PRESCRIBING FOR THE ELDERLY
Unpredictability of drug therapy
PRESCRIBING FOR THE ELDERLY
Unpredictability of drug therapy
The elderly differ from younger adults due to alterations in function of specific organ systems, in metabolic rate and capacities, in tissue and receptor sensitivities and in body composition and mass - these and other alterations introduce a larger element of unpredictability regarding drug efficacy and safety.
Two cardinal rules pertain in geriatric prescribing:
* Dose modification (reduction) is likely to be necessary;
* Susceptibility to toxic effects in general is increased, particularly
mental confusion.
Principles of geriatric pharmacotherapy
Non-compliance with drug therapy may be the result of inadequate doctor/patient communication, mental disability (poor memory, apathy or confusion), physical disability (poor vision or loss of manual dexterity) and the inappropriate use of complex dosage regimens.
Some elderly patients also find it difficult to swallow large tablets or to break small tablets in half. Child-proof containers may be difficult for arthritic fingers to manipulate.
Polypharmacy may result from self- medication with over-the-counter (OTC) drugs such as analgesics, cough and cold medicines (often polycomponent), antacids and purgatives.
* Preparations containing significant amounts of alcohol are potentially hazardous if used in combination with many agents especially CNS active drugs.
* The use of a record card, on which details of drug treatment are noted, is recommended. This card should be kept by the patient and should be available at every visit to a doctor, clinic or pharmacy.
Adverse drug reactions increase in incidence as age increases. Furthermore they may present in atypical ways and be ascribed to "old age" by the unwary. For example:
* Digoxin toxicity may present with confusion or psychosis.
* Patients given powerful diuretics for gravitational oedema may present with repeated falls because of postural hypotension.
* Loop diuretics may provoke acute urinary retention or severe incontinence.
* A common mistake is to add another drug to treat the adverse effects of one given earlier, e.g. using prochlorperazine to treat drug-induced hypotension and "dizziness".
Poly-clinic and poly-doctor management of patients, with additional intervention by other health-care providers can further complicate the patients' comprehension of their disorders and treatments, and increase the problems of management. In this situation it is crucial that one doctor, preferably the patient's family practitioner, assumes over-all responsibility for pharmacotherapy. The total use of medicines should be critically scrutinised at regular intervals so that the risk-benefit ratio may be constantly optimised.
Patients who seek medical help for non-medical problems such as loneliness, loss of independence, or personality and domestic problems may convincingly "medicalise" these problems. Health-care providers must be alert to this and endeavour to uncover, and invite full discussion of, the underlying problems.
The elderly are a heterogeneous group with the "old" elderly (those over 75 years) showing greater age-related changes in drug disposition and response than the "young" elderly. The sick elderly patient also appears to handle drugs differently from the well elderly person. Other factors are diet, physical inactivity, bowel motility, state of hydration and unhealthy habits such as smoking and excessive alcohol consumption.
Drug absorption is slightly reduced as age advances due to reduction in small bowel surface area and changes in gastric acidity. Enteric-coated preparations may be unpredictably absorbed. The possible interactions of different drugs are important: among these are the anticholinergic effects of many agents (e.g. antidepressants, antihistamines) which may delay gastric emptying and thereby affect absorption of certain drugs.
Body composition changes with ageing, the older person often being a smaller person, resulting in an increase in body fat initially, and a reduction in body water. Therefore drugs that are distributed mainly in body water (e.g. digoxin) require to be given in reduced dosages. There may be accumulation and prolongation of action of highly lipid-soluble drugs (e.g. diazepam and nitrazepam). Plasma albumin levels are generally lower, especially in the sick elderly, which means that more free drug may be available - this is particularly important in the case of highly protein-bound drugs such as phenytoin and the sulphonylurea group of oral antidiabetic agents.
Hepatic clearance: Phase I hepatic metabolism (oxidative pathways) may be impaired in the elderly, while phase II metabolism (conjugative pathways) remains unchanged. Drugs requiring extensive phase I metabolism (e.g. diazepam) are best avoided or should be used with extreme caution. First-pass extraction of certain orally administered drugs such as propranolol and tricyclic antidepressants may also be variably reduced, resulting in higher blood levels. It is likely that poor nutrition, a common problem in the urban elderly population, may adversely affect hepatic metabolism.
Renal clearance, i.e. the renal excretion, of drugs is the most important parameter that changes. There is a predictable decline in renal function of about 10% per decade after the age of 30. This is aggravated by acute illness, dehydration and hypotension, resulting in diminished ability to excrete water-soluble drugs, many of which have a narrow therapeutic range (e.g. digoxin and the aminoglycosides). Toxic blood levels are thus rapidly reached.
The serum creatinine is, unfortunately, a poor guide to renal function in the very old because of reduced muscle bulk in this age group. A convenient rule of thumb is to reduce the dose of drugs excreted by the kidneys by 50%, and then to monitor levels and clinical response.
Increased tissue sensitivity to drugs such as the benzodiazepine group, morphine and warfarin, has also been demonstrated, while paradoxically there may be decreased sensitivity to propranolol and beta2- adrenoceptor agonist bronchodilator agents, despite higher blood levels, because of reduced receptor sites.
Although the elderly are a heterogeneous group and treatment should always be individualised, the following general guidelines are appropriate:
* Assess whether the patient really needs drug therapy, e.g. salt restriction is the first step in treating hypertension, and the blood glucose of the elderly diabetic need not be normalised.
* Always obtain a comprehensive drug history, including the use of OTC medication; rationalise therapy before attempting to introduce new drugs.
* Use a reduced dose initially, and increase later if clinically indicated.
* Use as few drugs as possible. Dosage schedules should be uniformly timed to improve compliance. Write out the therapeutic schedule for the patient, to enhance compliance. Many drugs can be given once or twice daily instead of three or four times a day.
* Drugs with very long half-lives, e.g. chlorpropamide, are best avoided.
* Tablets that need to be broken in half should be avoided; low-dose "geriatric" formulations are often available, e.g. digoxin 0.0625 mg and glibenclamide 2.5 mg tablets.
* Review long-term therapy from time to time. Drugs should be discontinued if they are no longer indicated.
* Use suitable containers with clearly legible instructions.
* Educate the patient (or a responsible relative) about the drug and what to expect - e.g. that transient dizziness may occur with antihypertensive medication, if one gets up too quickly. All (including elderly) patients have the right to know why they are being treated and with which medication.
* Be aware of the possibility of drug-induced illness.