Heart Rate Variability: New Perspectives on Physiological Mechanisms, Assessment of Self-regulatory Capacity, and Health risk

Abstract

Heart rate variability, the change in the time intervals between adjacent heartbeats, is an emergent property of interdependent regulatory systems that operates on different time scales to adapt to environmental and psychological challenges. This article briefly reviews neural regulation of the heart and offers some new perspectives on mechanisms underlying the very low frequency rhythm of heart rate variability. Interpretation of heart rate variability rhythms in the context of health risk and physiological and psychological self-regulatory capacity assessment is discussed. The cardiovascular regulatory centers in the spinal cord and medulla integrate inputs from higher brain centers with afferent cardiovascular system inputs to adjust heart rate and blood pressure via sympathetic and parasympathetic efferent pathways. We also discuss the intrinsic cardiac nervous system and the heart-brain connection pathways, through which afferent information can influence activity in the subcortical, frontocortical, and motor cortex areas. In addition, the use of real-time HRV feedback to increase self-regulatory capacity is reviewed. We conclude that the heart's rhythms are characterized by both complexity and stability over longer time scales that reflect both physiological and psychological functional status of these internal self-regulatory systems.

Keywords: Heart rate variability; health risk; physiological mechanisms; self-regulatory capacity.

Figure 1

An example of the heart rate (HR) tachogram, a plot of the sequence of time intervals between heartbeats over an 8-hour period in ambulatory recording taken from a 36-year-old male. Each of the traces is 1 hour long, with the starting time of the hour on the left hand side of the figure. The time between each vertical line is 5 minutes. The vertical axis within each of the hourly tracings is the time between heartbeats (inter-beat-intervals) ranging between 400 and 1200 milliseconds (label shown on second row). The hours beginning at 10:45 through 12:45 were during a time when he was in a low-stress classroom setting. His overall HR increased, and the range of the HRV is considerably less during the hour starting at 13:45 (public speaking), when he was presenting to the class. In this case, the relative autonomic nervous system balance is shifted to sympathetic predominance due to the emotional stress around presenting to a group of his peers. Once the presentation completed near the end of the hour, his HR dropped and normal HRV was restored. In the following hours, he was listening to others present and providing feedback. In the hour starting at 17:45, he was engaged in physical exercise (walking up a long steep hill) starting about 20 minutes into the hour where his HR is increased and the HRV is reduced due to cycle-length dependence effects.

Figure 2

Long-term single-neuron recordings from an afferent neuron in the intrinsic cardiac nervous system in a beating dog heart. The top row shows neural activity. The second row is the actual neural recording. The third row is the left ventricular pressure. This intrinsic rhythm has an average period of 90 seconds with a range between 75 to 100 seconds (0.013 Hz - 0.01 Hz), which falls within the VLF band. Used with permission from Dr J. Andrew Armour.

Figure 3

A typical heart rate variability (HRV) recording over a 15-minute period during resting conditions in a healthy individual. The top tracing shows the original HRV waveform. Filtering techniques were used to separate the original waveform into VLF, LF, and HF bands as shown in the lower traces. The bottom of the figure shows the power spectra (left) and the percentage of power (right) in each band. Abbreviations: HF, high frequency; LF, low frequency; PSD: power spectral density; VLF, very low frequency.

Figure 4

Schematic diagram showing the relationship of the principal descending neural pathways from the insular and prefrontal cortex to subcortical structures and the medulla oblongata as outlined by Oppenheimer and Hopkins. The insular and prefrontal cortexes are key sites involved in modulating the heart's rhythm, particularly during emotionally charged circumstances. These structures alone with other centers such as the orbitofrontal cortex and cingulate gyrus can inhibit or enhance emotional responses. The amygdala is involved with refined integration of emotional content in higher centers to produce cardiovascular responses that are appropriate for the emotional aspects of the current circumstances. Imbalances between the neurons in the insula, amygdala and hypothalamus may initiate cardiac rhythm disturbances and arrhythmias. The structures in the medulla represent an interface between incoming afferent information from the heart, lungs and other body systems and outgoing efferent neuronal activity.

Figure 5

Microscopic image of interconnected intrinsic cardiac ganglia in the human heart. The thin, light blue structures are multiple axons that connect the ganglia. Used with permission from Dr J. Andrew Armour.

Figure 6

The neural communication pathways interacting between the heart and the brain are responsible for the generation of heart rate variability. The intrinsic cardiac nervous system integrates information from the extrinsic nervous system and from the sensory neurites within the heart. The extrinsic cardiac ganglia located in the thoracic cavity have connections to the lungs and esophagus and are indirectly connected via the spinal cord to many other organs such as the skin and arteries. The vagus nerve primarily consists of afferent fibers that connect to the medulla after passing through the nodose ganglion. Used with permission from the Institute of HeartMath, Boulder Creek, California.