Fifty percent nitrous oxide does not increase the risk of venous air embolism in neurosurgical patients operated upon in the sitting position

Although nitrous oxide (N₂O) should theoretically increase the severity of venous air embolism (VAE), data confirming this hazard in clinical situations are not available. The effect of 50% N₂O on the incidence and severity of VAE and on the emergence time from anesthesia was evaluated in 300 neurosurgical patients operated upon while in the sitting position. Of these, 110 patients underwent craniectomy for posterior fossa pathology and 190 patients underwent cervical spine surgery (CSS). Patients were randomized to receive either 50% N₂O in oxygen (O₂) (N₂O group) or O₂ (no-N₂O group) as part of an isoflurane-fentanyl-based anesthetic. In patients in the N₂O group, N₂O administration was discontinued immediately upon Doppler- detection of VAE and was reinstituted in not less than 30 min after resolution of the episode. The incidence of Doppler-detected VAE was significantly greater in the craniectomy group than the CSS group (43% vs. 7%, respectively; P < 0.001). N₂O had no effect on the incidence of VAE or the severity of VAE as judged by the magnitude of the reduction in blood pressure during hemodynamically significant episodes of VAE, the volume of gas aspirated from the right atrial catheter during episodes of VAE, or the magnitude of the decrease in end-tidal carbon dioxide tension during episodes of VAE. Hemodynamically significant episodes of VAE (i.e., episodes associated with a reduction in systolic blood pressure of ≥ 15 mmHg) occurred in 17 of the 61 patients experiencing VAE (28%) and was not different between the N₂O and no-N₂O groups. Similarly, hemodynamically significant episodes of VAE (n = 18) accounted for 15% of all episodes of VAE (n = 118) and was not different between the N₂O and no-N₂O groups. Emergence time was not significantly different between the N₂O and no-N₂O groups, with mean times of 2 ± 6 and 3 ± 7 min (± SD), respectively. Emergence time was significantly longer in the craniectomy group than in the CSS group (5 vs. 1 min, respectively; P < 0.001). Within the craniectomy group, the incidence of Doppler-detected VAE was significantly less in patients with previous surgery at the operative site (21%) compared to patients without previous surgery at the operative site (47%). Postoperatively, no complications could be related to the use of N₂O or directly attributed to the occurrence of VAE. The authors conclude that in neurosurgical patients operated upon while in the sitting position, 50% N₂O has no measurable effect on the incidence or severity of VAE if its administration is discontinued immediately upon Doppler detection of VAE. In addition, when the anesthetic agents are administered so as to have the patient awake at the completion of surgery, use of 50% N₂O to supplement an isoflurane-fentanyl-based anesthetic has no measurable effect on the emergence time from anesthesia.