The relationship between end tidal carbon dioxide and arterial carbon dioxide during controlled hypotensive anaesthesia.
OBJECTIVE
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 (Vdphys/Vt) during controlled hypotensive anaesthesia, and to evaluate whether or not ventilatory requirements remain unaltered during this procedure.


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 Vdphys/Vt ratio (41.17%) (p < 0.01) during the same period.


CONCLUSIONS
The decrease in ETCO2 does not reflect the changes in PaCO2. The larger decrease in ETCO2 is mainly due to the increase in the Vdphys/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.
Ali
Syed Shujat
SS
Department of Anaesthesia and Intensive Care, Al Sabah Hospital, PO Box 4078, Safat 13041, Kuwait. drshujatali@yahoo.com
Dubikaitis
Alexander
A
al Qattan
Abdul Raheem
AR
D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000032:analysis* / Q000097:blood; D004432:Ear, Middle / Q000601:surgery; D005260:Female; D006801:Humans; D007023:Hypotension, Controlled; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012119:Respiration; D012121:Respiration, Artificial; D012129:Respiratory Function Tests
D002245:Carbon Dioxide