Model for the administration of low-flow anaesthesia.

A general (multiple-gas) three-compartment mass-balance model of the circle-absorber breathing circuit with intermittent positive-pressure ventilation has been developed. We propose it as a tool to determine flowmeter and vapourizer settings for inhalation anaesthesia by low-flow methods (less than 1 litre min-1 total fresh gas flow). This model reproduces the results of various previously-published mass-balance models, but it appears to underestimate slightly the delivery of fresh gases to the inspired limb of the breathing circuit when tested with clinical data from surgical cases. Low-flow dosing schedules for 35% inspired oxygen and 1% inspired halothane were computed with the model and tested in vitro with a circle-absorber breathing circuit and an active gas-exchange lung at nine values of simulated patient gas exchanges. The mean inspired oxygen concentration over all trials was 32.8% (SD = 1.9%), while the mean inspired halothane concentration was 1.2% (SD = 0.3%). The flow meter and vapourizer settings calculated from the model appear to have sufficient accuracy to be useful in the clinical setting in conjunction with active oxygen and anaesthetic agent monitoring.

Comparison of impedance and inductance ventilation sensors on adults during breathing, motion, and simulated airway obstruction.

The goal of this study was to compare the relative performance of two noninvasive ventilation sensing technologies on adults during artifacts. We recorded changes in transthoracic impedance and cross-sectional area of the abdomen (abd) and rib cage (rc) using impedance pneumography (IP) and respiratory inductance plethysmography (RIP) on ten adult subjects during natural breathing, motion artifact, simulated airway obstruction, yawning, snoring, apnea, and coughing. We used a pneumotachometer to measure air flow and tidal volume as the standard. We calibrated all sensors during natural breathing, and performed measurements during all maneuvers without changing the calibration parameters. No sensor provided the most-accurate measure of tidal volume for all maneuvers. Overall, the combination of inductance sensors [RIP(sum)] calibrated during an isovolume maneuver had a bias (weighted mean difference) as low or lower than all individual sensors and all combinations of sensors. The IP(rc) sensor had a bias as low or lower than any individual sensor. The cross-correlation coefficient between sensors was high during natural breathing, but decreased during artifacts. The cross correlation between sensor pairs was lower during artifacts without breathing than it was during maneuvers with breathing for four different sensor combinations. We tested a simple breath-detection algorithm on all sensors and found that RIP(sum) resulted in the fewest number of false breath detections, with sensitivity of 90.8% and positive predictivity of 93.6%.