Effect of Short-Term and Long-Term Stress on Heart Rate Variability

Heart Rate Variability as a Measure of Adaptive Ability

Exposure to chronic stress is a good predictor of CV diseases. If stress lasts long and a person cannot cope with negative emotional states such as anger or anxiety, an imbalance occurs between the sympathetic and parasympathetic nervous systems (SNS, PNS). An increase in the ratio between SNS and PNS activity is associated with higher CV morbidity and mortality.

The cardiovascular system can adapt to various internal and external influences by changing heart rate. High variability in heart rate is a sign of a healthy organism with better adaptability. Heart rate variability (HRV) analysis involves assessing the fluctuation of intervals between normal myocardial contractions, most commonly between R waves on an ECG.

Methodology and Course of the Study, Assessed Population

The authors of the presented study examined two situations and their impact on heart rate variability: the effect of chronic stress (over the past month according to the HUS questionnaire) and the response to an acute stressor in the form of a psychologically demanding speech. The assessed group consisted of 50 participants (22 men and 28 women; 37 from the white population and 13 from the black population). The average age was 30 years (25–39), the average BMI was 23.4 kg/m² (18–29), and none had a blood pressure measured higher than 140/90 mmHg. Anamnestically, no diseases or medications affecting the activity of the autonomic nervous system were proven in any of them.

To assess chronic stress, the HUS questionnaire (Hassles and Uplift Scale) containing 53 questions focused on positive and negative events of everyday life over the past month was used. The task of the probands was also to prepare a spoken presentation, which would be visibly recorded by a camera and evaluated by experts for quality and eloquence. Heart rate and RR intervals were obtained from the ECG, and respiratory rate was also measured, with these parameters further analyzed, for example, in the form of the so-called D2 correlation dimension.

Results and Discussion

During the spoken presentation, the average respiratory rate decreased from 0.24 to 0.19 Hz (p < 0.001). The observed change in respiratory rate did not interfere with the associations between the acute stressor and various heart rate and heart rate variability measurements. The average heart rate and HRV increased during acute stress, and D2 decreased from 3.5 to 3.2.

Under normal circumstances, the heart rate generation system fluctuates in a set of states prepared to adapt to the external conditions of the constantly changing environment. The higher the D2, the more degrees of freedom the heart pacemaker has, and therefore the greater the range of possible adaptive responses. A decrease in the complexity of heart rate during stress conditions may thus indicate lesser adaptability and condition of the heart pacemaker and functional limitation of the involved CV elements.

When analyzing the effect of chronic stress, a decrease in D2 was also found. This finding is clinically significant since CV diseases are associated with lower HRV. A chronically reduced HRV due to stress can thus be associated with chronic failure to return heart rate to baseline and a chronically reduced ability to respond to slowing heart rate with increasing blood pressure.

Conclusion

The authors of the study observed a decrease in D2 heart rate due to acute and chronic stress in 50 healthy individuals. Among other things, they provided further evidence that the effect of stress on the function of the heart pacemaker is more complex than can be detected by conventional heart rate measurements.

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Source: Schubert C., Lambertz M., Nelesen R. A. et al. Effects of stress on heart rate complexity – a comparison between short-term and chronic stress. Biol Psychol 2009; 80 (3): 325–332, doi: 10.1016/j.biopsycho.2008.11.005.