The effect of isoflurane 0.6% on respiratory mechanics in anesthetized-paralyzed humans is not increased at concentrations of 0.9% and 1.2%.
OBJECTIVE
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 x kg(-1) iv, the trachea was intubated and an esophageal balloon was placed optimally by the occlusion test. After introduction of N(2)O 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 x kg(-1), 10 breaths x 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 H(2)O.L(-1), respectively.


CONCLUSIONS
Isoflurane concentrations between 0.6-1.2% are not associated to a dose-dependent effect on respiratory mechanics.
Ruiz
Pedro
P
Department of Anesthesia, McGill University, Montreal, Quebec, Canada. pedroruizmd@hotmail.com
Chartrand
Daniel
D
D000704:Analysis of Variance; D000768:Anesthesia, General; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D018686:Anesthetics, Intravenous / Q000627:therapeutic use; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007530:Isoflurane / Q000494:pharmacology*; D009467:Neuromuscular Depolarizing Agents / Q000627:therapeutic use; D010243:Paralysis / Q000139:chemically induced* / Q000503:physiopathology*; D015656:Respiratory Mechanics / Q000187:drug effects*; D013390:Succinylcholine / Q000627:therapeutic use; D013874:Thiopental / Q000627:therapeutic use; D014673:Vecuronium Bromide / Q000627:therapeutic use
D018685:Anesthetics, Inhalation; D018686:Anesthetics, Intravenous; D009467:Neuromuscular Depolarizing Agents; D014673:Vecuronium Bromide; D007530:Isoflurane; D013390:Succinylcholine; D013874:Thiopental