In respiratory research a breathing apparatus consisting of mouthpiece, filter, Pneumotachometer, and non-rebreathing valves plus some connectors are usually used. Although a non-rebreathing valve is used to reduce dead space, each of these devices has its own dead space. Though small, adding together they build a relatively large dead space sometimes. What is the max acceptable dead space in a breathing apparatus, for a study including healthy adults?
it depends on the type of study. Large dead space can effect some gas exchange studies.Instrumental response in studies over short time intervals can be affected by dead space.
if on the other hand, only ventilatory responses over longer time periods are studied, dead space is less relevant, unless the studies are dependent on equilibria being reached in certain times, eg some rebreathing studies
I fully agree with the first answer. I would however like to suggest using two pneumotachographs, one in the inspiratory limb, one in the expiratory limb. The sum of flow rates of both gives you the total flow rate. This way, the pneumotachographs do not contribute to dead space. You may consider this principle also for filters etc.
Thanks for the responses and guides.
The study is on the effects of slow deep breathing on cardiovascular regulatory system. Each breathing trial is for 3 minutes. With about 250 ml of dead space we had some cases with higher than 6% PetCO2. This is while we expect to see lower PetCO2 with slow deep breathing, sometimes even lower than 4.6%. It seems that 250 ml is too large for such study.
That we can use two PNTs/filters, one at inspiration one at expiration port of the non-rebreathing valve, is a very good idea. Of course, there are some versions of such device with lower housing dead space but similar ID/OD of the orifices which doesn't increase the resistance.
Dear Ali
I would aim for 2 mL/kg bodyweight - this is the recommendation for (inert gas washout) lung function testing in young children (see the consensus work of Urs Frey and Paul Robinson)
Please be aware that if you reduce dead space, apparatus resistance may increase.
BTW slow deep breahting may lead to hypoventilation and thus higher end-tidal CO2... this however could be also an effect of CO2 re-breahting
Cheers
Florian
Dear Florian,
Thanks for your response.
The 2 ml/kg is a good suggestion and this is what we usually see in the literature; a breathing apparatus with dead space of between 100 and 140 for adults which makes sense.
Some devices like the non-rebreathing valve and the Pneumotachometer may have large housing dead space. Reducing the housing volume decreases dead space but keep the resistance similar as the orifices are similar. Also, those connectors must be taken into account and must be reduced as much as possible.
Usually slow deep breathing (higher Vt) improves ventilation and decreases PetCO2. Indeed one of the concerns during slow deep breathing is hyperventilation and we always give feedback to the subjects to not breath too deeply.
By modifying the apparatus now I have ~ 150 ml dead space and I don't see those high values of PetCo2 anymore.
It's really important for psycho-physiological studies of respiration to take care of the dead space and measure, analyze, and report it in the publications. Something which is missing in many reports.
Here the results of our study on the effect of using inspiratory threshold load during slow deep breathing on cardiovascular parameters, in this study dead space was small as possible ~ 150 ml
https://doi.org/10.1111/psyp.13447
the type of study affect the dead space .Instrumente used also affect. response in studies over short time intervals can be affected by dead space.
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