Difficult airways can’t be reliably predicted

This paper1 proves what Rich Levitan has been saying (and writing) for years – that there is no method of prediction of difficult intubation that is both highly sensitive (the test wouldn’t miss many difficult airways) and highly specific (meaning those predicted to be difficult would indeed turn out to be difficult). Most importantly, this means one should always have a plan for failure to intubate and failure to mask-ventilate regardless of how ‘easy’ the airway may appear.

This study of a large prospectively collected database captured anaesthetists’ clinical assessment of likelihood of difficult intubation and difficult mask-ventilation, and compared them with actual findings. These studies are always difficult, due in part to the lack of standard definitions of difficult airways, but the take home was clear – the large majority of difficulties were unanticipated and not suspected from pre-operative clinical assessment.

This issue was brilliantly summed up by Yentis in a 2002 Editorial2:

I dare to suggest that attempting to predict difficult intubation is unlikely to be useful – does that mean one shouldn’t do it at all? To this I say no, for there is another important benefit of this ritual: it forces the anaesthetist at least to think about the airway, and for this reason we should encourage our trainees (and ourselves) to continue doing it.”

1. Diagnostic accuracy of anaesthesiologists’ prediction of difficult airway management in daily clinical practice: a cohort study of 188 064 patients registered in the Danish Anaesthesia Database
Anaesthesia. 2014 Dec 16. doi: 10.1111/anae.12955. [Epub ahead of print]


Both the American Society of Anesthesiologists and the UK NAP4 project recommend that an unspecified pre-operative airway assessment be made. However, the choice of assessment is ultimately at the discretion of the individual anaesthesiologist. We retrieved a cohort of 188 064 cases from the Danish Anaesthesia Database, and investigated the diagnostic accuracy of the anaesthesiologists’ predictions of difficult tracheal intubation and difficult mask ventilation. Of 3391 difficult intubations, 3154 (93%) were unanticipated. When difficult intubation was anticipated, 229 of 929 (25%) had an actual difficult intubation. Likewise, difficult mask ventilation was unanticipated in 808 of 857 (94%) cases, and when anticipated (218 cases), difficult mask ventilation actually occurred in 49 (22%) cases. We present a previously unpublished estimate of the accuracy of anaesthesiologists’ prediction of airway management difficulties in daily routine practice. Prediction of airway difficulties remains a challenging task, and our results underline the importance of being constantly prepared for unexpected difficulties.

2. Predicting difficult intubation–worthwhile exercise or pointless ritual?
Anaesthesia. 2002 Feb;57(2):105-9

High flow systems for apnoeic oxygenation

nascaniconApnoeic oxygenation during laryngoscopy via nasal prongs has really taken off in the last couple of years in emergency department RSI, and is associated with decreased desaturation rates in out-of-hospital RSI.

More effective oxygenation and a small amount of PEEP can be provided by high flow nasal cannulae with humidified oxygen (HFNC)

A logical step in the progression of this topic is to consider HFNC for apnoeic oxygenation, and Reuben Strayer wrote about this nearly three years ago.

In a Twitter conversation today, Dr Pete Sherren highlighted a new article describing its use in anaesthesia for patients with difficult airways. This is labelled Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE). A reply from Dr Neil Brain points out that when used in kids, the bulkiness of the apparatus may get in the way of bag-mask ventilation (if that becomes necessary).

But does HFNC apnoeic oxygenation confer any advantages over standard nasal cannulae?

In an apnoeic patient, 15l/min via standard cannulae should fill the pharyngeal space with 100% oxygen, and you can’t improve on 100%.

HFNC provide some continuous positive pressure, but this may be cancelled by the necessary mouth opening for laryngoscopy.

One issue with apnoea is of course a rise in carbon dioxide with consequent acidosis. The authors of the THRIVE paper (abstract below) point out that in previous apnoeic oxygenation studies, the rate of rise of carbon dioxide levels was between 0.35 and 0.45 kPa/min (2.7-3.4 mmHg/min), whereas with THRIVE the rise was 0.15 kPa/min (1.1 mmHg/min). They suggest that continuous insufflation with high flow oxygen facilitates oxygenation AND carbon dioxide clearance through gaseous mixing and flushing of the deadspace.

So should we switch from standard nasal cannula to high flow cannulae for apnoeic oxygenation? I think not routinely, but perhaps consider it in patients:

(1) with pressure-dependent oxygenation (eg. ARDS) although I’m not sure any CPAP effect would be sustained during laryngoscopy

and

(2) in patients with significant acidosis in whom a significant rise in carbon dioxide could be detrimental (eg. diabetic ketoacidosis).

I look forward to reading more studies on this, and to hearing from anyone with experience of this technique in the comments section.

Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE): a physiological method of increasing apnoea time in patients with difficult airways.
Anaesthesia. 2014 Nov 10. doi: 10.1111/anae.12923. [Epub ahead of print]


Emergency and difficult tracheal intubations are hazardous undertakings where successive laryngoscopy-hypoxaemia-re-oxygenation cycles can escalate to airway loss and the ‘can’t intubate, can’t ventilate’ scenario.

Between 2013 and 2014, we extended the apnoea times of 25 patients with difficult airways who were undergoing general anaesthesia for hypopharyngeal or laryngotracheal surgery. This was achieved through continuous delivery of transnasal high-flow humidified oxygen, initially to provide pre-oxygenation, and continuing as post-oxygenation during intravenous induction of anaesthesia and neuromuscular blockade until a definitive airway was secured. Apnoea time commenced at administration of neuromuscular blockade and ended with commencement of jet ventilation, positive-pressure ventilation or recommencement of spontaneous ventilation. During this time, upper airway patency was maintained with jaw-thrust.

Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) was used in 15 males and 10 females. Mean (SD [range]) age at treatment was 49 (15 [25-81]) years. The median (IQR [range]) Mallampati grade was 3 (2-3 [2-4]) and direct laryngoscopy grade was 3 (3-3 [2-4]). There were 12 obese patients and nine patients were stridulous. The median (IQR [range]) apnoea time was 14 (9-19 [5-65]) min. No patient experienced arterial desaturation < 90%. Mean (SD [range]) post-apnoea end-tidal (and in four patients, arterial) carbon dioxide level was 7.8 (2.4 [4.9-15.3]) kPa. The rate of increase in end-tidal carbon dioxide was 0.15 kPa.min(-1) .

We conclude that THRIVE combines the benefits of ‘classical’ apnoeic oxygenation with continuous positive airway pressure and gaseous exchange through flow-dependent deadspace flushing. It has the potential to transform the practice of anaesthesia by changing the nature of securing a definitive airway in emergency and difficult intubations from a pressured stop-start process to a smooth and unhurried undertaking.

London Cardiac Arrest Symposium 2014

The focus of the entire day is cardiac arrest and this is the second day of the London Cardiac Arrest Symposium.

Professor Niklas Nielsen kicked off with a presentation of his Targeted Temperature Management trial.  It seems that even now there is uncertainty in the interpretation of this latest study. I take heart from the knowledge that Prof Nielsen has changed the practice of his institution to reflect the findings of his study – I have certainly changed my practice. But we need to remain aware that there is more work to be done to answer the multiple questions that remain and the need for further RCTs is recognised.

The management of Cardiac arrest after avalanche is not a clinical scenario that I imagine I’ll ever find myself in. The management is well documented in the ICAR MEDCOM guidelines 2012. Dr Peter Paal reminded us that you’re not dead until you’re rewarmed and dead unless: with asystole, CPR may be terminated (or withheld) if a patient is lethally injured or completely frozen, the airway is blocked and duration of burial >35 min, serum potassium >12 mmol L(-1), risk to the rescuers is unacceptably high or a valid do-not-resuscitate order exists.

The age old question about prognostication after cardiac arrest was tackled by Prof Mauro Oddo. He covered the evidence for clinical examination, SSPE, EEG, and neurone specific enolase. Bottom line, all of these modalities are useful but none are specific enough to be used as a stand alone test so multiple modalities are required.

SAMU is leading the way with prehospital ECMO. They have mastered the art of cannulation (in the Louvre no less!) but there haven’t enough cases to demonstrate a mortality benefit. The commencement of ECMO prehospital reduces low flow time and theoretically should improve outcomes. This is begging for a RCT.

The experience of the Italians with in hospital ECMO shoes a better survival rate for in-hospital rather than out of hospital cardiac arrests, explained Dr Tomasso Mauri. They treat patients with a no flow time of <6min and low flow rate of <45min and had a 31% ICU survival rate. If you want to learn more about ED ECMO go to http://edecmo.org.

VA-ECMO

The Douglas Chamberlain lecture this year was Selective aortic arch perfusion presented by Prof James Manning. He spoke about the use of this technique in cardiac arrest and also in trauma (where it is known to you as Zone 1 REBOA).

image-1

In cardiac arrest the aim is to improve coronary perfusion, to preserve perfusion to the heart and the brain, offer a route of rapid temperature control and offer a direct route of administration of adrenaline. Coronary perfusion is seen to be supra normal after SAAP. And the suggested place for SAAP is prior to ECMO.

image-5

It’s more familiar ground talking about SAAP in trauma. This Zone 1 occlusion preserves cerebral and cardiac perfusion while blood loss is limited and rapid fluid resuscitation can occur.

image-3

You can hear Prof Manning on SAAP over at EMCrit (of course!). 

It’s been another great conference. Put the dates for next year’s London Trauma & Cardiac Arrest Conferences in your diary: 8th-10th December 2015!

Happy Holidays & Keep Well

Louisa Chan

 

 

 

 

 

London Trauma Conference 2014 Part 2

Day three is Air Ambulance and pre-hospital day and the great and the good are here en mass.
The heavy weights are coming out to make their points…..

selfUnarguably the best lecture of the day was delivered by our very own Cliff Reid on prehospital training. Using Sydney HEMS induction training he highlighted the challenges posed to prehospital services training doctors and paramedics rotating through the service.

Turning a good inhospital doctor into a great prehospital one in the space of an induction program requires focus. Knowledge is therefore not the focus of training, performance is. Often doctors already possess the clinical skills and knowledge and it is the application of these pre existing skills in challenging environments when cognitively overloaded that is the key.

 

The Sydney HEMS program provides the mindware and communication skills the practitioner needs to do this and drills these skills in simulated environments. He uses perturbation, so like the Bruce protocol exercise test the simulations just get harder until you are at the very limits of your bandwidth. Debriefing of course is important but the recommended protracted debrief is often impractical and unnecessary so simulations designed with cognitive traps are used to highlight learning points and are drilled until the message is received. In this way tress exposure enhances cognitive resilience. And importantly they use cross training, so the doctors and the paramedics undergo the same program so each member of the team understands the challenges faced by the other.

Does this sound like fun? For the shrinking violets out there it could be seen as threatening. But for the adrenaline junkies…….hell yeah!

It’s truly a training ethos that I buy into and I’d love to be able to achieve that standard of training in my own service.

SydneyHEMStops
Sydney HEMS Friends and Colleagues at the LTC

 

mwaveMicrowaves seem to be the future if diagnostic testing. This modality is fast, is associated with a radiation dose lower than that of a mobile phone, non invasive, portable and has been shown to provide good information. It can be used on heads for intracranial haemorrhage and stroke or chests for pneumothorax detection. It’s all in the early stages but seems like it will be a viable option in the future.
For further reading check out:
Diagnosis of subdural and intraparenchymal intracranial hemorrhage using a microwave-based detector
Clinical trial on subdural detection
Pneumothorax detection

 

How would you transfer a psychotic patient requiring specialist intervention that can only be received after aeromedical transfer? Stefan Mazur of MedSTAR, the retrieval service in South Australia shared their experience with ketamine to facilitate the safe transfer of these patients with no reports of adverse effects on the mental state of the patient, as first described by Minh Le Cong and colleagues. Is there no end to the usefulness of this drug? No wonder we’re experiencing a supply issue in the UK!

And finally, the ultimate reflective practice should include the post mortem of our critically sick patients. The approach the forensic pathologist takes is similar to a clinician (with the time pressure removed). They read the scene and use this information to predict injuries (sound familiar?). Post mortem CT scanning with recon provides yet another layer of information. We are missing a trick if we don’t seek this feedback to correlate with our clinical findings. Even better, rare practical skills are often routinely performed as part of the post mortem – we should be making use of this opportunity to train.

London Trauma Conference 2014 Part 1

I’ve travelled almost the entire length of England to get to the London Trauma Conference this year. What could be more important than attending one of the best conferences of the year? Examining for the DipRTM at the Royal College of Surgeons in Edinburgh

So was it worth the 4am start? Absolutely!

tomMy highlights would be Tom Evens explaining why trauma can be regarded like an elite sport. His background is as a sports coach in addition to his medical accomplishments and walking us through the journey he went through with the athlete he was coaching demonstrates the changes that need to occur when cultivating a performance culture and the results speak for themselves.

I can see similarities in the techniques used by athletes and those we are using in medicine now. Developing a highly performing team isn’t easy as anyone involved in the training of these teams will know.

 

 

jerry3Dr Jerry Nolan answered some questions about cervical spine movement in airway management. The most movement is seen in the upper cervical spine and there is no surprise that there is an increased incidence of cervical spine injury in unconscious patients (10%). The bottom line is that no movement clinicians will make of the cervical spine is greater than that at the time of injury. And whether it be basic airway manoeuvres, laryngoscopy or cricoid pressure the degree of movement is in the same ball park and unlikely to cause further injury. He states that he would use MILS like cricoid pressure and have a low threshold for releasing it if there are difficulties with the intubation. Of course many of us don’t use cricoid pressure in RSI anymore………..

 

After watching Tom and Jerry we heard that ATLS has had its day. Dr Matthew Wiles implores us to reserve ATLS for the inexperienced and move away from this outdated system and move to training in teams using local policies. The Cochrane reviewers found an increase in knowledge but no change in outcomes.

And finally Dr Deasy has convinced me that I will be replaced by a robot roaming around providing remote enhanced care. On the up side I might be the clinician providing that support.

More from me on this fantastic conference soon. In the meantime follow it on Twitter!

telemed