Surgical site infection occurred in 321 ( 9 % ) patients assigned to nitrous oxide , and in 311 ( 9 % ) patients in the no-nitrous oxide group ( p = 061 ) , and severe nausea and vomiting occurred in 506 patients ( 15 % ) assigned to nitrous oxide and 378 patients ( 11 % ) not assigned to nitrous oxide ( p < 00001 ) .