[Transcranial Doppler sonography: halothane increases average blood flow velocity in the middle cerebral artery]

Transcranial Doppler sonography (TCD) can non-invasively measure the blood flow velocity of basal cerebral arteries, especially of the middle cerebral artery (MCA). TCD has been used for monitoring the cerebral circulation during cardiopulmonary bypass or cross-clamping of the carotid artery. To date, systemic investigation of the effects of anesthetic agents on mean blood flow velocity in the MCA (vMCA) has been neglected, although their effects on cerebral blood flow are well known. Two groups of 10 patients each with minor surgical or gynecological operations were studied using TC2-64 (EME) TCD equipment. Constant artificial ventilation with N2O:O2 = 2:1 was controlled by capnometry; capillary pCO2 ranged from 35 to 40 mmHg in both groups. In group I, patients underwent general anaesthesia with approximately 0.8 vol% halothane. vMCA, blood pressure, and heart rate were determined before and during anesthesia as well as after skin incision. Group II patients received fentanyl and droperidol. Measurements were performed at time points comparable to those in group I. Halothane caused an increase in vMCA by more than one third of the preanesthetic value (from 54.4 +/- 12.2. to 84.2 +/- 23.9 (P less than 0.01]. In group II (neuroleptanesthesia), no significant alterations of vMCA or hemodynamic parameters were observed. These results show that anesthetic agents can influence vMCA as well as cerebral blood flow. Therefore, these data should be taken into account while monitoring vMCA during operative procedures. However it has to be considered that TCD only permits measurement of blood flow velocity, while the diameter of the investigated vessel remains unknown. Clinical applications of this fascinating noninvasive monitoring device are discussed.