The Relationship between End Tidal Carbon Dioxide and Arterial Carbon Dioxide during Controlled Hypotensive Anaesthesia

Abstract

Objectives: To prospectively assess the magnitude of changes in the arterial-to-end tidal carbon dioxide gradient [P(a-ET)CO₂] as well as in the ratio of physiological dead space to tidal volume (Vd_phys/Vt) during controlled hypotensive anaesthesia, and to evaluate whether or not ventilatory requirements remain unaltered during this procedure. Subjects and Methods: Twelve adult patients with American Society of Anesthesiologists’ physical status I and II undergoing middle ear surgery were selected. A standard anaesthetic procedure was followed for all cases, using thiopental sodium, succinylcholine, fentanyl, atracurium and 60% N₂O in 40% oxygen supplemented with isoflurane. Mean arterial blood pressure (MAP) was reduced to 60 ± 5 mm Hg in all patients using a sodium nitroprusside infusion. The end tidal (ET) CO₂, PaCO₂, MAP, peak airway pressure, plateau pressure and expiratory minute volume were recorded during a period of normal arterial blood pressure (time 1) and during hypotension (time 2). Results: A significant decrease in PaCO₂ (7%) and ETCO₂ (17%) from time 1 to time 2 (p < 0.01) was noted, as was a significant increase in P(a-ET)CO₂ (48%) and in the Vd_phys/Vt ratio (41.17%) (p < 0.01) during the same period. Conclusion: The decrease in ETCO₂ does not reflect the changes in PaCO₂. The larger decrease in ETCO₂ is mainly due to the increase in the Vd_phys/Vt ratio. During anaesthesia, once normocapnia is achieved with normal arterial blood pressure, there is hardly any need to change the ventilation after initiation of controlled hypotension.