[Low-flow anesthesia: isoflurane increment kinetics as a function of the rotatory circuit volume]

Study of the increment of isoflurane volume concentration in two rotatory circuits having different capacity during low flow narcosis.

Descriptive comparison of different halogenate concentration in circuits connected to two patient groups and evaluation of the significance of the observed differences.

Surgery theatres of a district Public Hospital.

16 female patients included in the same age, weight and body surface group, showing neither obstructive nor restrictive lung diseases. According to the surgical treatment used (abdominal or major gynecological surgery) the patients were then divided into two groups of 8 each.

After opening the halogenate nebulizer, dry measurement of the isoflurane concentration detected for 30 minutes in the inspiratory and expiratory circuit segment by means of monitors at pre-determined intervals, before abdominal or major gynecological surgery. Simultaneous measurement of isoflurane uptake by means of indirect methods (heart frequency, average arterial pressure, use of muscle relaxing agents). The analysis of the measured values showed higher isoflurane concentrations and a faster percent increase in the small circuit than in the big one (resp. 0.9% vs 0.71% in the inspiratory segment and 0.59% vs 0.4% in the expiratory one, with p < 0.005 and I.C. = 99.00%).

This study demonstrates how the different kinetics of the isoflurane increment in the rotatory circuit is mainly related to the circuit volume. To eliminate the problems due to the "low compliance" of the small circuit (collection of toxic gases) and to the "high compliance" of the big circuit rotatory (awareness), circuits with varying volume might be introduced.