Nitrous oxide (N₂O) holds a unique position as the oldest anesthetic gas still in regular use. Despite its long history, it remains a complex agent with both advantages and potential risks, particularly for neurosurgical and neurologically vulnerable patients. Inhaled nitrous oxide produces dose-dependent analgesia, dissociative amnesia, anxiolysis, and somnolence. However, due to its low potency at atmospheric pressure, it cannot induce general anesthesia on its own. Under hyperbaric conditions, nitrous oxide can induce general anesthesia, though this often results in significant hemodynamic instability. The anesthetic effects of nitrous oxide are primarily mediated by antagonism of NMDA receptors, which gives the gas its unique properties.
Impact on Cerebral Perfusion
From a cerebral perfusion perspective, nitrous oxide has traditionally been understood to increase cerebral blood flow (CBF), cerebral metabolic rate of oxygen consumption (CMRO₂), and intracranial pressure (ICP). These effects, combined with its tendency to enter surgically induced pneumocephalus or exacerbate venous air embolism, historically led to recommendations to avoid its use in neurologically vulnerable patients. Much of this understanding comes from its role as a MAC-sparing agent, where nitrous oxide is used alongside other anesthetics. Studies show that in this context, CBF increases while changes in CMRO₂ can vary—enhancing, blunting, or showing no change in metabolism.
The practical teaching that follows is still that nitrous oxide raises CBF, CMRO2 and ICP, that the rise is largest when it is added to a volatile agent, and that it is attenuated by hypocapnia and by co-administered intravenous agents such as propofol, barbiturates and opioids. Because nitrous oxide is about 34 times more soluble in blood than nitrogen, it also expands any closed gas space, so it is stopped before dural closure and avoided after a recent craniotomy, with pneumocephalus, or when venous air embolism is suspected.
Safety in Neurosurgery
However, when used as a single agent, nitrous oxide increases CBF but either causes no change or decreases CMRO₂. This suggests that nitrous oxide’s true impact on cerebral perfusion is context-dependent, influenced by whether it is used as a primary or adjunct agent, and by the timing of administration. Importantly, cerebral autoregulation, which allows the brain to maintain stable CBF despite fluctuations in systemic blood pressure, remains intact whether nitrous oxide is used alone or in combination with other anesthetics. This suggests nitrous oxide does not interfere with the brain’s natural ability to regulate blood flow in response to changes in perfusion pressure, adding a layer of safety in neurosurgical contexts.
Clinical studies, including the IHAST trial (International Hypothermia for Aneurysm Surgery Trial), present mixed findings on the safety of nitrous oxide in neurosurgery. While long-term neurological deficits were not observed in patients with ruptured subarachnoid hemorrhage, nitrous oxide was linked to transient perioperative neurological deficits in patients undergoing temporary occlusion of cerebral vessels. These deficits were resolved within three months post-surgery, indicating a temporary rather than permanent impact. This has led to renewed interest in using nitrous oxide in neurosurgical anesthesia, though careful consideration of patient-specific factors remains essential.
