Cardiovascular Anatomy

"HRV is the organized fluctuation of time intervals between successive heartbeats defined as interbeat intervals" (Shaffer, Meehan, & Zerr, 2020). The oscillations of a healthy heart are complex. Heart rate variability (HRV) indexes how efficiently we mobilize and utilize limited self-regulatory resources to maintain homeostasis. HRV plays a vital role in regulatory capacity, executive functions, health, and performance. A healthy heart can rapidly adjust to sudden challenges due to the cooperation of interlocking and better-calibrated control systems. HRV is crucial to health, performance, and resilience. Behavioral interventions like aerobic exercise, healthy breathing, compassion, and mindfulness meditation are powerful strategies for increasing HRV.

Biofeedback interventions like heart rate variability biofeedback (HRVB) target the cardiovascular system to treat disorders as diverse as anxiety, depression, essential hypertension, migraine, post-traumatic stress disorder, Raynaud's disorder, and stress.

Biofeedback monitors blood pressure (BP), heart rate (HR), heart rate variability (HRV), pulse wave velocity, temperature, and blood volume pulse modalities. There has been a paradigm shift in treating disorders like depression and heart failure. Clinicians increasingly teach clients to enhance HRV through exercises that strengthen parasympathetic nervous system (PNS) tone. PNS activity is also called vagal tone because the vagus is the primary component of this autonomic branch (Breit et al., 2018).

The brain receives more afferent projections from the heart than any other organ. Emerging evidence suggests that the heart's intrinsic nervous system has extensive bidirectional connections with the brain (MacKinnon et al., 2013; Shaffer, McCraty, & Zerr, 2014).

Researchers increasingly recognize the importance of HRV as an index of vulnerability to stressors and disease.

Here is a critical point to remember: the PNS and baroreceptor system produce brief (≤ 5 minutes) resting HRV without a sympathetic contribution. Patients can learn to increase the healthy variability of their hearts to treat disorders like anxiety, asthma, depression, hypertension, and irritable bowel syndrome. HRV biofeedback training can help patients restore a healthy dynamic balance between the sympathetic and parasympathetic nervous systems.

Temperature is one of the most widely trained modalities. Our understanding of the mechanisms underlying hand-warming and hand-cooling, while still incomplete, has radically changed due to landmark studies by researchers like Robert Freedman. These findings underscore the complexity of the cardiovascular system.

Two patterns, coupling and fractionation, describe changes observed when monitoring subjects. In response coupling, responses change together (HR up, BP up). In response fractionation, responses change independently (HR down, BP up).

Coupling and fractionation reflect the multiple, independent processes that jointly produce these physiological measures. Healthy systems operate nonlinearly (unpredictably) to adapt to rapidly changing demands. Whether responses couple or fractionate during a specific observation period depends on the complicated interplay of participant, task, and environmental variables.

Arteries: Your Body's High-Pressure Delivery System

Arteries carry blood away from the heart. Think of them as the express highways of your circulatory system, designed to handle high-pressure, high-speed blood flow.

Check out the Khan Academy YouTube video Arteries vs. Veins - What's the Difference?

Arteries come in different sizes, each with a specialized job. Elastic arteries are large arteries like the aorta that distribute blood from the heart to muscular arteries. Medium-sized muscular arteries (like the brachial artery you feel when taking a pulse at your inner elbow) circulate blood throughout the body. They branch into small resistance arteries, which divide into arterioles.

Here is where things get interesting for biofeedback: about 400 million arterioles regulate capillary blood flow by controlling resistance. The main source of resistance is the friction between blood and blood vessel walls. These microscopic vessels (15-300 μm in diameter) deliver blood to capillaries and anastomoses. Arterioles divide into microscopic capillaries as they penetrate tissue. There are 3-5 levels of branching between arterioles and capillary beds, varying with the supplied organ or tissue (Jackson, 2021).

Resistance arteries and arterioles help create peripheral vascular resistance, which is a major component of blood pressure (Segal, 2000). For example, arterioles contribute 50% of skeletal muscle vascular resistance (Fronek & Zweifach, 1975). Arterioles are responsible for most pulmonary vascular resistance due to their greater rigidity compared to larger arteries (Chaudhry et al., 2020).

The video below shows red blood cells traveling through a pulsating arteriole.

The control of arteriole diameter, which is crucial to regulating BP and hand temperature, is highly complex. Neural, hormonal, and local controls regulate blood flow through arterioles. These control mechanisms play varying roles across our body's organs. Modest degrees of vasoconstriction or vasodilation can greatly affect systemic vascular resistance (Tortora & Derrickson, 2021).

The Three Layers of Blood Vessels

Generalized blood vessels have three layers or tunics surrounding a hollow lumen or center. Understanding these layers is essential for grasping how hand-warming and hand-cooling work.

The tunica interna (innermost layer) directly contacts circulating blood and responds to epinephrine (E) and norepinephrine (NE) with vasodilation (increase in lumen diameter and blood flow) in digits like the fingers.

The tunica media (middle layer) is composed of smooth muscle and elastic fibers controlled by sympathetic constrictor fibers (C-fibers). This is a site of neurally-controlled vasoconstriction (decrease in lumen diameter and blood flow) in the digits. However, reduced sympathetic firing, exposure to nitric oxide, H⁺, lactic acid, or blood pressure changes relax the smooth muscle layer (Tortora & Derrickson, 2021).

Finally, the tunica externa or external layer is composed of a connective tissue sheath. It supplies vessel walls with nerves and self-vessels and connects vessels to tissues.

Here is a key insight for biofeedback clinicians: separate mechanisms produce hand-warming and hand-cooling. Hand-warming involves releasing a beta-adrenergic hormone and nitric oxide at the tunica interna. Hand-cooling is mediated by vasoconstrictor hormones and the firing of sympathetic C-fibers at the tunica media.

Human studies have demonstrated that hand warming involves the release of beta-adrenergic hormones and nitric oxide (NO). Beta-adrenergic receptors, particularly the β3 subtype, are present in the cardiovascular system and are activated by catecholamines at high concentrations.

Activation of β3-adrenergic receptors leads to the production of NO via endothelial NO synthase (eNOS), resulting in vasodilation and increased blood flow (Moens et al., 2010). Epinephrine is likely the more significant catecholamine for β3 receptor activation, as it induces vasodilation through direct receptor activation and NO-mediated pathways (Arch, 2002; Gauthier, Langin, & Balligand, 1996).

Three Measures of Peripheral Blood Flow

Temperature, blood volume pulse, and pulse wave velocity all depend on blood movement through arteries. Temperature and blood volume pulse provide relative measures of peripheral blood flow. Here is an important distinction to remember: temperature responds to stimuli in 20-30 seconds, while blood volume pulse reacts in a blazingly fast 0.5-2 seconds.

Temperature: The Slow but Steady Indicator

Skin temperature indirectly indexes peripheral blood flow, which is primarily regulated by cutaneous arterioles (Peek, 2016).

Temperature is a gradual tonic index of blood flow. Following a stressor, it may take temperature 20-30 seconds to fall since arterioles must constrict, tissue perfusion with blood must drop, and a sensor called a thermistor must register this change.

"Temperature is the modality most vulnerable to effort" (Khazan, 2019, p. 90). Since blood volume pulse and temperature monitor the same underlying physiology, "trying" can also produce large-scale disruptions in this modality.

What can trigger hand-cooling? Exposure to cold temperatures, overbreathing, trying too hard, stressors, and worrying can all do it. CO₂ loss reduces nitric oxide release, which is needed to relax arteriole walls.

Blood Volume Pulse: The Fast Responder

Blood volume pulse (BVP) is the phasic (momentary) change in blood volume with each heartbeat. It is the vertical distance between the maximum value (peak) and the minimum value (trough) of a pulse wave and is measured by the photoplethysmograph (PPG).

BVP responds to a stressor in 0.5-2 seconds. Why is BVP so much faster than temperature? Because it shines an infrared light on the skin surface instead of relying on a sluggish temperature sensor.

The large-scale BVP changes in hands that are not cold can help clients when hand-warming stalls since BVP provides higher-resolution feedback.

The shape of the BVP waveform can indicate loss of arterial elasticity, and decreased pulse transit time is associated with aging, arteriosclerosis, and hypertension (Izzo & Shykoff, 2001). Peper, Harvey, Lin, Tylova, and Moss (2007) compared BVP waveforms and BP values for two parents and their teenage daughter in the recordings below.

BVP amplitude can provide valuable information about a client's cognitive and emotional responses, as shown in the recording below from Peper, Harvey, Lin, Tylova, and Moss (2007).

Below is a BioGraph® Infiniti blood volume pulse (BVP) display. Note the small dicrotic notch following the peak of each waveform. The reduction or disappearance of a dicrotic notch may indicate the loss of arterial flexibility seen in arteriosclerosis.

When a client is successful in hand-warming, temperature has two advantages over BVP: it is measured in absolute units and gradually changes.

BioGraph® Infiniti blood volume pulse and temperature display © BioSource Software LLC.

Pulse Wave Velocity: The Indirect Blood Pressure Measure

The ejection of blood from the left ventricle during systole produces a pulse wave. Pulse wave velocity (PWV) is the rate of pulse wave movement through the arteries. Practitioners measure PWV by placing pressure transducers (motion sensors) at two points of the arterial system (like the brachial and radial arteries of the same arm). The interval required for the pulse wave to move between these points is called transit time (TT). Pulse wave velocity is used as an indirect measure of BP change. During stress tests, researchers have reported correlations with average and systolic (but not diastolic) BP changes.

Veins: The Return Journey to Your Heart

Veins are blood vessels that route blood from tissues back to the heart. Veins contain the same three layers found in arteries.

These layers are thinner in veins due to lower pressure.

Smooth muscle allows veins to adjust diameter actively. Systemic veins and venules serve as blood reservoirs that store 64% of blood volume at rest.

When we exercise, sympathetic signals to veins constrict them and increase blood flow to skeletal muscles. Venoconstriction helps to compensate for falling blood pressure (Tortora & Derrickson, 2021).

A venule is a small vein (10-200 μm in diameter) that collects blood from capillaries and delivers it to a vein. The low return pressure in these vessels requires valves that prevent backward blood flow. Venules play an essential role in controlling return blood flow to the heart due to narrow diameter, contractility, and extensive surface area. The smallest postcapillary venules (10-50 μm in diameter) exchange nutrients and wastes and allow the departure of white blood cells (Tortora & Derrickson, 2021).

Capillaries: Where the Real Exchange Happens

Capillaries may directly connect arterioles with venules or form extensive networks to rapidly exchange many substances (nutrients and waste products). "The mission of the entire cardiovascular system is to keep blood flowing through capillaries to allow capillary exchange, the movement of substances between blood and interstitial fluid" (Tortora & Derrickson, 2021, p. 781).

Capillaries are the smallest blood vessels (5-10 μm in diameter) and generally consist of a single layer of endothelium and basement membrane. Change in capillary diameter is passive due to the absence of a smooth muscle layer. True capillaries extend from arterioles or metarterioles. A precapillary sphincter functions as a valve that controls blood flow to the tissues at the arterial end of a capillary.

Capillaries exchange nutrients and metabolic end-products between blood vessels and cells. This exchange is aided by 1-micron-thick walls, extensive branching, and massive surface area. Capillary distribution is densest where tissue activity is highest.

Arteriovenous Anastomoses (AVAs): The Body's Temperature Shortcuts

Arteriovenous anastomoses (AVAs) are junctions of two or more vessels that supply the same region.

These vessels contain the three layers seen in both arterioles and venules. Smooth muscle allows anastomoses to adjust diameter actively. AVAs between arteries reroute blood to tissues or organs when ordinary movements compress a vessel or circulation is obstructed by disease, injury, or a surgical procedure. AVAs also form between veins, arterioles, and venules (Tortora & Derrickson, 2021).

AVAs bypass capillaries, directly shunt blood from arterioles to venules, and help regulate temperature (Walløe, 2016).

AVAs are all closed when a naked human body is exposed to temperatures around 79 degrees F. They all open as the temperature approaches 97 degrees F. AVA dilation transfers blood from the epidermis to the interior, cooling the skin. This mechanism is implicated in Raynaud's disease and phenomenon.

Blood Pressure: The Force That Keeps You Going

Blood pressure is the force exerted by blood as it presses against blood vessels. In clinical practice, BP refers to the pressure in arteries. Cardiac output is the amount of blood pumped by the heart in a minute calculated by multiplying stroke volume by heart rate. A typical value for a resting adult is 5.25 liters/minute (70 milliliters x 75 beats/minute). Stroke volume is the amount of blood ejected by the left ventricle during one contraction. Heart rate is the number of contractions per minute.

Blood leaving the left ventricle meets resistance or friction due to blood viscosity (thickness), length of blood vessels, and radius. Blood pressure equals cardiac output times resistance. Self-regulation skills that lower BP reduce cardiac output, resistance, or both.

Clinicians measure both systolic and diastolic BPs. Systolic blood pressure (SBP) is the force exerted by blood on arterial walls during contraction of the left ventricle (called systole). SBP is the upper value when BP is reported and is about 120 mmHg in young adult males at rest. Diastolic blood pressure (DBP) is the force applied against arteries during ventricular relaxation (called diastole). DBP is the lower value and is about 80 mmHg at rest.

The Heart: Your Tireless Four-Chambered Pump

The heart is a hollow muscular organ about the size of a closed fist that pumps 1,500 to 2,000 gallons of blood each day in the adult cardiovascular system.

Review the External Structure of the Heart

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The Heart's Four Chambers

The heart contains four chambers: two atria and two ventricles. The right atrium is the upper chamber that receives deoxygenated blood and pumps it into the right ventricle. The right ventricle is the lower chamber that receives deoxygenated blood from the right atrium and pumps it into the pulmonary artery. The left atrium is the upper chamber that receives oxygenated blood from the pulmonary veins and pumps it to the left ventricle. The left ventricle is the lower chamber that receives oxygenated blood from the left atrium and pumps it through the aorta.

The Heart's Electrical Conduction System

The heart beats because of electrical impulses generated by its conduction system. The sinoatrial (SA) node initiates each cardiac cycle through spontaneous depolarization of its autorhythmic fibers. The atrioventricular (AV) node is located between the atria and ventricles. The atrioventricular (AV) bundle conducts electrical impulses from the AV node to the top of the septum. Bundle branches descend along both sides of the septum and conduct the action potential over the ventricles about 0.2 seconds after the appearance of the P wave. Conduction myofibers extend from the bundle branches into the myocardium, depolarizing contractile fibers in the ventricles.

The Electrocardiogram (ECG): Reading Your Heart's Electrical Activity

The electrocardiogram (ECG) is a recording of the heart's electrical activity using an electrocardiograph. A cardiac cycle consists of systole (ventricular contraction) and diastole (ventricular relaxation).

The P wave is produced as contractile fibers in the atria depolarize and culminates in the atria's contraction (atrial systole). The QRS complex corresponds to the depolarization of the ventricles. The R-spike is the initial upward deflection in the QRS complex of the ECG. The S-T segment connects the QRS complex and the T wave; ventricular contraction continues through the S-T segment. The T wave represents ventricular repolarization.

Check out the YouTube video 15 Second EKG.

Considerations for HRV Biofeedback Training

Clinicians should examine ECG morphology for evidence of arrhythmias, ischemia, and prolonged Q-T intervals that could endanger client safety as part of client assessment for HRV biofeedback training (Drew et al., 2004).

Regulation by the Cardiovascular Center

The cardiovascular (CV) center located in the medulla regulates blood pressure and tissue perfusion by changing HR, stroke volume, systemic vascular resistance, and blood volume. Interconnected neuronal, hormonal, and local negative feedback systems are responsible for immediate and long-term BP regulation and redistribution of blood flow.

Autonomic Control of the Heart

The CV center regulates the heart and vasomotor tone. Sympathetic cardiac accelerator nerves target the SA node, AV node, and the bulk of the myocardium (heart muscle). Action potentials conducted by these motor neurons release NE and E. These neurotransmitters bind to beta-adrenergic (β1) receptors located on cardiac muscle fibers. This speeds spontaneous SA and AV node depolarization, increasing HR and strengthening atria and ventricle contractility.

Parasympathetic vagus (X) nerves also innervate the SA node, AV node, and atrial cardiac muscle.

Firing by these motor neurons triggers acetylcholine release and binding to muscarinic (mainly M2) receptors. Cholinergic binding decreases the rate of spontaneous depolarization in the SA and AV nodes (slowing heart rate). Since there is sparse vagal innervation of the ventricles, vagal tone minimally affects the ventricular contractility (Tortora & Derrickson, 2021).

Autonomic Balance: The Dance Between Branches

There is a dynamic balance between sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) influences in a healthy heart. The synergistic relationship between these autonomic branches is complex: sometimes reciprocal, additive, or subtractive (Gevirtz, Schwartz, & Lehrer, 2016).

PNS control predominates at rest, resulting in an average HR of 75 beats per minute (bpm) that is significantly slower than the SA node's intrinsic rate, which decreases with age, from an average 107 bpm at 20 years to 90 bpm at 50 years (Opthof, 2000).

The PNS can slow the heart by 20 or 30 beats per minute or briefly stop it (Tortora & Derrickson, 2021). This control illustrates the response called accentuated antagonism (Olshansky et al., 2011). Parasympathetic nerves exert their effects more rapidly (< 1 second) than sympathetic nerves (> 5 seconds) (Nunan et al., 2010; Shaffer, McCraty, & Zerr, 2014; Tortora & Derrickson, 2021).

Heart Rate: More Than Just a Number

Heart rate (HR; also called stroke rate) is the number of heartbeats per minute. This value is 75 beats/minute for a resting young adult male. Resting rates slower than 60 beats/minute (bradycardia) and faster than 100 beats/minute (tachycardia) may indicate a cardiovascular disorder. Typical non-athlete HRs are 60-80 bpm. Athletes may have HRs between 40-60 bpm (Khazan, 2019).

Abnormal or irregular rhythms are called arrhythmias or dysrhythmias (Tortora & Derrickson, 2021).

Here is a sobering finding: Imahori et al. (2021) conducted a cohort study of 2147 adults ≥ 60 who were free of dementia when they entered the study. Resting heart rates (RHR) ≥80 (compared with 60-69 bpm) were associated with a greater risk of dementia and more rapid cognitive decline, independent of cardiovascular disease.

Heart rate is significant because a high rate can reduce heart rate variability. Faster HRs allow less time between successive heartbeats for HR to vary. This lowers HRV.

Heart Rate Variability (HRV): The Rhythm of Resilience

Heart rate variability (HRV) consists of changes in the time intervals between consecutive heartbeats (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996). BCIA's Biofeedback Blueprint covers HRV briefly since it offers a separate HRV Biofeedback Certification and Certificate of Completion.

We measure the time intervals between successive heartbeats in milliseconds.

HRV is produced by interacting regulatory mechanisms that operate on different time scales (Moss, 2004). Circadian rhythms, core body temperature, and metabolism contribute to 24-hour HRV recordings, representing the "gold standard" for clinical HRV assessment. The parasympathetic, cardiovascular, and respiratory systems produce short-term (e.g., 5-minute) HRV measurements.

Sources of Heart Rate Variability

Respiratory sinus arrhythmia, the heart rate baroreflex, and the vascular tone rhythm are the primary sources of HRV, in descending order of importance (Hayano & Yuda, 2019).

🎧 Listen to a mini-lecture on Sources of Heart Rate Variability

Respiratory sinus arrhythmia (RSA), heart rate speeding and slowing across each breathing cycle, is the primary and entirely parasympathetic source of HRV (Gevirtz, 2020).

Inhalation partially disengages the vagal brake, speeding heart rate. This is purely parasympathetic.

Exhalation reapplies the vagal brake, slowing heart rate.

The baroreceptor reflex, which exerts homeostatic control over acute BP changes, is the second-most-important and entirely parasympathetic source of HRV (Gevirtz, 2020).

Slow-paced breathing increases RSA by stimulating the baroreceptor system at its unique resonance frequency (~ 0.1 Hz). The resonance frequency is caused by the delay in the baroreflex (Lehrer et al., 2004). Before HRVB, respiration and the baroreflex are usually out of phase resulting in weak resonance effects.

Visualize pushing a child on a swing. A single frequency pushes the child the highest and elicits the most squeals of excitement. The best pushing rate is analogous to the resonance frequency (Khazan, 2020).

Resonance is simple physics (Lehrer, 2020). The baroreflex system exhibits resonance since it is a feedback system with a fixed delay. Inertia due to blood volume in the vascular tree accounts for most of this delay.

When clients breathe at their resonance frequency, heart rate and respiration are in perfect phase (0°); their peaks and valleys coincide.

Slowing breathing to rates between 4.5-6.5 bpm for adults and 6.5-9.5 bpm for children increases RSA (Lehrer & Gevirtz, 2014). Increased RSA immediately "exercises" the baroreflex without changing vagal tone or tightening BP regulation. Those changes require weeks of practice. HRV biofeedback can increase RSA 4-10 times compared to rest (Lehrer et al., 2020b; Vaschillo et al., 2002).

Why Is Heart Rate Variability Important?

A healthy heart is not a metronome. When the time intervals between heartbeats significantly change across successive breathing cycles, this shows that the cardiovascular center can effectively modulate vagal tone.

"The complexity of a healthy heart rhythm is critical to the maintenance of homeostasis because it provides the flexibility to cope with an uncertain and changing environment...HRV metrics are important because they are associated with regulatory capacity, health, and performance and can predict morbidity and mortality" (Shaffer, Meehan, & Zerr, 2020).

Vagal tank theory (Laborde et al., 2018) argues that vagal traffic to the heart indicates how efficiently we mobilize and use scarce self-regulatory resources.

Heart rate variability biofeedback is extensively used to treat various disorders (e.g., asthma and depression) and enhance performance in various contexts (e.g., sports; Gevirtz, 2013; Lehrer et al., 2020a; Tan et al., 2016).

Lehrer et al. (2020) observed that "HRVB has the largest effect sizes on anxiety, depression, anger, and athletic/artistic performance and the smallest effect sizes on PTSD, sleep, and quality of life" (p. 109).

Reduced HRV Is Associated with Disease and Loss of Adaptability

In the early 1960s, researchers found that changes in HRV preceded fetal distress (Hon & Lee, 1963).

Reduced HRV is associated with vulnerability to physical and psychological stressors and disease (Lehrer, 2007). Prospective studies have shown that decreased HRV is the strongest independent predictor for the progression of coronary atherosclerosis (McCraty & Shaffer, 2015).

Low HRV is a marker for cardiovascular disorders, including hypertension, especially with left ventricular hypertrophy; ventricular arrhythmia; chronic heart failure; and ischemic heart disease (Bigger et al., 1995; Casolo et al., 1989; Maver, Strucl, & Accetto, 2004; Nolan et al., 1992; Roach et al., 2004). Low HRV predicts sudden cardiac death, especially due to arrhythmia following myocardial infarction and post-heart attack survival (Bigger et al., 1993; Bigger et al., 1992; Kleiger et al., 1987).

Reduced HRV may predict disease and mortality because it indexes reduced regulatory capacity, which is the ability to surmount challenges like exercise and stressors. Patient age may be an essential link between reduced HRV and regulatory capacity since both HRV and nervous system function decline with age (Shaffer, McCraty, & Zerr, 2014).

Heart-Brain Interactions: The Two-Way Street

Thayer and Lane (2000) outline a neurovisceral integration model that describes how a central autonomic network (CAN) that links the brainstem NST with forebrain structures (including the anterior cingulate, insula, and ventromedial prefrontal cortex, and the amygdala and hypothalamus) through feedback and feed-forward loops.

Shaffer, McCraty, and Zerr (2014) propose that interconnected cardiac ganglia create an intrinsic nervous system within the heart that influences the S-A and A-V node pacemakers and forms reciprocal connections with the extrinsic cardiac ganglia found in the chest cavity and the medulla. The sensory, interconnecting, afferent, and motor neurons within the heart can function independently and constitute a "little brain" on the mammalian heart.

The ascending afferent nerves help to regulate the heart and its rhythms physiologically and influence efferent SNS and PNS activity. From 85-90% of vagus nerve fibers are afferents and more afferents from the heart target the brain than any other major organ.

Cutting-Edge Topics in Cardiovascular Biofeedback

Glossary

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Assignment

Now that you have completed this module, describe how this module has changed your understanding of hand-warming. Also, explain when blood volume pulse feedback could be more useful than temperature biofeedback.

References