8 Vital Capacity Breaths is Better for Pre-Oxygenation Than 3 Minutes of Tidal Volume Breathing

Remi

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Interesting article here from the Journal of Anaesthesiology Clinical Pharmacology: Comparison of margin of safety following two different techniques of pre oxygenation. The full text is free.

This is a simple, well-done RCT that shows a very significantly prolonged time to desaturation (over 3 minutes longer) among patients who were pre oxygenated by taking 8 vital-capacity breaths, vs. those who breathed oxygen for 3 minutes at normal tidal volume.

I would not have expected such a dramatic difference, but apparently the few VC breaths are just more effective at replacing the nitrogen in the FRC with oxygen.

Abstract

Background and Aims: Shortening the duration of efficacious preoxygenation would provide benefit in emergency situations like fetal distress etc. This study aims to compare the margin of safety following preoxygenation using 8 vital capacity breaths (VCB) in 1 min and tidal volume breathing (TVB) for 3 min, by assessing changes in PaO2 and apnea induced desaturation time.

Material and Methods: Patients were randomly divided into Group A and B. In Group A, 3 min of TVB using O2 flow of 5 l/min and in Group B, 8 VCB in 60 s using O2 flow of 10 l/min were used. Anesthesia was induced in all patients with propofol followed by succinylcholine 2 mg/kg intravenously. Mask ventilation was not done and following intubation endotracheal tube was kept open to atmosphere. The time taken for the patients to desaturate to 90% was noted and immediately ventilation was resumed. Arterial blood gas samples were taken while patients were breathing room air, immediately after preoxygenation and at 90% desaturation.

Results: Baseline PaO2 of both the groups were comparable. After preoxygenation Group B had a significantly high PaO2 value than Group A (439.05 ± 62.20 vs. 345.16 ± 20.80). At 90% desaturation there was no significant difference between groups. Group B showed a significantly high apnea induced desaturation time when compared to Group A (6.87 ± 1.78 vs. 3.47 ± 0.38 min).

Conclusions: Preoxygenation by 8 VCB in 1 min provides a greater margin of safety, as it results in a significantly high PaO2 with an almost doubled apnea induced desaturation time, in comparison with TVB for 3 min.
 
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blindsideflank

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TVB was only at 5lpm. I think every breath would draw in ambient air so you would never have the opportunity to really denitrogenate the lungs...
I still believe 3 mins on a non rebreather would be at least equal to the vital capacity breaths.

I think denitrogenate is a word?!?!...
 

Remi

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TVB was only at 5lpm. I think every breath would draw in ambient air so you would never have the opportunity to really denitrogenate the lungs...
In a closed circuit like the one used in the study, 5lpm is plenty of flow to provide >0.95 Fi02. With a BVM you would probably have to double or triple the flow to get a similar Fi02, but you could still come close.

I still believe 3 mins on a non rebreather would be at least equal to the vital capacity breaths.
So what do you think about the VCB group having almost double the time to desaturation?

Do you think the authors are lying about their findings?
 
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blindsideflank

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I definitely misread the methods regarding a 3L reservoir bag. Does this mean there wasn't any outside air entrained into the system? If so then I wonder what would explain this. Greater alveolar recruitment? I wonder how CPAP would compare.

And no, I do not think the authors are lying. I had thought that at 5lpm on a system that was delivering an fio2 closer to 0.35 would be inferior to a non rebreather at 10-15lpm. I'm not sure why you would come at me like that
 

Remi

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I wasn't "coming at" you at all. Your statement about TV breathing being just as good made it seem like you were rejecting the findings of this study, and given the straightforwardness of the data presented, I couldn't think of any reason why you would do that aside from simply not believing the authors for some reason. So I suppose I was being a bit of a smart*** with my reply, but I wasn't trying to be quarrelsome - just making a point with my normal sarcastic style. My apologies.

With a rebreathing circuit ( or "closed circuit", or "circle system") like the ones used in the OR, exhaled oxygen is rebreathed by the patient rather than being simply vented to the atmosphere, as it is with a non-rebreathing system. This means that even when flow demands and minute volume are high, you can still provide a very high Fi02 even with relatively low gas flows. In the OR it's common to run 2 liters of oxygen during a case and still have an Fi02 greater than 95%.

Of course, this does assume that you have a good enough mask seal that you aren't entraining large amounts of room air, but that's a problem no matter what type of breathing circuit or pre-oxygenation method you are using. If the patient is taking in more than just a little bit of room air with each breath, then the oxygen is being diluted and the Fi02 is going to be significantly lower.

The fact that we don't have rebreathing systems in the field but doesn't really matter, because the whole point is just to supply a high Fi02 and enough flow to match the patient's demand. And while you do need a lot more flow to do that with a BVM or NRB than with a rebreathing system, you should be able to do it with a high enough flow rate (15-25 l/min).

I doubt alveolar recruitment was a factor here, because these were healthy, non-obese patients. So while CPAP would probably have resulted in a higher Pa02, I doubt it would have extended the time to desaturation significantly as compared to the VC group, because I doubt it would do a better job of oxygenating the FRC than the VC breaths did.
 
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