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 heartbeats that correspond 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.
SDNN is a time-domain parameter of Heart Rate Variability. SDNN means the Standard Deviation of normal to normal R-R Intervals and it denotes the median of the variability of heart rate. The SDNN value is recorded over a minimum period of 30 seconds. It represents an overall variability and total power of all HRV frequency components. It is measured in milliseconds.
SDNN is the most representative parameter of HRV. Sometimes the term ‘HRV’ in medical papers indicates ‘SDNN’ among many parameters of HRV analysis. Thus low SDNN is low HRV, which primarily indicates a reduction in dynamic complexity.
Higher SDNN is a sign of good heart health and indicates good adaptability of the autonomic nervous system, low SDNN is a sign of poor health and is an indicator of insufficient adaptability.
Healthy individuals have more irregular and complex HRV signals. Decrease in HRV has received increasing attention as a prognostic indicator of risk associated with a variety of chronic diseases, behavioral disorders, mortality and aging.
The clinical meaning of low SDNN can be as follows-
Weakened ANS’s ability to keep homeostasis against internal/external environmental challenges.
Lowered coping ability to various emotional/physical stressors.