Spirometer Oxygen Consumption Rate Calculator
Calculate VO₂ from volume-time traces with STPD correction
How to Calculate Rate of Oxygen Consumption from a Spirometer Trace
Calculating the rate of oxygen consumption ($\dot{V}O_2$) is a fundamental procedure in physiology and metabolic testing. When using a closed-circuit spirometer containing a carbon dioxide ($CO_2$) absorbent (such as soda lime), the total volume of gas in the system decreases over time. This decrease occurs because the subject consumes oxygen from the system, while the exhaled $CO_2$ is chemically removed, preventing it from returning to the spirometer bell.
The resulting spirometer trace (kymograph) displays a tidal breathing pattern with a noticeable downward slope. This calculator helps you determine the rate of oxygen uptake by analyzing the gradient of this slope and applying the necessary physics corrections.
Step 1: Analyzing the Trace (ATPS)
The raw data obtained from the spirometer is known as ATPS (Ambient Temperature and Pressure, Saturated). To calculate the uncorrected rate:
- Select two points: Choose two peaks (end-inspiration) or two troughs (end-expiration) on the trace that are significantly far apart to minimize measurement error.
- Measure Volume Change ($\Delta V$): Calculate the vertical difference between these two points on the Y-axis. This represents the volume of oxygen consumed.
- Measure Time Elapsed ($\Delta t$): Calculate the horizontal difference between the points on the X-axis.
- Calculate Slope: Divide the volume change by the time elapsed ($\Delta V / \Delta t$) to get the rate in mL/sec or mL/min.
Step 2: Correcting to STPD
Gas volumes are sensitive to temperature and pressure (Charles's Law and Boyle's Law). Because the gas inside the spirometer is at room temperature and ambient pressure, it occupies a different volume than it would under standard scientific conditions. To report standardized metabolic rates, you must convert the ATPS value to STPD (Standard Temperature, Pressure, Dry).
The STPD correction factor is calculated using the following variables:
- Ambient Temperature ($T$): The temperature of the room/spirometer in Celsius.
- Barometric Pressure ($P_{bar}$): The atmospheric pressure in mmHg.
- Water Vapor Pressure ($P_{H2O}$): The pressure exerted by water vapor at the given temperature (calculated automatically by this tool).
The formula used is: $$V_{STPD} = V_{ATPS} \times \left( \frac{273}{273 + T} \right) \times \left( \frac{P_{bar} – P_{H2O}}{760} \right)$$
Clinical Significance
The rate of oxygen consumption is directly related to the metabolic rate. Under resting conditions, it represents the Basal Metabolic Rate (BMR). A typical resting $\dot{V}O_2$ for a healthy adult is approximately 250 mL/min, though this varies significantly with body size, age, and gender.