Changes In Median Frequency Research Articles

Cerebral Blood Flow Velocity in Relation to Cerebral Blood Flow, Cerebral Metabolic Rate for Oxygen, and Electroencephalogram Analysis During Isoflurane Anesthesia in Dogs

The purpose of this study was to correlate changes in cerebral blood flow velocity (Vmean) with cerebral blood flow (CBF) during isoflurane anesthesia in dogs. The relation between cerebral oxygen consumption (CMRO2) and electroencephalogram (EEG) analysis also was investigated. Blood flow velocity was measured in the middle cerebral artery using a pulsed transcranial Doppler (TCD). CBF was measured with radioactive microspheres. EEG was measured over both hemispheres and median EEG frequency (median frequency) was calculated after fast Fourier transformation. Baseline anesthesia was maintained with 50% nitrous oxide in oxygen and 50 μg·kg−1·h−1 fentanyl. Animals of Group I (control, n = 6) were not given isoflurane. Data were recorded at baseline, and at 30, 60, and 90 min. There was no significant change in any variable over time. In Group II (n = 7), data were recorded at baseline and at 1%, 2%, and 3% end-tidal isoflurane. Mean arterial pressure was maintained at baseline levels by phenylephrine infusion. CBF increased from 70.8 ± 10.6 mL·100 g−1·min−1 at baseline to 146.1 ± 36.9 mL·100 g−1·min−1 with 3% isoflurane (P < 0.01). Vmean increased from 38.3 ± 6.7 cm/s to 65.6 ± 9.7 cm/s (P < 0.01). The correlation between relative changes in CBF and Vmean was r = 0.94 (P < 0.01). With 1% isoflurane the EEG shifted to slow-wave, high-voltage activity, and median frequency decreased from 5.9 ± 0.7 Hz to 1.4 ± 0.4 Hz (P < 0.05). Median frequency was not decreased further during 2% and 3% isoflurane anesthesia. In contrast, CMRO2 did not change with 1% isoflurane, but decreased 50% with 3% isoflurane (P < 0.05). The correlation between median frequency and CMRO2 was not significant (r = 0.19, P > 0.05). A negative correlation was found between periods of electrical silence and CMRO2 (r = −0.73, P < 0.05). We conclude that relative increases in CBF can be assessed by Vmean. In contrast, even though EEG patterns indicate changes in anesthetic depth, depression of cerebral metabolism is not closely related to changes in median frequency. (Anesth Analg 1993;76:1222-6)

Effects of electrode location on myoelectric conduction velocity and median frequency estimates.

The effect of surface electrode location on the estimates of the median frequency and conduction velocity of the myoelectric signal was investigated. The locations were identified with respect to the innervation zone and the tendonous portion of the tibialis anterior muscle. Considerable modifications in the median frequency and conduction velocity parameters were noted. The highest values of the median frequency occurred at the region of the innervation zone and tendonous insertion of the muscle, and decreased proportionally with distance from these areas. The rate of change of median frequency was not effected by electrode location. Estimates of conduction velocity were most stable in a region between the distal tendon and the adjacent innervation zone. This region also provided the best linear fit when comparing conduction velocity to median frequency estimates. The implications for signal detection procedures are discussed.