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Anatomy & Physiology

- Overview: Discusses the anatomy of the heart and its basic physiological functions.

  • Normal Heart Structure:

    • Major Components:

      • Aorta (AO): Supplies oxygen-rich blood to the body.

      • Pulmonary Arteries (PA): Carries oxygen-poor blood to the lungs.

      • Superior Vena Cava: Returns oxygen-poor blood from the upper body to the heart.

      • Inferior Vena Cava: Returns oxygen-poor blood from the lower body to the heart.

      • Atrial Septum and Ventricular Septum: Divides the left and right sides of the heart.

      • Valves:

        • Tricuspid Valve (RA to RV)

        • Pulmonary Valve (RV to PA)

        • Mitral Valve (LA to LV)

        • Aortic Valve (LV to AO)

    • Blood Flow Dynamics:

      • Oxygen-rich blood flows from the lungs via pulmonary veins to the left atrium (LA), then to the left ventricle (LV), and out to the body via the aorta (AO).

      • Oxygen-poor blood flows from the body to the right atrium (RA), through the tricuspid valve into the right ventricle (RV), and out to the lungs via the pulmonary arteries.

Electrical Conduction System

  • Main Components:

    • Sinoatrial (SA) Node:

      • Known as the main pacemaker of the heart.

      • Generates impulses leading to atrial contraction.

      • Has an inherent firing rate of 60-100 bpm.

    • Atrioventricular (AV) Node:

      • Acts as the "gatekeeper" of the heart, delaying impulses to allow for ventricular filling.

      • Contains junctional tissue with pacemaker cells.

      • Fires at an inherent rate of 40-60 bpm.

    • Bundle of His and Bundle Branches:

      • Connects to both the right and left ventricles. The right bundle branch extends to the right ventricle while the left splits into two pathways.

      • Has an inherent rate of 40-45 bpm.

    • Purkinje Fibers:

      • Last backup pacemaker system extending into myocardial tissue and conducting impulses to ventricles.

      • Fires at 20-40 bpm.

Properties of Cardiac Cells

  • Automaticity: Ability to initiate an electrical impulse.

  • Excitability: Ability to respond to stimuli.

  • Rhythmicity: Ability to generate impulses at regular intervals.

  • Conductivity: Ability to transmit impulses.

  • Contractility: Ability to contract when stimulated.

Depolarization and Repolarization

  • Depolarization:

    • The process where inside of the cardiac cell becomes positive, leads to contraction (ventricular systole).

    • Systole (S): Represented by the contraction phase.

  • Repolarization:

    • The inside of the cell becomes negative, leading to relaxation (ventricular diastole).

    • Diastole (R): Represents the resting phase and filling of the heart chambers.

Arrhythmias

  • Definition: Disorders of formation or conduction of electrical impulses in the heart, affecting heart rate and rhythm.

  • Potential Causes:

    • Arise from electrical conduction pathways, irritable sites outside the pathways, or abnormal conduction pathways.

  • Naming: Based on the originating part of the conduction system.

  • Key Note:

    • An ECG solely represents electrical activity and should be interpreted alongside patient assessment, especially in conditions like Pulseless Electrical Activity (PEA).

Basic Steps to ECG Interpretation

  • ECG Leads: Essential for conducting cardiac monitoring.

    • Ensure leads are attached correctly and skin is prepared for optimal adhesion.

  • Understanding Waveforms:

    • Positive Deflection: Above the baseline.

    • Negative Deflection: Below the baseline.

    • Isoelectric Line: Indicates no electrical activity.

ECG Timing and Lead Measurement

  • Time Measurements:

    • A large box on the ECG graph represents 0.20 seconds.

    • A small box represents 0.04 seconds.

  • 6-Second Strip: Total length is 30 large boxes, used for heart rate determination.

  • Heart Rate Calculation: Count R waves in a 6-second strip and multiply by 10.

The 10 Steps of ECG Interpretation

  1. “Wide” or “Narrow”?

  2. Rhythm: Regular or irregular patterns.

  3. Rate: Count of beats per minute.

  4. P-Waves: Is there a consistent morphology?

  5. PR Interval: Duration measured from the start of the P-wave to the start of the QRS. Normal range: 0.12 - 0.20 seconds.

  6. QRS: Duration measuring ventricular depolarization. Normal range: 0.04 - 0.12 seconds.

  7. QT Interval: Time from ventricular depolarization to repolarization. Normal range: 0.40 seconds or less.

  8. ST Segment: Initial phase of repolarization.

  9. T-Waves: Appearance and regularity.

  10. U-Waves: Rarely seen, may need to differentiate from P-waves.

Ectopic Beats and Irregularities

  • Ectopic Impulses: Generated outside the conduction system, often appear early and are marked by different morphology on ECG.

  • Escape Beats: Occur late, generated by a secondary pacemaker in the conduction system, and often display junctional or ventricular morphology.

  • Premature Beats: Occur too early in the cycle and can be complete or incomplete, potentially leading to arrhythmias.

  • Pauses: Periods where no beats are present, sometimes followed by different beats.

Electrolytes and the ECG

  • Hypokalemia: Can cause bradydysrhythmias and conduction blocks.

  • Hyperkalemia: May lead to long PR intervals, wide QRS complexes, asystole, and tall peaked T-waves (notably of concern in critical conditions).

  • Hypocalcemia: Associated with tachydysrhythmias and possible cardiac arrest.

  • Hypercalcemia: Can lead to bradydysrhythmias, conduction blocks, and cardiac arrest.

  • Hypomagnesemia: Linked to Torsades, usually treated with magnesium.

  • Hypermagnesemia: Symptoms include long PR, wide QRS, bradycardia, and tall peaked T-waves.