5: Cardiac Cycle

Cardiac Cycle Lecture Notes

• The cardiac cycle encompasses the events of one complete heartbeat, encompassing contraction and relaxation phases.
• The lecture discusses the sequence of cardiac contraction and relaxation, valve operations, chamber volumes, and pressures, along with pulmonary and aortic pressures.
• Normal valve operation is crucial for proper blood flow.
• Common measurements like LVEDV, LVESV, EF, and atrial/ventricular pressures are examined, as are the various phases within the cardiac cycle (atrial and ventricular systole/diastole).
• Heart sounds (S1, S2, S3, S4) are analyzed in relation to their origins and locations within the cardiac cycle.
• Students need to be able accurately to draw and label cardiac cycle components from ventricular pressure-volume loops.
• This lecture also explains the significance and measurement of pulmonary capillary wedge pressure.
• The lecturer provided resources like a website (cvphysiology.com) and a textbook (Cardiovascular Physiology Concepts, Wolters Kluwer) for additional learning.

Cardiac Chambers and Valves

• The lecture features a diagram illustrating the heart's chambers and valves.
• Abbreviations are used to describe the various chambers (e.g. RA, LA, LV, RV, IVC, SVC, Ao, PA).
• The role of different valves (tricuspid, mitral, pulmonary, aortic) and their significance in regulating blood flow is discussed.

Basic Phases of the Cardiac Cycle

• The cycle encompasses systole (ventricular contraction, ejection) and diastole (ventricular relaxation, filling).
• Systole marks the beginning of ventricular contraction and concludes when ventricular ejection stops (relaxation occurs before complete ejection ends).
• Diastole signifies ventricular relaxation, initiating ventricular filling after relaxation.

Atrial Systole (Phase 1)

• Atrial contraction is near the end of ventricular diastole, initiated by atrial depolarization (P wave).
• Atrial pressure increases momentarily, pushing more blood into the ventricles (~10% of filling).

Ventricular Systole (Phases 2-4)

• Isovolumetric Contraction: Ventricular contraction occurs, mitral valve closes. Left Ventricular Pressure (LVP) rises but volume doesn't change.
• Ejection: When LVP exceeds aortic pressure (AP), blood is ejected.
• Pressure development rate (dP/dtMAX) is maximal during this phase.

Ventricular Systole (Phases 3-4)

• Ejection: Aortic valve opens, maximal ejection velocity occurs early, and blood is ejected into the aorta.
• Repolarization: (T-wave) slows the ejection rate and leads to relaxation.
• Residual blood volume becomes end-systolic volume (ESV).

All Heart Valves

• Heart valves open and close passively based on pressure differences across them.
• Aortic valve opens when LVP is higher than aortic pressure (AP), and closes when LVP drops below AP.
• Mitral valve opens when left atrial pressure (LAP) is higher than LVP, closing when LAP falls.

Papillary Muscle Contraction

• Contraction of papillary muscles tenses chordae tendineae, preventing AV valve leaflets from bulging into atria, maintaining valve competency.

Ventricular Diastole (Phases 5-7)

• Isovolumetric Relaxation: Decreasing LVP leads to aortic valve closure and signifies the beginning of diastole.
• Diastole is initially isovolumetric, with decreasing LVP and constant volume.
• The mitral valve opens when ventricular pressure drops below atrial, initiating filling.

Passive Filling

• During ventricular diastole, most ventricular filling (~90% at rest) happens before atrial systole.
• Mitral valve opening occurs when ventricular pressure slips below atrial.

Ventricular Stroke Volume

• Ventricular stroke volume (SV) is the difference between EDV (end-diastolic volume) and ESV (end-systolic volume).
• In normal hearts, SV represents the volume of blood ejected into the aorta per contraction.

Ventricular Ejection Fraction (EF)

• EF gauges the percentage of blood ejected from the ventricle relative to available volume (EDV)
• EF is a clinical indicator of LV systolic function.
• A normal ejection fraction is typically in the 55-60% range.

Pulmonary Capillary Wedge Pressure

• An indirect measurement of left atrial pressure.
• Measured using a right-sided cardiac catheter with a balloon that occludes the pulmonary artery branch, representing left atrial pressure.

Swan-Ganz Catheter

• A type of right-sided cardiac catheter used to measure pulmonary capillary wedge pressure, reflecting left atrial pressure.

Intracardiac Pressure Measurements

• Diagrams illustrate pressure changes in different parts of the heart (right and left atria, ventricles, pulmonary artery and pulmonary artery wedge).

Basic Heart Sounds

• S1: Mitral and tricuspid valve closure, occurring at the start of isovolumetric ventricular contraction.
• S2: Aortic and pulmonary valve closure, reflecting the onset of isovolumetric ventricular relaxation
• S3: Associated with highly compliant ventricles, indicative of early, rapid filling and found in children.
• S4: Linked to stiff ventricles, typically observed in older adults or those with cardiac hypertrophy.

Summary of Major Concepts

• The cardiac cycle has systole and diastole phases regulated by specific events determined by heart events occurring within specific time points.
• Systole is initiated by ventricular depolarization, resulting in isovolumetric pressure generation and blood ejection into the aorta.
• Ventricular repolarization brings about ventricular relaxation, reducing pressure for ejection.
• Ejection culminates with aortic valve closure, initiating ventricular filling.
• Ventricular filling, also including passive filling, begins with the opening of the mitral valve after the isovolumetric relaxation phase.
• Pressure-volume loops are helpful in analyzing ventricular function.

Questions and Answers

• Some sample questions about the cardiac cycle and answers are included in the provided materials to help reinforce concepts.

This lecture covers the cardiac cycle, highlighting the phases of contraction and relaxation during a heartbeat. It explores valve functions, chamber pressures, and measurements such as LVEDV and EF, alongside heart sounds. Students will learn to diagram the cardiac cycle from ventricular pressure-volume loops and understand clinical implications.