Warning: Cannabis Can Mess with Going Under
Cannabis consumption just prior to surgery can mean you need more anesthetic to be put under.
It’s almost two decades ago that Dr. Symons, a UK anesthesiologist at Barnet General Hospital told a story about a patient that preoperatively appeared to him as a healthy guy in his thirties. The patient was scheduled for the removal of wisdom teeth, which is normally a routine procedure, a day’s case after which the patients are discharged within hours of operation.
Initial induction of inspired anesthetic ran smoothly, but the anesthetist noticed that the patient was not settling, even though he had a normal respiratory pattern. He had to give additional boluses of the volatile anesthetic, as well as an intravenous anesthetic in order to fully induce anesthesia in this patient. The procedure went well, and Dr. Symons went to visit the patient after he woke up. After Dr. Symons told the patient that it was very difficult to induce and maintain anesthesia, the patient disclosed to him reluctantly that he had smoked cannabis the night before. The anesthetist advised him in the future to refrain from smoking cannabis as long as he can if he needed to have any other procedures done that required anesthesia.
Why did this happen? What possible mechanisms could be at play to interfere with anesthesia in individuals who smoke cannabis? Firstly, due to the slow elimination of cannabinoids from the body, they could remain present in the tissue for days or weeks. As such, cannabinoids could interfere with the action of the anesthetic.
Secondly, cannabinoids exert complex actions on seizure activity, whereby they have been known to be proconvulsant in some cases and anticonvulsant in other cases. Additionally, it is now recommended that a corticosteroid, such as dexamethasone be given to patients who smoke cannabis pre-operatively, further underlining the importance of disclosing cannabis smoking.
Majority of the clinical evidence describing effects of cannabis on general anesthesia and in the perioperative setting are anecdotal case reports, and firm concrete evidence arguing either good or bad side is still lacking. In fact, some of cannabis effects such as sedation, bronchodilation, dryness of respiratory secretions, vein dilation, and increase in heart rate without inducing increase in blood pressure actually make it a great option for pre-medication.
Likewise, relieving nausea and its analgesic potential, without affecting respiratory depression make it an attractive candidate as a post-operative drug. In order to shed some light on this issue, investigators in Israel initiated a clinical trial to evaluate the use of cannabis in a perioperative setting. The agent they are using is Nabiximols (Sativex®), which contains THC and CBD in a 1:1 ratio.
The trial is still ongoing, and the investigators just published their first finding regarding anesthesia depth. The depth of anesthesia is routinely measured by the bispectral index (BIS) monitoring, which measures the electrical activity of the brain and the sedative effects of anesthesia. Computer-generated BIS scores range from 0 to 100, with 0 being coma, 40–60:general anesthesia, 60–90: sedation, and 100 being awake.
The investigators found that indeed, the patients who were receiving 21.6mg THC and 20mg CBD had higher BIS values under same dose of anesthesia. According to the BIS score scale, this would indicate a shallower depth of anesthesia. However, the investigators suggest that the higher BIS score may indicate a different pattern of brain electrical activity rather than shallower anesthesia, a hypothesis that still needs to be further explored.
According to the available data, cannabinoids affect anesthesia in one of two ways: either by frankly decreasing the depth of anesthesia or by altering the brain wave patterns such that it appears by the routinely used BIS monitoring that the anesthesia is shallower while it may not be. In any event, disclosing cannabis use with the doctor and anesthetist is likely a good idea, since there are some prophylactic measures already in place for individuals using it (such as dexamethasone administration).