
- ^ Scacci R (1979). "Air entrainment masks: jet mixing is how they work; the Venturi and Bernoulli principles are how they don't". Respir. Care. 24: 928–931.