Apnoeic Oxygenation and THRIVE

27/11/2014

I remember in the early days of my anaesthetic training (not that long ago) when I first heard about the concept of apnoeic oxygenation. It was in fact our very own John Weeks that pointed me in the direction of NODESAT (Nasal Oxygenation During Efforts to Secure a Tube), and the paper by Drs Weingart and Levitan discussing approaches to reduce the incidence of desaturation occurring during intubation of critically ill patients (1). Preoxygenation/denitrogenation was one of the first things taught to us when we started to learn about rapid sequence induction of anaesthesia (RSI), giving that extra time to secure an airway before the tone of the sats probe started its precipitous decline, but this was the only real technique used to extend this time. But then I read this paper that describes how your can maintain oxygen saturations for (potentially) ridiculous lengths of time without the need to ventilate patients. Mind blown. Unfortunately, nothing really happened about it. I have still yet to see the techniques described used by other clinicians I was working with, and I was certainly a little too junior to start trying some maverick techniques. RSI was thio, sux, tube. As such, it was a rather pleasant surprise to spot this paper being mentioned on Twitter recently: Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE). Indeed it is a pretty prominent anaesthetic journal (in the form of Anaesthesia that) has published it:

Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): a physiological method of increasing apnoea time in patients with difficult airways

What's it About?

Apnoeic oxygenation is not a particularly new concept in the anaesthetic literature, apparently first described over a century ago. During apnoea oxygen is still being taken up by the pulmonary capillaries at rate of about 250ml/min. When compared to the mere 8-20ml/min of carbon dioxide going the other way, it’s clear that a pressure gradient will be created by this imbalance. This negative pressure can pull in gas from the upper airway and if this is sufficiently oxygen rich, oxygenation of the blood can be maintained without ventilation (up to 100 mins has been described). The lack of CO2 clearance results in an inevitable worsening hypercapnia and acidaemia, but it seems this has minimal impact in the short term.

The paper looks at a particular method of providing this in the form of the OptiFlow system, providing transnasal humidified oxygen at high flow rate (70 L/min). The study looked at a group of 25 patients who were known or suspected high risk of difficult intubation and/or rapid desaturation at induction. They were preoxygenated, had anaesthesia induced, and apnoeic oxygenation continued whilst the airway was secured, with airway opening maneuvers still being maintained. Because of the difficult nature of the airways, this resulted in fairly significant apnoea times, but this wasn’t artificially extended beyond clinical need. They measured a variety of parameters through this, most notably the lowest O2 sats that resulted and the end tidal CO2 recorded at the recommencement of ventilation.

What Did They Find?

They recruited 25 patients who were undergoing a variety of head and neck procedures, including some with stridor on presentation. There were no desaurations below 90% despite an average apnoea time of 17 mins. Indeed two patients undergoing pharyygolaryngeal surgery had their whole procedure using THRIVE, with apnoea durations of 32 and 65 mins.

The authors also used the end tidal CO2 (or in a few cases arterial PCO2) values to estimate the rate of climb in arterial CO2 levels during this apnoea, coming up with a value of 0.15 kPa/min, somewhat less than several earlier studies that they describe. There were, reassuringly, no episodes of complications arising from the raised PaCO2 or acidaemia

Is It Any Good?

I think it’s clear from a quick read through that this isn’t the most rigorous of clinical studies. Indeed it is essentially just a case series with a brief description and discussion of their results without any control group for comparison. They have simply gone ahead with what appears to be their normal clinical practice for these difficult to intubate patients and added this technique to see if it stops them needing to bag the patients in between attempts to secure the airway. In fairness they have done their best to standardise some aspects of the anaesthetic they gave; positioning, duration of preoxygenation, and drug doses (roughly) were standardised between patients. They have also made some description of the patient characteristics, though this probably could have been more extensive. Overall though, both the size and the design mean that this is a pretty low quality study, potentially limiting what we can take from it.

Final Thoughts

I can hear the question; why have we just spent the past few paragraphs talking about an article that is perhaps as weak as you get in terms of scientific rigor? A fair enough question, but I would argue that these are still exciting results. No matter how limited the study, the authors are providing some evidence that you can do a pretty good job of oxygenating patients, temporarily at least, without ventilating them. 17 minutes is not exactly a marginal gain either. Even better is that you can do this whilst you faff around trying to get a tube in. It’s clear how this has been such an attractive prospect in the critical care world where there are patients that are hypoxic on a high FiO2 even before you make them apnoeic with your RSI, and there are definitely patients that we come across in general anaesthetic practice (chest pathology, high BMI, pregnancy) who can desaturate in the blink of an eye and might not be a straightforward laryngoscopy.

Perhaps even more exciting than this trial getting the platform of a major anaesthetic journal is the talk coming from the difficult airway society (DAS) recently (3). They are in the process of updating their intubation guidelines and have dropped hints that nasal oxygenation is something that might become recommended practice (amongst some other exciting changes). To me it seems the potential benefits are great, even if the cases when it will be important are small in number, as an extra layer of safety is never a bad thing. I can think of many theoretical scenarios, and indeed cases that I have had, where an extra bit of time would have been a great help in reducing the stress of a difficult intubation. I am excited that this may become a much more widespread approach to RSI in the future, even if I'm a bit surprised it hasn't already made there already.

Thanks again for reading and please let me know your thoughts on both apnoeic oxygenation and this paper, especially if it is something you are using currently in critical care of general anaesthetic practice. If you’ve not heard much about it before some of the links below are well worth a read. Bye for now.

Tom Heaton