Cardiac Muscle Physiology and Function

Cardiac Muscle Physiology

  • Aims of the Study

    • Learn about the basic physiology of cardiac muscle function.
    • Appreciate the heart's physiological responses to demand and stress.
    • Understand the pathophysiology of myocardial dysfunction and its effects.

  • Objectives

    • Describe molecular biology of muscular contraction and energy generation.
    • Outline hydraulics involved in cardiac pump function.
    • Understand the concept of cardiac reserve.
    • Appreciate factors influencing myocardial contractility and their relevance to cardiac physiology and pathology.

Case Example (Patient Scenario)

  • Scenario
    • Patient: Mabel, an elderly individual, has become breathless and can’t lie flat, presenting with low oxygen saturation and rapid shallow breathing.

Cardiac Cycle Dynamics

  • Pressure Ranges
    • Aortic Pressure: 120 mmHg (systole), 80 mmHg (diastole).
    • LA (left atrial) Pressure and LV (left ventricular) Pressure vary during the cardiac cycle, with corresponding phases of diastole and systole.

Cardiac Muscle Structure

  • Cellular Composition
    • Muscles fibers include thin (actin) and thick (myosin) filaments, organized into sarcomeres.
    • Critical components: light and dark bands, nuclei, mitochondria, and sarcoplasmic reticulum play roles in contraction and energy production.

Mechanism of Cardiac Contraction

  • Calcium Role

    • Calcium binds to ryanodine receptors on the sarcoplasmic reticulum, leading to calcium release into the cytosol, initiating muscle contraction.
    • Binding of calcium to troponin induces a conformational change, exposing binding sites on actin for myosin heads to attach, requiring ATP.

  • Contraction Process

    • Myosin heads pull actin, causing muscle contraction and force generation, transforming chemical energy from ATP into mechanical energy for pumping blood.

Cardiac Energetics

  • Energy Utilization
    • Myocardial cells use 6kg of ATP daily to generate force and drive contraction.
    • The hydraulic function results from longitudinal shortening and circumferential thickening of muscle. This optimizes blood ejection from the heart.

Cardiac Functional Reserve

  • Definition

    • Cardiac reserve refers to the heart's ability to increase output in response to stress or increased demand (e.g., exercise, pregnancy).
    • Augmented cardiac output is crucial during activities requiring enhanced performance.

  • Mechanisms to Augment Cardiac Output

    • Increased heart rate (positive chronotropy, SA node stimulation).
    • Enhanced conduction and contractility (positive dromotropy and inotropy).
    • Attributes include effects of sympathetic stimulation, renin-angiotensin-aldosterone system (RAAS), and catecholamines.

Frank-Starling Mechanism

  • Length-Tension Relationship
    • Small increases in sarcomere length lead to significant tension increases at physiological stretch levels.
    • Enhanced preload (LV end-diastolic volume) increases contraction efficiency, confirming that more actin-myosin interactions occur when muscle is appropriately stretched.

Heart Failure Pathophysiology

  • Consequences of Myocardial Disease
    • Conditions like ischemia, wall thinning, and chronic overload lead to reduced contractility and cardiac reserve.
    • Symptoms may include fluid overload and reduced ejection fractions leading to changes in preload and ultimately affecting cardiac output.

Treatment Strategies

  • Patient Care

    • Approach to Mabel includes:
    • Beta-blockers for heart rate control.
    • Diuretics to manage fluid overload.
    • Inotropic agents to support contraction as needed.

  • Key Interventions

    • Addressing preload and optimizing contractility can help shift patients back towards better performance on the Frank-Starling curve.

Summary

  • An efficient heart must be strong, recoverable, and adaptable, relying on sympathetic nervous system (SNS), RAAS, and intrinsic regulation to maintain function.
  • Understanding the Frank-Starling law is crucial as it illustrates how changes in preload affect stroke volume, with proper management crucial for heart failure patients.