Heart Rate Variability

Heart Rate Variability (HRV) was first used clinically in 1965 when Hon and Lee (01)noted that fetal distress was accompanied by changes in beat-to-beat variation of the fetal heart, even before there was detectable change in heart rate (HR).

In the near future measurements of HRV in the clinical setting will be as common as pulse, blood pressure or temperature measurements.

Since 1967 there have been over 11,600 scientific articles on HRV published in the English language alone. HRV has been used as a screening tool to predict significant changes in many disease processes. Many healthcare specialties have adopted HRV. In diabetes and heart disease is a stable, repeatable and proven predictor of long term outcomes and patient prognosis.
In 1996, a special task force was formed between the US and European Physiological associations to outline current finds on HRV and set specific standards on using HRV in medical science and future practice. Since then a steady stream of new information and value continues to come out of HRV research.

Physiological basics of HRV

The heartbeat originates in specialized tissue in the heart called the sino-atrial (SA). Cells in the SA node continuously generate an electrical impulse that spreads throughout the heart muscle through specialized electrical conduction pathways. This initiates process of heart muscle contraction, a well-synchronized pump that sequentially constricts all 4 chambers of the heart (two atria and two ventricles.)

The SA node generates approximately 100-120 impulses per minute when the heart is at rest. However, healthy individuals do not have a resting heart rate (HR) of 100-120 beats per minute. In health, the heart rate is never that high. The autonomic nervous system (ANS) governs SA node activity by inhibiting some of the electrical impulses. The net effect results in the normal resting heart rate of 55 to 70 beats per minute we observe in healthy people.

Autonomic nervous system: The autonomic nervous system is the "automatic" nervous system: that part of the nervous system that is not under conscious control. It controls the organs and systems of the body that are rhythmic, regular and automatic such as breathing, digestion and heart rate. There are two branches of the autonomic nervous system: sympathetic and parasympathetic. These two branches work to keep the organs they control in perfect balance.

Sympathetic nervous system: This nervous system is responsible for "survival." It is the "fight, flight or fright" nervous system that shuts down digestion and funnels blood directly to large muscles for vigorous contraction (for escape, attack or other forms of self-preservation). An increase in sympathetic stimulation causes increased cardiac output by increasing HR and the vigor of heart muscle contraction.
The response of the heart rate to normally encountered levels of sympathetic stimulation is slow by comparison to parasympathetic stimulation. It takes about 5 seconds to increase HR after actual onset of sympathetic stimulation and almost 30 seconds to reach its peak steady level.

Parasympathetic nervous system: In contrast, the parasympathetic nervous system is the "recovery" nervous system, or the "rest and digest" system. While parasympathetic stimulation decreases heart rate and cardiac output, it facilitates nerve impulses that drive digestive processes and repair of tissues and organs. The HR responds rapidly to parasympathetic stimulation. The deceleration of the heartbeat is almost instantaneous. It only takes 1 or 2 heartbeats to see these changes take place, slowing the heart rate.
In healthy individuals the HR is variable. It fluctuates. Greater variability (or HRV) correlates with better health. Higher HRV indicates a healthy autonomic nervous system, and in particular, healthy balance between the sympathetic and parasympathetic systems. A decreased HRV is an early, accurate indicator that the autonomic nervous system is out of balance. The lower the HRV, the greater the imbalance in autonomic control and the greater the likelihood of poor health, both now and in the future.
There are several ways to measure and analyze HRV. Heart rate signals are obtained through Electrocardiogram (ECG) or by pulse wave measurement called "Photoplethysmography" (PPG). The most accurate clinical determination of HRV is derived from measuring the duration of the intervals between contractions of the heart, called interbeat intervals, on ECG (or EKG). In contrast, PPG is less invasive but simpler to apply. PPG conveniently accesses capillaries in a fingertip or the earlobe. Using differential light absorption characteristics and an optical sensor PPG detects changes in the pulse waves generated by blood flow through the microcirculation. In this way an accurate estimate of HRV can be obtained.

01. Hon EH, Lee ST. Electronic evaluations of the fetal heart rate patterns preceding fetal death, further observations. Am J Obstet Gynae 1965; 87:814–26


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