Time Constants
Time Constant = Resistance x Compliance
A time constant is the time it takes for a 63% change in lung volume. Three time constants account for 95% of the volume change. Time constants can be used for inspiration as well as exhalation; however, expiratory time constants tend to be more accurate and useful for determining compliance due to exhalation typically being a passive phase. Increased compliance or resistance leads to longer or shorter time constant values and different lung areas may have varying time constants. This is especially important for patients with obstructive diseases like BPD and COPD.
Calculating time constants helps you:
- Estimate the maximum respiratory rate
- Optimize ventilator settings
- Determine lung mechanics
How to calculate:
- Record flow and volume scalars.
- Identify v1 (inspiratory termination point) on the volume scalar.
- Multiply v1 by 0.37 to find v2.
- Find the time values (t1 and t2) for v1 and v2.
- Subtract t1 from t2 to find the time constant (Tc).
- Multiply Tc by 3 to find the minimum required expiratory time.
To further optimize you can:
- Add your set inspiratory time to the required expiratory time to find total cycle time.
- Divide 60 by total cycle time for the maximum respiratory rate before air trapping becomes a concern.
Keep in mind:
- You must use passive mechanical breaths and avoid breaths that have active exhalation as this will interfere with the accuracy of your calculations.
- Suction the patient before recording waveforms.
- Avoid using points affected by circuit recoil (spikes).
I will be posting a video tutorial on this soon. I hope this helps!