Once in the cardiovascular circulation, the medication is carried to its site of action. Again, there are areas of nursing knowledge that are important for consideration. It is useful to understand how the treatment is carried in the blood, since many medicines are bound tightly to plasma proteins while others are not. If a medicine has a high affinity for plasma proteins it may be necessary to give a large dose of the medication to achieve a therapeutic effect since only the proportion of the medicine that is not bound to the plasma proteins can be used effectively. Commonly used medicines with a high affinity for plasma proteins (more than 80% bound) include the antidepressant amitriptyline, and the anxiolytic diazepam. Other medicines with a high affinity for plasma proteins are propanolol, warfarin, furosemide and the antibiotics erythromycin and rifampicin. A comprehensive review of protein bound medications is offered by Kee & Hayes (2005).

When planning to administer medicines, a factor that may affect the client’s concentration of plasma proteins should be considered. This is because an individual with a reduced albumin concentration may be at risk of toxicity if a dose of medication with a high affinity for plasma protein is given. Other issues that should be considered in relation to the distribution of medication via the cardiovascular system include the rate and volume of perfusion to the desired area as any reduction in access to the required site may reduce the impact of the treatment offered. Additionally some areas of the body are protected from receiving many medications as a result of physiological barriers. The brain is one such area as the meninges around the central nervous system create a blood–brain barrier. Another area that selectively reduces the passage of substances is evident during pregnancy when the placenta offers a barrier between mother and baby. However, it should be noted that some substances may pass through these barriers. In nursing, the implications of the effectiveness of such barriers must be addressed when making informed decisions about nursing care. It is critical to assess the possibility of pregnancy in all premenopausal women who require medicinal treatment, with particular awareness of the potentially hazardous effects in causing fetal abnormalities of drugs such as phenytoin and tetracyclines, which can pass the placental barrier. Finally, the distribution of medication to infants via maternal breast milk must also be acknowledged. In some cases medication given to the mother may be safe for her, but can have toxic effects on the baby. For more information about the effects of specific medications in breastfeeding please refer to the Annotated Further Reading at the end of the chapter.

After a variable period of time in the body the medication given to the client will be excreted. This is mostly via the kidneys, although other routes of excretion include the lungs, via bile into faeces, and in lactating mothers, breast milk. There may also be some excretion through sweat glands onto the skin surface (Downie et al 2007). Perhaps the most important factor in medicine excretion, however, is the speed at which it is lost from the body. The balance between absorption and excretion determines the half-life of a medicine. This is the time it takes for the concentration of the medicine in the plasma to fall by 50% (Downie et al 2007). Clearly, a medicine with a long half-life can accumulate easily within the plasma. Those medicines that are particularly problematic are monitored carefully through checking blood plasma levels to ensure that plasma levels do not become dangerously high.

An important consideration for nurses, therefore, relates to factors about the client that may affect the response and half-life of a medicine. For example, clients who are at the extremes of the age continuum or have renal impairment require particular consideration. The infant does not develop full renal and urinary tract function until after the first year of life, while the renal function of the elderly (over 80 years of age) deteriorates to about half the capacity of a young adult. Renal impairment can affect the excretion of medicines from the body, resulting in a build-up of metabolites, or in some instances medicines, in the circulation. A knowledge of such possibilities may alert you to making a decision to include specific points for observation related to medication in the client’s plan of care. A full introduction to factors that may affect the response of medication can be found in Kee and Hayes, 2005 and Downie et al., 2007: Ch. 6).