Seasonal variations of arterial blood pressure in normotensive and essential hypertensives.

Marked seasonal variations in environmental fluid losses and arterial blood pressure (BP) have been observed by us. Factors causing these changes in BP have been investigated. Effect of seasonal variation on BP was studied in 15 controls and 15 essential hypertensives. Mean temperature and relative humidity in well defined 5 local seasons was recorded. Monthly observations included the plasma levels and 24 hours urinary excretion of norepinephrine (NE), epinephrine (E), sodium (Na+) and potassium (K+). Average systolic, diastolic and mean BP were higher in winter season in both the groups (P less than 0.01). In hypertensives this variation was observed despite a significant increase in drug consumption during winter season (P less than 0.001). Both the groups revealed higher plasma levels and daily urinary excretion of NE and E during winter months, (P less than 0.05 - less than 0.001). 24 hrs urinary volume, Na+ and K+ were significantly higher in winter season (P less than 0.05 - less than 0.001). These parameters showed a negative correlation with mean ambient temperature. Increased sympathetic nervous activity as documented by increased NE and E in plasma and urinary, and decreased environmental loss of fluids and sodium may be contributory to this rise in blood pressure during winter season.