The Cardiac Cycle

All of the parts of your heart, like the chambers and valves, work together to ensure that blood always flows on the same path on its way through your heart and lungs.

How The Heart is Controlled

• The brain is able to modulate the heart rate via the Autonomic Nervous System (ANS). This system adapts

the strength and timing of the heartbeat during rest and exercise or intense emotion.

Autonomic Nervous System

Responsible for control of involuntary or visceral bodily functions

CardiovascularRespiratory

DigestiveUrinary

Reproductive functionsKey role in the bodies response to stress

• Although the heartbeat arises in the SA Node it can be altered by the brain via nerve fibres. These nerve fibres are subdivided into two groups

• Parasympathetic Nerves• Sympathetic Nerves

The autonomic nervous system (ANS)

Automonic Nervous System

Sympathetic nervous system

allow body to function under stressfight or flight

Parasympathetic nervous system

controls vegetativefeed or breed or rest and repose

constant opposition to sympathetic system

A little video…

The autonomic nervous system (ANS)

Normal HR• Heart rate is normally determined by the pacemaker activity

of the sinoatrial node (SA node) located in the posterior wall of the right atrium. The SA node exhibits automaticity that is determined by spontaneous changes in Ca++, Na+, and K+ conductances. This intrinsic automaticity, if left unmodified by neurohumoral factors, exhibits a spontaneous firing rate of 100-115 beats/min. This intrinsic firing rate decreases with age.

The medulla, located in the brainstem above the spinal cord, is the primary site in the brain for regulating sympathetic and parasympathetic (vagal) outflow to the heart and blood vessels. The nucleus tractus solitarius (NTS) of the medulla receives sensory input from different systemic and central receptors (e.g., baroeceptors and chemoreceptors).

The medulla also receives information from other brain regions (e.g., hypothalamus). The hypothalamus and higher centers modify the activity of the medullary centers and are particularly important in stimulating cardiovascular responses to emotion and stress (e.g., exercise, thermal stress).

Autonomic outflow from the medulla is divided principally into sympathetic and parasympathetic (vagal) branches. Efferent fibers of these autonomic nerves travel to the heart and blood vessels where they modulate the activity of these target organs.

baroreceptors• How do the baroreceptors respond to a sudden decrease in arterial pressure and

how is cardiovascular function altered? A decrease in arterial pressure (mean, pulse or both) results in decreased baroreceptor firing. The "cardiovascular center" within the medulla responds by increasing sympathetic outflow and decreasing parasympathetic (vagal) outflow.

• Under normal physiological conditions, baroreceptor firing exerts a tonic inhibitory influence on sympathetic outflow from the medulla. Therefore, acute hypotension results in a disinhibition of sympathetic activity within the medulla, so that sympathetic activity increases. These autonomic changes cause vasoconstriction (increased systemic vascular resistance, SVR), tachycardia and positive inotropy. The latter two changes increase cardiac output. The increases in cardiac output and SVR lead to a partial restoration of arterial pressure.

• Arterial blood pressure is normally regulated within a narrow range, with a mean arterial pressure typically ranging from 85 to 100 mmHg in adults. It is important to tightly control this pressure to ensure adequate blood flow to organs throughout the body. This is accomplished by negative feedback systems incorporating pressure sensors (i.e., baroreceptors) that sense the arterial pressure.

baroreceptors• The most important arterial baroreceptors are located in the

carotid sinus (at the bifurcation of external and internal carotids) and in the aortic arch.

• These receptors respond to stretching of the arterial wall so that if arterial pressure suddenly rises, the walls of these vessels passively expand, which stimulates the firing these receptors. If arterial blood pressure suddenly falls, decreased stretch of the arterial walls lead to a decrease in receptor firing.

Parasympathetic Nerves

• These DECREASE Heart Rate.• Without any influence from the nervous system the inherent

rate of heart beat is 100bpm. This is somewhat higher than the normal resting heart rate which is around 70bpm.

• The reason for this is the parasympathetic activity (via vagus nerve) slows down the rate of SA node impulse generation.

• At rest the heart is said to be ‘vagal tone’. This allows the brain to increase the heart rate by reducing the activity of the vagus nerve.

Sympathetic Nerves• These INCREASE Heart Rate.• During increased demands on the circulatory system such as occurs in

exercise, sympathetic fires release noradrenalin which speeds up the SA impulse generation.

• In addition the sympathetic activity increases the speed of the electrical conduction through the AV node, allowing the ventricles to be excited and therefore beat more frequently

• As well as exercise, intense emotional states such as fear can increase the HR through the sympathetic system.

• The sympathetic nervous system supply to the heart leaves the spinal cord at the first four thoracic vertebra, and supplies most of the muscle of the heart. Stimulation via the cardiac beta-1 receptors causes the heart rate to increase and beat more forcefully

• Circulating catecholamines, epinephrine and norepinephrine, originate from two sources. Epinephrine is released by the adrenal medulla upon activation of preganglionic sympathetic nerves innervating this tissue. This activation occurs during times of stress (e.g., exercise, heart failure, hemorrhage, emotional stress or excitement, pain). Norepinephrine is also released by the adrenal medulla (about 20% of its total catecholamine release is norepinephrine).

• The primary source of circulating norepinephrine is spillover from sympathetic nerves innervating blood vessels. Normally, most of the norepinephrine released by sympathetic nerves is taken back up by the nerves (some is also taken up by extra-neuronal tissues) where it is metabolized. A small amount of norepinephrine, however, diffuses into the blood and circulates throughout the body. At times of high sympathetic nerve activation, the amount of norepinephrine entering the blood increases dramatically.

The fight-or-flight response, also called hyperarousal or the acute stress response, was first described by Walter Cannon in 1915

His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing. This response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms

• The SA node is richly innervated by parasympathetic nervous system fibers (CN X: Vagus Nerve) and by sympathetic nervous system fibers (T1-4, Spinal Nerves). This makes the SA node susceptible to autonomic influences.

• Stimulation of the vagus nerve (parasympathetic fibres) causes a decrease in the SA node rate (thereby decreasing the heart rate and force of contraction).

• Stimulation via sympathetic fibres causes an increase in the SA node rate (thereby increasing the heart rate and force of contraction).

• Exercise increases blood flow through the heart so that the cardiac cycle accelerates to accommodate the increased demand for oxygen.

• The normal cycle is around 0.8 seconds. This accelerates with faster and more powerful atrial and ventricular contraction, which is stimulated by the cardiac centre in the brain.

• Heart rate:- is defined as the number of heart contractions in each minute.

• There are two distinct periods in the cardiac cycle- one of the heart muscle relaxation (cardiac diastole), the other of contraction (cardiac systole)

• What is the Cardiac Cycle.

• The cardiac cycle is the sequence of events that occur when the heart beats. There are two phases of this cycle:

• Diastole – Heart Muscles are relaxed. • Systole – Heart Muscles contract.

Diastole

• Diastole is the period of time when the heart relaxes after contraction. Ventricular diastole is the period during which the ventricles are relaxing, while atrial diastole is the period during which the atria are relaxing.

• During the first phase (diastole), deoxygenated blood enters the right atrium and oxygenated blood enters the left atrium. As these near capacity blood flows into the ventricles.

Atrial Systole

• Atrial systole is the contraction of the heart muscle (myocardia) of the left and right atria. Normally, both atria contract at the same time. The term systole is synonymous with contraction (movement or stretching) of a muscle.

• As the atria contract, the blood pressure in each atrium increases, forcing additional blood into the ventricles. The additional flow of blood is called atrial kick.

• Atrial kick is absent if there is loss of normal electrical conduction in the heart, such as during atrial fibrillation, atrial flutter, and complete heart block. Atrial kick is also different in character depending on the condition that the heart is in like stiff heart, that is found in patients with diastolic dysfunction.

Ventricular systole

• Ventricular systole is the contraction of the muscles (myocardia) of the left and right ventricles forcing blood through the semi lunar valves.

• The closing of the mitral and tricuspid valves (known together

as the atrioventricular valves) at the beginning of ventricular systole cause the first part of the "lub-dub" sound made by the heart as it beats. Formally, this sound is known as the First Heart Tone,. This first heart tone is created by the closure of mitral and tricuspid valve and is actually a two component sound.

• The second part of the "lub-dub" (the Second Heart Tone, or is caused by the closure of the aortic and pulmonic valves at the end of ventricular systole.

How the Heart Beats

• The hear contains specialised tissue that generates an intrinsic rhythmic beat. The brain controls the heart rate by sending nervous impulses that alter this inherent rhythm.

• A remarkable feature of the heart is that as long as it is bathed in solution containing vital nutrients, it will continue to beat when removed form the body. This is because the heartbeat originates from inside the heart itself, rather than impulses from the brain.

• Specialised pacemaker tissue and an electrical conducting system are responsible for generating the electrical spark underlying the heart beat and transmitting it in an orderly sequence across the upper and lower chambers.

SA Node• The sinoatrial node (abbreviated SA node or SAN, also called

the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava

AV Node• The AV node functions as a critical delay in the conduction

system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles.

Bundle of His• The distal portion of the AV node is known as the Bundle of His.

The Bundle of His splits into two branches in the interventricular septum, the left bundle branch and the right bundle branch. The fascicular branches then lead to the Purkinje fibers which innervate the ventricles, causing the cardiac muscle of the ventricles to contract at a paced interval.

The fascicular branches then lead to the Purkinje fibers which innervate the ventricles, causing the cardiac muscle of the

ventricles to contract at a paced interval.

During the ventricular contraction portion of the cardiac cycle, the Purkinje fibers carry the contraction impulse from the left

and right bundle branches to the myocardium of the ventricles. This causes the muscle tissue of the ventricles to contract and

force blood out of the heart — either to the pulmonary circulation (from the right ventricle) or to the systemic

circulation (from the left ventricle).

Purkinje fibres

Pulse...• The pulse rate (which in most people is identical to the heart rate)

can be measured at any point on the body where an artery's pulsation is transmitted to the surface - often as it is compressed against an underlying structure like bone. Some commonly palpated sites are as listed.

• The ventral aspect of the wrist on the side of the thumb (radial artery), and less commonly ulnar artery on the pinky side which is deeper and harder to palpate

• The neck (carotid artery),• The inside of the elbow, or under the biceps muscle (brachial artery)• The groin,

• Behind the medial malleolus on the feet (posterior tibial artery)• Middle of dorsum of the foot (dorsalis pedis).• Behind the knee (popliteal artery)• Over the abdomen (abdominal aorta)• The chest (aorta). This can be felt with one's hands or fingers but it

is possible to auscultate the heart by utilizing a stethoscope.• The temple• NOTE: The thumb should never be used for measuring another

person's heart rate, as its strong pulse may interfere with discriminating the site of pulsation, and you may count the thumb's pulse accidentally when measuring' (Of course, this is a non-issue when measuring your own pulse).

Maximum heart rate• Maximum heart rate (also called STD, or HRmax) is the highest number

of times your heart can contract in one minute, or the heart rate that a person could achieve during maximal physical exertion. It is not the maximum one should obtain often during exercise. MHR is used as a base number to calculate target heart rate for exercise (see below). The heart beats about 60 to 80 times a minute when we're at rest. Resting heart rate usually rises with age, and it's generally lower in physically fit people.

• Resting heart rate is used to determine one's training target heart rate. Athletes sometimes measure their resting heart rate as one way to find out if they're over trained. The heart rate adapts to changes in the body's need for oxygen, such as during exercise or sleep

Conducting a maximal exercise test can require expensive equipment. If you are just beginning an exercise regimen, you should only perform this test in the presence of medical staff due to risks associated with high heart rates. Instead, people typically use a formula to estimate their individual Maximum Heart Rate. The most common formula encountered is:

HRmax = 220 − age (caution: can vary significantly!) This is attributed to various sources, often "Fox and Haskell." While the most common (and easy to remember and calculate), this particular formula is not considered by some to be a good predictor of HRmax.

A 2002 study of 43 different formulae for HRmax (including the one above) concluded the following:

1) No "acceptable" formula currently existed, (they used the term "acceptable" to mean acceptable for both prediction of , and prescription of exercise training HR ranges) 2) The most decent formula of those examined was: HRmax = 205.8 − (0.685 × age)

Measuring HRmax

Fox and Haskell formula• The most accurate way of measuring HRmax for an individual is via a cardiac

stress test. In such a test, the subject exercises while being monitored by an electrocardiogram (EKG). During the test, the intensity of exercise is periodically increased (if a treadmill is being used, through increase in speed or slope of the treadmill), or until certain changes in heart function are detected in the EKG (at which point the subject is directed to stop). Typical durations of such a test range from 10 to 20 minutes.

Recovery Heart Rate• This is the heart rate measured at a fixed (or reference) period after

ceasing activity; typically measured over a 1 minute period.

• Heart-Rate Recovery Immediately after Exercise as a Predictor of Mortality• The increase in heart rate that accompanies exercise is due in part to a

reduction in vagal tone. Recovery of the heart rate immediately after exercise is a function of vagal reactivation. Because a generalized decrease in vagal activity is known to be a risk factor for death, it has been hypothesized* that a delayed fall in the heart rate after exercise might be an important prognostic marker.

• Study by: Christopher R. Cole, M.D., Eugene H. Blackstone, M.D., Fredric J. Pashkow, M.D., Claire E. Snader, M.A., and Michael S. Lauer, M.D. ; Art. ref. from the NEJM, Volume 341:1351-1357 October 28, 1999 Number 18