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Misleading end-tidal CO2 tensions
1Department of Anaesthesia, SMBD-Jewish General Hospital, Montreal, Canada.
Purpose:
The aim of this study was to determine if the literature supported the assumption that the values and changes in end-tidal PCO2 (PETCO2) during anaesthesia accurately reflect the values and changes in arterial PCO2 (PaCO2) is tenable.
Methods:
The information was obtained by (a) a Medline literature search and the appropriate references quoted in the list generated; (b) appropriate abstracts in recent issues of the annual meeting supplements of Anesth Analg, Anesthesiology, Br J Anaesth and Can J Anaesth. We specifically sought information obtained during major operations, in sick patients, and reports of serial measurements. The information obtained is summarized in graphic form, with a discussion of the mechanisms and clinical implications.
Results:
(1) Patients with systemic disease, or when placed in the lateral position, or with haemodynamic instability have an increased Pa-PETCO2 gradient. The values during surgery are probably due to marked alterations of ventilation: perfusion relationships. (2) In a number of reports, the gradient varied widely during the procedure. (3) The gradient may be reduced due to an alteration of the configuration of the alveolar plateau. (4) The magnitude and direction of change in PaCO2 and PETCO2 can be disproportionate and in the opposite direction.
Conclusion:
End-tidal PCO2 is often not indicative of PaCO2. Also, changes in PETCO2 do not always accurately indicate the direction and extent of the change in PaCO2.