HYPOTHESIS: POSTURE IS ONE OF THE DETERMINANTS OF THE CIRCADIAN RHYTHM OF URINE FLOW AND ELECTROLYTE EXCRETION IN ELDERLY FEMALE PATIENTS

In view of recent investigations suggesting that the circadian rhythm of urine output is reversed in the elderly, the present study was designed to see if increased urine flow at night may be related to change in posture. Urine flow, urea and electrolyte excretion were measured 4-hourly for 6 consecutive days and nights in four catheterized female patients, mean age 83 years. Two were nursed in chairs for 8 h daily for 3 days, followed by total bed-rest for 3 days, and the other two in reverse order. None of the patients showed the morning rise of water and electrolyte excretion which has been described in young people. On full bed-rest, there was no significant day-night difference in water, sodium, potassium, urea or creatinine excretion. Chair nursing produced a negative mean day-night difference in water excretion (-321 ml: P less than 0.001) and in sodium (-11.7 mmol: P less than 0.002), urea (-26.6 mmol: P = 0.005) and creatinine excretion (-0.6 mmol: P = 0.008), but had no significant effect on potassium excretion. These results suggest that sitting upright may have important effects on water and electrolyte balance in elderly patients and may contribute to nocturia and nocturnal incontinence.