Cardiac Cycle Summary Questions

Cardiac Cycle

By the end of this section, you will be able to:

• Describe the relationship between blood pressure and blood flow
• Summarize the events of the cardiac cycle
• Compare atrial and ventricular systole and diastole
• Relate heart sounds detected by auscultation to action of heart’s valves

 

The period of time that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle.

• The period of contraction that the heart undergoes while it pumps blood into circulation is called systole.
• The period of relaxation that occurs as the chambers fill with blood is called diastole.
  • Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.

  Overview of the Cardiac Cycle The cardiac cycle begins with atrial systole and progresses to ventricular systole, atrial diastole, and ventricular diastole, when the cycle begins again. Correlations to the ECG are highlighted.

 

Pressures and Flow

Fluids, whether gases or liquids, are materials that flow according to pressure gradients—that is, they move from regions that are higher in pressure to regions that are lower in pressure.

• Accordingly, when the heart chambers are relaxed (diastole), blood will flow into the atria from the veins, which are higher in pressure.
  • As blood flows into the atria, the pressure will rise, so the blood will initially move passively from the atria into the ventricles.
• When the action potential triggers the muscles in the atria to contract (atrial systole), the pressure within the atria rises further, pumping blood into the ventricles.
• During ventricular systole, pressure rises in the ventricles, pumping blood into the pulmonary trunk from the right ventricle and into the aorta from the left ventricle.
  • Again, as you consider this flow and relate it to the conduction pathway, the elegance of the system should become apparent.

 

Phases of the Cardiac Cycle

At the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole).

• Blood is flowing into the right atrium from the superior and inferior venae cavae and the coronary sinus.
• Blood flows into the left atrium from the four pulmonary veins.
• The two atrioventricular valves, the tricuspid and mitral valves, are both open, so blood flows unimpeded from the atria and into the ventricles.
  • Approximately 70–80 percent of ventricular filling occurs by this method.
  • The two semilunar valves, the pulmonary and aortic valves, are closed, preventing backflow of blood into the right and left ventricles from the pulmonary trunk on the right and the aorta on the left.

 

Atrial Systole and Diastole

Contraction of the atria follows depolarization, represented by the P wave of the ECG.

• As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves.
• At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole.
• Atrial contraction, also referred to as the “atrial kick,” contributes the remaining 20–30 percent of filling.
  • Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.

 

Ventricular Systole

Ventricular systole follows the depolarization of the ventricles and is represented by the QRS complex in the ECG.

• It may be conveniently divided into two phases, lasting a total of 270 ms.
• At the end of atrial systole and just prior to atrial contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position.
  • This volume is known as the end diastolic volume (EDV) or preload.

 

Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart.

• However, blood pressure quickly rises above that of the atria that are now relaxed and in diastole.
• This increase in pressure causes blood to flow back toward the atria, closing the tricuspid and mitral valves.
• Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant.
  • Consequently, this initial phase of ventricular systole is known as isovolumic contraction, also called isovolumetric contraction.

 

In the second phase of ventricular systole, the ventricular ejection phase, the contraction of the ventricular muscle has raised the pressure within the ventricle to the point that it is greater than the pressures in the pulmonary trunk and the aorta.

• Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves.
  • Pressure generated by the left ventricle will be appreciably greater than the pressure generated by the right ventricle, since the existing pressure in the aorta will be so much higher.
  • Nevertheless, both ventricles pump the same amount of blood.
  • This quantity is referred to as stroke volume.
    • Stroke volume will normally be in the range of 70–80 mL.
  • Since ventricular systole began with an EDV of approximately 130 mL of blood, this means that there is still 50–60 mL of blood remaining in the ventricle following contraction.
    • This volume of blood is known as the end systolic volume (ESV).

 

Ventricular Diastole

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented by the T wave of the ECG.

• It too is divided into two distinct phases and lasts approximately 430 ms.

 

During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on the remaining blood within the ventricle begins to fall.

• When pressure within the ventricles drops below pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in blood pressure tracings.
• The semilunar valves close to prevent backflow into the heart.
• Since the atrioventricular valves remain closed at this point, there is no change in the volume of blood in the ventricle, so the early phase of ventricular diastole is called the isovolumic ventricular relaxation phase, also called isovolumetric ventricular relaxation phase.

 

In the second phase of ventricular diastole, called late ventricular diastole, as the ventricular muscle relaxes, pressure on the blood within the ventricles drops even further.

• Eventually, it drops below the pressure in the atria.
• When this occurs, blood flows from the atria into the ventricles, pushing open the tricuspid and mitral valves.
  • As pressure drops within the ventricles, blood flows from the major veins into the relaxed atria and from there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, and the semilunar valves remain closed.
• The cardiac cycle is complete.

  Relationship between the Cardiac Cycle and ECG Initially, both the atria and ventricles are relaxed (diastole). The P wave represents depolarization of the atria and is followed by atrial contraction (systole). Atrial systole extends until the QRS complex, at which point, the atria relax. The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. The T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.

 

Heart Sounds

One of the simplest, yet effective, diagnostic techniques applied to assess the state of a patient’s heart is auscultation using a stethoscope.

 

In a normal, healthy heart, there are only two audible heart sounds: S1 and S2.

• S1 is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as a “lub,” or first heart sound.
• The second heart sound, S2, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a “dub”.
  • In both cases, as the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed.
• There is a third heart sound, S3, but it is rarely heard in healthy individuals.
  • It may be the sound of blood flowing into the atria, or blood sloshing back and forth in the ventricle, or even tensing of the chordae tendineae.
  • S3 may be heard in youth, some athletes, and pregnant women.
  • If the sound is heard later in life, it may indicate congestive heart failure, warranting further tests.
  • Some cardiologists refer to the collective S1, S2, and S3 sounds as the “Kentucky gallop,” because they mimic those produced by a galloping horse.
• The fourth heart sound, S4, results from the contraction of the atria pushing blood into a stiff or hypertrophic ventricle, indicating failure of the left ventricle.
  • S4 occurs prior to S1 and the collective sounds S4, S1, and S2 are referred to by some cardiologists as the “Tennessee gallop,” because of their similarity to the sound produced by a galloping horse with a different gait.
  • A few individuals may have both S3 and S4, and this combined sound is referred to as S7.

  Heart Sounds and the Cardiac Cycle In this illustration, the x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.

 

The term murmur is used to describe an unusual sound coming from the heart that is caused by the turbulent flow of blood.

• Murmurs are graded on a scale of 1 to 6, with 1 being the most common, the most difficult sound to detect, and the least serious.
  • The most severe is a 6.
• Phonocardiograms or auscultograms can be used to record both normal and abnormal sounds using specialized electronic stethoscopes.

 

During auscultation, it is common practice for the clinician to ask the patient to breathe deeply.

• This procedure not only allows for listening to airflow, but it may also amplify heart murmurs.
  • Inhalation increases blood flow into the right side of the heart and may increase the amplitude of right-sided heart murmurs.
  • Expiration partially restricts blood flow into the left side of the heart and may amplify left-sided heart murmurs.

  Stethoscope Placement for Auscultation Proper placement of the bell of the stethoscope facilitates auscultation. At each of the four locations on the chest, a different valve can be heard.

 

Cardiac Physiology

Heart Rates

HRs vary considerably, not only with exercise and fitness levels, but also with age.

• Newborn resting HRs may be 120 bpm.
• HR gradually decreases until young adulthood and then gradually increases again with age.

 

Maximum HRs are normally in the range of 200–220 bpm, although there are some extreme cases in which they may reach higher levels.

• As one ages, the ability to generate maximum rates decreases.
• This may be estimated by taking the maximal value of 220 bpm and subtracting the individual’s age.
  • So a 40-year-old individual would be expected to hit a maximum rate of approximately 180, and a 60-year-old person would achieve a HR of 160.

 

Heart: Abnormal Heart Rates

For an adult, normal resting HR will be in the range of 60–100 bpm.

• Bradycardia is the condition in which resting rate drops below 60 bpm, and tachycardia is the condition in which the resting rate is above 100 bpm.
• Trained athletes typically have very low HRs.
  • If the patient is not exhibiting other symptoms, such as weakness, fatigue, dizziness, fainting, chest discomfort, palpitations, or respiratory distress, bradycardia is not considered clinically significant.
  • However, if any of these symptoms are present, they may indicate that the heart is not providing sufficient oxygenated blood to the tissues.
• The term relative bradycardia may be used with a patient who has a HR in the normal range but is still suffering from these symptoms.
  • Most patients remain asymptomatic as long as the HR remains above 50 bpm.

 

Bradycardia may be caused by either inherent factors or causes external to the heart.

• While the condition may be inherited, typically it is acquired in older individuals.
  • Inherent causes include abnormalities in either the SA or AV node.
    • If the condition is serious, a pacemaker may be required.
  • Other causes include ischemia to the heart muscle or diseases of the heart vessels or valves.
  • External causes include metabolic disorders, pathologies of the endocrine system often involving the thyroid, electrolyte imbalances, neurological disorders including inappropriate autonomic responses, autoimmune pathologies, over-prescription of beta blocker drugs that reduce HR, recreational drug use, or even prolonged bed rest.
• Treatment relies upon establishing the underlying cause of the disorder and may necessitate supplemental oxygen.

 

Tachycardia is not normal in a resting patient but may be detected in pregnant women or individuals experiencing extreme stress.

• In the latter case, it would likely be triggered by stimulation from the limbic system or disorders of the autonomic nervous system.
  • In some cases, tachycardia may involve only the atria.
• Some individuals may remain asymptomatic, but when present, symptoms may include dizziness, shortness of breath, lightheadedness, rapid pulse, heart palpations, chest pain, or fainting (syncope).
  • While tachycardia is defined as a HR above 100 bpm, there is considerable variation among people.
  • Further, the normal resting HRs of children are often above 100 bpm, but this is not considered to be tachycardia.
• Many causes of tachycardia may be benign, but the condition may also be correlated with fever, anemia, hypoxia, hyperthyroidism, hypersecretion of catecholamines, some cardiomyopathies, some disorders of the valves, and acute exposure to radiation.
• Elevated rates in an exercising or resting patient are normal and expected.
  • Resting rate should always be taken after recovery from exercise.
• Treatment depends upon the underlying cause but may include medications, implantable cardioverter defibrillators, ablation, or surgery.

 

Heart: Broken Heart Syndrome

Extreme stress from such life events as the death of a loved one, an emotional break up, loss of income, or foreclosure of a home may lead to a condition commonly referred to as broken heart syndrome.

• This condition may also be called Takotsubo cardiomyopathy, transient apical ballooning syndrome, apical ballooning cardiomyopathy, stress-induced cardiomyopathy, Gebrochenes-Herz syndrome, and stress cardiomyopathy.
  • The recognized effects on the heart include congestive heart failure due to a profound weakening of the myocardium not related to lack of oxygen.
  • This may lead to acute heart failure, lethal arrhythmias, or even the rupture of a ventricle.
  • The exact etiology is not known, but several factors have been suggested, including transient vasospasm, dysfunction of the cardiac capillaries, or thickening of the myocardium—particularly in the left ventricle—that may lead to the critical circulation of blood to this region.
• While many patients survive the initial acute event with treatment to restore normal function, there is a strong correlation with death.
• Careful statistical analysis by the Cass Business School, a prestigious institution located in London, published in 2008, revealed that within one year of the death of a loved one, women are more than twice as likely to die and males are six times as likely to die as would otherwise be expected.

 

Other Factors Influencing Heart Rate

Using a combination of autorhythmicity and innervation, the cardiovascular center is able to provide relatively precise control over HR.

• However, there are a number of other factors that have an impact on HR as well, including epinephrine, NE, and thyroid hormones; levels of various ions including calcium, potassium, and sodium; body temperature; hypoxia; and pH balance.
• After reading this section, the importance of maintaining homeostasis should become even more apparent.

 

Major Factors Increasing Heart Rate and Force of Contraction

Factor	Effect
Cardioaccelerator nerves	Release of norepinephrine
Proprioreceptors	Increased rates of firing during exercise
Chemoreceptors	Decreased levels of O2; increased levels of H+, CO2, and lactic acid
Baroreceptors	Decreased rates of firing, indicating falling blood volume/pressure
Limbic system	Anticipation of physical exercise or strong emotions
Catecholamines	Increased epinephrine and norepinephrine
Thyroid hormones	Increased T3 and T4
Calcium	Increased Ca2+
Potassium	Decreased K+
Sodium	Decreased Na+
Body temperature	Increased body temperature
Nicotine and caffeine	Stimulants, increasing heart rate

 

Factors Decreasing Heart Rate and Force of Contraction

Factor	Effect
Cardioinhibitor nerves	Release of acetylcholine
Proprioreceptors	Decreased rates of firing following exercise
Chemoreceptors	Increased levels of O2; decreased levels of H+ and CO2
Baroreceptors	Increased rates of firing, indicating higher blood volume/pressure
Limbic system	Anticipation of relaxation
Catecholamines	Decreased epinephrine and norepinephrine
Thyroid hormones	Decreased T3 and T4
Calcium	Decreased Ca2+
Potassium	Increased K+
Sodium	Increased Na+
Body temperature	Decrease in body temperature