Advanced technologies and devices for inhalational anesthetic drug dosing.

Technological advances in micromechanics, optical sensing, and computing have led to innovative and reliable concepts of precise dosing and sensing of modern volatile anesthetics. Mixing of saturated desflurane flow with fresh gas flow (FGF) requires differential pressure sensing between the two circuits for precise delivery. The medical gas xenon is administered most economically in a closed circuit breathing system. Sensing of xenon in the breathing system is achieved with miniaturized and unique gas detector systems. Innovative sensing principles such as thermal conductivity and sound velocity are applied. The combination of direct injection of volatile anesthetics and low-flow in a closed circuit system requires simultaneous sensing of the inhaled and exhaled gas concentrations. When anesthetic conserving devices are used for sedation with volatile anesthetics, regular gas concentration monitoring is advised. High minimal alveolar concentration (MAC) of some anesthetics and low-flow conditions bear the risk of hypoxic gas delivery. Oxygen sensing based on paramagnetic thermal transduction has become the choice when long lifetime and one-time calibration are required. Compact design of beam splitters, infrared filters, and detectors have led to multiple spectra detector systems that fit in thimble-sized housings. Response times of less than 500 ms allow systems to distinguish inhaled from exhaled gas concentrations. The compact gas detector systems are a prerequisite to provide "quantitative anesthesia" in closed circuit feedback-controlled breathing systems. Advanced anesthesia devices in closed circuit mode employ multiple feedback systems. Multiple feedbacks include controls of volume, concentrations of anesthetics, and concentration of oxygen with a corresponding safety system. In the ideal case, the feedback system delivers precisely what the patient is consuming. In this chapter, we introduce advanced technologies and device concepts for delivering inhalational anesthetic drugs. First, modern vaporizers are described with special attention to the particularities of delivering desflurane. Delivery of xenon is presented, followed by a discussion of direct injection of volatile anesthetics and of a device designed to conserve anesthetic drugs. Next, innovative sensing technologies are presented for reliable control and precise metering of the delivered volatile anesthetics. Finally, we discuss the technical challenges of automatic control in low-flow and closed circuit breathing systems in anesthesia.

Time course of inhaled anaesthetic drug delivery using a new multifunctional closed-circuit anaesthesia ventilator. In vitro comparison with a classical anaesthesia machine.

BACKGROUND: The aim of this study was to detail the time-course, defined as the changes in end-tidal drug concentration with time, and consumption of inhaled anaesthetics when using a multifunctional closed-circuit anaesthesia machine in various drug delivery modes, and to compare it with a classical anaesthesia machine using an out-of-circle vaporizer under high and low fresh gas flow conditions. METHODS: Using an artificial test lung, sevoflurane and desflurane time-course and consumption were compared when using the Zeus apparatus (Dräger, Lubeck, Germany) with direct injection of inhaled anaesthetics or the Primus apparatus (Dräger, Lubeck, Germany) using a classical out-of-circle vaporizer. Anaesthetics were targeted at 1 and 2 MAC end-tidal during 15 min. For both apparatus, out-of-circle high and low fresh gas control (FGC) and for Zeus, auto-control (AC) modes (fixed fresh gas flow at 6 and 1 litre min(-1) and uptake mode) were compared. Time to reach target, initial overshoot and stability at target, and wash-out times were compared. RESULTS: In FGC, an initial overshoot in end-tidal drug concentration is seen when using 6 litre min(-1) fresh gas flow and a slower time course is observed when using only 1 litre min(-1) in both apparatus. In auto-control mode, the time course of both sevoflurane and desflurane was very fast and not influenced by the changes in fresh gas flow. No overshoot at target was seen. At all settings, the wash-out times were faster when using Zeus than Primus. Inhaled anaesthetic consumption was lowest with the Zeus ventilator in uptake AC mode. CONCLUSION: A combination of the fastest time course and lowest consumption of sevoflurane and desflurane was found when using the Zeus apparatus in AC uptake mode.