The Venturi valve increases the flow rate of oxygen and air to above the patient's inspiratory flow rate which is why these masks are more noisy (Figure 2.2). By doing this it can provide a constant oxygen concentration no matter how quickly or slowly the patient breathes. Venturi masks have colour-coded valves (labelled 24%, 28%, 35%, 40%, and 60%), which are
Total gas flow 30 litres per minute t
Total gas flow 30 litres per minute
Jet of O2 2 litres per minute
^ Entrained 14 litres per minute air each side
Figure 2.2 Velocity or flow of gas in a Venturi mask. Bernoulli's equation for incompressible flow states that 0-5 pv2 + P = constant (where p is density), so, if the pressure (P) of a gas falls, it gains velocity (v). When gas moves through the Venturi valve, there is a sudden pressure drop, owing to the increase in area. The velocity or flow of the gas increases according to this equation designed to deliver a fixed percentage of oxygen when set to the appropriate flow rate. To change the oxygen concentration, both the valve and flow have to be changed. At oxygen requirements over 40%, the mask may not have enough total flow to meet high inspiratory demands. Venturi masks are the mask of choice in acutely ill patients who require controlled oxygen therapy.
Flow is not the same as concentration. Low flow masks can deliver high concentrations of oxygen and high flow masks can deliver low concentrations of oxygen. Therefore the terms "high concentration" and "low concentration" are preferred when speaking about the amount of oxygen a patient is receiving. Humidified oxygen should be given when prolonged high concentrations are used as the airways may become dry. This impedes the expectoration of secretions, which can be extremely important in certain situations.
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