Cardiac Impulse and Cardiac Cycle

Cardiac Impulse 

The cardiac impulse refers to the electrical signals that are generated in the heart and travel through specialized cardiac muscle fibers, resulting in the coordinated contraction of the cardiac muscle. This process is crucial for maintaining a normal heart rhythm and pumping blood efficiently throughout the body.

Origin of the impulse: 

The cardiac impulse begins in the sinoatrial (SA) node, located in the right atrium of the heart. The SA node generates electrical impulses that set the rhythm for the heartbeat, and is therefore known as the pacemaker of the heart.

Pathway of the impulse: 

From the SA node, the electrical impulses spread through the atria, causing them to contract. The impulses then travel to the atrioventricular (AV) node, located at the junction between the atria and ventricles. The impulses then travel through the bundle of His and its branches, the Purkinje fibers, which deliver the impulses to the ventricles, causing them to contract.

Regulation of the impulse: 

The cardiac impulse is regulated by the autonomic nervous system, which consists of the sympathetic and parasympathetic branches. The sympathetic nervous system can increase the heart rate and force of contraction, while the parasympathetic nervous system can slow the heart rate and decrease the force of contraction.

Clinical significance: 

Disorders of the cardiac impulse can lead to abnormal heart rhythms, or arrhythmias, which can be potentially life-threatening. Examples of arrhythmias include atrial fibrillation, ventricular tachycardia, and bradyarrhythmias. Treatment for arrhythmias may include medications, medical devices such as pacemakers or defibrillators, or in some cases, surgery.

Diagnostic tests: 

Diagnostic tests that may be used to assess the cardiac impulse include electrocardiograms (ECGs), Holter monitoring, and exercise stress tests. These tests can help diagnose arrhythmias and other cardiac disorders.

Cardiac Cycle

The cardiac cycle is the sequence of events that occur during one complete heartbeat. It involves the contraction and relaxation of the atria and ventricles, which allows the heart to pump blood throughout the body. The cardiac cycle consists of the following phases:

1. Atrial systole

Atrial systole is the phase of the cardiac cycle where the atria contract, pushing blood into the ventricles. This is the first phase of the cardiac cycle and it is initiated by an electrical signal generated by the sinoatrial (SA) node, the natural pacemaker of the heart.

During atrial systole, the atrioventricular (AV) valves - the mitral and tricuspid valves - are open, and the semilunar valves - the aortic and pulmonary valves - are closed. The contraction of the atria increases the pressure in the atria, which causes blood to flow into the ventricles.

The timing of atrial systole is important for the efficient filling of the ventricles. Normally, the duration of atrial systole is about 0.1 seconds, or one-tenth of a second. This allows for enough time for the atria to contract and push blood into the ventricles, but not so much time that the ventricles overfill with blood.

After atrial systole, the ventricles begin to contract in isovolumic contraction, during which the volume of the ventricles does not change, and the pressure in the ventricles increases. When the pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, the aortic and pulmonary valves open, and blood is ejected from the ventricles during ventricular systole.

2. Isovolumic contraction

Isovolumic contraction is a phase of the cardiac cycle that occurs after the atria have contracted and the ventricles have begun to contract. During this phase, the ventricles contract isometrically, which means that the volume of the ventricles does not change even though the pressure within them increases.

The isovolumic contraction phase begins when the ventricles start to contract, and the pressure within them increases rapidly. The pressure eventually becomes higher than that within the atria, causing the atrioventricular (AV) valves - the mitral and tricuspid valves - to close. This prevents any backflow of blood into the atria and creates the first heart sound (S1) that can be heard on auscultation.

At this point, the aortic and pulmonary valves remain closed, which means that no blood is being ejected from the ventricles yet. The ventricles continue to contract isometrically during this isovolumic contraction phase until the pressure within them becomes higher than the pressure in the aorta and pulmonary artery.

The duration of the isovolumic contraction phase is relatively short, lasting about 0.05 to 0.1 seconds. This phase is followed by the ventricular ejection phase, during which the pressure within the ventricles continues to increase, causing the aortic and pulmonary valves to open and blood to be ejected into the systemic and pulmonary circulations.

3. Ventricular Ejection

Ventricular ejection is the phase of the cardiac cycle during which blood is ejected from the ventricles into the pulmonary and systemic circulations. This phase begins when the pressure within the ventricles becomes higher than the pressure in the aorta and pulmonary artery, causing the aortic and pulmonary valves to open.

During ventricular ejection, blood is ejected out of the ventricles and into the circulatory system due to the pressure gradient created by ventricular contraction. The pressure within the ventricles reaches its peak during this phase, and the ventricular walls continue to contract forcefully to maintain a high pressure and eject blood from the heart.

The duration of ventricular ejection varies depending on the individual's heart rate and other physiological factors. On average, it lasts for about 0.3 to 0.4 seconds during each cardiac cycle. The amount of blood ejected during each ventricular ejection is called the stroke volume and is typically around 70 mL per beat.

Towards the end of ventricular ejection, the pressure within the ventricles decreases as the ventricular walls relax. As the pressure in the ventricles drops below the pressure in the aorta and pulmonary artery, the aortic and pulmonary valves begin to close. The closure of these valves creates the second heart sound (S2) that can be heard on auscultation.

The end of ventricular ejection marks the beginning of the isovolumic relaxation phase, during which the ventricles relax and the pressure within them drops, allowing blood to flow into them from the atria.

4. Isovolumic Relaxation

Isovolumic relaxation is a phase of the cardiac cycle during which the ventricles are relaxed and the pressure within them decreases. This phase begins when the aortic and pulmonary valves close at the end of ventricular ejection, marking the end of systole.

During isovolumic relaxation, the ventricles are momentarily closed chambers, as both the mitral and tricuspid valves are also closed. This means that the volume of blood within the ventricles does not change, despite the ventricular walls relaxing. The relaxation of the ventricular walls allows the pressure within them to decrease, which creates a pressure gradient between the ventricles and the atria.

The duration of isovolumic relaxation is relatively short, lasting about 0.06 to 0.08 seconds. The pressure within the ventricles drops rapidly during this phase, allowing blood to flow into the ventricles from the atria. However, the volume of blood within the ventricles remains constant during this phase, as the mitral and tricuspid valves remain closed until the pressure within the ventricles drops below the pressure in the atria.

The closure of the aortic and pulmonary valves and the opening of the mitral and tricuspid valves mark the beginning of the next phase of the cardiac cycle, which is ventricular filling. As the pressure within the ventricles drops, the pressure within the atria becomes higher, causing the mitral and tricuspid valves to open and allowing blood to flow into the ventricles.

5. Rapid filling

Rapid filling is a phase of the cardiac cycle during which the ventricles are filled with blood from the atria. This phase occurs after the end of isovolumic relaxation, when the mitral and tricuspid valves open and blood begins to flow from the atria into the ventricles.

During rapid filling, the pressure within the ventricles is low, and the pressure within the atria is higher, creating a pressure gradient that allows blood to flow rapidly into the ventricles. This phase is called "rapid filling" because the initial filling of the ventricles occurs very quickly due to the high pressure gradient.

The duration of rapid filling varies depending on the individual's heart rate and other physiological factors. On average, it lasts for about 0.15 to 0.2 seconds during each cardiac cycle. The amount of blood that enters the ventricles during rapid filling is influenced by the compliance of the ventricular walls, the volume of blood in the atria, and the pressure gradient between the atria and the ventricles.

Towards the end of rapid filling, the pressure within the atria begins to decrease as the ventricles continue to fill with blood. This decrease in pressure reduces the pressure gradient between the atria and the ventricles, causing the rate of blood flow to slow down. This slower flow of blood marks the beginning of the next phase of the cardiac cycle, which is called diastasis.

Heart Sounds

The heart is a vital organ that functions to pump blood throughout the body. During each cardiac cycle, the heart undergoes a sequence of events that are reflected in the sounds that can be heard with a stethoscope. These sounds are commonly known as heart sounds and are heard as "lub-dub."

The first heart sound, or S1, is the sound heard at the beginning of the cardiac cycle. It is caused by the closure of the atrioventricular (AV) valves, which separate the atria from the ventricles. The closure of the valves is due to the contraction of the ventricles, which increases the pressure in the ventricles, causing the AV valves to snap shut. S1 is typically described as a low-pitched, "lub" sound.

The second heart sound, or S2, is the sound heard at the end of the cardiac cycle. It is caused by the closure of the semilunar valves, which separate the ventricles from the pulmonary artery and aorta. The closure of these valves is due to the relaxation of the ventricles, which causes the pressure in the arteries to exceed that in the ventricles, causing the valves to close. S2 is typically described as a higher-pitched, "dub" sound.

In addition to S1 and S2, there may be additional heart sounds heard during the cardiac cycle. These additional sounds may be due to abnormalities in the heart's structure or function. For example, a third heart sound, or S3, may be heard in some individuals with heart failure. S3 is typically heard during the rapid filling phase of the ventricles and is caused by the vibration of the ventricular walls due to the inflow of blood. S3 is described as a low-pitched, "ta" or "dull thud" sound.

Similarly, a fourth heart sound, or S4, may be heard in individuals with stiff ventricles, such as those with hypertension or aortic stenosis. S4 is heard just before S1 and is caused by the contraction of the atria pushing blood into the stiff ventricles. S4 is described as a low-pitched, "atrial kick" sound.