Adaptation of the Quadrupole Mass Spectrometer to Multipatient Anaesthesia Gas Monitoring

Abstract

A series of modifications designed to increase the suitability of the quadrupole mass spectrometer for multipatient gas monitoring in the operating suite are presented and evaluated. The adaptations include a pressure-stabilised long capillary inlet system and computer control of the quadrupole filter together with tuning, calibration and subsequent multitheatre analysis and display of clinical information. A pilot study of three months clinical monitoring provided documentation of contributions towards the safety and effectiveness of anaesthesia. These included indication of equipment malfunction, and fluctuations in gas exchange associated with inappropriate ventilation, haemodynamic instability and variations in body temperature. Limited accuracy for carbon dioxide and a low signal-to-noise ratio together with rapid ageing of electron multipliers were identified as problems still only partially solved.

Model-based administration of inhalation anaesthesia. 3. Validating the system model

“…Carbon dioxide readings lower than expected have been reported. 24 We found that carbon dioxide levels, as measured by the mass spectrometer, were invariably lower than those assessed by the infrared analyser incorporated in the anaesthesia system, at least during controlled ventilation. It is puzzling, however, why calibration mixtures showed no error.…”

“…The ventilation in the model was matched to that measured for each patient as follows. A target value for the`ideal' alveolar carbon dioxide tension (PA CO 2 , assumed equal to Pa CO 2 ) was obtained from: (i) the ventilation measured (averaged over the period of administration of des¯urane); (ii) a chosen value of 50% for wasted ventilation; 1 and (iii) a speci®c, rearranged form of equation (24) in Appendix 2 of reference 1. The value obtained was used by the model to calculate alveolar ventilation (see below).…”