¹³C-stable isotope analyses technique for ICU patients

Full text

Introduction

By ¹³C isotope spectroscopy, a noninvasive diagnostic tool for specific functions of different organs is available [1]. A ¹³C labelled tracer is administered and is metabolised by a specific pathway to ¹³CO₂. The increase of exhaled ¹³CO₂ is analysed and gives an indication of a specific function of the organ under test. The availability of sensitive (resolution 0.3??) and stable isotope-selective infra-red (IR) spectrometers has replaced expensive mass-spectrometers. The present IR analysers rely on two conditions: 1) on co-operating patients who collect breath samples into containers and; 2) on equilibrated test gas samples.

Methods

Online monitoring of ventilated ICU patients over a long period requires an automated collection of the exhaled gases and an equilibration of the CO₂ content without compromising the lung ventilator set-up. The sample should only contain exhaled gases, a dilution would increase the necessary sensitivity of the analyser. A collection and mixing device has been developed that collects the exhaled gases and does not interfere with the safety and monitoring concept of the lung ventilator. The unit collects the sample gas from an Evita ventilator (Dräger) and supplies it to an Infra-Red Isotope-selective Spectrometer (IRIS, Wagner) which has been modified to analyse a continuous gas stream. The expiratory concentrations of ¹³CO₂ and ¹²CO₂, the ratio of ¹³CO₂ / ¹²CO² as well as the cumulated hourly percentage of the recovery of the administered ¹³C label are determined by evaluation IRIS software (V2.3 1998, Wagner) and stored on a computer.

Results

The risk analysis and technical set-up have been reviewed and certified by the TÜV-Rheinland. The collection unit connecting the lung ventilator to the analyser has proven not to interfere with the functionality of the therapeutic device. The set-up was validated in volunteers and in ICU patients. Results of a first study determining the gastric emptying by isotope techniques will be discussed in parallel on this congress.

Conclusion

The stable IR isotope spectroscopy can be applied to ventilated patients. This allows a non-invasive focus e.g. on liver enzyme activities and on other organ specific problems.