Understanding Venturi Mask Flow Calculations
This calculator determines the Total Flow delivered to a patient when using an Air-Entrainment Mask (AEM), commonly known as a Venturi mask. Unlike standard nasal cannulas, Venturi masks are high-flow devices designed to deliver a precise Fraction of Inspired Oxygen (FiO2) regardless of the patient's respiratory pattern.
To ensure the patient receives the exact prescribed oxygen concentration, the total flow delivered by the mask must exceed the patient's peak inspiratory flow demand (typically 40-60 LPM during respiratory distress).
The Magic Box Formula
To calculate the total flow, we first determine the Air-to-Oxygen entrainment ratio using the standard formula:
Note: While some clinical shortcuts use 20 for room air, this calculator uses 21 for greater mathematical precision.
How to Calculate Total Flow
- Calculate the Ratio: Determine how many parts of air are entrained for every 1 part of oxygen.
- Sum the Parts: Add the air parts and the oxygen part (1) together to get the Total Parts.
- Multiply by Flow: Multiply the Total Parts by the Oxygen Flowmeter setting (LPM).
Example Calculation
If a patient requires 40% FiO2 and the flowmeter is set to 8 LPM:
- Step 1 (Ratio): (100 – 40) / (40 – 21) = 60 / 19 ≈ 3.16 parts air to 1 part O2.
- Step 2 (Total Parts): 3.16 (Air) + 1 (O2) = 4.16.
- Step 3 (Total Flow): 8 LPM (Input) × 4.16 = 33.3 LPM Total Flow.
Standard Air Entrainment Ratios
Below are common Venturi mask settings and their approximate entrainment ratios:
| FiO2 (%) | Air : O2 Ratio | Total Parts |
|---|---|---|
| 24% | 25 : 1 | 26 |
| 28% | 10 : 1 | 11 |
| 31% | 7 : 1 | 8 |
| 35% | 5 : 1 | 6 |
| 40% | 3 : 1 | 4 |
| 50% | 1.7 : 1 | 2.7 |
| 60% | 1 : 1 | 2 |
Why is Total Flow Important?
If the Total Flow calculated is less than the patient's inspiratory demand, the patient will draw in room air around the mask ports during inspiration. This dilutes the oxygen concentration, meaning the patient receives a lower FiO2 than intended. To correct this, the Respiratory Therapist must increase the oxygen flow rate on the flowmeter, which proportionally increases the entrained air and total flow while maintaining the same FiO2.