Heart rate variability
Heart Rate Variability (HRV) is a measure of the variation in time between each heartbeat. HRV measures the specific changes in time (or variability) between successive heartbeats. The time between beats is measured in milliseconds (ms) and is called an R-R interval or inter-beat interval (IBI). Our heart does not tick evenly like a metronome, but instead, there is constant variation. In a normal, healthy situation, HRV is typically high when the heart is beating slowly, and low when the heart is beating faster. HRV is influenced by our respiration, exercise, stress, recovery, and other factors like our hormonal reactions, metabolic processes, and cognitive processes.
Measurement of HRV
HRV is quantified by analysis of variations of intervals between consecutive normal heart beats that corresponds to two consecutive heartbeats. Heart Rate Variability can be measured by acquiring the intervals between successive heartbeats from various techniques, e.g. Electrocardiography (ECG), Photoplethysmography (PPG), Ballistocardiography (BCG), etc. The heartbeat intervals obtained from any of these sources are then used to determine the parameters representing HRV. These parameters are predominantly segregated into 2 categories- Time Domain Parameters and Frequency Domain Parameters.
The medical significance of HRV parameters
Apart from quantifying the physical restoration and rebuilding of the body as mentioned above, there are numerous studies conducted worldwide by renowned cardiological societies, universities and institutions that show clear and significant relationship between HRV parameters and various risks associated with diseases ranging from blood pressure to mortality due to CVDs (CardioVascular Disorders). HRV parameters are a part of many medical scores used to find the risk level of a patient.
rMSSD is a time-domain parameter of Heart Rate Variability. rMSSD is referred to as the square root of the mean of the squares of the successive differences between adjacent NN’s. It reflects beat to beat variance in heart rate and it is used to estimate vagally mediated changes that are reflected in high-frequency HRV components. It is measured in milliseconds.
Higher rMSSD is a sign of good heart health and indicates good adaptability of the autonomic nervous system, low rMSSD is a sign of poor health and is an indicator of insufficient adaptability.
The clinical meaning of low rMSSD can be as follows-
Weakened ANS’s ability to keep homeostasis against internal/external environmental challenges
Lowered coping ability to various emotional/physical stressors
General weakness of health
Sudden cardiac death
Coronary artery disease
Congestive heart failure
Diabetic autonomic neuropathy
Chronic fatigue syndrome
Anxiety disorder (Panic disorder)