Control of Carbon Dioxide Levels during Neuroanaesthesia: Current Practice and an Appraisal of our Reliance upon Capnography

Abstract

With the widespread availability of capnography, many anaesthetists have swung away from formally verifying hypocapnia by intraoperative arterial blood gas analysis and, instead, have come to rely upon capnography as an acceptable and constant predictor of arterial CO 2 tension (PaCOJJ during neurosurgery. However, the nature of the arterial-endtidal CO 2 gradient is complex, and is frequently unexpectedly large, or even negative. The importance of close intraoperative CO 2 control during neurosurgery-more specifically, routine hyperventilation, and our reliance upon capnography to guide intraoperative management-is reappraised. There is a growing appreciation of the adverse effects of hyperventilation and hypocarbia, especially upon abnormal or ischaemic brain, and it is clear that capnography alone cannot be used to confidently predict the true P a C0 2 during neuroanaesthesia.

A Retrospective Observational Study Examining the Admission Arterial to End-Tidal Carbon Dioxide Gradient in Intubated Major Trauma Patients

“…While a negligible gradient exists in health, it is an increase in alveolar deadspace in the hypovolaemic trauma patient that contributes to regional ventilation-perfusion mismatch and the observed Pa-etCO 2 gradient. The data from our study confirm earlier comments on the potential for PetCO 2 to be a misleading guide for adequacy of ventilation 6,13,14 . This is relevant as ventilation (via a self-inflating Laerdel TM or Ambu TM bag) in the prehospital setting is typically titrated to PetCO 2 and not with a ventilator with spirometry analysis.…”

Noninvasive monitoring of PaCO2 during one-lung ventilation and minimal access surgery in adults: End-tidal versus transcutaneous techniques

“…Alternatively, areas of low ventilation-perfusion ratios (shunt) result in ineffective gas exchange and the addition of blood with a high partial pressure of CO 2 to the arterial circulation contributing to the increased ET-CO 2 to PaCO 2 gradient. [ 17 – 18 ]…”

Dynamic optic nerve sheath diameter responses to short-term hyperventilation measured with sonography in patients under general anesthesia

“…Carbon dioxide (CO 2 ), a potent cerebral vasodilator, is associated with CBF, which may contribute to ICP change [2,3]. It has been reported that the CBF can be affected dynamically, within a few minutes, in response to arterial carbon dioxide concentration (PaCO 2 ) change and tight control of PaCO 2 can be associated with neurological outcomes [4], as well as important to improve surgical outcomes even in non-neurosurgical patients [5,6]. Therefore, PaCO 2 should be deliberately monitored during mechanically ventilated patients, particularly in neurocritically-ill patients.…”