1 (edited by rachael.stickland 2019-05-24 11:57:15)

Topic: Understanding Prep Stage

Can anyone point me towards the best materials/publications that I can read that will help me better understand the prep stage?

I understand that a good prep involves checking the end-tidal peaks are found adequately, and getting consistent breathing from your subject, but I would also like to better understand the algorithm used to arrive at the final baseline parameters, and wasn't sure where best to look for that. Typically, I'm seeing about +5mmHg increase from the first initially recorded end-tidal CO2 values (at start of prep) compared to the final targeted end-tidal CO2, e.g. 37mmHg at start of prep -> 42mmHg final target. I wondered if there were any recommendations for spotting whether the targeted baseline parameters were more - or less - plausible.



Re: Understanding Prep Stage

The prep stage is designed to measure parameters for the model of the subject’s physiology. There isn’t a reference to that stage that I can point to for you to read specifically on the prep stage. However, I can explain why the end-tidal PCO2 rises at the end.

With a resting calm subject, in the initial prep the gas exchange in the lungs is set by the subject’s tidal volume, dead space, breathing frequency and the metabolic production of CO2.  This stage should be with the subject relaxed. The key factor is the alveolar ventilation, which is tidal volume – dead space.  We breathe in and out via the same airways, so the conducting airways constitute a volume that contains expired (used) gas on inspiration.  Only after this airways volume (dead space) has been inspired do we get fresh air.

During sequential gas delivery (SGD) the RespirAct controls alveolar ventilation and the gas composition of the fresh air (alveolar air). In doing so it controls arterial PO2 and PCO2.  To see how sequential gas delivery works, look at Fisher, J.A., Iscoe, S., and Duffin, J. (2016). Sequential gas delivery provides precise control of alveolar gas exchange. Respir Physiol Neurobiol 225, 60-69.
When the RespirAct initiates SGD it sets the alveolar volume for each breath based on the measured tidal volume and the estimated subject’s dead space.  To allow for errors in the dead space estimate the alveolar volume is lowered by a fraction.  It is therefore less than the subject’s resting alveolar ventilation before SGD. Hence PCO2 rises because the elimination of CO2 via the lungs is less than it was.  To allow the RespirAct to best control PCO2, have the subject increase their tidal volume from what it was during the relaxed phase of prep before SGD.  I recommend the kind of slow deep breathing practiced for meditation. Implementing that voluntary control (if possible) after the SGD is established will improve targeting during the sequence.

3 (edited by rachael.stickland 2019-06-26 10:56:36)

Re: Understanding Prep Stage

Thanks for your reply. Just so I'm clear on the best recommendation to get best targeting:
- During relaxed stage of prep, consistent breathing from the subject but at their normal pace?
- During SGD stage of prep, ask participant to do slow deeper breathing to increase their tidal volume
- During SGD for a specific sequence, is it still best to advise participants to do slow deep breathing, or is that not necessary as the system should be able to adapt to that?


Re: Understanding Prep Stage

Sorry for the late reply,
Yes for the first two recommendations.
If it was me I would carry on with the slow deep breathing during the sequence but you don't have to except for targeting PCO2 below resting.
I know some investigators talk to their subjects all through the sequence coaching a regular deep breathing but others only during the hypocapnic portion.