Lesson 92- Cardiac cycle and heart murmurs

Systems Overview

Cardiovascular and Respiratory Systems
Cardiac Cycle and Heart Murmurs

Dr. Hector Zerpa, St. George's University, School of Veterinary Medicine, Grenada, West Indies

Learning Outcomes

By the end of this session, students will be able to:

  • Distinguish simultaneous events (pressure, volume, electrical activity, heart sounds) of systole and diastole in the heart, providing a comprehensive understanding of the cardiac cycle.

  • Describe the components of the cardiac cycle in detail:

    • Diastole: This phase is characterized by isovolumetric ventricular relaxation, ventricular filling, and atrial contraction (atrial systole), during which the heart chambers fill with blood.

    • Systole: The ventricles contract to eject blood into circulation, involving isovolumetric ventricular contraction and ventricular ejection, representing the heart's active phase.

  • Explain the cardiac cycle using the left ventricle pressure/volume loop, elaborating on its phases and the mechanical events involved.

  • Interpret the Frank-Starling Mechanism, which describes how cardiac output is influenced by the degree of myocardial stretch, enhancing contractility in response to increased preload.

  • Locate heart sounds accurately in the cardiac cycle and correlate them with electrical events using various diagnostic tools, such as phonocardiograms and ECG recordings.

  • Identify audible heart sounds in dogs, cats, and horses, recognizing the variations among species and potential clinical implications.

  • Analyze heart murmur development and timing during the cardiac cycle, offering examples of conditions like insufficiency (valve leakage) and stenosis (narrowed blood flow), which are crucial for diagnosing heart diseases.

  • Describe heart murmur development using specific pathology cases, such as mitral regurgitation and patent ductus arteriosus (PDA), to illustrate the impact of abnormal hemodynamics on heart sounds.

Effects of Systole on Intraventricular Functions

Affects:

  • Increases Pressure: During systole, intraventricular pressure rises sharply to overcome systemic vascular resistance.

Cardiac Cycle Phases Review

Systole

  • The ventricles contract, leading to the ejection of blood into the pulmonary and systemic circulation.

  • It is an active phase where both AV valves (atrioventricular) close to prevent backflow, and semilunar valves open to allow blood flow into the aorta and pulmonary artery.

Diastole

  • The ventricles relax, allowing for passive filling with blood from the atria.

  • This passive phase sees both semilunar valves closed to prevent backflow while the AV valves are open, permitting blood flow from large veins into the ventricles.

Atria Functions

  • Atrial systole and diastole occur, but these phases are comparatively shorter than ventricular actions and serve to fill the ventricles efficiently prior to contraction.

Cardiac Cycle Phases Breakdown

Diastole

  • Isovolumetric ventricular relaxation: The ventricles are closed off from the great vessels and relax without changing volume.

  • Ventricular filling: Blood flows from the atria into the ventricles.

  • Atrial contraction (atrial systole): Ensures the last portion of blood is pushed into the ventricles before systole begins.

Systole

  • Isovolumetric ventricular contraction: Both sets of valves are closed, and the ventricles contract, raising pressure internally.

  • Ventricular ejection: Blood is pumped out to the lungs and the rest of the body.

Cardiac Cycle Timing and Heart Sounds

Diastole

  • Isovolumetric Ventricular Relaxation: Marks the transition from systole and corresponds to the second heart sound (S2).

  • Ventricular Filling: Associated with the third heart sound (S3), which can be pathological in small animals.

  • Atrial Contraction: Corresponds to the fourth heart sound (S4), less commonly heard but significant in identifying certain heart conditions.

Systole

  • Isovolumetric Ventricular Contraction: Precedes the actual ejection of blood from the heart.

  • Ventricular Ejection: Marked by the first heart sound (S1), arising from the closure of the AV valves, loudest over mitral and tricuspid areas.

Pressure-Volume Loop

  • This graph displays left ventricular pressure against left ventricular volume, depicting the efficiency of the heart in pumping blood.

  • Components:

    • A-B: Diastole where ventricular filling occurs, coupled with atrial systole.

    • B-C: Systole marked by isovolumetric contraction.

    • C-D: Systole characterized by ventricular ejection.

    • D-A: Transition back to diastole marked by isovolumetric relaxation.

Preload and Afterload

Preload

  • Refers to the degree of stretch of cardiac muscle fibers before contraction and is crucial for determining the strength of the following contraction (akin to preparing a balloon for blowing).

Afterload

  • Refers to the pressure the heart must work against to open the aortic valve, similar to the effort needed to push the knot of a balloon to create an opening.

Frank-Starling Mechanism

  • Describes how increased left ventricular end-diastolic pressure (preload) leads to stronger contractions, essential for maintaining adequate cardiac output despite fluctuations in venous return, independent of neurohumoral influences.

  • This relationship optimizes the length-tension properties of myocardial fibers, enhancing contractility and calcium sensitivity to ensure effective pumping actions.

Cardiac Cycle Impact Scenarios

  • Increased preload from IV crystalloid infusion may enhance stroke volume but requires careful monitoring to avoid overload.

  • Increased afterload in cats with chronic kidney disease and hypertension poses risks that can lead to heart failure.

  • Positive inotropic drugs like pimobendan can increase myocardial contractility, beneficial especially in cases of heart failure.

Heart Sounds and Murmurs

Heart Sounds

  • Physiological noises generated from blood oscillation, as well as muscle and valve vibrations, are critical for diagnosing heart conditions.

    • S1: Arises from the closure of AV valves, loudest over mitral and tricuspid areas.

    • S2: Results from the closure of semilunar valves, audible over corresponding valve areas.

    • S3: A low-frequency sound indicating rapid ventricular filling, may indicate pathological conditions in small animals.

    • S4: Often a pathological sound linked to atrial contraction, significant in certain diagnostic scenarios.

Heart Murmurs

  • Abnormal sounds resulting from turbulent blood flow through altered valves or abnormal openings.

  • Grading Scale: Ranges from 1/6 to 6/6, based on the intensity and significance of findings during auscultation.

Causes of Murmurs

  • Stenosis: Condition characterized by narrowed valve or vessel, leading to abnormal sounds during both systole or diastole.

  • Insufficiency: Occurs when valves leak, producing abnormal sounds during the closure phases, leading to regurgitation.

Mitral Valve Issues

  • Stenosis: Affects blood flow from the left atrium to the left ventricle, requiring analysis during auscultation for potential interventions.

  • Insufficiency (Regurgitation): Leakage from the left ventricle to the left atrium during systole can lead to volume overload in the atria.

Patent Ductus Arteriosus (PDA)

  • Definition: A condition where a muscular blood vessel remains patent (open) after birth; it creates a machinery-type murmur audible during both systole and diastole.

  • Common congenital defect in dogs: Results from changes associated with birth and hypoxia-induced vasoconstriction, requiring prompt diagnosis and management.

Lesson Summary

  • Key phases of the cardiac cycle, heart sounds, and murmurs discussed with clinical relevance.

  • Examples provided to enhance understanding and applicability in real-life scenarios.

Next Steps

  • Prepare for the next session focusing on the exploration of circulatory systems and cardiovascular pathophysiology, including case studies and management options for various cardiac conditions.