A LOOK AT CARDIAC PHYSIOLOGY

Events of the cardiac cycle

The cardiac cycle consists of the following five events.

1. Isovolumetric ventricular contraction:

In response to ventricular depolarization, tension in the ventricles increases. The rise in pressure within the ventricles leads to closure of the mitral and tricuspid valves. The pulmonic and aortic valves stay closed during the entire phase.

2.Ventricular ejection: When ventricular pressure exceeds aortic and pulmonary arterial pressure, the aortic and pulmonic valves open and the ventricles eject blood.

3.Isovolumetric relaxation: When ventricular pressure falls below pressure in the aorta and pulmonary artery, the aortic and pulmonic valves close. All valves are closed during this phase. Atrial diastole occurs as blood fills the atria.

4.Ventricular filling: Atrial pressure exceeds

ventricular pressure, which causes the mitral and tricuspid valves to open. Blood then flows passively into the ventricles. About 70% of ventricular filling takes place during this phase.

5. Atrial systole: Known as the atrial kick, atrial systole (coinciding with late ventricular diastole) supplies the ventricles with the remaining 30% of the blood for each heartbeat.

2. Ventricular ejection

3. Isovolumetric relaxation

4. Ventricular filling

A balancing act

The cardiac cycle produces cardiac output, which is the amount of blood the heart pumps in 1 minute. It’s measured by multiplying heart rate times stroke volume. (See Understanding preload, afterload, and contractility, page 12.) The term stroke volume refers to the amount of blood ejected with each ventricular contraction.

Normal cardiac output is 4 to 8 L/minute, depending on body size. The heart pumps only as much blood as the body requires. Three factors affect stroke volume—preload, afterload, and myocardial contractility. A balance of these three factors produces optimal cardiac output.

Preload

Preload is the stretching of muscle fibers in the ventricles and is determined by the pressure and amount of blood remaining in the left ventricle at the end of diastole.

Quick facts about circulation

•It would take about 25 capillaries laid end-to- end to fill 1 (2.5 cm).

•The body contains about 10 billion capillaries.

•On average, it takes

a red blood cell less than 1 minute to travel from the heart to the capillaries and back again.

Afterload

Afterload is the amount of pressure the left ventricle must work against to pump blood into the circulation. The greater this resistance, the more the heart works to pump out blood.

Contractility

Contractility is the ability of muscle cells to contract after depolarization. This ability depends on how much the muscle fibers are stretched at the end of diastole. Overstretching or understretching these fibers alters contractility and the amount of blood pumped out of the ventricles. To better understand this concept, picture trying to shoot a rubber band across the room. If you don’t stretch the rubber band enough, it won’t go far. If you stretch it too much, it will snap. However, if you stretch it just the right amount, it will go as far as you want it to.

Nerve supply to the heart

The heart is supplied by the two branches of the autonomic nervous system—the sympathetic, or adrenergic, and the parasympathetic, or cholinergic.

The sympathetic nervous system is basically the heart’s accelerator. Two sets of chemicals—norepinephrine and epinephrine— are highly influenced by this system. These chemicals increase heart rate, automaticity, AV conduction, and contractility.

Contractility is the heart’s ability to stretch — like a balloon!

Preload

Preload is the passive stretching of muscle fibers in the ventricles. This stretching results from blood volume in the ventricles at end-diastole. According to Starling’s law, the more the heart muscles stretch during diastole, the more forcefully they contract during systole. Think of preload as the balloon stretching as air is blown into it. The more air the greater the stretch.

Contractility

Contractility refers to the inherent ability of the myocardium to contract normally. Contractility is influenced by preload.

The greater the stretch the more forceful the contraction—or, the more air in the balloon, the greater the stretch, and the farther the balloon will fly when air is allowed to expel.

Afterload

Afterload refers to the pressure that the ventricular muscles must generate to overcome the higher pressure in the aorta to get the blood out of the heart. Resistance is the knot on the end of the balloon, which the balloon has to work against to get the air out.

The vagus nerve is stimulated by baroreceptors, specialized nerve cells in the aorta and the internal carotid arteries. Conditions that stimulate the baroreceptors also stimulate the vagus nerve.

For example, a stretching of the baroreceptors, which can occur during periods of hypertension or when applying pressure to the carotid artery, stimulates the receptors. In a maneuver called carotid sinus massage, baroreceptors in the carotid arteries are purposely activated in an effort to slow a rapid heart rate.

Transmission of electrical impulses

The heart can’t pump unless an electrical stimulus occurs first. Generation and transmission of electrical impulses depend on four characteristics of cardiac cells:

•Automaticity refers to a cell’s ability to spontaneously initiate an impulse. Pacemaker cells possess this ability.

•Excitability results from ion shifts across the cell membrane and indicates how well a cell responds to an electrical stimulus.

•Conductivity is the ability of a cell to transmit an electrical impulse to another cardiac cell.

•Contractility refers to how well the cell contracts after receiving a stimulus.

Memory jogger

To help you remember

the difference between depolarization and repolarization, think of the R in repolarization as standing for REST. Remember that repolarization is the resting phase of the cardiac cycle.

“De”-cycle and “re”-cycle

As impulses are transmitted, cardiac cells undergo cycles of depolarization and repolarization. (See Depolarization-repolar- ization cycle, page 14.) Cardiac cells at rest are considered polarized, meaning that no electrical activity takes place. Cell membranes separate different concentrations of ions, such as sodium and potassium, and create a more negative charge inside the cell. This is called the resting potential. After a stimulus occurs, ions cross the cell membrane and cause an action potential, or cell depolarization.

When a cell is fully depolarized, it attempts to return to its resting state in a process called repolarization. Electrical charges in the cell reverse and return to normal.

A cycle of depolarization-repolarization consists of five phas- es—0 through 4. The action potential is represented by a curve that shows voltage changes during the five phases. (See Action potential curve, page 15.)

Many phases of the curve

During phase 0, the cell receives an impulse from a neighboring cell and is depolarized. Phase 1 is marked by early, rapid repolarization. Phase 2, the plateau phase, is a period of slow repolarization.

During phases 1 and 2 and at the beginning of phase 3, the cardiac cell is said to be in its absolute refractory period. During

Those impulses really get around!

Phase 4:

Resting phase

•Cell membrane is impermeable to sodium.

•Potassium moves out of the cell.

Na+

K+

The AV node’s main function is to delay impulses by 0.04 second to keep the ventricles from contracting too quickly. This delay allows the ventricles to complete their filling phase as the atria contract. It also allows the cardiac muscle to stretch to its fullest for peak cardiac output.

Branch splitting

The bundle of His, a tract of tissue extending into the ventricles next to the interventricular septum, resumes the rapid conduction of the impulse through the ventricles. The bundle eventually divides into the right and left bundle branches.

The right bundle branch extends down the right side of the interventricular septum and through the right ventricle. The left bundle branch extends down the left side of the interventricular septum and through the left ventricle.

The left bundle branch then splits into two branches, or fasciculi: the left anterior fasciculus, which extends through the anterior portion of the left ventricle, and the left posterior fasciculus, which runs through the lateral and posterior portions of the left ventricle. Impulses travel much faster down the left bundle branch (which feeds the larger, thicker-walled left ventricle) than the right bundle branch (which feeds the smaller, thinner-walled right ventricle).

The difference in the conduction speed allows both ventricles to contract simultaneously. The entire network of specialized nervous tissue that extends through the ventricles is known as the

His-Purkinje system.

Those perky Purkinje fibers

The bundle of His eventually divides into the right and left bundle branches. This branch works just fine for me!

CARDIAC ANATOMY AND PHYSIOLOGY

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Abnormal impulses

Now that you understand how the heart generates a normal impulse, let’s look at some causes of abnormal impulse conduction, including automaticity, backward conduction of impulses, reentry abnormalities, and ectopy.

When the heart goes on “manual”

Automaticity is a special characteristic of pacemaker cells that generates impulses automatically, without being stimulated to do so. If a cell’s automaticity is increased or decreased, an arrhythmia can occur. Tachycardia, for example, is commonly caused by an increase in the automaticity of pacemaker cells below the SA node. Likewise, a decrease in automaticity of cells in the SA node can cause the development of bradycardia or an escape rhythm (a compensatory beat generated by a lower pacemaker site).

Out of synch

Impulses that begin below the AV node can be transmitted backward toward the atria. This backward, or retrograde, conduction usually takes longer than normal conduction and can cause the atria and ventricles to beat out of synch.

Coming back for more

Ages

and stages

Pediatric pacemaker rates

In children younger than age 3, the atrioventricular node may discharge impulses at a rate of 50 to 80 times per minute; the Purkinje fibers may discharge at a rate of 40 to 50 times per minute.

The heart’s valves

•Tricuspid — AV valve between the right atrium and right ventricle

•Mitral — AV valve between the left atrium and left ventricle

•Aortic — semilunar valve between the left ventricle and the aorta

•Pulmonic — semilunar valve between the right ventricle and the pulmonary artery

Blood flow

•Deoxygenated blood from the body returns to the right atrium and then flows to the right ventricle.

•The right ventricle pumps blood into the lungs where it’s oxygenated. Then the blood returns to the left atrium and flows to the left ventricle.

• Oxygenated blood is pumped to the aorta and the body by the left ventricle.

Coronary arteries and veins

•Right coronary artery — supplies blood to the right atrium and ventricle and part of the left ventricle

•Left anterior descending artery — supplies blood to the anterior wall of the left ventricle, interventricular septum, right bundle branch, and left anterior fasciculus of the left bundle branch

•Circumflex artery — supplies blood to the lateral walls of the left ventricle, left atrium, and left posterior fasciculus of the left bundle branch

•Cardiac veins — collect blood from the capillaries of the myocardium

•Coronary sinus — returns blood to the right atrium

Cardiac cycle dynamics

•Atrial kick — atrial contraction, contributing about 30% of the cardiac output

•Cardiac output — the amount of blood the heart pumps in 1 minute, calculated by multiplying heart rate times stroke volume

•Stroke volume — the amount of blood ejected with each ventricular contraction (it’s affected by preload, afterload, and contractility)

•Preload — the passive stretching exerted by blood on the ventricular muscle at the end of diastole

•Afterload — the amount of pressure the left ventricle must work against to pump blood into the aorta

•Contractility — the ability of the heart muscle cells to contract after depolarization

Innervation of the heart

Two branches of the autonomic nervous system supply the heart:

•Sympathetic nervous system — increases heart rate, automaticity, AV conduction, and contractility

through release of norepinephrine and epinephrine

•Parasympathetic nervous system — vagus nerve stimulation reduces heart rate and AV conduction through release of acetylcholine.

Transmission of electrical impulses

•Conductivity — the ability of a cell to transmit an electrical impulse to another cardiac cell

•Contractility — how well the cell contracts after receiving a stimulus.

Depolarization-repolarization cycle

Cardiac cells undergo the following cycles of depolarization and repolarization as impulses are transmitted:

•Phase 0: Rapid depolarization — the cell receives an impulse from a nearby cell and is depolarized

•Phase 1: Early repolarization — early rapid repolarization occurs

•Phase 2: Plateau phase — a period of slow repolarization occurs

•Phase 3: Rapid repolarization — the cell returns to its original state

•Phase 4: Resting phase — the cell rests and readies itself for another stimulus.

Cardiac conduction

•The electrical impulse begins in the SA node and travels through the internodal tracts and Bachmann’s bundle to the AV node.

•From the AV node, the impulse travels down the bundle of His, along the bundle branches, and through the Purkinje fibers.

Intrinsic firing rates

•SA node — 60 to 100/minute

•AV junction — 40 to 60/minute

•Purkinje fibers — 20 to 40/minute

Generation and transmission of electrical impulses depend on these cell characteristics:

•Automaticity — a cell’s ability to spontaneously initiate an impulse, such as found in pacemaker cells

•Excitability — how well a cell responds to an electrical stimulus

Abnormal impulses

•Automaticity — the ability of a cardiac cell to initiate an impulse on its own

•Retrograde conduction — impulses that are transmitted backward toward the atria

•Reentry — when an impulse follows a circular, rather than the normal, conduction path