Cardiac cycle

Cardiac Sinoatrial Node Cells

• Cardiac sinoatrial (SA) node cells produce slow-response action potentials, as shown in the image.
• The upstroke in phase 0 of the action potential is due to the influx of Ca2+.

Ventricular Cardiac Action Potential

• During a continuing medical education session for cardiologists, a presenter uses the ventricular cardiac action potential to explain functional electrical reentry, which can lead to arrhythmias in a structurally normal heart.
• Ventricular cardiomyocytes are responsive to atypical restimulation and initiation of functional reentry circuits during periods P2 and P3 of the cardiac action potential.

Ventricular Myocyte Stimulation

• A typical action potential of a ventricular cardiac myocyte is shown when stimulated at time zero.
• If the cell is stimulated again during the time labeled P3, the second action potential is noted to have a reduced amplitude when compared to the first.

L-type Ca2+ Channels

• L-type Ca2+ channels do not contribute to the pacemaker potential.
• L-type Ca2+ channels are in cardiac ventricular muscle cells open during the plateau of the action potential and allow Ca2+ entry, which triggers the sarcoplasmic reticulum Ca2+ release, and open in response to membrane depolarization.
• These channels are found in the T-tubule membrane.

Cardiac Cycle Phases

• The cardiac cycle's individual phase names are derived from ventricular events.
• Systole is the contraction phase, including isovolumic contraction and ejection.
• Diastole is the relaxation phase, including isovolumic relaxation and the filling phase, which consists of rapid filling, slow filling, and atrial systole.

Cardiac Cycle Events Overview

• Closing of the AV valves corresponds to isovolumic contraction during systole.
• Opening of the semilunar valves corresponds to ventricular ejection during systole.
• Closing of the semilunar valves corresponds to isovolumic relaxation during diastole.
• Opening of the AV valves corresponds to rapid ventricular filling, slow ventricular filling, and atrial contraction, all during diastole.

Isovolumic Contraction

• Isovolumic contraction begins after the onset of the QRS wave, representing electrical activation of the ventricles.
• AV valves close when ventricular pressure exceeds atrial pressure.
• SL valves are closed the entire time.
• There's a rapid increase in intraventricular pressure, and the apex-to-base length decreases.
• Pressure increases to the value of the diastolic pressure in the aorta or pulmonary artery and lasts 0.06 to 0.08 seconds.
• dp/dt is 1000 mmHg/second, and the volume in the ventricles is the end-diastolic volume (EDV).
• This phase ends with the opening of the semilunar valves.

Ejection Phase

• The ejection phase occurs after the QRS complex and during the T wave.
• AV valves are closed, and semilunar valves are open.
• Pressure rises to its maximum value in the first third and then decreases in the remaining two-thirds.
• Blood is ejected into the aorta and pulmonary artery, causing the ventricular volume to decrease.
• The apex-to-base length decreases, atrial volume increases, and intraatrial pressure falls.
• When pressures in the aorta and pulmonary artery equilibrate, the ventricles start to relax, and the semilunar valves close.
• Stroke volume(SV) is ejected.

Isovolumic Relaxation

• Repolarization of the ventricles is complete after the T wave on the ECG.
• AV valves are now closed.
• SL valves are also closed.
• Pressure decreases rapidly.
• When pressures in the ventricles become less than in the atria, the AV valves open.
• The volume in ventricles is the end-systolic volume (ESV).

Rapid Ventricular Filling

• Repolarization of the ventricles is complete after the T wave on the ECG.
• AV valves are open.
• SL valves are closed.
• Pressures in the aorta and pulmonary artery decrease.
• Ventricular volume increases.

Slow Ventricular Filling

• The longest phase of the cardiac cycle occurs.
• AV valves are open.
• SL valves are closed.
• Pressures in the aorta and pulmonary artery continue to decrease.
• Ventricular volume increases.

Atrial Contraction

• Atrial contraction occurs after the P wave on the ECG.
• Blood is forced through the open AV valves.
• There's an increase in atrial pressure.
• It contributes 20% of the ventricular end-diastolic volume.
• Atrial contraction is the last subphase of ventricular diastole, and the ventricles are filled up to EDV.

Ventricular Systole

• Ventricular systole includes isovolumetric ventricular contraction and ventricular ejection when blood flows out of the ventricle.
• During isovolumetric ventricular contraction, the atrium is relaxed, the ventricle contracts, and the AV and aortic/pulmonary valves are closed.
• During ventricular ejection, the atrium is relaxed, the ventricle contracts, and the AV valve is closed while the aortic and pulmonary valves are open.

Ventricular Diastole

• Ventricular diastole includes isovolumetric ventricular relaxation, ventricular filling, and atrial contraction.
• During isovolumetric ventricular relaxation, the atrium and ventricle are relaxed, the AV is closed, and the aortic and pulmonary valves are closed.
• During ventricular filling, the atrium and ventricle are relaxed and the AV is open while the aortic and pulmonary valves are closed.
• During atrial contraction, the atrium contracts while the ventricle is relaxed, the AV is open, and the aortic and pulmonary valves are closed.

Blood Flow in the Ascending Aorta

• Blood flow oscillates between slightly negative to highly positive values.
• Blood flow increases and decreases during the ejection phase.
• The peak flow corresponds to the beginning of the reduced ejection phase.
• Blood flow is negative during isovolumic relaxation.

Aortic Blood Pressure

• Aortic blood pressure ranges from 80-120 mmHg.
• The aortic blood pressure waveform is an arterial sphygmogram.
• It exhibits anacrotic and catacrotic phases.
• The dicrotic notch reflects the closure of the semilunar valves.

Arterial Blood Pressure

• Arterial blood pressure is reflected in the arterial sphygmogram.
• It shows anacrotic and catacrotic phases, with the dicrotic notch reflecting SL valve closure.

Blood Pressure Waveform

• Blood pressure waveforms change as blood flows through different vessels from the proximal aorta to capillaries.

Pressure Changes in the Atria

• Positive waves during atrial contraction (a), isovolumic contraction and bulging of the AV valve (c), and isovolumic relaxation as the atrium fills against a closed AV valve (v).
• Dips (x, y) occur during the ejection phase and rapid ventricular filling.

Central Venous Pressure

• CVP is also known as right atrial pressure (RAP).
• It is the pressure of blood in the thoracic vena cava near the right atrium.
• CVP reflects the amount of blood returning to the heart and the heart's ability to pump blood into the arterial system.
• It is a good approximation of the right atrial pressure, typically around 5 mmHg (0.6 kPa).

Jugulogram

• Jugulogram involves placing a transducer over the right jugular vein to measure pressure changes.
• The a wave results from venous distention during right atrial systole.
• The c wave results from increased filling of the right atrium during isovolumic contraction (cusp).
• The x descent is due to atrial relaxation and descent of the floor of the right atrium during right ventricular ejection.
• The v wave results from the rise in right atrial pressure when blood flows into the right atrium during isovolumic relaxation when the tricuspid valve is shut.
• The y descent, or downslope of the v wave, is related to the decline in right atrial pressure when the tricuspid valve reopens during rapid filling.

Heart Sounds

• Heart sounds are auscultated using a stethoscope and recorded via phonocardiogram.
• Opening and closing of valves are accompanied by heart sounds.
• S1 occurs during isovolumic contraction.
• S2 occurs during isovolumic relaxation.

Heart Sounds - S1

• S1 corresponds to the "lub" of "lub-dub".
• Composed of components M1 (mitral) and T1 (tricuspid), with M1 slightly preceding T1.
• It is associated with the closure of the atrioventricular valves.
• Results from reverberation within the blood associated with the sudden block of flow reversal by the valves.
• Duration is about 150 msec.
• Occurs 50-70 msec after the onset of the QRS complex on the ECG.
• It has a frequency of 25-45 Hz.
• Has three components: 1. low amplitude vibrations of blood, ventricular walls, and cusps, 2. higher amplitude vibrations of blood and ventricular walls when intraventricular pressure abruptly rises, 3. lower amplitude opening of semilunar valves.

Heart Sounds - S2

• S2 corresponds to the "dub" of "lub-dub".
• Composed of components A2 (aortic) and P2 (pulmonic).
• Normally, A2 precedes P2, especially during inspiration with increased filling of the right ventricle, leading to increased preload and ejection time.
• It is caused by vibrations of the aortic and pulmonary valves at the end of slow ejection/onset of isovolumic relaxation.
• Shorter in duration than S1.
• Occurs after the end of the T wave on the ECG.

Heart Sounds - S3

• S3 occurs at the beginning of diastole, specifically during rapid filling, approximately 0.15 seconds after S2.
• It is normal under 40 years of age.
• S3 is low-pitched and faint, and is a sign of disease in older adults.

Heart Sounds - S4

• S4 is a heart sound associated with atrial contraction.
• S4 occurs at the end of the P wave on the ECG.
• Is usually faint.
• S4 is rare in healthy adults and indicates increased intra-atrial pressure or lower ventricle compliance, often due to ventricular hypertrophy.

Summary of Heart Sounds

• S1 is the first heart sound, "lub", with mitral and tricuspid components, associated with closing of the mitral and tricuspid valves.
• S2 is the second heart sound, "dub," with aortic and pulmonary components, associated with closing of the aortic and pulmonary valves.
• S3 is the third heart sound and occurs during diastolic filling.
• S4 is the fourth heart sound and occurs during atrial sound.

Changes in Ventricular Volumes

• Phase 1 corresponds to ventricular filling.
• Phase 2 corresponds to Isovolumetric contraction.
• Phase 3 corresponds to ventricular ejection.
• Phase 4 corresponds to isovolumetric relaxation.

Pressure Ranking During Isovolumetric Contraction

• The pressures during isovolumetric contractions are ranked as follows
• Left ventricular > aortic > left atrial

Blood Pressures in the Right and Left Circulations

• Pressures in mmHg
• Right Atrium: mean is 2
• Left Atrium: mean 8, a wave 13, c wave 12, v wave 15
• Right Ventricle: peak systolic 25, minimum diastolic 0
• Left Ventricle: peak systolic 120, minimum diastolic 0
• Pulmonary Artery: mean 15, peak systolic 25, end diastolic 8
• Aorta: mean 95, peak systolic 120, end diastolic 80
• Pulmonary Capillares: mean 10
• Systemic Capillaries: mean 25