Air-Entrainment-Calculations

Air-Entrainment Calculation Outline Air-Entrainment looks at how many parts of oxygen there is and how many parts to air there is to combine and make a specific FiO 2 1. Air-Entrainment Ratio (there are 3 ways to approach this) a. First is to know the formula i. Liters air Liters of O 2 = 100 − % O 2 %O2 − 21 ii. So, if I wanted an FiO 2 of .50 I would calculate: 1. Ratio = (100-50%)/(50%-21%) 2. Ratio = 50/29 = 1.7 3. Ratio = 1.7:1 b. Second way is to use the magic box a. This is can be less accurate but plug the same thing in for 50% 50/30 = 1.7 and ratio is 1.7:1 c. The third way is to just memorize the most common ones – either the ratios or just the “total parts.” The total parts are the two sides of the ratio added together The bolded ones are ones to know but 35, 40, and 50 are probably the most common. Even if you just remember total parts that will work, because most will just ask you find total flow. Take total parts and x flow. 50-20 = 30 100-50 = 50 50%

2. Total Flow a. Total flow = the total amount of flow that is being created by the air-entrainment device based on the FiO 2 and what you have the flow meter set to b. Total flow = Total ratio parts x Liter flow on meter i. If you have 50% and a liter flow of 8 L/min, what is the total flow? 1. Ratio is 1.7:1 2. Total parts = 2.7 (which is ratio parts added: 1.7 + 1) 3. Total flow = 2.7 x 8 L/min = 21.6 L/min ii. If you have 40% and liter flow of 10 L/min, what is the total flow? 1. Ratio is 3:1 2. Total parts = 4 3. Total flow = 4 x 10 L/min = 40 L/min 3. Patient inspiratory demand a. This can be estimated by taking VE and multiplying by 3 i. If the patient has a VE of 8 L/min, what is their inspiratory demand? 1. 8 L/min x 3 = 24 L/min ii. Technically the equation is VE x (sum of I:E) 1. The 3 comes from a normal I:E assumed of 1:2 (1+2=3) 2. If you are given a different I:E ratio, you should use that 3. If you have a VE of 8 L/min and an I:E of 1:4, then do: a. 8 x (1+4) = b. 8 x 5 = 40 L/min b. Inspiratory flow can also be determined by Vt/I time , but is less accurate when flow isn’t constant i. If the patient has a Vt of 500 mL and an I time of .8 sec, what is the inspiratory flow? 1. I’m going to convert from mL to L because I want the end result to be L/min. You can convert flow before or after. I’m going to it after here. 2. Flow = .500 L/.8 sec = 0.625 L/sec 3. Convert flow to L/min = 0.625 x 60 = 37.5 L/min 4. Determining what flow rate you need a. If you want to deliver 40% FiO 2 to a patient with a VE of 10 L/min, what is the minimum flow the meter should be set at? i. Minimum flow = Inspiratory Demand/Total Ratio Parts 1. Inspiratory demand = 10 x 3 = 30 L/min 2. Ratio = 3:1 and Total Parts = 4 3. Minimum flow = 30 L/min divided by 4 = 7.5, so really 8 L/min b. If you want to deliver 60% FiO 2 to a patient with a Vt of 600 mL and an I time of 1 sec, what flow needs to be set on the flowmeter? i. Inspiratory demand = Vt/I time = .600 L/1 sec = .600 L/sec = .600 L/sec x 60 = 36 L/min ii. Inspiratory demand = 36 L/min iii. Ratio = 1:1 and Total Parts = 2 iv. Minimum flow = 36 L/min divided by 2 = 18 L/min