Subarachnoid local anesthetic block does not affect morphine absorption from paired intramuscular and subcutaneous injection sites in the elderly patient.

Morphine absorption from subcutaneous and intramuscular injection sites is dependent on local skin and muscle blood flow. Normally, drug absorption from intramuscular and subcutaneous injections at the same anatomical site is comparable, but spinal anesthesia alters muscle and skin blood flow in the blocked area. This study measured morphine absorption from pairs of intramuscular and subcutaneous injection sites at vastus lateralis and deltoid following a single dose of morphine sulfate in the presence of spinal anesthesia. Differences between the maximum concentration achieved after injection (Cmax) and the time at which this occurred (Tmax) were not significant when absorption for each "same-site" pair of injections was compared, but Tmax after intramuscular injection at vastus lateralis was significantly longer than that observed following either intramuscular or subcutaneous injection at the deltoid (P < 0.05 for both comparisons). Cmax/Tmax ratios and the area under concentration time curve (AUC) also were analyzed, but there were no statistically significant differences among any of the four groups.