Cardiac Cycle

Definition and Anatomy Review

Definitions

The cardiac cycle describes 1 complete contraction and relaxation of all 4 chambers of the heart during a standard heartbeat. The 7 phases of the cardiac cycle all occur in < 1 second.

• Systole: cardiac muscle Cardiac muscle The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Muscle Tissue: Histology contraction
• Diastole: cardiac muscle Cardiac muscle The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Muscle Tissue: Histology relaxation

Anatomy review

Blood sequentially flows through the heart in 1 direction through the following structures: 

• Deoxygenated blood enters the heart via the superior vena cava Superior vena cava The venous trunk which returns blood from the head, neck, upper extremities and chest. Mediastinum and Great Vessels: Anatomy (SVC) and the inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy ( IVC IVC The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy) → 
• Right atrium (RA) → tricuspid valve Tricuspid valve The valve consisting of three cusps situated between the right atrium and right ventricle of the heart. Heart: Anatomy → right ventricle (RV) → pulmonary valve Pulmonary valve A valve situated at the entrance to the pulmonary trunk from the right ventricle. Heart: Anatomy → 
• Pulmonary trunk Pulmonary Trunk Truncus Arteriosus → pulmonary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs: Anatomy (blood is oxygenated) → pulmonary veins Pulmonary veins The veins that return the oxygenated blood from the lungs to the left atrium of the heart. Lungs: Anatomy → 
• Left atrium (LA) → mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy → left ventricle (LV) → aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy → aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy → 
• Systemic arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology (blood is deoxygenated) → veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology → SVC/ IVC IVC The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy → back to the heart

Phases of the Cardiac Cycle

Phases 6, 7, and 1: ventricular filling

Ventricular filling occurs in 3 parts:

• Ventricles are in diastole (relaxed).
• Pressure in the ventricles is lower than the pressure in the atria.
• = Atrioventricular (AV) valves (tricuspid and mitral valves) are open.
• Phase 6:
  • Blood rapidly fills the ventricles.
  • Result of the high-pressure gradient between the atria and ventricles
• Phase 7:
  • Marked by slower passive filling
  • P wave P wave Electrocardiogram (ECG) of the ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG) occurs marking atrial depolarization Depolarization Membrane Potential.
  • 90% of the ventricular volume is obtained through passive filling in phases 6 and 7.
• Phase 1 Phase 1 Skin: Structure and Functions:
  • Atrial systole (i.e., atrial contraction) occurs.
  • Supplies the remaining 10% of the ventricular volume at rest (40% during exercise)
• End-diastolic volume (EDV):
  • The volume of blood in a single ventricle at the end of ventricular filling
  • Approximately 130 mL in each ventricle

Phase 2 Phase 2 Skin: Structure and Functions: isovolumetric contraction

Isovolumetric means “same volume”. Phase 2 Phase 2 Skin: Structure and Functions represents ventricular contraction while both the AV valves and the semilunar valves are closed, which = no change in ventricular volume.

• Ventricles depolarize: = QRS complex QRS complex Electrocardiogram (ECG) on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)
• Ventricular systole (i.e., contraction) begins.
• Ventricular pressure rises sharply:
  • Ventricular pressure > atrial pressure
  • Tricuspid and mitral valves close as blood surges back against the valves (heart sound S1 S1 Heart Sounds).
• Semilunar valves (aortic and pulmonic valves) remain closed because:
  • Aortic pressure Aortic pressure Cardiac Mechanics > LV pressure 
  • Pulmonary trunk Pulmonary Trunk Truncus Arteriosus pressure > RV pressure
• = No blood is ejected during phase 2 Phase 2 Skin: Structure and Functions of ventricular contraction.

Phases 3 and 4: ventricular ejection

• Blood is ejected from the ventricle during ventricular contraction (i.e., systole):
  • Phase 3 Phase 3 Skin: Structure and Functions: rapid ejection
  • Phase 4: slower ejection
• Begins when the semilunar valves open, which occurs when:
  • LV pressure > aortic pressure Aortic pressure Cardiac Mechanics
  • RV pressure > pulmonary trunk Pulmonary Trunk Truncus Arteriosus pressure
• Changes in ventricular pressure:
  • Initially, a sharp increase continues in the 1st half of phase 3 Phase 3 Skin: Structure and Functions.
  • Decrease as blood is ejected during the 2nd half of phase 3 Phase 3 Skin: Structure and Functions and continues through phase 4
• End-systolic volume (ESV):
  • Residual volume Residual volume The volume of air remaining in the lungs at the end of a maximal expiration. Common abbreviation is rv. Ventilation: Mechanics of Breathing left in the ventricles at the end of systole
  • Approximately 60 mL
• Stroke volume: 
  • The amount of fluid ejected during systole
  • Stroke volume = EDV ‒ ESV
• Ejection fraction (EF):
  • The percentage of the EDV ejected during ventricular systole
  • EF = stroke volume / EDV
  • Average EF: approximately 55%–60% 
• Ventricular repolarization Repolarization Membrane Potential occurs during phases 3 and 4: = T wave T wave Electrocardiogram (ECG) on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

Phase 5: isovolumetric relaxation

• Beginning of ventricular diastole
• Aortic and pulmonary valves close due to decreasing intraventricular pressures.
• Intraventricular pressure sharply decreases as the ventricular muscles relax and the cavity expands.
• T wave T wave Electrocardiogram (ECG) resolution on ECG ECG An electrocardiogram (ECG) is a graphic representation of the electrical activity of the heart plotted against time. Adhesive electrodes are affixed to the skin surface allowing measurement of cardiac impulses from many angles. The ECG provides 3-dimensional information about the conduction system of the heart, the myocardium, and other cardiac structures. Electrocardiogram (ECG)

Atrial pressures

• Atrial systole: contraction of the atria ( phase 1 Phase 1 Skin: Structure and Functions)
• Atrial diastole: relaxation of the atria (phases 2–7)
• Atrial pressure waveforms:
  • A wave: represents atrial contraction (i.e., atrial systole (also known as the “atrial kick”))
  • C wave: reverberation of pressure into the atrium during ventricular contraction
  • V wave: 
    • During phases 3–5, the atria fill and the AV valves are closed.
    • The V wave marks the sudden drop in pressure as the AV valves open at the beginning of ventricular filling.

Normal Pressures in the Cardiac Chambers and Great Vessels

The normal pressures found in the heart chambers and great vessels during ventricular systole and diastole are shown in the table and image below:

• Differences in pressure between 1 chamber and another cause:
  • Valves to open or close
  • Blood to move forward in the circuit
• Low-pressure areas: right side of the heart
• High-pressure areas: left side of the heart

Pressure-Volume Loops

• A graphic demonstration of how volumes and pressures change in the left ventricle throughout the cardiac cycle:
  • Removes “time” from the cardiac cycle graphs
  • Result: The diagram appears as a loop.
• X-axis: LV volume
• Y-axis: LV pressure
• Point A: mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy opens
• Point B: mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy closes
• Point C: aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy opens
• Point D: aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy closes
• Ventricular filling (phases 6, 7, and 1): 
  • Segment A → B
  • Seen as the “flat-ish” line across the bottom of the graph, moving from left to right
  • Volume increases.
  • Because the mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy is open, pressure increases minimally.
• Isovolumetric contraction ( phase 2 Phase 2 Skin: Structure and Functions): 
  • Segment B → C
  • Seen as the vertical line going straight up
  • The mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy closes at point B.
  • The aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy doesn’t open until point C: = Volume change is not possible with both valves closed.
  • The ventricular contraction causes an increase in LV pressure without changes in volume.
• Ventricular ejection (phases 3 and 4): 
  • Segment C → D
  • Seen as the curved line along the top, moving from right to left
  • The aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy opens allowing blood to leave: = volume falls
  • Ventricles contract: Pressure increases until volume falls, causing pressure to also fall.
• Isovolumetric relaxation (phase 5): 
  • Segment D → A
  • Seen as the vertical line going straight down
  • The aortic valve Aortic valve The valve between the left ventricle and the ascending aorta which prevents backflow into the left ventricle. Heart: Anatomy closes.
  • The mitral valve Mitral valve The valve between the left atrium and left ventricle of the heart. Heart: Anatomy does not open until point A: = Volume change is not possible with both valves closed.
  • Ventricular relaxation causes a drop in LV pressure without changes in volume.

Clinical Relevance

The following are common conditions affecting the cardiac cycle.

• Heart failure Heart Failure A heterogeneous condition in which the heart is unable to pump out sufficient blood to meet the metabolic need of the body. Heart failure can be caused by structural defects, functional abnormalities (ventricular dysfunction), or a sudden overload beyond its capacity. Chronic heart failure is more common than acute heart failure which results from sudden insult to cardiac function, such as myocardial infarction. Total Anomalous Pulmonary Venous Return (TAPVR) (HF): refers to the inability of the heart to supply the body with the normal cardiac output Cardiac output The volume of blood passing through the heart per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with stroke volume (volume per beat). Cardiac Mechanics required to meet metabolic needs. HF may result in chest pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, exertional dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea, and episodes of hypotension Hypotension Hypotension is defined as low blood pressure, specifically < 90/60 mm Hg, and is most commonly a physiologic response. Hypotension may be mild, serious, or life threatening, depending on the cause. Hypotension, dizziness Dizziness An imprecise term which may refer to a sense of spatial disorientation, motion of the environment, or lightheadedness. Lateral Medullary Syndrome (Wallenberg Syndrome), and/or syncope Syncope Syncope is a short-term loss of consciousness and loss of postural stability followed by spontaneous return of consciousness to the previous neurologic baseline without the need for resuscitation. The condition is caused by transient interruption of cerebral blood flow that may be benign or related to a underlying life-threatening condition. Syncope. Ejection fraction of the LV is used to clinically categorize HF into either HF with preserved EF (≥ 50%) or HF with reduced EF (≤ 40%). Severity, prognosis Prognosis A prediction of the probable outcome of a disease based on a individual’s condition and the usual course of the disease as seen in similar situations. Non-Hodgkin Lymphomas, and treatment regimens differ for each category.
• Cardiomyopathies Cardiomyopathies A group of diseases in which the dominant feature is the involvement of the cardiac muscle itself. Cardiomyopathies are classified according to their predominant pathophysiological features (dilated cardiomyopathy; hypertrophic cardiomyopathy; restrictive cardiomyopathy) or their etiological/pathological factors (cardiomyopathy, alcoholic; endocardial fibroelastosis). Cardiomyopathy: Overview and Types: a group of myocardial diseases associated with structural changes of the myocardium Myocardium The muscle tissue of the heart. It is composed of striated, involuntary muscle cells connected to form the contractile pump to generate blood flow. Heart: Anatomy and impaired systolic and/or diastolic function in the absence of other heart disorders. Cardiomyopathies Cardiomyopathies A group of diseases in which the dominant feature is the involvement of the cardiac muscle itself. Cardiomyopathies are classified according to their predominant pathophysiological features (dilated cardiomyopathy; hypertrophic cardiomyopathy; restrictive cardiomyopathy) or their etiological/pathological factors (cardiomyopathy, alcoholic; endocardial fibroelastosis). Cardiomyopathy: Overview and Types are classified as dilated, restrictive, hypertrophic, or arrhythmogenic. With abnormal ventricular structure, the pumping action of the ventricles can be severely impaired, resulting in heart failure Heart Failure A heterogeneous condition in which the heart is unable to pump out sufficient blood to meet the metabolic need of the body. Heart failure can be caused by structural defects, functional abnormalities (ventricular dysfunction), or a sudden overload beyond its capacity. Chronic heart failure is more common than acute heart failure which results from sudden insult to cardiac function, such as myocardial infarction. Total Anomalous Pulmonary Venous Return (TAPVR) and/or volume overload.