Systemic recirculation assessed in apnoeic anaesthetized patients using carbon dioxide concentration measurements during stepwise expiration.
BACKGROUND
Mixed venous partial pressure of carbon dioxide has often been estimated in anaesthesia, usually by rebreathing techniques. This assumes equilibrium between respired gas and mixed venous blood before significant recirculation can occur, and requires vigorous rebreathing and precise identification of equilibrium. Modern clinical sidestream capnometers do not measure gas composition as rapidly as the devices used in previous studies, and cannot measure end-tidal values adequately during vigorous rebreathing. In contrast, a single-staged exhalation provides a stable sample for more accurate measurement of gas composition, and clinical measurements then allow examination of the evidence that equilibration occurs before recirculation. Theoretically, this assumption is optimistic, although it forms the basis of a method for non-invasive measurement of cardiac output. We used measurements of staged exhalation to study the evidence for equilibrium more closely.


METHODS
We studied 19 patients during general anaesthesia. The lungs were inflated with mixed exhaled gas. Stepwise expiration allowed exhaled gas to be analysed over approximately 1 min. The rate of increase in exhaled carbon dioxide fraction was related to the duration of expiration.


RESULTS
Carbon dioxide concentration continued to change throughout the study period. The lack of equilibrium of carbon dioxide concentration over this time supports simulation studies which predict recirculation from well-perfused body compartments within 20 s, and that the subsequent increase of carbon dioxide reflects the wash in of multiple body compartments.


CONCLUSIONS
This method allows adequate time for full response of sidestream analysers. Recirculation is an important early feature affecting breath-hold estimates of mixed venous carbon dioxide. Carbon dioxide accumulation during apnoea or rebreathing will have prompt effects on arterial carbon dioxide values and a constant mixed venous composition cannot be assumed when methods based on partial rebreathing are used over this time period.
Department of Anaesthesia, Critical Care and Pain Medicine, University of Edinburgh, Royal Infirmary, Edinburgh EH16 4SA, UK.
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Pizzichetta
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Drummond
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000769:Anesthesia, Inhalation; D001049:Apnea / Q000503:physiopathology*; D019296:Capnography; D002245:Carbon Dioxide / Q000097:blood / Q000502:physiology*; D045853:Exhalation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D010313:Partial Pressure; D011650:Pulmonary Alveoli / Q000378:metabolism; D011659:Pulmonary Gas Exchange / Q000502:physiology
D002245:Carbon Dioxide