Weather and Heart Rate Variability (HRV)

Ambient temperature and air pollution are associated with cardiovascular diseases. One of the concepts associated with weather is Heart Rate Variability (HRV). In a layman language HRV is simply a measure of the variation in time between each heartbeat or a physiological phenomenon of the variation in the time interval between consecutive heartbeats. Some situations result in high HRV and some may cause low HRV.

Why HRV measurement is important?

The variation in HRV is controlled by a primitive part of the nervous system called the Autonomic Nervous System (ANS). ANS regulates our internal organs such as heart rate, blood pressure, breathing, and digestion. The ANS is subdivided into two components, the sympathetic and the parasympathetic nervous system functions. The sympathetic nervous system (SNS) prepares the body for intense physical activity. On the other hand, the parasympathetic nervous system (PNS) relaxes the body and inhibits or slows many high energy functions. HRV is a non-invasive electrocardiographic method that can be used to measure the ANS in a variety of clinical situations. 

           “HRV provides additional physiological information which cannot be obtained through current invasive cardiac monitoring and current vital signs”

Low and High HRV

Low HRV indicates a monotonously regular heart rate. Reduced HRV is associated with the impaired regulatory and homeostatic autonomic nervous system. This reduces the body’s ability to cope with internal and external stressors.  The main mechanisms of homeostasis are body temperature, body fluid composition, blood sugar, gas concentration, and blood pressure.

If a person’s system is more sympathetic the variation between subsequent heartbeats is low. The variation between beats is high if one is in a more relaxed state. People with high HRV have greater cardiovascular fitness and be more resilient to stress.

Role of Weather in HRV

Climate factors lead to changes in HRV due to the physiological reaction of the ANS. A physiological reaction is an automatic physical response to a stimulus.  Heat increases sympathetic nervous system activity by reducing HRV. Long term exposure to cold has not found to have an influence on HRV due to adaptation effects.

Sensitive fluctuations of HRV occur in response to heat stress. In healthy adult males, a short 15 -30 minutes’ period of passive (35⁰ -75⁰ Celsius) is sufficient to induce a decrease in HRV. The influence of heat exposure on HRV was reported to be stronger as the duration of heat stress progressed. Seasonal change in HRV reflects SNS activation and vagal (the vagus nerve runs from the brain through the face and thorax to the abdomen. It stimulates certain muscles in the heart that help to slow heart rate) withdrawal that may increase the risk of cardiovascular-related complications during the summer season.

A study was done on elderly males in Greater Boston during the period (2008-09) found that ambient temperature was associated with a decrease HRV during the warm season (May-September) but not during the cold season (November-March).

Other factors that Reduce HRV

The other factors which affect ANS function and thus decreases HRV are age, high insulin level, reduced baroreflex sensitivity, physical inactivity, rapid and shallow breathing, smoking, depression etc.