What are the side effects of opioids?

For over fifty years intraoperative opioid administration has been recognized as a standard of care to complement general anesthesia for patients undergoing surgery. During general anesthesia, opioids’ main mechanism of action is the blockade of pain receptors (nociceptor) through binding of opioid receptors in the brainstem and the spinal cord which reduce sympathetic reaction to stimulus (i.e. surgical incision, endotracheal intubation, etc.). Clinically, this can help maintain overall patient stability (heart rate, blood pressure, metabolic). For instance, during general anesthesia opioid use reduces the risk of tachycardia (increased heart rate) by enhancing cholinergic input to the sinoatrial node in the heart which slows down the heart rate.

There are however potential adverse effects associated with the use of opioids during surgery, such as, but not limited to respiratory depression, nausea, constipation, dizziness, difficulty passing urine, and hyperalgesia (i.e. higher pain intensity for the same stimulus). Opioids have an effect on the respiratory system by three main mechanisms, namely a decrease in the respiratory rate, sedation and a decrease in upper airway muscle tone leading to upper respiratory tract obstruction. Respiratory depression is generally of little importance during general anesthesia as ventilatory parameters can be controlled; however, opioids given while the patients are under general anesthesia can have sustained effects following the anesthesia. Opioids that are used during the intraoperative period are extremely potent (e.g. fentanyl is approximately 100 times more potent than morphine), thus leading to an increased risk hyperalgesia (i.e. higher pain intensity for the same stimulus), which in turn, can increase short-term opioid needs after surgery and potentially reduce the patient comfort during their recovery.

For patients under general anesthesia, specific clinical guidelines and recommendations on effective and safe use of opioids do not currently exist. In general, opioids are controlled drugs that should be administered only when necessary, for the shortest amount of time possible. For patients receiving general anesthesia, this recommendation is challenging to apply, as there is no valid monitoring to evaluate nociception (i.e. pain transmission) or reliable clinical signs of nociception to guide opioid administration. There is thus a need for further studies to guide the use of opioids and opioid alternatives (other pharmacologic and non-pharmacologic strategies that can be used) during anesthesia.