The Wiggers’ Diagram and Heart Sounds Notes

Learning Objectives

• Describe the Wiggers diagram and identify the 6 phases of the cardiac cycle.
• Describe the volume-pressure diagram of the cardiac cycle.
• List cardiac sounds ($S1$ to $S4$) and what they represent.
• Describe systolic and diastolic murmurs.

Heart Pressures and Volume Changes

• Aorta:     * Systolic Pressure (S): $110-130\,mmHg$     * Diastolic Pressure (D): $70-80\,mmHg$
• Pulmonary Artery:     * Systolic Pressure (S): $15-25\,mmHg$     * Diastolic Pressure (D): $8-15\,mmHg$
• Left Atrium:     * Mean Pressure: $4-12\,mmHg$
• Right Atrium:     * Mean Pressure: $0-8\,mmHg$
• Right Ventricle:     * Systolic Pressure (S): $15-25\,mmHg$     * Diastolic Pressure (D): $0-8\,mmHg$
• Left Ventricle:     * Systolic Pressure (S): $110-130\,mmHg$     * Diastolic Pressure (D): $4-12\,mmHg$

Anatomy of Heart Sounds and Cardiac Structures

• Heart Sound Generation: Sounds are generated with the closure of the Atrioventricular (AV) and Semilunar valves.
• Semilunar Valves:     * Pulmonary Valve.     * Aortic Valve.
• Atrioventricular (AV) Valves:     * Tricuspid Valve (Right side).     * Bicuspid or Mitral Valve (Left side).
• Internal Structures:     * Chordae Tendineae.     * Papillary Muscle.
• First Heart Sound ($S1$):     * Known as "LUB."     * Caused by the closure of the Mitral and Tricuspid valves.
• Second Heart Sound ($S2$):     * Known as "DUB."     * Caused by the closure of the Aortic and Pulmonary valves.

Electrocardiogram (ECG) Foundations

• Definition: A graphical record of electric potentials generated by the heart muscle during each cardiac cycle.
• Detection: Detected on the body surface using electrodes on the limbs and chest wall.
• Display: Amplified by the ECG machine and displayed on special graph paper representing voltage and time.
• Utility: Characterizes arrhythmias and conduction disturbances.
• Specific Waves:     * P Wave: Represents atrial depolarization.     * QRS Complex: Represents ventricular depolarization.     * T Wave: Represents ventricular repolarization.

The Wiggers’ Diagram: Phases of the Cardiac Cycle

• Overview: A standard diagram correlating pressures (atrial, ventricular, aortic), ventricular volume, ECG, and heart sounds (usually focusing on the left side of the heart).

1. Atrial Systole

• Atria start contraction after the atrial depolarization (visualized around the middle of the P wave).
• Filling Contribution: Causes an additional $20\%$ filling of the ventricles.
• ‘a’ Wave: Occurs in the atrial pressure curve due to atrial contraction.
• Pressure Changes: Left Atrial (LA) pressure increases by approximately $7-8\,mmHg$.
• Ventricular Effects: Ventricular pressure and volume increase due to the injection of blood from the atria.

2. Isovolumetric Contraction (1st Phase of Systole)

• Ventricles start to contract after atrial contraction.
• ECG Timing: Occurs around the middle of the QRS complex ($ventricular\,depolarization$).
• Valve Action: Closure of the AV valves occurs, producing the first heart sound ($S1$).
• Pressure/Volume: The Left Ventricular (LV) pressure curve increases sharply, but there is no change in volume because all valves are closed.
• ‘c’ Wave: An atrial pressure wave occurring when ventricles begin to contract. It results from a slight backflow of blood into the atria and the bulging of AV valves toward the atria due to high ventricular pressure.

3. Ventricular Ejection (2nd Phase of Systole)

• Trigger: Ventricular pressure exceeds the aortic pressure ($80\,mmHg$).
• Action: The aortic valve opens, and blood is pumped out of the LV.
• Volume/Pressure: Ventricular volume decreases rapidly. Aortic pressure increases to approximately $120\,mmHg$ (systolic pressure).
• ‘v’ Wave: Atrial pressure wave occurring at the end of ventricular contraction. It results from slow blood flow into the atria from the veins while AV valves are closed. Once the AV valves open after isovolumetric relaxation, stored blood flows into the ventricles and the ‘v’ wave disappears (atrial pressure drops).

4. Isovolumetric Relaxation (1st Phase of Diastole)

• Suddenly, ventricles start to relax (diastole begins).
• ECG Timing: Occurs at the end of the T wave ($ventricular\,repolarization$).
• Valve Action: Aortic valve closes, producing the second heart sound ($S2$).
• Pressure/Volume: Ventricular pressure decreases rapidly. No change in ventricular volume occurs because both AV and Semilunar (SL) valves are closed.
• Incisura (Dicrotic Notch): A short period of backward blood flow in the aorta occurring when the aortic valve closes.
• Diastolic Pressure: Aortic pressure decreases slowly throughout diastole to about $80\,mmHg$ before the next contraction.

5. Rapid Ventricular Filling (2nd Phase of Diastole)

• Trigger: Ventricular pressure drops below atrial pressure, causing AV valves to open.
• Action: Blood flows rapidly from the atria into the ventricles.
• Volume: Ventricular volume curve increases significantly.

6. Diastasis (3rd Phase of Diastole)

• Characteristics: Occurs in the middle third of diastole where only a small amount of blood flows into the ventricles.
• Volume: Ventricular volume curve increases slightly.
• Duration: Persists until the SA node initiates an Action Potential (AP), leading to the next atrial contraction.

Ventricular Volumes

• End Diastolic Volume (EDV): The volume of blood in the ventricle at the end of filling (cycle peak).
• End Systolic Volume (ESV): The volume of blood remaining in the ventricle after ejection.
• Stroke Volume (SV): The amount of blood pumped out per beat. Calculated as:     $SV = EDV - ESV$

Volume-Pressure Diagram Analysis

• Phase A-B (Period of Filling):     * Starts with the opening of the mitral valve.     * LV volume increases as blood flows from the LA.     * Atria contract at the end, increasing volume to about $120\,ml$ (EDV) and pressure to $5-7\,mmHg$.
• Phase B-C (Isovolumic Contraction):     * LV pressure rises to equal aortic pressure ($80\,mmHg$).     * No change in volume; mitral valve closes at point B.
• Phase C-D (Period of Ejection):     * Aortic valve opens at point C.     * Blood flows into the aorta, and LV volume decreases.
• Phase D-A (Isovolumic Relaxation):     * Aortic valve closes at point D.     * LV pressure falls to diastolic levels with no change in volume until the mitral valve opens at point A.

Heart Sounds and Phonocardiogram

• Sound S1: Closing of AV valves ("LUBB"). Caused by vibrations in blood and cardiac walls.
• Sound S2: Closing of Semilunar valves ("DUB").
• Sound S3: Early diastolic sound. Caused by the rush of blood into the ventricles during rapid filling.
• Sound S4: End diastolic sound. Caused by atrial systole pushing blood into a stiff or non-compliant ventricle.
• Species Variation: $S3$ and $S4$ are normal in large animals (horses/cattle) and some dogs.
• Auscultation Regions:     * Pulmonic Valve Region: Left sternal border.     * Aortic Valve Region: Left sternal border.     * Mitral Valve Region: Left sternal border.     * Tricuspid Valve Region: Right sternal border.

Cardiac Murmurs

• Definition: Abnormal heart sounds caused by turbulent blood flow through cardiac defects or valvular issues.
• Categories:     * Valve Regurgitation: Backward flow due to incompetent valves.     * Valve Stenosis: Narrowing of the valve orifice causing turbulence during forward flow.     * Extra Sounds: Caused by Ventricular Septal Defects (VSD) or Patent Ductus Arteriosus (PDA).

Systolic Murmurs

• Occur during ventricular systole (more common).
• Mitral or Tricuspid Incompetence: Regurgitation into the atria.
• Aortic or Pulmonic Stenosis: Turbulence as the valve fails to open fully.
• Ventricular Septal Defect (VSD): Hole in the interventricular septum.
• Patent Ductus Arteriosus (PDA): Persistence of the fetal connection between the aorta and pulmonary artery. Often results in a continuous murmur because aortic pressure is higher than pulmonary pressure throughout the entire cycle.

Diastolic Murmurs

• Occur during ventricular diastole (less common).
• Tricuspid or Mitral Stenosis: Narrowing preventing filling.
• Pulmonic and Aortic Insufficiency: Regurgitation back into the ventricles.

Murmur Intensity Grading (Levine Six-Level Scheme)

• Grade I/VI (Soft): Difficult to hear, not immediately apparent.
• Grade II/VI (Soft): Quieter than normal heart sounds but detected immediately.
• Grade III/VI (Moderate): Similar intensity to normal heart sounds.
• Grade IV/VI (Loud): Louder than normal heart sounds; no palpable thrill.
• Grade V/VI (Thrilling): Very loud; palpable thrill on the chest wall.
• Grade VI/VI: Very loud; audible without a stethoscope or with the stethoscope lifted off the chest; palpable thrill present.

Canine Breed Cardiovascular Predispositions

• Mitral Valve Disease (Acquired): Cavalier King Charles Spaniel, Dachshund, small breeds.
• Mitral Valve Disease (Congenital): Bull Terrier, Rottweiler.
• Myocardial Failure: Boxer, Cocker Spaniel, Doberman Pinscher, Great Dane, Irish Wolfhound, Labrador Retriever.
• Patent Ductus Arteriosus (PDA): Cocker Spaniel, English Springer Spaniel, German Shepherd, Maltese Terrier, Poodle.
• Pulmonary Stenosis: Boxer, Cocker Spaniel, English Bulldog, Mastiff, Miniature Schnauzer, Samoyed, West Highland White Terrier.
• Subvalvular Aortic Stenosis: Boxer, German Shepherd, Golden Retriever, Newfoundland, Rottweiler.
• Tricuspid Valve Disease (Congenital): Labrador Retriever.
• Ventricular Septal Defect (VSD): English Springer Spaniel.