Figure 3-6 The relationship between concentration, expressed as a fraction of K_m (Michaelis constant), and drug metabolism, expressed as a fraction of V_m (maximum rate). As long as the outflow concentration is less than one-half K_m , a change in concentration is matched by a proportional change in metabolic rate. This is true of nearly all anesthetic drugs. (Adapted from Shafer S: Principles of pharmacokinetics and pharmacodynamics. In Longnecker DE, Tinker JH, Morgan GE [eds]: Principles and Practice of Anesthesiology, 2nd ed. St. Louis, Mosby-Year Book, 1997.)