Oxido nitroso: uma boa opçåo como gás para pneumoperitôneo nas colescistectomias por videolaparoscopia sob anestesia geral / Nitrous oxide: good option as pneumoperitoneum gas in videolaparoscopic cholecystectomy under general anesthesia

RESUMO

Backgoruind and oblectives - Nitrous oxide is less iritating than carbon dioxide when injected into the abdominal cavity, besides inducing less hemodynamic and respiratory changes. The objective of this study was to demonstrate that a) N2O used as pneumoperitoneum gas is little absorbed by the splancnic circulation; b) Intraperitoneal N2), associated or not with inhaled N2O, does not change PETCO2, and cannot be responsible for postoperative vomiting; c) the association of inhaled and intraperitoneal N2O reduces alfentanil requirement as compared to ventilation with 100 per cent oxygen. Methods - After Institutional approval, thirty two patientes of both sexes, ASA physical status 1 or 2, submmited to laparoscopic cholecystectomy, gave their informed consent to participate. Patients were premedicated with intramuscular midazolan (0.1mg.Kg-1). Monitoring included blood presure, ECG, gas analyser, pulse oximeter, oxygen analyser and peripheral nerve stimulator. Anhestesia was included with droperidol (0.15 mg.Kg-1), alfentanil (40ug.Kg-1), propofol (0.5 mg.Kg-1) in continuous infusion, atracurium (0.5 mg.Kg-1) and 100 per cent oxygen by mask, followed by tracheal intubation. Ventilation was controlled with 7 ml.Kg-1 tidal voleme and the ventilatory rate was adjusted to maintain PETCO2 at 32-36 mmHg, 97-98 per cent HbO2 saturation, 33-35 per cent FlO2 and intratracheal pressure of 10-12 cmH2O. Metropolol, 0.05-1 mgKg-1, was injested after tracheal intubation. The patients were allocated into two groups Group O, ventilated with 100 per cent O2; Group N, ventilated with N2O 66-67 per cent in O2. Maintenance of anesthesia was done with propofol infusion (35 ug.Kg-1) and intermitent doses of alfentanil in both groups. Alfentanil doses systolic and diastolic arterial pressures and heart rate were registered. The PETCO2 was adjusted to 32-36 mmHg with constant tidal volume and changes in ventilatory rate if necessary. The N2O was insuflated into the abdominal cavity up to obtaining 10-12 mmHg pressure and the initial volume was registered. Samples were colected from the gastric cavity and the N2O concentration was measured in the abdominal cavity at the end of the procedure. In Group O, the expired N2O was monitored to detect absorption by splacnin circulation. In Group N, the time after discontinuation of N2O required for the expires concentration to reach 15 per cent and zero, were registered. Data were statisticaly analyzed with Student's "t" test, ANOVA and chi square test. Results - In Group O, alfentanil doses were higher than in Group N (p<0.001). Initial intraperitoneal insuflation volume of N2O were 1.76 +- 0.47 Land l.92 +- 0.59 L respectively in Groups O and N. In both groups, the N2O concentration in the desinsuflation gas was near 100 per cent and N2O was not detected in the gastric cavity gas. In Group O, N2O was not detected in the expiratory gases. In 3.38 +- 1.09 min the N2o concentration was reduced to 15 per cent and 8.38 +- 3.36 min it was zero in Group N. Conclusions - The authors concluded that 1- N2O used by peritoneal route is not absorbed by splancnic circulation; 2- the association of inhalation and intraperitoneal N2O does not induce significant hemodynamic or respiratory changes; 3- because it is little absrbed and unlikely to produce explosive mixtures, the authors recommend N2O to be used as pneumoperitoneum gas in laparoscopic cholecistectony