Reduced vagal cardiac control variance in exhausted and high strain job subjects.

OBJECTIVES This paper has two primary objectives. First, the paper proposes methodological strategies for analyzing multiscale vagal cardiac control based on the Stress Disequilibrium Theory (SDT) using high frequency power of heart rate variability (HFP) and short term variance of HFP. Second, the paper provides evidence of reduced vagal cardiac control range and variability in high strain job and exhausted subjects. MATERIALS AND METHODS Job Strain was measured using the Job Content Questionnaire, 8/day diary reports, and a nationally standardized occupational code linkage in 36 healthy mid-aged males with varying strain jobs. Subjects were Holter-monitored for 48 hours, including a work and rest day. Subjects responded to questions on a daily diary as well as on the Job Content Questionnaire to test for exhaustion as a dichotomous state variable. Vagal cardiac control was measured by components of electrocardiograph: heart rate variability based measures of high frequency power (HFP). We assessed range of vagal cardiac control using extreme value analysis (data in upper tail); and short term vagal variability using Poincaré plots of HFP. Comparisons were made between high (N = 10) and low job strain (N = 22) jobs. Furthermore, subjects categorized as exhausted (N = 4) were separately analyzed. RESULTS Exhausted subjects displayed a reduced range of vagal cardiac control on the workday; and both high strain and exhausted subjects displayed reduced short-term variance in vagal cardiac control. A repeated measures ANOVA controlling for age confirms reductions in variance of cardiac vagal activity in high job strain subjects (0.01), with further reductions in subjects reporting exhaustion (p = 0.001). CONCLUSION This analysis supports the hypothesis that (a) job strain is associated with reductions in cardiac vagal - or system level - variance; and (b) that reduced system variability may be a characteristic of exhaustion.