Dehydration is a reduction in total body water volume and may be defined as significant when over 3% of body weight is lost. However, it is often difficult to determine precisely how much weight has been lost and whether it is all due to water loss. It is usually regarded as present when it is accompanied by changes in biochemical indices and by clinical features (see below, under-diagnosis).

Prevalence of dehydration in institutional care

Studies within nursing homes have found that dehydration is frequent. One prospective study found dehydration events occurred in 31% of residents over 6 months and another found that 98% of residents consumed less than the daily recommended fluid intake. In another study, some 91 of 339 elderly nursing home residents who became ill had biochemical features of dehydration. Many hospital admissions of nursing home residents are associated with dehydration and the electrolyte disturbances that may indicate it. In one study, 34% of nursing home patients admitted to the hospital were diagnosed with dehydration. Another study found 84% of hypernatraemic patients developed this during admission to the hospital – only 16% were hypernatraemic on admission.

Consequences of dehydration

Dehydration is associated with increased hospitalization and mortality. It may not be easy to distinguish between poor outcomes due to an underlying illness and poor outcomes from it itself. In one study of 130 nursing home residents, there were 48 febrile episodes over a 4 month period and 14 febrile residents had biochemical markers of dehydration. Of the 5 febrile residents who died, all had markers of hypohydration. This unintended dehydration and associated increased mortality should be distinguished from the hypohydration that frequently accompanies terminal illnesses such as cancer and renal failure.

Risk factors for dehydration

The greatest risk factor for dehydration is poor oral intake. In the study of 48 febrile episodes in nursing home residents, 11 patients were noted by staff to have poor oral intake and nine of these (82%) developed biochemical markers of hypohydration. In a study of hospitalized patients, 86% of patients who developed hypernatraemia in hospitals lacked free access to water.

Other significant risk factors for dehydration in a study of 339 elderly residents who became ill included female gender, age over 85, four or more chronic conditions, use of more than four medications, and being bedridden. Among those who were most severely dehydrated, inability to feed oneself and impaired functional status were additional risk factors. In another study, diuretic use was a risk factor for electrolyte disturbances in elderly people requiring hospitalization. Diuretic use is a recognized risk factor in the genesis of renal impairment and electrolyte disturbances, especially in older people, and is likely to increase the risk of dehydration in nursing home residents.

The availability of appropriately skilled staff to assist residents is also a factor that contributes to the risk of dehydration. Inadequate staffing, lack of assessment, and disregard for personal and cultural preferences contribute to inadequate fluid intake and dehydration in residential care. These findings have been replicated by others – in a recent study weight loss and hypohydration were 17% less likely in facilities that provided residents with at least 3 hours of nursing assistant care daily compared with those providing less than 3 hours daily.

Older people have a reduced thirst in response to fluid deprivation and their hormonal response to hypohydration (secretion of anti-diuretic hormone) may also be impaired. These changes maybe even more pronounced in residents with Alzheimer’s disease, a common condition in residential care. These factors both make older people more prone to dehydration and also indicate that thirst cannot be relied upon as an indicator of dehydration.

Normal Fluid Intake

Fluid intake must replace measurable losses (urine, feces, and occasionally others such as drain tubes) and insensible (not easily measured) losses from respiration and through the skin. The recommended minimum total fluid intake is 1500–2000 mL, (equivalent to 6–8 250 mL cups) a day. This comes from all sources including soups and beverages.

Signs of dehydration include dry mucous membranes, reduced tissue turgor (elasticity), reduced sweating, sunken eyes, tachycardia, low blood pressure, and postural blood pressure drop, altered consciousness including confusion, increasing functional impairment, weakness, constipation, reduced urine output, and more concentrated (darker) urine. Unfortunately, many of these signs are quite subjective, with no defined “normal” ranges, and thus poor positive and negative predictive values for the diagnosis of dehydration. Some of these signs can be present in other conditions – for instance, low blood pressure can be due to overtreatment with medications that lower blood pressure, in cardiac failure, and when there is autonomic neuropathy. Indeed, over-reliance on low blood pressure as a sign may lead to overdiagnosis of dehydration.

In a study of 102 consecutive medical admissions in people older than 65 with a diagnostic coding of dehydration (16% admitted from nursing homes), only 17% had biochemically confirmed dehydration (serum osmolarity above 295 mOsmol) suggesting over-diagnosis by physicians, probably due to over-reliance on physical signs or other less accurate biochemical indices. Another study of 150 elderly patients and residents with dehydration (defined as hypernatraemia) found that most of the classical signs of hypohydration were irregularly present. There were four signs that were significantly and independently associated with hypernatraemia – abnormal subclavicular and thigh skin turgor, dry oral mucosa, and recent changes in consciousness.

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