The Cardiac Cycle

The cardiac cycle refers to the series of events that happen in the heart during one heartbeat. The cardiac cycle functions to maintain blood flow through the heart and the body. Each cardiac cycle is initiated by electrical signal that originates in the SA node and travels in a wave of depolarization through the heart. The average heart rate is 60-90 beats per minute, which means the cardiac cycle repeats 60-90 times per minute.

The cardiac cycle has two main phases:

• Diastole: When the heart muscle relaxes and fills with blood, and
• Systole: When the heart muscle contracts and pumps blood forward.

It is important to note that the terms "diastole" and "systole" are typically used to describe phases of the ventricles. The heart chambers and their roles during diastole and systole are discussed in more detail below.

First, let's start off with an overview of blood flow through the heart:

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There are four chambers in the heart that fill with blood and function to move that blood a.) to the pulmonary vasculature (lungs) to be oxygenated, and b.) out to the systemic vasculature (rest of body):

1. Right atrium: Receives deoxygenated blood; the veins of the body empty into the inferior vena cava and superior vena cava, which both empty into the RA. The RA has some of the lowest pressures in the heart, because it needs to create a favorable pressure gradient, in order for blood from the body's venous system to readily flow into the RA.
2. Right ventricle: Fills with blood from the RA during diastole and pumps it forward into the pulmonary arteries during systole (to go to the lungs for oxygenation).
3. Left atrium: Receives oxygenated blood from lungs via the pulmonary veins.
4. Left ventricle: Fills with blood from the LA during diastole and pumps it forward to the aorta during systole (to go out to the rest of the body). The LA has some of the highest pressures in the heart, because it has to generate enough force to eject blood through the aorta and to the body.

Systole and Diastole

"Systole" and "diastole" typically refers to "ventricular systole" and "ventricular diastole". xxx.

Heart chambers summary chart:

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There are four heart valves that control the flow of blood through the heart, preventing the backward flow of blood. The opening and closing of the heart valves relies on the relative pressures in the heart chambers. The closing of heart valves create the typical heart sounds ("lub-dub") that we hear on auscultation of the heart. There are four important heart valves:

1. Tricuspid valve: Between the right atrium (RA) and right ventricle (RV). Opens during diastole, when blood is flowing out of the RA and filling the RV. Closes when the RV is done filling and is starting to contract to push blood into pulmonary vasculature. Tricuspid valve closure prevents the backward flow of blood into the RA while the RV contracts. Thus, tricuspid valve closure is part of the sound you hear in S1, and it signals the start of systole.


2. Pulmonic valve: Between the RV and pulmonary artery, taking blood from the RV to the lungs to be oxygenated. Opens during systole, as the RV contracts and creates the pressure to open the valve and eject blood out into the pulmonary artery (to go to the lungs). Closes after the RV is done contracting and starting to relax again (RV pressure decreases, valve closes). Thus, pulmonic valve closure is part of the sound you hear in S2 and signals the start of diastole.


3. Bicuspid (mitral) valve: Between left atrium (LA) and left ventricle (LV). Opens during diastole, when blood is flowing out of the LA and filling the LV (LA is filled with blood and creates small contraction to increase LA pressure and push open valve). Closes when the LV is done filling and is starting to contract to push blood into the aorta and out to the body. Mitral valve closure prevents the backward flow of blow into the LA while the LV contracts. Thus, mitral valve closure is part of the sound you hear in S1 and signals the start of systole.


4. Aortic valve: Between left ventricle (LV) and aorta. Opens during systole, as the LV contracts and creates the pressure necessary to open the valve and eject blood out into the aorta (to go to the body). Closes after the LV is done contracting and starting to relax again. Aortic valve closure prevents the backward flow of blood into the LV during diastole, when the LV relaxes and LV pressure decreases again. Thus, aortic valve closure is part of the sound you hear in S2 and signals the start of diastole.

Diseases or abnormalities of heart valves can lead to heart murmurs, which are due to abnormal blood flow across diseased heart valves.

Heart valves summary chart:

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The classic heart sounds that we hear on physical exam are created by the closing of heart valves at various points during one round of a heartbeat. The heart sounds are classically described as a "lub-dub" sound, where the first heart sound ("lub") is S1 and the second heart sound ("dub") is S2.

1. S1 (first heart sound): Sound made when the tricuspid valve and bicuspid (mitral) valve close. S1 signifies the end of diastole and the beginning of systole.
2. S2 (second heart sound): Sound made when the pulmonic valve and aortic valve close. S2 signifies the end of systole and the beginning of diastole.

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Sometimes, we can hear extra heart sounds on auscultation. Sometimes these can occur in normal hearts, but they sometimes indicate disesae.

S3

S3 occurs in early diastole (right after S2) when there is really rapid or forceful filling of the LV (the S3 sound is essentially blood slamming against the LV wall during early filling, right after the mitral valve opens). This usually happens in the context of a high LA pressure, which leads to a swift rush of blood from the LA to the LV during diastole. An S3 can be present in the following situations:

• Mitral regurgitation: xxx.
• Acute heart failure: xxx.
• Can be normal in young patients and pregnant women: xx.

S4

S4 occurs in late diastole (right before the next S1) when the LV pressures are so high that the LA needs to give a little extra contraction at the end of diastole in order to empty all its blood into the high-pressure LV (this is why it is called an "atrial kick"). This usually happens in the context of a stiff LV, which creates a high-pressure LV even during diastole, when it should be more relaxed. An S4 can be present in the following situations:

• LV hypertrophy: xxx.
• Hypertrophic cardiomyopathy: xxx.
• Diastolic heart failure: xxx.

Note: Left-sided S3 and S4 are associated with left heart abnormalities. Right-sided S3 and S4 are associated with right heart abnormalities.

Splitting of S2

The second heart sound (S2) can sometimes seem to be "split" into two separate sounds. S2 is normally created by the simultaneous closure of the pulmonic and aortic valves. So, S2 splitting occurs if either the pulmonic or aortic valve closure is delayed, so that the pulmonic valve and aortic valve do not close at the same time.

• Normal delay of pulmonic valve closure (normal S2 splitting): xxx.
• Abnormal delay of pulmonic valve closure: xxx.
• Abnormal delay of aortic valve closure: xxx.

Other sounds that can be heard during auscultation include heart murmurs, which usually signify diseases or abnormalities of heart valves or blood flow.

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***.

Here is some common vocabulary used when describing murmurs:

• Stenosis: ***.
• Regurgitation: ***.
• Systolic murmur: ***.
• Diastolic murmur: ***.

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Pressure-volume loops ***.

Wiggers diagram ***.