Anesthesia in the News
  • Prior Cannabis Use Reduces Need for BP Support During General Anesthesia

    Nonacute cannabis exposure decreases the need for blood pressure support during general anesthesia, according to a retrospective study of adult patients undergoing orthopedic trauma surgery at the University of California, Irvine Medical Center (UC Irvine), in Orange, California.

    Significant differences

    The study published in the July 2021 issue of the journal Cannabis and Cannabinoid Research found that patients who were tetrahydrocannabinol (THC)-positive were significantly less likely to receive intraoperative vasopressors, had significantly higher mean arterial blood pressure and mean diastolic blood pressure, required significantly less total fluid input, and had a lower overall fluid balance, compared to controls that screened negative for THC.

    “As cannabis use continues to increase in popularity and is encountered in patients undergoing surgery, it is important to consider the effect it may have on anesthetic management,” wrote the authors.

    Patients and protocol

    The study evaluated all 9,373 emergency department (ED) visits at UC Irvine, a level 1 trauma center, from November 2017 to January 2020. The start date coincided with the institution’s debut of a new electronic health record (EHR) system and the cutoff date corresponded with subsequent widespread changes in anesthetic management practices due to the onset of the COVID-19 pandemic.

    Among the data elements extracted from the EHR system were intraoperative blood pressure; anesthesia event times (for induction, intubation, and extubation); intraoperative medications such as anesthetic gases and fluids; intraoperative urine output; and intraoperative blood loss.

    This was followed by a manual chart review of intraoperative fluids (crystalloids, colloids, urine output, total blood loss and blood products) and all intraoperative medications (induction agents, muscle paralytics and vasoactive medications).

    Biologically validated use

    This study distinguished patients via urinalysis — an objective biological sample — and compared the physiologic differences in reaction to anesthesia between the THC-positive and THC-negative patient groups. This identification of preoperative cannabis users and noncannabis users in an objective manner is distinct from most prior investigations which have solely relied upon patient reports.

    Typically, the pharmacodynamic effects of cannabis resolve 10 hours or less after consumption (Liu et, 2018), but detectable levels of THC and its liver first-pass metabolites can be found in urine for up to 24 days. Patients in this study experienced an average interval of 20 hours between ED visit and entrance into the OR. Therefore, the study patients were objectively confirmed to have used cannabis within 24 days of admission and were reliably not acutely intoxicated with the substance at the time of anesthetic management.

    Patients underwent urgent but not emergent orthopedic trauma surgery within 48 hours of their ED visit. Other inclusion criteria were an anesthesia time greater than 1 hour, and anesthesia induction and intubation while in the operating room.

    A total of 221 adult patients met inclusion criteria, of whom 55 tested positive for THC and 165 tested negative.

    What accounts for the differences?

    In the THC-negative group, 65.1% of patients were given vasoactive medications compared to only 36.4% in the THC-positive group. No significant differences existed between the two groups for induction doses of propofol or fentanyl, total doses of propofol or fentanyl, or age-adjusted minimum alveolar concentration (MAC) of volatile gas during the middle third of cases. Thus, differences in anesthetic management did not account for the fluctuations in measured blood pressure parameters or interventions to target blood pressure.

    “Chronic exposure to THC has been shown to downregulate cannabinoid 1 receptors and cause alterations in endocannabinoid tone,” wrote the authors. “These are two potential mechanisms by which the THC positive group in our study may have become more resistant to the typically observed hypotensive effects of general anesthesia.”

    Caution against extrapolation

    Because the study was exploratory, the authors note that it is premature to change clinical decision-making at this time. They observe that, despite its findings, they may not be applicable to other patient populations due to the study’s very defined subset of patients, and the fact that the study was limited to a single center with a small population and was retrospective in nature.

    The researchers recommend that future studies include a larger sample size and be designed to ascertain if the beneficial effect of THC on blood pressure support is dose-dependent and if acute intoxication causes similar or contrary effects.

    The complete paper, “Cannabis Exposure Decreases Need for Blood Pressure Support During General Anesthesia in Orthopedic Trauma Surgery.” Cannabis Cannabinoid Res. 2021 Jul 5. may be accessed at no cost at: