[Analysis of induction with 5% isoflurane and 60% nitrous oxide in premedicated adults]

To analyze rapid inhalatory anesthetic induction in premedicated adults (fentanyl, droperidol, diazepam and atropine) with an inspired mixture of O2, N2O (60%) and 5% isoflurane as a means for accelerating induction time and reducing complications.

We studied a series of 35 adult ASA-I patients at four times: baseline (1), premedication (2), induction (3) and postintubation (4). Measurements taken were arterial hemoglobin oxygen saturation (SpO2), arterial pressure (AP), heart rate (HR), end-tidal CO2 pressure (PETCO2) and respiratory frequency. We also recorded maximum end-tidal isoflurane pressure (FEtiso) at time 3; presence of apnea, cough and mask rejection at time 3; arrhythmia at times 3 and 4, and induction recall at time 3. Mean time of induction (time from start of induction to laryngoscopy) and the double product (HR x systolic AP) were calculated.

Coughing was observed in 20% (7/35) and was significantly associated with smoking; mask rejection was observed in 5.71% (2/35). The lowest SpO2 measurements occurred during period 2 and the highest during 3 (98.2 +/- 0.76). The incidence of apnea (77.14%, 27/35) was high during period 3, with mean PETCO2 values (+/- SD) of 40.71 +/- 4.19 mmHg and mean maximal FEtiso values of 1.6 +/- 0.53%. Mean time of induction was 96.28 +/- 5.19 s. There were no cases of recalled induction of arrhythmia in periods 3 and 4. The highest levels of AP and HR were recorded during period 4. The only difference in AP and HR between periods 2 and 3 was in diastolic AP, which was higher (p < 0.05) in period 3.

Rapid induction with 5% isoflurane is achieved within 1.5 min, with a 20% incidence of coughing and a 5% incidence of mask refusal. The high frequency of apnea (77%) is caused by the rapid elevation of alveolar isoflurane, which has an impact on the apneic threshold. Anesthetic induction with 5% isoflurane and 60% nitrous oxide in premedicated adults is accomplished with good maintenance of hemodynamic stability. However, tachycardia and arterial hypertension appear after laryngoscopy and tracheal intubation.