Lesson 2: The Cardiac Cycle: Mechanics, Pathologies, and Clinical Insights

Part 1: Physiology Overview – The Cardiac Cycle

The cardiac cycle refers to the full sequence of mechanical, electrical, and hemodynamic events that occur in the heart during one heartbeat. This cycle coordinates the contraction and relaxation of the atria and ventricles to ensure efficient blood movement through the pulmonary and systemic circuits. The cardiac cycle forms the cornerstone of cardiovascular physiology and is critical for interpreting pathologies like murmurs, arrhythmias, and shock states.

Understanding the cardiac cycle is foundational for USMLE Step 1 preparation, as questions frequently assess your ability to recognize key phases using Wiggers diagrams, pressure-volume loops, and auscultation findings. Mastery of this topic enables quick clinical reasoning in diagnosing heart failure, identifying murmurs, and assessing preload/afterload changes.

High-Yield Tip:

Expect integrated questions involving auscultation, pressure tracings, valve pathology timing, and graphical labeling. Familiarity with key volume-pressure relationships and heart sounds during the cycle is essential.

Major Phases of the Cardiac Cycle

Diastole (Relaxation & Filling Phase):

• Definition: Diastole is the phase in which the ventricles are relaxed and fill with blood. It consists of passive and active filling.

• Key Events:

• Early (Rapid) Filling: Ventricular pressure < atrial pressure → AV valves open → passive inflow of blood.

• Diastasis: Reduced filling as pressures begin to equalize.

• Atrial Systole: Final push of blood into ventricles by atrial contraction.

• Clinical Associations:

• S3: Heard in rapid filling phase (can indicate volume overload or CHF).

• S4: Heard during atrial contraction, common in stiff ventricles (e.g., LVH).

Systole (Contraction & Ejection Phase):

• Definition: Systole begins with ventricular contraction and ends with blood ejection into the aorta and pulmonary artery.

• Key Events:

• Isovolumetric Contraction: Ventricles contract with all valves closed → rapid pressure increase.

• Ejection Phase: When ventricular pressure exceeds aortic/pulmonary pressure → semilunar valves open → blood is ejected.

• Clinical Associations:

• S1: Closure of mitral and tricuspid valves (start of systole).

• Murmurs: Aortic stenosis and mitral regurgitation occur here.

Step-by-Step Events of the Cardiac Cycle

1. Atrial Systole:

• Final 10–20% of ventricular filling.

• Important in patients with diastolic dysfunction (e.g., hypertensive heart disease).

2. Isovolumetric Contraction:

• All valves closed.

• Sharp rise in intraventricular pressure.

3. Ventricular Ejection:

• Ventricular pressure exceeds arterial pressure.

• Semilunar valves open, stroke volume ejected.

4. Isovolumetric Relaxation:

• All valves closed again.

• Ventricular pressure rapidly drops.

5. Rapid Filling:

• AV valves open → blood rushes in (may hear S3).

6. Reduced Filling:

• Passive phase slows down.

Memorization Mnemonic:

“AIM VERI” – Atrial contraction, Isovolumetric contraction, Max ejection, Ventricular relaxation, Early filling, Reduced filling, Isovolumetric relaxation.

Key Hemodynamic Parameters & Clinical Calculations

• End-Diastolic Volume (EDV): Max volume in ventricles before contraction; indicator of preload.

• End-Systolic Volume (ESV): Volume remaining after contraction.

• Stroke Volume (SV) = EDV − ESV

• Ejection Fraction (EF) = SV ÷ EDV → Normal ≥55%. Used to gauge systolic function.

• Cardiac Output (CO) = SV × HR → Reflects total blood flow per minute.

• Mean Arterial Pressure (MAP) = (2 × DBP + SBP) ÷ 3

USMLE Insight:

• EF <40% is diagnostic of HFrEF.

• EF >50% with symptoms = HFpEF.

• Stroke volume is directly proportional to preload and contractility, and inversely to afterload.

Part 2: Pathophysiology Connections – Cardiac Cycle Disorders

1. Heart Failure (HFrEF vs. HFpEF):

• Systolic Dysfunction (HFrEF):

• Dilated ventricles.

• ↓ Contractility, ↓ EF.

• Volume overload signs: S3, pulmonary rales.

• Diastolic Dysfunction (HFpEF):

• Hypertrophied/stiff ventricles.

• Normal EF but ↓ filling.

• Presents with exertional dyspnea, preserved contractility.

2. Valve Abnormalities:

• Aortic Stenosis (AS):

• Obstructs systolic ejection.

• Classic triad: Angina, Syncope, Dyspnea.

• Crescendo-decrescendo systolic murmur.

• Mitral Regurgitation (MR):

• Volume backflow into LA during systole.

• Holosystolic murmur at apex.

• Aortic Regurgitation (AR):

• Diastolic backflow → widened pulse pressure.

• Bounding pulses (“water hammer pulse”).

• Mitral Stenosis (MS):

• Impairs ventricular filling during diastole.

• Opening snap + diastolic rumble.

3. Arrhythmias:

• Atrial Fibrillation:

• No organized atrial contraction.

• No atrial kick → ↓ preload → ↓ CO.

• Common in mitral stenosis and aging hearts.

4. Pericardial Diseases:

• Cardiac Tamponade:

• Pericardial fluid compresses heart.

• ↓ Diastolic filling → ↓ SV & CO.

• Signs: Pulsus paradoxus, hypotension, distant heart sounds (Beck’s Triad).

Part 3: Clinical Integration & Heart Sound Correlations

Heart Sounds:

• S1: Closure of mitral/tricuspid valves → onset of systole.

• S2: Closure of aortic/pulmonic valves → onset of diastole.

• S3: Rapid filling phase. Normal in youth, abnormal in heart failure.

• S4: Atrial contraction into stiff ventricle (e.g., LVH, diastolic dysfunction).

Murmur Basics:

• Systolic murmurs:

• Aortic stenosis (crescendo-decrescendo).

• Mitral regurgitation (holosystolic).

• Mitral valve prolapse (late systolic + click).

• Diastolic murmurs:

• Aortic regurgitation (decrescendo early diastolic).

• Mitral stenosis (opening snap + rumble).

Jugular Venous Pressure (JVP):

• Estimate of RA pressure.

• Elevated in right heart failure, pericardial tamponade, fluid overload.

Pulse Pressure:

• Widened: Aortic regurgitation, anemia, hyperthyroidism.

• Narrowed: Aortic stenosis, shock, tamponade.

Preload & Afterload Dynamics:

• Preload = Stretch before contraction (↑ with fluids, ↓ with nitrates).

• Afterload = Resistance to ejection (↑ with HTN, ↓ with ACE inhibitors).

USMLE Memory Enhancers

• Graph Mastery: Learn pressure-volume loops, Wiggers diagram points (especially Aortic/Mitral valve timing).

• Mnemonic Match: AIM VERI mnemonic for phases; PROVe for TOF in congenital defects.

• Heart Sound Logic: Link murmur timing to valve phases; draw it if needed on test.

• Clinical Triggers: Know how murmurs change with maneuvers (e.g., Valsalva ↑ HCM murmur).