Plasma concentrations of noradrenaline and adrenaline and plasma renin activity during extradural blockade in dogs.

Anaesthetized dogs (pentobarbitone 25 mg kg-1 + 3 mg kg-1 h-1) with a Teflon catheter inserted to the femoral artery for pressure and heart rate measurement and blood sampling were studied. Extradural puncture was carried out at L7--S1 under x-ray control and 2% lignocaine (1 ml min-1) infused for approximately 10 min. During extradural block a decrease in arterial pressure was accompanied by a simultaneous decrease in plasma noradrenaline concentration (NA) and a compensatory increase in plasma renin activity (PRA), which became significant 15 min after commencing the block when the arterial pressure was at the minimum. Dihydroergotamine (DHE) led to a rapid reversal of the hypotension with no change in heart rate and a decrease in PRA. Hypovolaemia (blood loss 5 ml kg-1) was induced before and during block, respectively, in two further experimental groups. Increased NA and PRA were found under hypovolaemia without extradural block, but only PRA increased with hypovolaemia and a block. The effects of DHE in the later studies were comparable to those observed in the initial study.