Instrumentation for measuring continuous oxygen consumption of surgical patients

A xenon recirculating ventilator for the newborn piglet: developing clinical applications of xenon for neonates

CONTEXT

The clinical applications of xenon for the neonate include both anaesthesia and neuroprotection. However, due to the limited natural availability of xenon, special equipment is required to administer and recapture the gas to develop xenon as a therapeutic agent.

OBJECTIVE

In order to test the xenon recirculating ventilator for the application of neuroprotection in a preclinical trial, our primary objective was to test the efficiency, reliability and safety of administering 50% xenon for 24 h in hypoxic ischaemic piglets.

DESIGN

A prospective observational study.

SETTING

Institute for Women's Health, University College London, January 2008 to March 2008.

ANIMALS

Four anaesthetised male piglets, less than 24 h old, underwent a global hypoxic ischaemic insult for approximately 25 min prior to switching to the xenon recirculating ventilator.

INTERVENTION

Between 2 and 26 h after hypoxic ischaemia, anaesthetised piglets were administered a mixture of 50% xenon, air, oxygen and isoflurane.

MAIN OUTCOME MEASURES

The primary outcome measure was blood gas PaCO2 (kPa) and secondary outcome measure was xenon gas use (l h), over the 24-h duration of xenon administration.

RESULTS

The xenon recirculating ventilator provided effective ventilation, automated control of xenon/air gas mixtures, and stable blood gas PaCO2 (4.5 to 6.3 kPa) for 24 h of ventilation with the xenon recirculating ventilator. Total xenon use was minimal at approximately 0.6 l h at a cost of approximately &OV0556;8 h. Additional features included an isoflurane scavenger and bellows height alarm.

CONCLUSION

Stable gas delivery to a piglet with minimal xenon loss and analogue circuitry made the xenon recirculating ventilator easy to use and it could be modified for other large animals and noble gas mixtures. The technologies, safety and efficiency of xenon delivery in this preclinical system have been taken forward in the development of neonatal ventilators for clinical use in phase II clinical trials for xenon-augmented hypothermia and for xenon anaesthesia.

Bymixer system can measure O2 uptake and CO2 elimination in the anesthesia circle circuit

BACKGROUND

The ability to measure carbon dioxide elimination (Vco(2)), oxygen uptake (Vo(2)), and R (respiratory exchange ratio, Vco(2)/Vo(2)) during anesthesia may help the non-invasive detection of critical events (e.g., abrupt decrease in cardiac output) and metabolic upset (e.g., onset of anaerobic metabolism).

METHODS

We have developed a new clinical bymixer (inline mixing chamber) that can measure mixed inspired and expired gas fractions in the anesthesia circle circuit. The addition of a standard anesthesia gas analyzer and flowmeter, and a new airway temperature and humidity sensor, allow determinations of Vco(2) and Vo(2) at the airway opening of the circle circuit. Over a range of tidal volume and frequency, Vco(2) and Vo(2) were compared to reference values generated by the combustion of metered liquid ethanol in a new metabolic lung simulator.

RESULTS

By linear regression, bymixer-flow measurements of Vco(2) (slope = 1.02, Y-intercept = -5.31, coefficient of determination, R(2) = 0.998) and Vo(2) (slope = 1.05, Y-intercept = -4.34, R(2) = 0.993) correlated closely to the reference values generated by the metabolic lung simulator. Limits of agreement analysis generated percent errors (mean +/- 1.96 SD) of -1.2 +/- 7.2% for Vco(2) and 2.5 +/- 9.8% for Vo(2).

CONCLUSIONS

The new clinical bymixer is compact, lightweight, disposable, inexpensive, and has a fast and adjustable response time (time constant about 14 sec). Anesthesia circle circuit integrity is maintained. Bymixer-flow measurements of Vco(2) and Vo(2) are accurate and may add to clinical monitoring under anesthesia and surgery.

Model-based administration of inhalation anaesthesia. 2. Exploring the system model

We explored our model by displaying its new capabilities, testing its sensitivity to variations in input data and illustrating its use. Its multiple-gas character allows simulation of the mechanisms governing concentration and second gas effects. Simulating the volume of a standing bellows makes it possible to test algorithms for automated closed-circuit anaesthesia. Using desflurane, the model's sensitivity to changes in blood/gas partition coefficient (range 0.42-0.576), cardiac output and minute ventilation was analysed. The model was very sensitive to changes in blood solubility; other results agreed with those reported previously. An alveolar isoflurane tension of 1% atm was rapidly attained and maintained, even using 0.5 litres min(-1) of fresh gas, when isoflurane was 'co-administered' through a vaporizer set to 3.5 vol% and a single aliquot (1.25 ml liquid) injected into the expiratory limb. As a result of its credibility and capabilities, the model is to be tested in the clinical setting.

Closed-loop control for anesthesia breathing systems

Numerous medical applications of closed-loop control have been developed over the past 40 years. For the patient breathing system, appropriate sensors are available. Feedback controllers have been developed and tested. Gas and vapor delivery devices seem ready for use. With the sensors, controllers, and delivery devices developed and tested, it seems likely that closed-loop control will be an integral part of future anesthesia workstations. The convenience and improved stability and response time will be important advantages in future anesthesia delivery systems.

Continuous measurement of pulmonary gas exchange during general anaesthesia in man

We report a system for the continuous measurement of oxygen uptake (VO2) and carbon dioxide output (VCO2) during open-circuit anaesthesia. Gas concentrations were measured by a mass-spectrometer, and expired flow by a pneumotachograph. The values measured by the system were compared in vitro to values produced by a nitrogen-dilution technique. Excellent correlations were found. Continuous measurements were performed in 21 patients anaesthetized for abdominal surgery. Compared to pure intravenous anaesthesia (flunitrazepam-fentanyl), anaesthesia including the administration of nitrous oxide or nitrous oxide and halothane led to more pronounced and sustained decreases in VO2 and core temperature, with a better cardiovascular stability. Two hours postoperatively, VO2 was not different from preoperative values. After a transient increase at the onset of anaesthesia, the respiratory exchange ratio (VCO2/VO2) returned to preoperative values, and then remained subsequently unchanged. Based on these observations, the system described provides an accurate approach to noninvasively monitoring the pulmonary gas exchange in the operating room.

Microcomputer data acquisition and control

In medicine and biology there are many tasks that involve routine well defined procedures. These tasks are ideal candidates for computerized data acquisition and control. As the performance of microcomputers rapidly increases and cost continues to go down the temptation to automate the laboratory becomes great. To the novice computer user the choices of hardware and software are overwhelming and sadly most of the computer sales persons are not at all familiar with real-time applications. If you want to bill your patients you have hundreds of packaged systems to choose from; however, if you want to do real-time data acquisition the choices are very limited and confusing. The purpose of this chapter is to provide the novice computer user with the basics needed to set up a real-time data acquisition system with the common microcomputers. This chapter will cover the following issues necessary to establish a real time data acquisition and control system: Analysis of the research problem: Definition of the problem; Description of data and sampling requirements; Cost/benefit analysis. Choice of Microcomputer hardware and software: Choice of microprocessor and bus structure; Choice of operating system; Choice of layered software. Digital Data Acquisition: Parallel Data Transmission; Serial Data Transmission; Hardware and software available. Analog Data Acquisition: Description of amplitude and frequency characteristics of the input signals; Sampling theorem; Specification of the analog to digital converter; Hardware and software available; Interface to the microcomputer. Microcomputer Control: Analog output; Digital output; Closed-Loop Control. Microcomputer data acquisition and control in the 21st Century--What is in the future? High speed digital medical equipment networks; Medical decision making and artificial intelligence.

The effect of dopamine, atropine, phenylephrine and cardiac pacing on oxygen consumption during fentanyl-nitrous oxide anaesthesia in the dog

In animals deeply anaesthetized with fentanyl and nitrous oxide the arterial blood pressure and heart rate were increased using dopamine, atropine, electrical pacing and phenylephrine in order to study the accompanying change in whole body oxygen consumption. Seven dogs (16-24 kg) were anaesthetized with fentanyl 1 microgram . kg-1 . min-1. After completing instrumentation a dopamine infusion was started at a rate of 39 micrograms . kg-1 .min-1. After the mean blood pressure reached 18.6 kPa the infusion was reduced to 10 micrograms . kg-1 . min-1 and maintained for 10 minutes. After waiting 45 minutes an infusion of atropine 20 micrograms . kg-1 . min-1 was started and when the heart rate reached 120 b/min the infusion was slowed to 1.25 micrograms . kg-1. min-1 and maintained for 10 minutes. Twenty-five minutes later the heart rate was increased to 150 beats/min and maintained at that level for 10 minutes using electrical pacing. The pacing was removed and an infusion of phenylephrine 5 micrograms . kg-1 . min-1 was started. When the blood pressure reached 21.3 kPa the infusion was reduced to 2.5 micrograms . kg-1 . min-1 and maintained for 10 minutes. The results show increases in oxygen consumption of 14 per cent with dopamine, 19 per cent with atropine, 16 per cent, and 14 per cent with phenylephrine. All changes were significantly different from the control values. The magnitude of change in whole body oxygen consumption was best predicted by either the cardiac output X blood pressure product or by the cardiac output alone.

Changes in oxygen consumption induced by fentanyl and thiopentone during balanced anaesthesia

Changes in whole body oxygen consumption due to bolus infusions of fentanyl and thiopentone given during "balanced anaesthesia" have been examined in 17 patients. It has been shown that fentanyl causes an average decrease in oxygen consumption of 4.9, 8.4, and 5.2 per cent at 5, 10 and 15 minutes after its infusion. Thiopentone causes an average decrease of 7.0, 8.8, and 1.9 per cent at 5, 10 and 15 minutes after infusion. It is believed that the effects of fentanyl and thiopentone on oxygen consumption are caused mainly by decreases in cerebral and myocardial oxygen consumption.