the metablog

Posts tagged feature.

Five things I love about the C-MAC

Ah, videolaryngoscopes – wonderful toys increasingly common in operating rooms, intensive care units and emergency departments throughout the world.

After the first video laryngoscope was conceived by New York emergency physician Jon Berall in 1998, commercial success quickly followed with the 2001 Glidescope (designed by a vascular surgeon nonetheless!). Our love of the videolaryngoscope has grown ever since.

Like many anaesthetists and anesthesiologists I've used several different videolaryngoscopes over the past few years. The character that sets apart the videoscopes I like to use from those I do not is how well they enhance airway techniques I use every day rather than requiring a new technique specific to their device.

Overwhelmingly this comes down to how similar the laryngoscope is to a standard Macintosh blade rather than inventing a whole new shape. While there may be theoretical (or even real!) benefits to increasing the angle of the blade or adding extra conduits, when I have a difficult airway I want to augment the tools and techniques I use every day rather than change to something completely different. (When I want to change to something completely different I'll pick up a fibreoptic bronchoscope or something sharp!)

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Threshold concepts: a gateway drug for clinical teaching

I recently learned of something in educational theory that lead me to a small epiphany: threshold concepts.

Critical care specialties are full of threshold concepts.

What is a threshold concept?

The idea of a threshold concept in learning was first introduced by Meyer and Land1, focusing on economics education. They described this as concepts that "...once understood, transform perception of a given subject."

A threshold concept provides a gateway to a greater, more in-depth understanding of an area – but is often difficult to master. Meyer and Land described a threshold concept as having:

"...the notion of transformation (in which students change the way they perceive and practice aspects of their discipline), irreversibility (once learnt rarely forgotten or 'unlearnt'), integrative (whereby connections are made to concepts or knowledge of previously unknown or concealed areas), bounded (in that they help define the boundaries of a subject area) and potentially troublesome." K. Connan (2014)

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  1. Meyer J H F and Land R 2003 "Threshold Concepts and Troublesome Knowledge – Linkages to Ways of Thinking and Practising" in Improving Student Learning – Ten Years On. C.Rust (Ed), OCSLD, Oxford. [ pdf

The Cardiology Referral: Avoid hypoxia, avoid hypotension?

Recently I needed to refer a patient preoperatively to a cardiologist for review. This is not an uncommon situation – one which happens thousands of times every week throughout the world. And yet it is a referral that anesthesiologists and anesthetists often do very poorly.

Avoid hypoxia, avoid hypotension?

We sometimes roll our eyes at recommendations made by physician colleagues: either providing unhelpful physiological parameters that we normally aim to maintain anyway (“avoid hypoxia?”), or stepping outside their expertise and boxing the anesthetist in by suggesting specific anesthetic techniques (“okay for a spinal”).

It is easy to be annoyed at both the lack of value this adds to our perioperative planning, as well as to the nonchalant ignorance of intraoperative medicine that it betrays. At the end of the day though, it’s our fault.

As Dr Andrew Silvers, a Melbourne cardiac and neuro-anesthetist recently opined, if your cardiologist replies with “avoid hypoxia and avoid hypotension” then YOU the anesthetist or anesthesiologist are at fault for not asking your cardiology colleague specific questions that will materially effect your perioperative planning.

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Metoclopramide: it actually works!?

Metoclopramide had long been written off by many anesthetists and anesthesiologists, aware of trials and meta-analyses that show no or limited effect in treating or preventing nausea and vomiting – in particular limited ability to prevent post-operative nausea and vomiting (PONV). Most recently Henzi, Walder and Tramèr (1999) were able to show only very limited benefit for metoclopramide 10 mg in preventing vomiting and no significant effect in preventing nausea in adults.1,2,3

What is metoclopramide?

Metoclopromaide is a benzamide, predominately used for antiemesis and its gastric prokinetic effect. It is marketed under the names Maxalon®, Pramin® and Reglan® in various countries. Although considered an old drug its antiemetic action was first identified in 1964 by French doctors Justin-Besançon and Laville.3 (In contrast the analgesic tramadol is often considered a "modern" drug outside of Europe, but was launched by Grünenthal GmbH in 1977.)

Metoclopramide readily crosses the blood-brain barrier where it mediates anti-emetic effects primarily as a dopamine D2 antagonist in the chemoreceptor trigger zone (CTZ – located in the area postrema of the 4th ventricle). Metoclopramide also has mixed 5-HT3 receptor antagonist and 5-HT4 receptor agonist actions. The former may contribute to anti-emesis at higher doses and the later to its pro-kinetic effects. Muscarinic cholinergic actions have also been identified, both through increasing acetylcholine release and by increasing receptor sensitivity to acetylcholine in the upper GI tract – further contributing to the pro-kinetic effect.

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  1. Henzi I, Walder B, Tramèr MR. Metoclopramide in the prevention of postoperative nausea and vomiting: a quantitative systematic review of randomized, placebo-controlled studies. Br J Anaesth. 1999 Nov;83(5):761-71. 

  2. It's also interesting to note that there was no dose responsive effect regardless of route. NNT to prevent early (<6h) and late (<48h) vomiting were 9.1 (95% CI 5.5-27) and 10 (6-41) respectively. In children the best documented regimen was 0.25 mg/kg. NNT to prevent early vomiting was 5.8 (3.9-11); there was no effect on late vomiting. There was only a single documented case of extrapyramidal side effects out of 3260 patients, giving an incidence of 0.03%. 

  3. Justin-Besançon L, Laville C.C R Seances Soc Biol Fil. 1964;158:723-7. 

Sugammadex and rocuronium anaphylaxis

I have been intrigued since the first case reports appeared describing the use of sugammadex in rocuronium anaphylaxis. It sounds beautiful and elegant. A drug that magically mops up the offending molecule, removing it from circulation; quickly reversing the cardiovascular collapse as rapidly as it reverses muscle relaxation.

The little we know

  1. There have been case reports from 5 countries showing dramatic improvement of rocuronium-confirmed anaphylaxis after administration of sugammadex.
  2. One case study showed a dose-dependent effect of sugammadex on modifying anaphylaxis.
  3. There are not yet any published cases of rocuronium anaphylaxis where sugammadex was administered without clinical improvement (though beware).
  4. Sugammadex although incompletely encapsulating rocuronium, does prevent the rocuronium epitope from binding IgE.
  5. Cutaneous and in vitro models of hypersensitivity have shown no or limited ability of sugammadex to modify type 1 hypersensitivty after triggering.
  6. Our understanding of the pathophysiology of anaphylaxis is over-simplified and incomplete.

Unfortunately the truth is not quite as clear. Case reports showing impressive recovery of rocuronium anaphylaxis minutes after giving sugammadex are tempered by in vitro and in vivoimmunological studies suggesting an inability of sugammadex to modify a type 1 hypersensitivity reaction. The reality is likely somewhere in between, highlighting our limited understanding of anaphylaxis and our tendency to rush to over-simplified models of disease processes.

The story so far...

Jones and Turkstra first raised the possibility of using sugammadex to treat rocuronium anaphylaxis in 2010.1 One year later Nolan McDonnell and team published the first case report of a remarkable use of sugammadex to manage rocuronium anaphylaxis.2 McDonnell described a 33 year old having an elective diagnostic laparoscopy suffering anaphylaxis to rocuronium. After 19 min of conventional resuscitation, involving CPR, 3500 mL of intravenous fluids and 4 mg of epinephrine/adrenaline - 500 mg of sugammadex was given with remarkable effect:

"A dose of 500 mg (6.5 mg kg21) was given while chest compressions were in progress. The last dose of epinephrine had been given 4 min previously. Approximately 45 s after administration and while chest compressions were in progress, the patient suddenly opened her eyes and reached for her tracheal tube.

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  1. Jones PM, Turkstra TP. Mitigation of rocuronium-induced anaphylaxis by sugammadex: the great unknown. Anaesthesia. 2010 Jan;65(1):89-90; author reply 90. 

  2. McDonnell NJ, Pavy TJ, Green LK, Platt PR. Sugammadex in the management of rocuronium-induced anaphylaxis. Br J Anaesth. 2011 Feb;106(2):199-201. 

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