Cardiac Cycle Phases

Cardiac Cycle Overview

• The names of individual cardiac cycle phases are derived from ventricular events
• Systole involves contraction of the heart
  • Isovolumic contraction happens during systole
  • Ejection of blood also occurs during systole
• Diastole involves relaxation of the heart
  • Isovolumic relaxation occurs during diastole
  • The filling phase happens during diastole
    • Rapid filling is part of the filling phase
    • Slow filling is part of the filling phase
    • Atrial systole is part of the filling phase

Valve and Chamber Events

• The closing of AV valves causes isovolumic contraction
• Isovolumic contraction occurs during systole
• the opening of semilunar valves causes ventricular ejection
• Ventricular Ejection occurs during Systole
• The closing of semilunar valves causes isovolumic relaxation
• Isovolumic relaxation happens during diastole
• The opening of AV valves causes rapid ventricular filling, slow ventricular filling, and atrial contraction
• Rapid ventricular filling, slow ventricular filling, and atrial contraction all occur during diastole

Ventricular Contraction: Isovolumic contraction

• Isovolumic contraction starts after the QRS wave onset, representing electrical ventricular activation
• AV valves close when ventricular pressure exceeds atrial pressure
• Semilunar valves stay closed throughout the isovolumic contraction
• There is a rapid increase in intraventricular pressure during this phase
• The apex-to-base length of the ventricles decreases
• Ventricular pressure increases to the aorta or pulmonary artery's diastolic pressure value
• Isovolumic contraction lasts for 0.06 to 0.08 seconds
• The change in pressure over time (dp/dt) reaches 1000 mmHg/second
• Semilunar valves open at the end of this phase
• Ventricular volume is at end-diastolic volume (EDV)

Ventricular Contraction: Ejection

• Ejection occurs after the QRS wave and during the T wave on the ECG
• AV valves remain closed, while semilunar valves open
• Pressure initially rises to one-third of its maximum value, then decreases during the remaining two-thirds
• Ejection moves blood into the aorta and pulmonary artery
• Ventricular volume decreases during ejection
• Apex-to-base length decreases, atrial volume increases, and intra-atrial pressure falls
• When pressures in the aorta and pulmonary artery equilibrate, ventricles begin to relax, and semilunar valves close
• Stroke volume (SV) is ejected

Ventricular Relaxation: Isovolumic Relaxation

• Repolarization of the ventricles is complete after the T wave on the ECG
• Both atrioventricular (AV) valves and semilunar (SL) valves are closed
• Pressure decreases rapidly
• When pressures in the ventricles become lower than in the atria, AV valves open
• The volume in the ventricles is at end-systolic volume (ESV)

Ventricular Relaxation: Rapid Ventricular Filling

• Ventricular repolarization is complete after the T wave on the ECG
• AV valves are open
• Semilunar valves are closed
• Pressures in the aorta and pulmonary artery decrease
• Ventricular volume increases

Ventricular Relaxation: Slow Ventricular Filling

• Slow ventricular filling is the longest phase of the cardiac cycle
• AV valves are open
• Semilunar valves are closed
• Pressures in the aorta and pulmonary artery continue to decrease
• Ventricular volume increases

Ventricular Relaxation: Atrial Contraction

• Atrial contraction occurs after the P wave on the ECG
• Blood is forced through the open AV valves
• Atrial pressure increases
• Atrial contraction accounts for 20% of the ventricular end-diastolic volume
• Atrial contraction is the last subphase of ventricular diastole
• Ventricles are filled up to the end-diastolic volume(EDV)

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 blood flow corresponds to the beginning of the reduced ejection phase
• Blood flow is negative during the isovolumic relaxation phase

Aortic Blood Pressure

• Normal aortic blood pressure ranges from 80-120 mmHg
• The arterial sphygmogram is a recording of arterial pressure
• The arterial sphygmogram Includes anacrotic and catacrotic phases
• The dicrotic notch on the arterial pressure wave reflects the closure of the semilunar valves

Arterial Blood Pressure

• Arterial sphygmogram consists of Anacrotic and catacrotic phases
• Arterial sphygmogram has dicrotic notch reflects closure of SL valves

Pressure Changes in the Atria

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

Central Venous Pressure (CVP)

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

Jugulogram Waves

• A jugulogram uses a transducer over the right jugular vein to record pressure changes
• The a wave results from venous distention due to right atrial systole
• The c wave results from increased filling of the right atrium during isovolumic contraction in the RA(c = 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, the 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 typically auscultated using a stethoscope and recorded via phonocardiogram
• Heart sounds accompany the opening and closing of heart valves
• S1 heart sound occurs during isovolumic contraction
• S2 heart sound occurs during isovolumic relaxation

Heart Sounds: S1

• S1 is the "lub" part of the "lub-dub" sound
• S1 is composed of the M1 and T1 components, where M1 slightly precedes T1
• S1 is caused by the closure of the atrioventricular valves
• S1 results from reverberation within the blood associated with the sudden block of flow reversal by the valves
• S1 has a duration of about 150 msec
• S1 occurs 50-70 msec after the onset of the QRS complex on the ECG
• S1 has a frequency range of 25-45 Hz
• S1 consists of three components:
  • Low amplitude vibrations of blood, ventricular walls, and cusps
  • Higher amplitude vibrations of blood and ventricular walls when intraventricular pressure abruptly rises
  • Lower amplitude opening of semilunar valves

Heart Sounds: S2

• S2 is the "dub" part of the "lub-dub" sound
• S2 is composed of the A2 and P2 components
• Normally, aortic component A2 precedes pulmonic component P2, especially during inspiration because of increased preload. Increased filling of RV will increase ejection time
• S2 consists of vibrations of the aortic and pulmonary valves during the end of slow ejection/onset of isovolumic relaxation
• S2 is shorter in duration than S1
• S2 occurs after the end of the T wave on the ECG

Heart Sounds: S3

• S3 occurs at the beginning of diastole (during rapid filling), approximately 0.15 seconds after S2
• S3 may be normal in individuals under 40 years of age
• S3 has a low-pitched and faint sound
• S3 can indicate disease in middle-aged or older adults

Heart Sounds: S4

• S4 occurs during atrial contraction
• S4 occurs at the end of the P wave on an ECG
• S4 is faint
• S4 is rare in a healthy person
• S4 may indicate increased intra-atrial pressure or lower ventricle compliance (ventricular hypertrophy)

Heart Sounds and Associated Events

• S1, the first heart sound "lub", is associated with the closure of the mitral and tricuspid valves
• S2, the second heart sound "dub", is associated with the closure of the aortic and pulmonic valves
• S3, the third heart sound, is associated with diastolic filling
• S4, the fourth heart sound, is associated with an atrial sound

Right vs Left Heart Pressures

• Pressures are different in the right and left sides of the heart and pulmonary/systemic circulations
• The right atrium mean pressure normally around 2 mmHg
• The left atrium mean pressure normally around 8 mmHg
• The right ventricle peak systolic pressure normally around 25 mmHg
• The left ventricle peak systolic pressure normally around 120 mmHg
• The Pulmonary artery mean pressure normally around 15 mmHg
• The Aorta Artery mean pressure normally around 95 mmHg
• Pulmonary capillaries mean pressure is around 10 mmHg
• Systemic capillaries mean pressure is around 25 mmHg

Action Potentials in Cardiac Cells

• Cardiac sinoatrial (SA) node cells produce slow-response action potentials
• The upstroke of phase 0 in slow-response cardiac tissue is due to the influx of Ca2+

Ventricular Cardiomyocytes

• 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

• If a ventricular cardiac myocyte is stimulated again during time P3, the second action potential will have a reduced amplitude

L-type Ca2+ Channels

• L-type Ca2+ channels do open in response to depolarization of the membrane and are found in the T-tubule membrane
• L-type Ca2+ channels are open during the plateau of the action potential
• L-type Ca2+ channels allow Ca2+ entry, triggering sarcoplasmic reticulum Ca2+ release
• L-type Ca2+ channels DO NOT contribute to the pacemaker potential