The degree of motor blockade produced is dependent on the concentration of the solution. The duration of anesthesia also varies depending upon the technique and type of block, the concentration, and the individual. Procaine will normally provide anesthesia which is adequate for 1 hour. Local anesthetics are bound to plasma proteins in varying degrees. Generally, the lower the plasma concentration of drug, the higher the percentage of drug bound to plasma.
Local anesthetics appear to cross the placenta by passive diffusion.
The rate and degree of diffusion is governed by the degree of plasma protein binding, the degree of ionization, and the degree of lipid solubility. The extent of placental transfer is also determined by the degree of ionization and lipid solubility of the drug. Lipid, soluble nonionized drugs readily enter the fetal blood from the maternal circulation. Depending upon the route of administration, local anesthetics are distributed to some extent to all body tissues, with high concentrations found in highly perfused organs such as the liver, lungs, heart, and brain. Various pharmacokinetic parameters of the local anesthetics can be significantly altered by the presence of hepatic or renal disease, addition of epinephrine, factors affecting urinary pH, renal blood flow, the route of drug administration, and the age of the patient.