621. Zero-order elimination is independent on plasma concentration of the drug like ethanol. Phenytoin converts from 1st order to zero-order when enzyme saturated. 1st order is dependent on the levels of the drug; means increase conc. Increase elimination like 80 mg à 40 mgà 20 mgà 10 mg. Most drugs follow 1st order kinetics, so each has fixed half-life.
622. Dose = Vd*C◦(initial plasma conc.).
623. Loading dose = Vd*Css.
624. Infusion rate (IR) = clearance (CL)*Css.
625. Maintenance dose (MD) = CL*Css*(dosing interval).
PS, Vd is volume of distribution, Css is steady state concentration.
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