Intro to ECGs/EKGs Notes

Intro to ECGs/EKGs

Expectations and Objectives

  • The goal is not to become an expert in reading ECGs in one lecture.
  • The aim is to familiarize with identifying rhythms and determining heart rates for board exams and clinical practice.
  • This lecture covers basic physiology of the heart and components of rhythm strips.
  • Clinical applications and decision-making will be discussed in the lab due to multifactorial considerations.
  • The content is based on the textbook, providing background information on why certain rhythms occur.
  • Learning objectives focus on understanding and interpreting ECGs.

Cardiac Cycle

  • The cardiac cycle includes atrial contraction (extra cardiac kick), ventricular contraction, and valve opening/closing.
  • ECG correlates:
    • P wave: Atrial contraction
    • QRS complex: Ventricular contraction
    • T wave: Ventricular repolarization

Purpose of ECGs

  • Monitor heart activity.
  • Electrocardiogram provides the most cardiac function information non-invasively.
  • Monitors heart's electrical activity (action potential), not actual contraction force.
  • Methods:
    • Exercise ECGs (stress tests)
    • Holter monitors (continuous monitoring during exercise or at home)

Technical Terms

  • Aberrant: Abnormal, diverging from the expected rhythm pattern.
  • Arrhythmia: Absence of rhythm, pause, or impulse. Often referred to as dysrhythmia (bad rhythm).
  • Dysrhythmia: Abnormal rhythm, not necessarily bad, but often used to describe aberrant rhythms.
  • Ectopic: Abnormal place or position (e.g., ectopic heartbeat starts from somewhere other than the SA node).
  • Sinus: Related to the SA node (e.g., sinus rhythm is normal rhythm originating in the SA node).

Basic Principles

  • Automaticity: Heart's ability to contract without external nerve stimulation.
  • Rhythmicity: Regular pattern of heartbeats (regular is good; irregular may cause concern).
  • Conductivity: Nerve impulses travel through cardiac muscles.

Conduction System

  • Impulse originates at the SA node (right atrium).
  • Travels through:
    • Internodal pathways
    • Bachmann bundle (to left atrium)
    • AV node (provides pause for atrial kick)
    • Bundle of His
    • Right and left bundle branches (along the septum)
    • Purkinje fibers (ventricular muscles)

Components of Conduction System

  • SA node: Pacemaker cell in the right atrium.
    • Functions rhythmically.
    • At rest, beats in a regular rhythmic pattern.
  • Internodal tracts transmit from the SA node to the AV node.
  • AV node passes impulses to the ventricles.
  • AV bundle (bundle of His) splits into right and left bundle branches, then Purkinje fibers.

Depolarization and Repolarization

  • Depolarization: Changes in heart's electrical charges, leading to contraction.
    • Resting membrane potential is slightly negative.
    • Depolarization makes it less negative.
  • Repolarization: Restoration of membrane potential.
    • Involves movement of sodium, potassium ions, and sodium-potassium pump.

Action Potential in Conducting Cells

  • Resting state is approximately -60 millivolts.
  • Slow influx of sodium makes it less negative.
  • Rapid influx of calcium makes it sharply positive (depolarization).
  • Outflux of potassium restores the resting potential (repolarization).

Action Potential in Cardiac Muscles

  • Rapid depolarization followed by a plateau phase.
  • Plateau phase: Slow calcium channels open, leading to contraction.
  • Long repolarization period.
  • Absolute Refractory Period: Muscles will not contract, regardless of the strength of the impulse.
  • Relative Refractory Period: Muscles might contract if the impulse is strong enough.
  • Significance: Long period allows the heart to fully contract, maximizing stroke volume.

Skeletal Muscle vs. Cardiac Muscle

  • Skeletal muscle contraction is quick with a rapid tension release.
  • Cardiac muscle builds tension over time for a sustained contraction.
  • Skeletal muscle has a short refractory period, while cardiac muscle has a long absolute refractory period.
  • Cardiac muscle's long refractory period prevents off-rhythm contractions.

Measuring Depolarization with ECGs

  • Electrode placement (leads) is essential.
  • Positive lead: Wave of depolarization moving towards it causes an upward deflection.
  • Negative lead: Wave of depolarization moving away from it causes a negative deflection.
  • Repolarization has the opposite effect.
  • Law of Averages: The left ventricle is larger, so its impulse magnitude is greater and more towards the left.

ECG Leads

  • 12-lead ECGs: Multiple views of the heart.
  • Rhythm strip: Single lead view.
  • 12-lead ECG includes six chest leads and six limb leads.

12-Lead ECG vs. Single-Lead

  • 12-Lead ECG:
    • Determines ischemia or infarction.
    • Compares to previous recordings.
    • More descriptive.
    • Assesses heart rate, rhythm, hypertrophy, and infarction.
  • Single Lead:
    • Assesses heart rate, rhythm, and abnormal rhythms.

Electrode Placement

  • Electrode placement is different from lead placement.
  • Electrodes can be positive or negative, depending on the lead.
  • Standard placement: Right arm, left arm, right leg (ground), left leg, and precordial leads (V1-V6).
  • Avoid placing electrodes on large muscle portions to reduce noise.

Limb Leads

  • Lead I, II, III, AVR, AVL, AVF.
  • Lead I: Left lateral lead (left arm positive, right arm negative).
  • Lead II: Inferior lead (left foot positive, right arm negative).
  • Lead III: Inferior lead (left foot positive, left arm negative)
  • AVR: Right arm positive, left arm and left leg negative.
  • AVL: Left arm positive, right arm and left foot negative.
  • AVF: Left foot positive, arms negative.
  • Right foot acts as a ground.

Chest (Precordial) Leads

  • V1-V6 placed horizontally across the chest to view the heart in the transverse plane.
  • Each lead views cardiac activity from a different angle.
  • V1 is on the right side of the sternum, while V6 is towards the midaxillary line.

Understanding Deflections

  • If depolarization moves towards a positive lead, there's an upward deflection.
  • If it moves away, there's a downward deflection.
  • V1: Normally has a large downward deflection because the majority of the impulse goes towards the left ventricle.
  • V6: Normally has a large upward deflection because the impulse goes towards the left ventricle.

ECG Strip

  • Typically a six-second strip with tick marks at the top (each tick mark represents three seconds).
  • Each large box: 0.2 seconds.
  • Each small box: 0.04 seconds.
  • Amplitude: Each large box is 0.5 millivolts.

ECG Components

  • P wave: Atrial depolarization.
  • PR interval: Includes P wave and pause segment (0.12-0.2 seconds).
  • PR segment: End of P wave to beginning of QRS complex.
  • QRS complex: Ventricular depolarization.
    • R wave: First upward deflection.
    • Q wave: First downward deflection.
    • S wave: Downward deflection after the R wave.
    • R prime: Second upward deflection after the S wave.
  • ST segment: After the S wave to the beginning of the T wave.
  • T wave: Ventricular repolarization. Atrial repolarization is not seen because it occurs during the QRS complex.

Normal vs. Abnormal ECG

  • Normal P wave, PR interval, QRS complex, and T wave.
  • Expected time frame for each component.

Reading ECGs: Two Approaches

  • Approach 1 (Textbook): Evaluate P wave, PR interval, then heart rate, etc.
  • Approach 2 (Suggested): Start big (observe patient, rhythm) and move to small (evaluate waves and intervals).

Suggested ECG Reading Approach

  1. Observe the Patient: Is the patient in distress? Expect normal ECGs if the patient is stable; suspect abnormalities if distressed. Note any symptoms. (chest pains, fatigue, shortness of breath)
  2. Evaluate the Rhythm: Is it regular or irregular? Regular rhythms suggest normalcy, while irregular rhythms need further investigation.
  3. Estimate the Heart Rate:
    • Method 1: Count QRS complexes in a six-second strip and multiply by 10.
    • Method 2: Use bold boxes to estimate rate based on 300, 150, 100, 75, 60.
    • Method 3: Count small boxes between QRS complexes and divide 1500 by that number.

P Wave Abnormalities

  • Normal P wave: Rounded, symmetrical, upright, < 0.12 seconds, < 2.5 mm
  • Right Atrial Enlargement: Large amplitude.
  • Left Atrial Enlargement: Prolonged duration.

Ventricular Hypertrophy

Right Ventricular Hypertrophy

  • Increased R wave in V1 and increased S wave in V6.

  • R wave in V1 is larger than the S wave.

    Left Ventricular Hypertrophy

  • Increased R wave in left leads, increased S wave in right leads.

  • Use math to determine if the sum of R and S waves exceeds normal measurements.

Rhythm Abnormalities

  • Normal Sinus Rhythm: Normal rhythms originating from the SA node.

  • Sinus Pause: SA node fails to initiate an impulse for one cycle. Generally benign.

    PREMATURE ATRIAL COMPLEX

  • Premature Atrial Complex: The SA node fires early causes an irregular rhythm. Generally Benign.

    • No significant signs or symptoms

    WANDERING ATRIAL PACEMAKER

  • Wondering Atrial Pacemaker: The P waves look different, meaning the impulse is initiated outside the SA node.

    • May require medical treatment to prevent atrial fibrillation

    ATRIAL FLUTTER

  • Atrial Flutter: Rapid succession of atrial depolarization caused by an ectopic focus that depolarizes at a rate of 250-350 times per minute

    • Characterized by a sawtooth pattern. Treatment: Medication.

    ATRIAL FIBERLATION

  • Atrial Fibrillation: Erratic quivering, causes by multiple ectopic foci creating an irregular, and irregular heart rhythm.

    • Requires Treatment such as drugs or a pacemaker
    • May decrees cardiac output, but not necessarily life threatening

    HEART BLOCKS

    FIRST DEGREE AV BLOCK

  • First-Degree AV Block: Prolonged PR interval. Treatment usually not warranted.

    SECOND DEGREE AV BLOCK TYPE 1

  • Second-Degree AV Block Type 1 (Wenckebach/Mobitz I): Lengthening PR interval until a QRS complex is dropped. Treatment usually not necessary.

    SECOND DEGREE AV BLOCK TYPE 2

  • Second-Degree AV Block Type 2 (Mobitz II): Non-conduction of impulse to the AV node. Requires medical attention because leads to possibility of a complete heart block

    THIRD DEGREE AV BLOCK

  • Third-Degree AV Block: Medical Emergency, because the p waves and QRS, the ventricles depolarize from somewhere other then the AV node are not corrdinating.

    • Complete AV Block: Lack of coordination between P waves and QRS complexes.

Ventricular Arrhythmias

PREMATURE VENTRICULAR COMPLEX (PVC)

  • Premature Ventricular Complex (PVC): Can happen with healthy people

  • Paired together, Origin(multifocal) and frequent meaning more then 6 and triplets are more serious can causes, ventricular fibrillation.

    • Travels along the regular pathway

    UNIFOCAL PVC

  • Unifocal PVC: regular PVC

  • PVCs are identical. Means that the impulse starts from the same place with the same pathway

    MULTIFOCAL PVC

  • Multifocal PVCs: PVCs looks different, with different pathways. Starting different paths

  • Irregular Pvcs

    RUNNER OF V-TACH

  • three beat, R on T, Run of V tach: Medical Attention Required

  • The r happens before the repolarization through the, probably the relative refractory period, then can leads to ventricular fibrillation.

    VENTRICULAR TACHYCARDIA

  • V1 is where looks like relatively normal if V1 looks like the abnormal on Medical Attention Required

    TORSADES DE POINTES

    • Rare ventricular tachycardia, increasing/decreasing amplitude. Medical emergency, cardioversion is used.

    VENTRICULAR FIBERLATION

    • Erratic quivering of the ventricular muscle. No pulse or blood ejected. Medical emergency requiring resuscitation.

STEMI and NSTEMI

  • STEMI (ST-Elevation Myocardial Infarction) indicates ischemia or infarction.
  • ST Segment: Period from end of S wave to beginning of T wave.
    • ST-segment elevation: ischemic area
    • ST-segment depression: injured cardiac muscle
    • T-wave inversion: repolarization traveling differently than expected.

Extra Considerations

  • Tick marks indicate pacemaker: Atrial pacing or ventricular pacing.
  • Implanted cardioverter defibrillator (ICD): Detects tachycardia and delivers a shock.