Chapter 20

Understanding Cardiac Disorders

  • Focus on understanding the definitions and descriptions of cardiac disorders, without delving into causes or treatments, as these will be covered in nursing school.

Thrombosis and Coronary Artery Disease

  • Build-up of substances in coronary and potentially cerebral arteries leads to reduced blood flow:

    • Reduced lumen size can result in complications such as heart attacks.

Stents

  • A stent is a medical device used to open narrowed arteries to restore blood flow.

Function of Heart Valves

  • Valves play a crucial role in blood flow control within the heart:

    • There are four valves: Two atrioventricular valves and two semilunar valves.

      • Right Side:

        • Tricuspid valve separates the right atrium and right ventricle.

      • Left Side:

        • Bicuspid (Mitral) valve separates the left atrium and left ventricle.

    • Large vessels also have valves to prevent backflow.

    • Structure and function could be represented visually.

Valvular Anatomy

  • Atrioventricular valves are supported by chordae tendineae:

    • Chordae tendineae are connective tissue strands connecting valves to papillary muscles of the ventricle wall.

    • Papillary muscles contract and relax in synchrony with the ventricles:

      • Contraction pulls on chordae tendineae to keep valves closed during ventricular pumping.

      • Relaxation allows the valves to open during filling of the ventricles.

Cardiac Cycle

  • Understanding the contraction and relaxation phases of the heart:

    • It is crucial to know which valves are open and which are closed, how they operate, and the pressure changes during these phases.

Valvular Disorders

  • Septal Defects:

    • Holes present in atrial or ventricular septa, usually congenital, can close spontaneously or require surgical intervention.

  • Stenosis:

    • Refers to the narrowing of heart valves, impeding blood flow and causing pressure build-up in the heart.

  • Prolapse:

    • Mitral valve prolapse—common condition—occurs when the valve flips backward, leading to regurgitation of blood into the atria during ventricular contraction.

  • Endocarditis:

    • Infection leading to inflammation of the heart lining, particularly dangerous for individuals with pre-existing valve disorders.

      • Patients with mitral valve prolapse may need prophylactic antibiotics before dental procedures to prevent bacteria from entering the bloodstream.

Blood Flow Pathway

  • Understanding the pathway of blood through the heart and systemic and pulmonary circulation:

    1. Blood enters the heart via superior and inferior vena cavae into the right atrium.

    2. From the right atrium through the tricuspid valve to the right ventricle.

    3. Blood is pumped via the pulmonary valve into the pulmonary artery for oxygenation in the lungs.

    4. Oxygenated blood returns via pulmonary veins into the left atrium.

    5. From the left atrium through the bicuspid valve to the left ventricle.

    6. Blood is pumped through the aortic valve into the aorta, supplying systemic circulation.

Pressure Differences

  • The left side of the heart operates under higher pressure compared to the right:

    • The myocardium of the left ventricle is thicker than that of the right to accommodate higher pressure demands of systemic circulation.

Cardiac Muscle Cells

  • Two types of cardiac muscle cells exist:

    • Contractile Cells: Majority responsible for heart contractions.

    • Conducting Cells: Specialized cells that propagate electrical signals responsible for coordinating heart beats.

  • Cardiac muscle contracts in response to depolarization caused by ion flux, primarily sodium, potassium, and calcium ions.

  • Mechanical and electrical connectivity is facilitated through intercalated discs, including desmosomes (structural connections) and gap junctions (ion flow connections).

Electrical Activity and Action Potentials

  • Heartbeats are initiated by action potentials generated by the sinoatrial node (SA node), the heart's natural pacemaker, which maintains the rhythm:

    • Normal heart rhythm ranges from 70-80 beats per minute.

  • The conduction of impulses follows a specific path:

    1. SA Node → Atrial Contraction

    2. AV Node → Bundle of His → Right and Left Bundle Branches → Purkinje Fibers → Ventricular Contraction

  • Arrhythmias: Irregular heart rhythms can occur, requiring medical evaluation.

    • A pacemaker may be necessary for patients with bradycardia or irregular rhythms.

Resting Membrane Potential of Cardiac Cells

  • At resting state, sodium (Na+) ions are more concentrated outside the cardiac cell, and potassium (K+) ions are concentrated inside:

    • This results in a negative interior potential (-70 mV to -90 mV).

  • During depolarization, sodium ions enter the cell, reversing the charge.

  • Following depolarization, repolarization occurs, restoring resting conditions.

Electrolyte Balance

  • Electrolytes such as sodium and potassium are crucial for cardiac function. Disorders include:

    • Hyperkalemia: Excess potassium impacting heart function, potentially leading to arrhythmias.

    • Hypokalemia: Decrease in potassium, with similar risks for cardiac function.

    • Calcium channel blockers are used to manage certain cardiac conditions and arrhythmias.

Electrocardiogram (ECG)

  • An ECG records the electrical impulses of the heart, with the following waveforms critical to analysis:

    • P Wave: Atrial depolarization.

    • QRS Complex: Ventricular depolarization.

    • T Wave: Ventricular repolarization.

  • Proper interpretation aids in diagnosing cardiac issues:

    • Normal rhythm vs. arrhythmias including tachycardia and fibrillation.Overview of the Cardiac Cycle

      Definition of Cardiac Cycle

      • Cardiac Cycle: The sequence of events that occurs during one heartbeat, including diastole (relaxation) and systole (contraction).

        • Diastole: The heart muscle is relaxed, allowing blood to fill the chambers.

        • Systole: The heart muscle contracts to pump blood out.

      Phases of the Cardiac Cycle

      Diastole

      • Both Atria and Ventricles: In a state of diastole (relaxed).

      • Atrioventricular Valves (AV): Open, allowing blood flow into the ventricles (tricuspid and bicuspid valves).

      • Semilunar Valves (SL): Closed.

      • Ventricular Filling: Primarily occurs through passive filling due to the pressure difference in the heart and vessels.

        • Approximately 70-80% of ventricles are filled passively.

        • Followed by active filling when atria contract.

      Atrial Contraction

      • Atria contract (active filling):

        • Completes the filling of the ventricles which was at 70-80% due to passive filling before.

        • Active filling contributes an additional 20-30%.

      • Total Volume Called: The end diastolic volume (EDV).

      Systole (Contraction of Ventricles)

      • Early Systole: Initial contraction begins, pressure builds, AV valves close.

        • Major Event: Pressure rise leads to closing of AV valves.

      • Late Systole: Further contraction occurs, pressure continues to build, semilunar valves open.

        • Ejection Phase: Blood is ejected into the aorta and pulmonary trunk due to increased pressure in the ventricles.

      Isovolumetric Contraction

      • Definition: A phase where volume remains constant while pressure increases.

        • The pressure is higher in the left ventricle than in the right ventricle because the left side has to pump to the entire body, while the right side pumps only to the lungs.

      • End Systolic Volume (ESV): Amount of blood left in the ventricles after contraction.

        • Stroke Volume (SV): The volume of blood pumped out of each ventricle per heartbeat.

        • The heart does not pump all the blood out (not 100% efficiency), some remains (ESV).

      Summary of Cardiac Cycle Steps

      • Step 1: Both atria and ventricles relaxed; passive and active filling of ventricles.

      • Step 2: Atria contract (active filling); vents filled completely.

      • Step 3: Early systole begins with AV valves closing; late systole with pressure buildup causes SL valves to open.

        • Key Points: Early and late contraction are crucial in understanding blood flow dynamics.

      Electrocardiogram (ECG) Insights

      • P Wave: Represents atrial depolarization (contraction).

      • QRS Complex: Corresponds to ventricular depolarization (contraction).

      • T Wave: Indicates ventricular repolarization (relaxation).

      Heart Sounds During Cardiac Cycle

      • Lub-Dub Sounds: Result from valve closure.

        • Lub: Sound from the closing of AV valves at the beginning of systole.

        • Dub: Sound from closing of SL valves at the end of systole.

      • Additional Sounds: A third sound may be heard during rapid filling (not typical).

        • Heart Murmur: Sounds indicating issues like incompetent valves causing backflow or stenosis (narrowing) of valves, causing turbulent flow.

      Measurement Techniques

      • Blood Pressure Measurement: Using a stethoscope to detect heart sounds.

      • Cardiac Output Calculation: Determines efficiency of blood pumping:

        • Formula: Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR).

      • Monitoring Conditions: Important for understanding heart health, especially in conditions like shock and heart failure.

      Heart Regulation Factors

      Intrinsic Regulation

      • Preload: Amount of blood filling the ventricles; increased volume leads to stronger contractions (Starling's law).

        • Emptying rate affected by filling volume; more volume = stronger contraction.

      Extrinsic Regulation

      • Nervous System Role: Autonomic nerves help modulate heart rate:

        • Parasympathetic Nervous System: Decreases heart rate (via vagus nerve). Responsible for regular operations when body is at rest.

        • Sympathetic Nervous System: Increases heart rate (fight or flight response). Activates during stress or physical exertion.

      Additional Cardiac Physiology Insights

      • Sensors: Baroreceptors detect blood pressure fluctuations and respond accordingly to maintain homeostasis.

      • Chemoreceptors: Monitor pH, CO2, and O2 levels and send feedback to the nervous system for adjustments.

        • Located in the medulla oblongata and carotid/aortic arteries.