Tissue distribution and age
Changes in distribution
As we age, the distribution of drugs also changes. The illustration shows how we gain adipose tissue and lose tissue and intracellular water as we age. This should be considered when giving either hydrophilic or lipophilic drugs. For example, an older patient typically has a smaller Vd of hydrophilic antibiotics than a younger patient (and this is one of the reasons for a smaller doses). In obese patients, clinicians need to adjust the doses of lipid-soluble drugs. Conversely, patients who are volume overloaded secondary to heart failure have increased Vd for digoxin and theophylline. The same is true for the increased lean body mass seen in patients with cystic fibrosis. They have a noticeable increase in their Vd for aminoglycosides.
Notice that dose adjustments are not only made based on the distribution volume of a patient. The clearance of the drug in a patient (dependent on the metabolism and excretion of the drug) can also affect the blood plasma concentration, and dose regimens may need adjustment based on the condition of the patient (e.g. CYP polymorphisms or kidney function).