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The effect of isoflurane 0.6% on respiratory mechanics in anesthetized-paralyzed humans is not increased at concentrations of 0.9% and 1.2%

[L’effet de l’isoflurane à 0,6 % sur la mécanique respiratoire n’a pas augmenté chez l’humain anesthésie et paralysé à des concentrations de 0,9 % et 1,2 %]

From the Department of Anesthesia, McGill University, Montreal, Quebec, Canada.

Address correspondence to: Dr. Pedro Ruiz, Department of Anesthesia, McGill University, Montreal General Hospital, Room D8-132, 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada. Phone: 514-934-1934; E-mail: pedroruizmd{at}hotmail.com

Purpose: To assess the dose-dependent effect of low concentrations of isoflurane on respiratory mechanics in normal subjects.

Methods: We studied 12 non-premedicated ASA I patients scheduled for lower abdominal or extremity surgery. After thiopental 5–7 mg•kg^-1 iv and succinylcholine 1 mg•kg^-1 iv, the trachea was intubated and an esophageal balloon was placed optimally by the occlusion test. After introduction of N₂O and muscle paralysis with vecuronium, we studied 0, 0.6, 0.9 and 1.2% isoflurane. We recorded flow (F), airway opening and esophageal pressures. Signals were amplified, filtered, sampled at 100 Hz, and then fed in a 12-bit analogue-digital converter in a personal computer. Data were collected and analyzed using LABDAT and ANADAT software. Signals were acquired for 60–90 sec during mechanical ventilation (10 mL•kg^-1, 10 breaths•min^-1, I:E ratio 1:2). We estimated respiratory system (RS), lung (L) and chest wall (W) dynamic elastance (E) and resistance (R) by P(t) = EV_T(t) + RF(t) + K, where t is time, V_T tidal volume from integration of F, and K an estimation of end-expiratory pressure. ANOVA was used for comparing the basal state with the three concentrations.

Results: E and R were statistically lower at 0.6, 0.9 and 1.2% compared to basal values for RS, L and W. Concentrations equal to or higher than 0.6% did not further change respiratory mechanics, except for E_L1.2 compared to E_L0.6, 12.37 ± 5.72 and 13.52 ± 5.64 cm H₂O.L^-1, respectively.

Conclusion: Isoflurane concentrations between 0.6–1.2% are not associated to a dose-dependent effect on respiratory mechanics.