MONITORING ADEQUACY OF ALPHA-ADRENOCEPTOR BLOCKADE FOLLOWING SYSTEMICPHENTOLAMINE ADMINISTRATION

Systemic phentolamine administration has been suggested as a diagnosti c tool for identifying patients with sympathetically maintained pain ( SMP) (Raja et al. 1991). The dose of phentolamine to produce adequate blockade of peripheral alpha-adrenoceptor function has, however, not b een previously determined. In this study, the effects of two different doses of phentolamine on peripheral sympathetic vasoconstrictor funct ion were investigated. One-hundred and seventeen (117) patients with c hronic extremity pain underwent 130 phentolamine diagnostic tests usin g two different doses of phentolamine (0.5 mg/kg over 20 min (n = 60) and 1 mg/kg over 10 min (n = 59)). Eleven (11) patients did not receiv e phentolamine during the test. Cutaneous temperature was measured in the distal extremity before and after administration of phentolamine, In a subset of patients, baseline blood flow and sympathetically media ted vasoconstrictor response (SMR) to deep inhalation were measured on glabrous skin using laser Doppler flowmetry. SMR was elicited with a 5-sec maximal inspiratory gasp. A dose-related increase in cutaneous t emperature was observed. In addition, baseline blood flow increased an d SMR was attenuated after both doses of phentolamine, but to a greate r degree after the 1 mg/kg dose. However, SMR was not completely atten uated, even after administration of the higher phentolamine dose. Thes e results indicate that a phentolamine dose of 1 mg/kg over 10 min mor e completely blocks alpha-adrenoceptor function than a dose of 0.5 mg/ kg over 20 min. We therefore recommend that to ensure adequate alpha-a drenoceptor blockade the higher phentolamine dose be used in the phent olamine diagnostic test for SMP.