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 patients 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 estimated by the remaining postdialysis weight excess over dry weight, urea reduction ratio (URR) in dialysis, and monthly means for daylight span and outdoor temperature over the study period. Average BP in the population diminished 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 presented synchronous seasonal variations, with peaks in late autumn and early winter and troughs in summer. These biological rhythms were inversely related to the seasonal daylight span and outdoor temperature. Both BP and chronic OH periods were synchronous with the daylight annual cycle and preceded the seasonal variations of temperature by 1 month. Multiple regression analysis showed that chronic OH and daylight, but not URR or temperature, had a significant independent association with BP changes. These results show the existence of seasonal variations of BP in dialysis patients that are associated and synchronous with seasonal changes in chronic OH status. Both cycles depend on conditions influenced by the annual daylight span more than by external temperature.