Responses of EEG, cerebral oxygen consumption and blood flow to peripheral nerve stimulation during thiopentone anaesthesia in the dog.

The effects of sciatic nerve stimulation on the electroencephalogram (EEG), cerebral metabolic rate for oxygen (CMRO2) and cerebral blood flow (CBF) were investigated during thiopentone anaesthesia in dogs. Anaesthetic levels at 15, 35, 65, 95 and 125 minutes after the start of thiopentone infusion (23 mg X kg-1 X hr-1) were designated levels I, II, III, IV and V, respectively. The effects of stimulation for 5 min were tested at each level. At level I (plasma thiopentone concentration; 15 +/- 2 micrograms X ml-1), the EEG was activated with stimulation and CMRO2 and CBF increased by a maximum of 16 and 15 per cent, respectively. The increase in CMRO2 and CBF was significant for five and four minutes, respectively, though the increase became less with time. At level II (27 +/- 3 micrograms X ml-1), the CMRO2 and CBF increased at one minute by eight and nine per cent, the increase being accompanied by transient EEG activation. At the three deepest levels III, IV and V (37 +/- 6, 42 +/- 6, 49 +/- 6 micrograms X ml-1), the EEG, CMRO2 and CBF remained unchanged with stimulation. The results suggest the existence of the tight coupling between the EEG, CMRO2 and CBF and of a threshold level of thiopentone to block the response to peripheral stimulation during thiopentone anaesthesia.