- Remifentanil infusions above 0.20-0.25 μg/kg/min are associated with hyperalgesia (OIH = Opioid Induced Hyperalgesia) and tolerance (AOT = Acute Opioid Tolerance) respectively.
- Some of these effects can be mitigated by multimodal analgesia (notably ketamine), and possibly by gradual weaning of a remifentanil infusion.
- The findings have been predominately identified in rats and volunteer human studies. The clinical and longterm significance is still uncertain.
- Although OIH and AOT arise from different physiological mechanisms, they are clinically difficult (if not impossible) to differentiate.
- The clinical priority for management is prevention.
One of the earliest published case series linking post-operative hepatic necrosis to halothane anaesthesia.
We now know this occurs in about 1 in 10,000-30,000 adult halothane anaesthetics, and 1 in 60,000 in children, with a historical mortality of 30-70%. In 20% of cases the hepatitis is mild and self-limiting.
An infamous article from 1985 that investigated the relationship between perioperative myocardial ischaemia and postoperative myocardial infarction in 1,023 elective CABG patients. The study findings are broadly consistent with our understanding that increasing myocardial oxygen demand in those with coronary artery disease is undesirable.
Although there are unsurprising problems with this 35 year old article, it is best known for the infamous anesthesiologist number 7 who subjected his/her patients to disproportionately more postop infarcts, along with tachycardia and hypertension.
Final word? Don’t be a number 7 anesthesiologist...
Beware anesthesiologist number 7... the quality of intraoperative conduct of anesthesia effects patient outcomes.
What did they do?
Kowark and friends randomised 343 patients across four German hospitals to receive desflurane, sevoflurane or propofol for maintenance anesthesia using a laryngeal airway for surgery expected to be up to 2 hours.
And they found?
There was no difference in airway reactions among the three groups, and the desflurane patients emerged (statistically) significantly faster.
But the difference in emergence times was, i) at most only 2 minutes, and ii) was a surrogate marker for what actually matters – when a patient leaves the PACU or hospital – which wasn't reported.
Additionally, the study protocol very prescriptively defined when volatiles were decreased (50% at 5 min before expected surgical finish) and ceased – the same for both Des and Sevo. Yet it is common practice to begin weaning Sevo earlier than Des if trying to achieve comparable emergence.
Could this even be applied to my patients?
Probably not. Unless you are in the habit of using remifentanil infusions (0.15 mcg/kg/min) for surgery that almost certainly does not justify its use and have access to uniquely European analgesics piritramide and metamizole.
The elephant in the room...
This study demonstrates the well known faster pharmacokinetics of desflurane during an unnecessarily complex laryngeal mask anesthetic, and yet adds little to meaningful clinical outcomes.
Also see Carbon Footprint from Anaesthetic gas use [pdf] from the UK’s Sustainable Development Unit.
“Inhalational anaesthetic agents are chlorofluorocarbons, ‘greenhouse gases’ that have between 349 (sevoflurane) and 3714 (desflurane) times the global warming potential over a 20 year time horizon of carbon dioxide (isoflurane 1401), equivalent to driving a car 18 (sevoflurane) to ~350 miles (desflurane) per hour of anaesthetic use (isoflurane 30 miles); these figures do not account for the additional carbon cost of heating desflurane vaporisers. Together with nitrous oxide, inhalational anaesthetic agents contribute ~2.5% of the 22.8 million tonnes of carbon dioxide equivalents the NHS produces annually.” - White
Why is this important?
With obesity rates over 40% in many industrialised countries, and accelerating growth in bariatric surgery for more than a decade, there is need for evidence based guidelines to direct perioperative care.
This evidence review was conducted to identify protocols that achieve "superior outcomes, reduced length of hospital stay, and cost savings" for bariatric patients.
Many of the institutional protocols were founded on ERAS principals originating with colorectal surgery.
Ok, what did they identify?
The AHRQ made evidence-based anesthesia recommendations across three areas:
- Preoperative: reduce fasting; provide carbohydrate loading; multimodal preanesthesia medication.
- Intraoperative: standardised intraoperative anesthesia; protective ventilation; goal-directed fluid therapy (minimization); postop nausea and vomiting prophylaxis.
- Postoperative: multimodal analgesia.
These protocols largely reflect 'good quality modern anesthesia', and there is little here that is specific to bariatric patients.
This is not a critcism, but a reminder that it's consistent and holistic application of quality anesthesia across the perioperative period that improves outcomes – especially among higher risk patients. Interventions do not need to be fancy, just quality principles consistently applied.
Anesthesiologists have the ability to contribute either positively or negatively to the growing opioid misuse crisis.
Why should I care?
Misuse of opioids is a growing global problem, well established in the US and quickly appearing in many high-resource countries. One person dies every 15 minutes in the US from opioid overdose.
For many affected, the perioperative period is the first exposure event. In the US ~6% of previously opioid-naive patients progress to persistent opioid use after surgery.
What can anaesthetists and anesthesiologists do?
- Identify patients at risk of opioid dependence.
- Use multi-modal non-opioid analgesia perioperatively.
- Educate patients on realistic expectations for post-operative pain.
- Consider regional techniques intraoperatively when appropriate.
- Limit discharge prescribing of opioids (42-71% of all postop opioid tablets go unused!).
The bigger picture...
Although inidividual practice changes are important, real impact will come through anesthesiologists as integrators of care (eg. ERAS interventions) and contributions to institutional strategies, patient and provider education.
Take a long view, this problem is not going away in a hurry...