PAIN ON I.V. INJECTION OF SOME ANAESTHETIC AGENTS IS EVOKED BY THE UNPHYSIOLOGICAL OSMOLALITY OR pH OF THEIR FORMULATIONS

Abstract

We have studied the intensity and time-course of pain during and after injection into an isolated vein segment in seven normal subjects of saline or glucose of different osmolalities (0–6 osmol kg⁻¹) or pH (2–13). Pain scores were recorded continuously by a modified visual analogue scale apparatus. With osmolar stimulation, pain occurred at 1.0 osmol kg⁻¹ during perfusion and 3.0 osmol kg⁻¹ with rapid injection and increased with osmolar concentration of both saline and glucose solutions. Acidic and alkaline solutions evoked pain at a pH value < 4 or > 11. We conclude that pain on i.v. injection of some sedative and hypnotic drugs is likely to be caused by formulations of extremely unphysiological osmolalities or pH values.