SPECIAL ARTICLE Fluid and Electrolytes in the Aged Andrew E. Luckey, MD; Cyrus J. Parsa, MD Objective: To review the physiological changes in fluid population in the United States is individuals 65 years and electrolytes that occur in aging. or older. It is imperative that health care professionals review the physiological changes that manifest during the Data Sources: Data collected for this review were aging process. Fluids and electrolytes are important pe- identified from a MEDLINE database search of the rioperative factors that undergo age-related changes. These English-language literature. The indexing terms were changes include impaired thirst perception; decreased fluids, intravenous fluids, fluid resuscitation, fluid manage- glomerular filtration rate; alterations in hormone levels, ment, perioperative, electrolytes, aged, elderly, hemody- including antidiuretic hormone, atrial natriuretic pep- namics, hyponatremia, hypernatremia, hypocalcemia, tide, and aldosterone; decreased urinary concentrating hypercalcemia, hypomagnesemia, hypermagnesemia, hypo- ability; and limitations in excretion of water, sodium, po- phosphatemia, hypokalemia, and hyperkalemia. Relevant tassium, and acid. references from articles obtained by means of the above search terms were also used. Conclusions: There are age-related alterations in the ho- meostatic mechanisms used to maintain electrolyte and Study Selection: All pertinent studies were included. water balance. Health care providers must familiarize Only articles that were case presentations or did not spe- themselves with these alterations to guide treatment of cifically address the topic were excluded. this growing population. Data Synthesis: The fastest-growing segment of the Arch Surg. 2003;138:1055-1060 INCE THE fastest-growing seg- tration rate (GFR), decreased urinary con- ment of the population is centrating ability, and narrowed limits for people older than 65 years, pe- the excretion of water, sodium, potas- rioperative treatment of the sium, and acid. The homeostatic mecha- elderly has become a field of nisms for fluid and electrolyte balance in the Sincreased significance. Currently, 12.7% of elderly are capable of dealing with the nor- the population is 65 years or older. By 2040, mal daily variations in intake and output. the US Census Bureau estimates their con- Despite this fact, surgical illness and the po- tribution to double to compose 23% of the tential for iatrogenic stresses can disrupt US population. The segment of the popu- these labile mechanisms, resulting in added lation 85 years or older is the fastest- morbidity to the elderly patient undergo- growing portion of the population, com- ing surgery. To understand these changes, posing 5.4% of Americans by 2040.1 This it is important to review the normal physi- trend predicates older patients presenting ological changes in renal function and fluid- for surgical evaluation. Advances in medi- electrolyte balance associated with aging: cal science have permitted and mandated higher-risk patients in the operative suite; • Decrease in total body water close attention to fluid balance consti- • Decrease in GFR tutes a critical aspect of the perioperative • Decrease in urinary concentrating ability care of these patients. • Increase in antidiuretic hormone (ADH) Normal physiological changes of ag- • Increase in atrial natriuretic peptide From the Department of ing increase the likelihood of fluid- (ANP) Surgery, University of electrolyte disorders in the elderly surgical • Decrease in aldosterone California, San Francisco–East patient. The most important of these • Decrease in thirst mechanism Bay, Oakland. changes are a decrease in the glomerular fil- • Decrease in free-water clearance (REPRINTED) ARCH SURG/ VOL 138, OCT 2003 WWW.ARCHSURG.COM 1055 ©2003 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/26/2021 this decrease, the serum creatinine concentration re- 30-y-Old 160 mains within normal limits in the healthy aged. This para- 85-y-Old 140 dox is due to the decrease in muscle mass (the principal determinant of creatinine production) that accompa- 120 nies the aging process. The decrease in GFR parallels the 100 decrease in muscle mass; hence, serum creatinine level remains constant with age. Notably, an increased serum 80 creatinine in an elderly patient implies a much lower GFR 60 than in a younger patient with a similar creatinine level Figure 40 ( ). An elevated creatinine level in an elderly pa- Creatinine Clearance, mL/min tient implies a GFR low enough that any small physi- 20 ological stress could result in frank symptoms of ure- 10 0 mia. Hence, the elderly surgical patient is at increased 0.7 1.0 1.5 2.0 2.5 risk for virtually every cause of acute renal failure, an out- Serum Creatinine, mg/dL come associated with a mortality of greater than 50%.10 The normal renal response to anesthesia and sur- The relationship of age, serum creatinine level, and creatinine clearance. Curves are standardized for a 70-kg man with variable baseline creatinine gery does not appear to differ significantly between older 10 values by means of the Cockroft-Gault formula11 (adapted from Beck3). To and younger patients. Although GFR is directly de- convert serum creatinine to micromoles per liter, multiply by 88.4. pressed by inhalation anesthetics,12 the more deleteri- ous effects on GFR and renal blood flow arise from a de- FUNCTIONAL AND ANATOMIC CHANGES crease in cardiac output. Hypothermia and/or intravascular IN THE KIDNEYS loss will impact systemic blood pressure and cardiac out- put and result in a concomitant reduction in renal blood As with other organ systems, there is a progressive de- flow. Under elective surgical conditions, none of the above crease in the baseline function of the kidney after young factors should result in clinically significant alteration of adulthood. Longitudinal studies have shown significant the GFR. Most patients, however, are relatively oliguric variation in the rates of change in renal function among during surgery and in the immediate postoperative pe- otherwise similar individuals, so it remains uncertain riod. Urine tends to be concentrated, principally be- whether these common changes reflect subclinical dis- cause of the stimulation of ADH as a direct consequence ease or normal aging.2 In most individuals between the of the procedure and the anesthetic agents used. ages of 30 and 85 years, there is a 20% to 25% loss of Body fluid homeostasis can usually be maintained renal mass, most of which is cortex.3 The aging kidney under normal circumstances. Problems occur when the also exhibits hyalinization of blood vessel walls and a de- older patient is stressed by illness confounded with fluid crease in the number of glomeruli. This process progresses deprivation and/or iatrogenic insult. The most impor- to hyalinizing arteriosclerosis and scattered arteriolar oblit- tant principle in the elderly surgical patient is to main- eration with a resultant loss of nephrons secondary to tain normal intravascular volume.10 ischemia.2,4-8 A well-known comorbidity is that elevated blood pressure will advance the decline of renal func- FLUIDS tion.8 Functional changes parallel anatomic changes in the Total body water decreases with age. In a younger man kidney. The kidneys exhibit an impaired concentrating near his ideal body weight, total body water composes capacity over time and a 10% decline in renal blood flow 60% to 65% of his body mass. By age 80 years, this con- per decade after young adulthood. Functionally, the most tribution is reduced to 50%. Less arterial distensibility, studied change in the aged kidney is the decline in the decreased baroceptor reflexes, and sluggish homeo- GFR. static mechanisms result in increased susceptibility to he- Rowe et al9 in 1976 first showed a sequential fall in modynamic changes in the elderly.13 After ingestion of standardized GFR in an aging population. Subsequent re- water (via enteral or parenteral routes), the body fluid sults obtained from the Baltimore Longitudinal Study of compartments are diluted. Aging demonstrated that a declining GFR is not inevi- With less than a 1% decrease in osmolality, the hy- table. In a healthy cohort, examined regularly for 20 to pothalamus–posterior pituitary axis responds by inhib- 30 years, most individuals demonstrated a fall in GFR at iting ADH release. In the absence of ADH, the kidney ex- an average rate of about 10 mL/min per decade.2 How- cretes a dilute urine, hence the efficacious excretion of ever, 30% of healthy aging individuals showed no de- the water load. Even with GFR rates as low as 30 to 50 crease in GFR.2 Without more rigorous clinical testing mL/min, typical water loads can be excreted effectively. for a more precise estimation of GFR (eg, 24-hour crea- However, in the presence of inappropriately elevated ADH tinine clearance),10 it would be difficult to select the 30% levels or significant extracellular volume depletion, a rela- of aged patients who maintain a GFR comparable to that tive water excess can result in hyponatremia. of their younger years; therefore, it is clinically sound to assume that the older patient has a reduced GFR. Defect in Urinary Concentrating Ability Excluding individuals with inherent renal disease, it is generally accepted that there is a 50% to 63% de- The elderly are limited in their ability to tolerate water cline in GFR from the ages of 30 to 80 years.2 Despite deprivation and, contrarily, to tolerate water