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Postoperative cognitive dysfunction occurs in a proportion of patients after noncardiac surgery. Older patients are particularly vulnerable. We hypothesized that dehydration, a common perioperative problem in the elderly, may provoke cognitive dysfunction. We used a clinical scenario free of surgical/anesthetic intervention to determine whether dehydration caused by bowel preparation results in cognitive changes.Thirty-eight patients of an age associated with a significant incidence of postoperative cognitive dysfunction were recruited in a prospective observational study. A further control group of 14 patients undergoing sigmoidoscopy, who did not receive any bowel preparation, were matched for age, education, and gender.Loss of total body weight (1.5 kg [95% CI: 0.9–2.2]; P < 0.001) occurred in patients undergoing bowel preparation (2.0 [95% CI: 1.3–2.6] percent total body weight), whereas sigmoidoscopy patients’ weight did not change (0.17 kg [95% CI: −0.2–0.6 kg]; P = 0.26). Total body water, derived from foot bioimpedance, indicated dehydration in the bowel preparation group only (mean impedance change 36 [Omega] [95% CI; 25–46], P < 0.001) with a calculated decrease of 2.6% in total body water (95% CI: 1.1–4.8; P < 0.001). Hematocrit increased after bowel preparation only (prebowel prep 0.41 [0.40–0.43] versus postbowel prep 0.43 [0.42–0.45]; P = 0.003). Despite this degree of dehydration, all cognitive tests were within 1 sd of the population mean of normal values. Repeated measures analysis of variance did not reveal significant changes for within group comparisons over time for motor speed (P = 0.51), executive function (P = 0.57), Trail Making Tests and recall (P = 0.88), other than a 3 s slowing in learning ability (Rey Auditory Verbal Learning Test; P = 0.04). Hydration status did not affect learning (P = 0.42), recall (P = 0.30) motor speed (P = 0.36), or executive function tests (P = 0.26).Dehydration alone does not result in cognitive dysfunction.