Physicochemical Characteristics

Etomidate is an imidazole derivative (R-(+)-pentylethyl-1H-imidazole-5 carboxylate sulfate).^[576] Its chemical structure is illustrated in Figure 10-22 . Etomidate exists as two isomers, but only the (+) isomer is active as a hypnotic. ^[582] Its molecular weight is 342.36 kd.^[582] Etomidate is water insoluble and is unstable in a neutral solution. It therefore has been formulated with several solvents.^[583] In the United States it is supplied as a 2-mg/mL propylene glycol (35% by volume) solution with a pH of 6.9 and an osmolality of 4640 mOsm/L. In Europe, a new formulation in a lipid emulsion has been introduced in an attempt to reduce some of the side effects of etomidate.^[584] Unlike sodium thiopental, when etomidate is mixed with other commonly used anesthetic agents such as neuromuscular blockers, vasoactive drugs, or lidocaine, it does not cause precipitation.^[585]