Excretion
Excretion in pregnancy
The excretion rate of drugs in pregnant women is usually increased. The accelerated excretion is necessary to dispose the extra waste generated by the fetus. The body adapts by increasing the renal blood flow by 50% and the glomerular filtration rate by 60%. There is also up to a 20% increase in reabsorption into the tubules.
Example:
If the use of antibiotics such as penicillin is needed during pregnancy, these drugs will be eliminated quicker from the body and dose adjustments are required.
A woman has a drinking habit. You regularly monitor her blood alcohol level. As a doctor you know that below a blood level of 0.3 g/L the elimination of alcohol changes form zero-order to first-order. This means that at a blood level below 0.3 g/L the elimination rate
Extra info: Zero-order elimination implies a constant amount of drug eliminated per unit of time. Alcohol is eliminated mainly metabolically by an enzyme system called alcohol dehydrogenase. This system becomes saturated at blood alcohol levels higher than 0.3 g/L, and processes the alcohol at maximal constant speed. First-order elmination implies a constant fraction of drug eliminated per unit of time. At lower alcohol level this alcohol dehydrogenase system is non-saturated and not performing full speed. The elimina
A mother with a constant lithium-level of 0.7 mmol/L breastfeeds a baby (6 kg) with 200 ml breastmilk every 4 hr. The distribution of lithium over the blood-compartment equals the distribution over the breastmilk compartment. The kinetics of lithium in mother and child is first-order; the half-life of lithium is 24 hr. The Vd of lithium is 0.66 L/kg. What is the maximal lithium level that can be reached in the blood of the child ?
Extra info: Pharmacokinetically the child follows a repeated dose schedule. Therefore you may use the formula Csteady state = F x D/Cl x dose interval with F (biological availability) = 1 assuming that lithium is completely absorbed after oral ingestion and the dose interval is 4 h Calculate the total dose (Dtotal) Dtotal = 200 ml = 0.2 x 0.7 mmol lithium = 0.14 mmol lithium Calculate the total volume of distribution (Vdtotal) for the child Vdtotal = 0.66 L/kg x 6 kg = 4 L Calculate the clearance Cl = (0.693/T1/2) x V
