Twenty-four-hour ambulatory blood pressure in shift workers

Abstract

Blood pressure and heart rate of 15 male shift workers were measured every 15 minutes for 24 hours during three work shifts: morning, 4:00 am to noon; afternoon, noon to 8:00 pm; and night, 8:00 pm to 4:00 am. For each shift, 24-hour systolic and diastolic blood pressure showed a large 'through' (the low pressure span) and a continuous range of elevated pressure (the high pressure span). Fourier series were used to model the 24-hour blood pressure profiles. A careful examination of the residuals (measured minus predicted pressures) showed that four harmonics were necessary to describe the data accurately. The model enabled localization in each blood pressure profile of the high and low pressure spans that did not coincide with the subject's work and the rest periods. The time and slope of blood pressure entering and leaving these spans could also be individually determined. Mean blood pressure during the high pressure span was the same for the three shifts, but mean blood pressure during the low pressure span was lower when the subject worked in the afternoon. During that shift, the systolic blood pressure slopes entering and leaving the low pressure span were steeper than during the two other shifts. The high pressure span was longest during the night shift and shortest during the afternoon shift. Therefore, a change in the working time profoundly perturbed the 24-hour blood pressure profile. We conclude that internal regulation mechanisms, subjects' activities, and the circadian rhythm are the three main factors that govern a 24-hour blood pressure level; a complete change in the time of activities modifies the combined effects of these three factors.