A quick & easy way to initially set "T-Low" during Airway Pressure Release Ventilation is to use the "Expiratory Time Constant (RCexp)". The RCexp indicates alveolar emptying time and it takes at least 4 time constants for adequate alveolar emptying (~99%).
References state set T-Low to obtain a "Peak Expiratory Flow Rate Termination Point (T-PEFR)" at 50 to 75% of the measured "Peak Expiratory Flow Rate"
By setting T-Low based on one expiratory time constant one would believe that you would have obtained a T-PEFR 75% of the PEFR, (3/4 = 75%). However, in the 1st time constant the larger airways empty quicker due to decreased turbulent flow & the elastic recoil of the thorax.Thus contributes to ~ 63% of the total emptying of the lungs.
The additional 37% of the volume from the smaller airways empties out over the next three to four time constants.
So one can set the T-Low based on the RCexp and get close to the 50-75% range, however T-Low still needs to be continually reassessed & titrated based on T-PEFR & PEFR measurements.
The following example is from a Hamilton C2 ventilator, any ventilator may be used which calculates a RCexp or the practitioner can calculate.
note- all Hamilton Medical Platforms; C2, G5, & Galileo automatically calculates the RCexp
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| Figure 1: C2 monitoring screen showing a measured RCexp of 0.53 seconds. |
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| Figure 2: Setting T-Low based on the RCexp at 0.53 seconds. |
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| Figure 3: Viewing the pressure (yellow) & flow (purple) waveforms on the C2. |
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| Figure 4: Using the C2's screen freezing capabilities to analyze the expiratory flow waveform. |
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| Figure 5; Using the control knob to scroll & measure the PEFR -39. |
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| Figure 6: Scrolling back to the T-PEFR to obtain the measurement -17.2
In this example the T-PEFR is at ~ 44% of the PEFR (17 divided by 39), showing that ~ 56% of the delivered volume emptied in the first time constant. This gets the operator close to the ideal range at where T-low should be set at.
Setting T-Low based on the RCexp is a quick & easy way to initially set this parameter, however the operator still needs to continually assess the expiratory flow waveform for precise inputs.
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