Cardiac cycle

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Overview

Cardiac cycle is the term referring to all or any of the events related to the flow of blood that occur from the beginning of one heartbeat to the beginning of the next.^[1] The frequency of the cardiac cycle is the heart rate. Every single 'beat' of the heart involves three major stages: atrial systole, ventricular systole and complete cardiac diastole. The term diastole is synonymous with relaxation of a muscle. Throughout the cardiac cycle, the blood pressure increases and decreases.

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. Electrical systole is the electrical activity that stimulates the myocardium of the chambers of the heart to make them contract. This is soon followed by Mechanical systole, which is the mechanical contraction of the heart.

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.

Detection of atrial systole

Electrical systole of the atria begins with the onset of the P wave on the EKG.

Ventricular systole

Ventricular systole is the contraction of the muscles (myocardia) of the left and right ventricles.

Detection of ventricular systole

Heart sounds

Main article: Heart sounds

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, or S₁. This first heart tone is created by the closure of mitral and tricuspid valve and is actually a two component sound, M1, T1.

The second part of the "lub-dub" (the Second Heart Tone, or S₂), is caused by the closure of the aortic and pulmonic valves at the end of ventricular systole. As the left ventricle empties, its pressure falls below the pressure in the aorta, and the aortic valve closes. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonic valve closes. The second heart sound is also two components, A2, P2. The aortic valve closes earlier than the pulmonic valve and they are audibly separated from each other in the second heart sound.

Electrocardiogram

In an electrocardiogram, electrical systole of the ventricles begins at the beginning of the QRS complex.

Complete cardiac diastole

Cardiac Diastole is the period of time when the heart relaxes after contraction in preparation for refilling with circulating blood. Ventricular diastole is when the ventricles are relaxing, while atrial diastole is when the atria are relaxing. Together they are known as complete cardiac diastole.

During ventricular diastole, the pressure in the (left and right) ventricles drops from the peak that it reaches in systole. When the pressure in the left ventricle drops to below the pressure in the left atrium, the mitral valve opens, and the left ventricle fills with blood that was accumulating in the left atrium. Likewise, when the pressure in the right ventricle drops below that in the right atrium, the tricuspid valve opens, and the right ventricle fills with blood that was accumulating in the right atrium.

Regulation of the cardiac cycle

Cardiac muscle is myogenic, which means that it is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli for excitation. The heart's rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger. For example, the phrenic nerve accelerates heart rate and the vagus nerve decelerates heart rate.

The rhythmic sequence of contractions is coordinated by the sinoatrial (SA) and atrioventricular (AV) nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation that initiates atrial contraction by creating an action potential. Once the wave reaches the AV node, situated in the lower right atrium, it is delayed there before being conducted through the bundles of His and back up the Purkinje fibers, leading to a contraction of the ventricles. The delay at the AV node allows enough time for all of the blood in the atria to fill their respective ventricles. In the event of severe pathology, the AV node can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the pacemaker cells in the SA node and hence is overridden.

See also

• Apex beat
• Blood pressure
• Cardiac action potential
• Cardiac muscle
• Cardiac output
• Electrocardiogram
• Heart
• Systolic array (computer architecture)
• Ventricle

References

External Links

• Interactive cardiac cycle. Interactivephysiology.com
• Cardiac cycle. University of Utah School of Medicine
• Heart sounds. ULCA School of Medicine

v • d • e Anatomy of torso, cardiovascular system: heart
Structures	atria (interatrial septum, musculi pectinati) • ventricles (interventricular septum, trabeculae carneae, chordae tendinae, papillary muscle) • valves • cusps
Regions	base • apex • grooves (coronary/atrioventricular, interatrial, anterior interventricula, posterior interventricular) • surfaces (sternocostal, diaphragmatic) • borders (right, left)
Right heart	(vena cavae, coronary sinus) → right atrium (auricle, fossa ovalis, limbus of fossa ovalis, crista terminalis, valve of the inferior vena cava, valve of the coronary sinus) → tricuspid valve → right ventricle (conus arteriosus, moderator band/septomarginal trabecula)  → pulmonary valve → (pulmonary artery and pulmonary circulation)
Left heart	(pulmonary veins) → left atrium (auricle) → mitral valve → left ventricle → aortic valve (aortic sinus) → (aorta and systemic circulation)
Layers	pericardium: fibrous pericardium • serous pericardium (pericardial cavity, epicardium/visceral layer) • pericardial sinus myocardium • endocardium • cardiac skeleton (fibrous trigone, fibrous rings)
Conduction system	Cardiac pacemaker • SA node • AV node• bundle of His • Purkinje fibers

v • d • e WikiDoc Research Resources for Cardiac cycle
Articles on Cardiac cycle	Most recent articles on Cardiac cycle • Most cited articles on Cardiac cycle • Review articles on Cardiac cycle • Articles on Cardiac cycle in N Eng J Med, Lancet, BMJ
Media (Slides, Video, Images, MP3) on Cardiac cycle	Powerpoint slides on Cardiac cycle • Images of Cardiac cycle • Photos of Cardiac cycle • Podcasts & MP3s on Cardiac cycle • Videos on Cardiac cycle
Evidence Based Medicine Regarding Cardiac cycle	Cochrane Collaboration on Cardiac cycle • Bandolier on Cardiac cycle • TRIP on Cardiac cycle
Cost Effectiveness of Cardiac cycle	Cost Effectiveness of Cardiac cycle
Clinical Trials Involving Cardiac cycle	Ongoing Trials on Cardiac cycle at Clinical Trials.gov • Trial results on Cardiac cycle • Clinical Trials on Cardiac cycle at Google
Guidelines / Policies / Government Resources (FDA/CDC) Regarding Cardiac cycle	US National Guidelines Clearinghouse on Cardiac cycle • NICE Guidance on Cardiac cycle • NHS PRODIGY Guidance • FDA on Cardiac cycle • CDC on Cardiac cycle
Textbook Information on Cardiac cycle	Books and Textbook Information on Cardiac cycle
Pharmacology Resources on Cardiac cycle	Dosing of Cardiac cycle • Drug interactions with Cardiac cycle • Side effects of Cardiac cycle • Allergic reactions to Cardiac cycle • Overdose information on Cardiac cycle • Carcinogenicity information on Cardiac cycle • Cardiac cycle in pregnancy • Pharmacokinetics of Cardiac cycle •
Genetics, Pharmacogenomics, and Proteinomics of Cardiac cycle	Genetics of Cardiac cycle • Pharmacogenomics of Cardiac cycle • Proteomics of Cardiac cycle
Newstories on Cardiac cycle	Cardiac cycle in the news • Be alerted to news on Cardiac cycle • News trends on Cardiac cycle
Commentary on Cardiac cycle	Blogs on Cardiac cycle
Patient Resources on Cardiac cycle	Patient resources on Cardiac cycle • Discussion groups on Cardiac cycle • Patient Handouts on Cardiac cycle • Directions to Hospitals Treating Cardiac cycle • Risk calculators and risk factors for Cardiac cycle
Healthcare Provider Resources on Cardiac cycle	Symptoms of Cardiac cycle • Causes & Risk Factors for Cardiac cycle • Diagnostic studies for Cardiac cycle • Treatment of Cardiac cycle
Continuing Medical Education (CME) Programs on Cardiac cycle	CME Programs on Cardiac cycle
International Resources on Cardiac cycle	Cardiac cycle en Espanol • Cardiac cycle en Francais
Business Resources on Cardiac cycle	Cardiac cycle in the Marketplace • Patents on Cardiac cycle
Informatics Resources on Cardiac cycle	List of terms related to Cardiac cycle

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Acknowledgement and Attribution Regarding Sources of Content

Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .