TRANSCRANIAL DOPPLER SONOGRAPHY DURING ISOFLURANE N2O ANESTHESIA AND SURGERY - FLOW VELOCITY, VESSEL-AREA AND VOLUME-FLOW

Transcranial Doppler sonography (TCD) constitutes an advance in noninv asive monitoring of the cerebral circulation. However as long as the d iameter and cross-sectional area of the insonated middle cerebral arte ry (MCA) remain unknown, the derived flow velocities (v) are not infor mative. It is not known how the human MCA is influenced by anaesthetic agents. However, a TCD-modification allows noninvasive determination of ''vessel area'' (VA) and ''volume flow'' (VF) in MCA by analysing t he backscattered Doppler power. This investigation evaluates the effec ts of isoflurane (in combination with N2O and surgery) on v, VA and VF In 14 patients (ASA I) scheduled for minor surgical or gynaecological operations, anaesthesia was induced with droperidol, alfentanil, thio pentone and vecuronium. After intubation ventilation with N2O:O-2 = 3: 2 was adjusted, to maintain endexpiratory carbon dioxide (FECO(2)) con stant between 4 and 5%. Baseline values of heart rate (HR), oscillomet ric mean arterial pressure (MAP), and TCD variables (v, VA VF) were me asured before adding 2.4% isoflurane to the inspiratory mixture. Furth er measurements were made 3, 6, 10, and 20 min after starting isoflura ne. Surgery commenced between the sixth and tenth minute after isoflur ane application. The MAP FECO(2) and v showed only minor alterations; HR increased after 6, 10 and 20 min. Transcranial ''vessel area'' and ''volume flow'' showed increases after isoflurane inhalation. lte incr ease of ''vessel area'' supports the assumption that isoflurane greate r than 1 MAC dilates large human cerebral arteries, so that if flow ve locities are considered alone, alterations of cerebral blood flow may easily be underestimated.