Optimum duration of hyperventilation during electroencephalography

Abstract

Hyperventilation (HV) is carried out for 3 minutes as a standard activation procedure in most routine electroencephalographic (EEG) procedures. The cerebral blood flow (CBF) reduction and the accompanying cerebral vasoconstriction caused by HV is believed to be the mechanism of EEG activation during HV. Some advocate for 5 minutes of HV, although the optimum duration is unknown. In this study, we measured the CBF continuously over the anterior temporal lobes using subdural probes, which use thermal diffusion flowmetry to measure CBF directly from the cerebral cortex. We sought to determine the duration of HV that produces the maximum reduction in CBF during routine HV in our epilepsy monitoring unit and prolonged the procedure for an additional 2 minutes for this study. Flowtronics^® CBF probes were placed over the anterior temporal lobes in addition to the standard subdural strip placement for localization of their seizure focus in six patients who were candidates for epilepsy surgery. CBF was measured continuously for 2 minutes before and 5 minutes during HV for each patient. Time to reach maximum reduction of CBF for each attempt (11 temporal lobes) was computed. At 3 minutes, CBF reduction ranged from 11.6% to 40.0% from the pre-HV CBF level (mean 23.9%). At 5 minutes, CBF ranged from 14.3% to 42.0% (mean 25.7%). Six of the 11 measurements were steady or decreased slightly, and in the five other measurements, CBF showed a reverse trend after 3 minutes. A significant CBF reduction was attained in 3 minutes of HV in all trials. Continued HV after 3 minutes resulted in only a marginal (mean 1.8%) additional CBF reduction after 3 minutes. Thus, we propose that 3 minutes of HV is sufficient for EEG activation by the CBF criterion.

Keywords: Cerebral blood flow; electroencephalography; epilepsy; hyperventilation; seizure.

Figure 1.

(a) Placement of the medial, lateral, and frontal subdural strips and the continuous cerebral blood flow (CCBF) probe over the lateral surface of the anterior temporal lobe on the left side, as labeled for localization of the seizure focus in our Level 4 epilepsy monitoring unit. (b) The Flowtronics^® CCBF probe showing the distal platinum and proximal gold contacts, which serve as the thermistor as well as the thermocouple (sensor) used for the measurement of CCBF as it is being inserted into the temporal burr hole. (c) The modified Flowtronics^® CCBF probe showing the addition of the EEG leads to derive EEG information from the same contacts used as the thermocouple for measuring CCBF. (This modification was courtesy to the author by Flowtronics Corporation.)

Figure 2.

(a) Continuous cerebral blood flow (CBF) during 2 minutes of baseline (pre-hyperventilation [HV]) and 5 minutes of HV showing a relatively rapid reduction in CBF as HV starts. A trough level is reached by 3 minutes, with minimal changes noted in the last 2 minutes of HV. (b) Percentage reduction of CBF from baseline as HV starts with a rise (return toward) baseline in four instances after 4 minutes.