Anesthesia was my first clinical rotation in medical school and my introduction to pharmacology. One of my initial tasks was to administer a small dose of remifentanil, a fast-acting type of fentanyl, through a patient’s IV. As my mind raced with first-day nerves, one thought stood out: Isn’t fentanyl the same drug responsible for so many overdose deaths across Canada?
From January to June 2024, Health Canada reported an average of 21 opioid-related deaths per day, with 79% involving fentanyl—a potent opioid 100 times stronger than morphine. While the overall number of opioid-related deaths has slightly decreased since 2023, each loss is deeply felt by families, friends, and communities. The community where I attend medical school is no exception to the many cities facing the same opioid crisis across Canada. Patients coming in for routine surgeries such as joint replacements, hernia repairs, or varicose vein procedures often have a history of opioid use disorder. This raises an important question: How does previous or current opioid use affect anesthesia? Could a past opioid overdose make it more difficult to be “put under”?
The answer isn’t a simple yes or no. This is because general anesthesia is a combination of multiple effects, and different types of anesthesia exist. For this article, I’ll focus on general anesthesia, the classic “knocked out” state seen in medical dramas. However, depending on the procedure, deep sedation or regional anesthesia may also be used.
The different components of general anesthesia are often remembered as the “5 A’s”:
- Amnesia: Latin for memory loss
- Areflexia: loss of reflexes and muscle function
- Autonomic stability: keeping blood pressure and heart rate within normal limits
- Analgesia: stemming from the Greek word for pain relief
- Lack of awareness
A successful surgery requires all five components. For example, if a patient feels no pain and maintains a stable heart rate, remembering the surgery would still be traumatizing. Each of these effects is controlled by separate, specialized drugs, allowing anesthesiologists to fine-tune the depth of anesthesia and ensure a safe awakening. In many ways, anesthesiologists function like airplane pilots, adjusting multiple controls such as speed, rotation, and elevation, to ensure a smooth and safe journey.
Our unconscious brains are aware of pain
Why is pain management, or analgesia, so crucial during surgery? After all, specialized drugs already ensure that patients are unconscious, their muscles are relaxed, and they won’t remember the procedure. However, our bodies are incredibly smart and our brains can still register pain signals even when we are unconscious.
During a knee replacement, for example, nerves in the area still detect and send pain signals to the brain. The brain then triggers the sympathetic nervous system, our “fight or flight” response. This can lead to increased heart rate, rapid breathing, muscle tension, and a spike in blood pressure. While these responses are helpful for escaping danger, such as running from a lion gnawing at our knee, they are dangerous during surgery. For an anesthesiologist, prolonged increases in heart rate and blood pressure can increase the risk of complications like a heart attack. This is why adequate pain control is a critical part of anesthesia.
Opioids are a mainstay for analgesia during surgery. Their powerful pain killing and sedating effects have been used by humanity for thousands of years, with the earliest records of use entombed in Sumerian tablets from about 5000 BCE. Remember the nerves that communicate pain from a surgery in our knee to our brain? Opioids work by binding to receptors in these same nerves and switching them off. This interrupts the communication of pain from knee to brain. Unfortunately, opioids also switch off the neurons holding back dopamine – our “feel good” neurotransmitter – from flooding our brains. The result is a surge in dopamine which is what makes opioids so addictive.
Even more dangerously, chronic opioid use changes our brains at a microscopic level. Our brains are constantly searching for the perfect balance of excitement and inhibition. Long-term opioid use places the brain into a state of excessive excitement. Our brains respond by making the inhibitory neurons more powerful, which returns our brain to a perfect balance while flooded with opioids. But after the opioids wear off? The newly strengthened inhibitory neurons overpower our baseline excitatory neurons, causing the unpleasant symptoms of opioid withdrawal such as anxiety, elevated blood pressure, and sweating.
This presents a challenge during surgery. A standard dose of remifentanil that works for an opioid-naïve patient will be insufficient for someone with a history of opioid use disorder. Without adequate pain control, the “fight or flight” response can become overactive, leading to dangerously high blood pressure and an increased risk of heart attack. To prevent this, anesthesiologists must carefully adjust opioid doses and consider alternative pain management strategies.
Opioid use disorder affects people from all walks of life, and many will require surgery at some point. Understanding how past opioid use influences anesthesia is crucial, not just for patient comfort but for patient safety.
Resources for opioid use disorder
Health Canada resources for substance use
Maya McKeown graduated from McGill in 2023 with a B.Sc. in neuroscience. She is now a second-year medical student at McMaster.
