Seasonal variations of blood pressure and overhydration in patients on chronic hemodialysis

Blood pressure (BP) has a seasonal cycle in the general population and in p atients undergoing maintenance dialysis, but the causes remain unclear. We studied the BP measurements recorded at fixed hours three times weekly from 1994 to 1997 in 102 hemodialysis patients. We obtained monthly averages of the following variables: predialysis mean BP, greatest overhydration (OH) estimated by predialysis body weight excess over dry weight, chronic OH est imated by the remaining postdialysis weight excess over dry weight, urea re duction ratio (URR) in dialysis, and monthly means for daylight span and ou tdoor temperature over the study period. Average BP in the population dimin ished over the 48-month period, associated with a decrease in chronic OH (r = 0.66; P < 0.0005) but independent of greatest OH. BP and chronic OH pres ented synchronous seasonal variations, with peaks in late autumn and early winter and troughs in summer. These biological rhythms were inversely relat ed to the seasonal daylight span and outdoor temperature, Both BP and chron ic OH periods were synchronous with the daylight annual cycle and preceded the seasonal variations of temperature by 1 month. Multiple regression anal ysis showed that chronic OH and daylight, but not URR or temperature, had a significant independent association with BP changes. These results show th e existence of seasonal variations of BP in dialysis patients that are asso ciated and synchronous with seasonal changes in chronic OH status. Both cyc les depend on conditions influenced by the annual daylight span more than b y external temperature, (C) 2000 by the National Kidney Foundation, Inc.