Electrocardiography Essentials

Introduction to Electrocardiography

  • Electrocardiogram (ECG / EKG) = graphic recording of the heart’s electrical activity obtained by a sensitive galvanometer.
    • Performed to:
      • Evaluate electrical activity.
      • Investigate chest pain (MI, pericarditis, angina).
      • Clarify symptoms: dyspnea, dizziness, syncope, palpitations.
      • Detect chamber hypertrophy.
  • Conduction system generates impulses ➔ electrodes pick them up ➔ machine converts to a waveform giving a 3-D view of electrical function.
  • A complete picture uses a 12-lead ECG (12 angles) recorded with 10 physical lead wires.

ECG Hardware

Electrodes

  • Small paper/plastic/metal sensors containing conductive gel.
  • Disposable; prevent cross-contamination.
  • Lower-leg electrodes face upward; all others downward.

Cables & Lead Wires

  • Color-coded, letter-coded, insulated.
  • Color mnemonics (snow over grass / smoke over fire):
    • RA – White (snow) • RL – Green (grass)
    • LA – Black (smoke) • LL – Red (fire)
    • V1-V6 – Browns (various shades)

The 12 Leads

1. Standard (Bipolar) Leads – 3

  • Measure current in two directions; all have positive deflection.
    • Lead I = RA(–) ➔ LA(+)
    • Lead II = RA(–) ➔ LL(+)
    • Lead III = LA(–) ➔ LL(+)
  • Together form \text{Einthoven's Triangle}.
  • RL = ground (not recorded).

2. Augmented (Unipolar) Limb Leads – 3

  • Machine amplifies tracing by 50\%.
    • aVR : heart ➔ RA(+); negative deflection.
    • aVL : heart ➔ LA(+).
    • aVF : heart ➔ LL(+).

3. Chest / Precordial Leads – 6 (Unipolar, horizontal plane, all positive)

  • V1 : 4th ICS RSB (right sternal border).
  • V2 : 4th ICS LSB.
  • V3 : midway V2–V4.
  • V4 : 5th ICS LMCL (left mid-clavicular line).
  • V5 : same level as V4 at anterior axillary line.
  • V6 : same level as V4/V5 at mid-axillary line.

Modifications to Lead Placement

  • Stress test : limb electrodes just below clavicles / lower abdomen.
  • Amputation : place above stump at equal levels bilaterally.
  • Parkinson’s tremor : arms electrodes under clavicles; patient sits on hands/buttocks.
  • Dextrocardia : maintain V1 left; move V2–V6 to right chest mirror-image.
  • Pediatrics : shift V5 & V6 posteriorly to avoid crowding.

ECG Machine Controls

Paper Speed

  • Standard 25\;\text{mm/s}.
  • Tachycardia / pediatrics : 50\;\text{mm/s}.
  • Bradycardia : 12.5\;\text{mm/s}.

Gain (Amplitude)

  • Standard 10\;\text{mm/mV}.
    • Strong voltage (tall waves) ➔ reduce to 5 or 2.5\;\text{mm/mV}.
    • Weak voltage (short waves) ➔ increase to 20\;\text{mm/mV}.

Artifact Filter

  • Suppresses extraneous marks.

Graph Paper Characteristics

  • Heat- & pressure-sensitive grid.
  • Small box = 1\;\text{mm} = 0.04\;\text{s} horizontally.
  • Large box = 5\times5 small = 0.20\;\text{s}.
  • Vertical scaling: 10\;\text{mm} = 1\;\text{mV}.

Artifacts & Corrections

  1. Muscle/Somatic tremor
    • Parkinson’s – reposition leads; sit on hands.
    • Shivering – warm blanket.
    • Anxiety – reassure patient.
  2. AC interference (60 Hz)
    • Remove electronics, jewelry; move bed from wall.
  3. Wandering baseline
    • Re-secure cables, clean/dry skin, replace loose/dirty electrodes.

Patient Preparation & Positioning

  • Clean skin with alcohol; avoid mixing electrode types.
  • Routine ECG: Semi-Fowler’s (≈45^{\circ}) helpful for COPD, asthma, obesity, large breasts.

Cardiac Electrical States

  • Polarization : resting.
  • Depolarization : contraction.
  • Repolarization : recovery/restoration.

ECG Waveforms, Segments & Intervals

  • P wave : atrial depolarization (SA node trigger).
  • QRS complex : ventricular depolarization (septal Q, early R, late S).
  • T wave : ventricular repolarization (may invert with MI, BBB, LVH).
  • U wave : Purkinje repolarization; prominent in hypokalemia.
  • Baseline / Isoelectric line : no activity.

Segments & Intervals

  • PR interval (PRI) : start P ➔ start QRS; AV nodal delay; normal 0.12–0.20\;\text{s}.
  • ST segment : end S ➔ start T; early ventricular repolarization; elevation/depression ⇒ ischemia/MI.
  • QT interval : start QRS ➔ end T; total ventricular activity.

Box Timing Reference

  • 1 small box =0.04\;\text{s}.
  • 1 large box =0.20\;\text{s}.
  • Normal QRS width =0.06–0.10\;\text{s} (≈1.5–2.5 small boxes).

Heart-Rate Calculation Methods

  1. R–R (300) Method (regular rhythms)
    • \text{HR}=\dfrac{300}{\text{# large boxes between R waves}}.
    • Example: \dfrac{300}{4}=75\,\text{bpm}.
  2. 1500 Method (most accurate, regular rhythms)
    • \text{HR}=\dfrac{1500}{\text{# small boxes between R waves}}.
  3. 6-Second Method (irregular rhythms)
    • Count 30 large boxes (6 s strip), count R waves, multiply ×10.
    • Example: 5 R waves ⇒ 5\times10=50\,\text{bpm}.

Systematic Rhythm Interpretation (use Lead II strip)

  1. Rate : NSR / tachy / brady / arrhythmic.
  2. Rhythm : regular vs irregular (R-R spacing).
  3. P waves : presence, shape, one-to-one with QRS, upright/inverted.
  4. PRI : >0.20\;\text{s} suggests heart block.
  5. QRS : width <0.12\;\text{s}? morphology?
  6. ST & T : elevation/depression, inversion.

Basic Sinus Rhythms

  • Normal Sinus Rhythm (NSR): 60–100\,\text{bpm}, regular, normal PRI/QRS.
  • Sinus Bradycardia: <60\,\text{bpm}; may be normal in athletes/elderly.
  • Sinus Tachycardia: >100\,\text{bpm}; shortened diastole.
  • Sinus Arrhythmia: overall 60–100 bpm but irregular variation.

Atrial Rhythms

  • Supraventricular Tachycardia (SVT): 100–250\,\text{bpm}, hidden P, wide QRS; emergency.
  • Atrial Flutter: atrial 250–350\,\text{bpm}, “saw-tooth” F waves; emergency.
  • Atrial Fibrillation: atrial >350\,\text{bpm}, no P waves, fibrillatory baseline; emergency.

Ventricular Rhythms

  • PVC: premature, wide/inverted QRS (>0.12 s).
  • Ventricular Tachycardia: >150\,\text{bpm}, no P, wide QRS; emergency.
    • Polymorphic form = Torsades de Pointes (variable QRS size/shape).
  • Ventricular Fibrillation: chaotic baseline, no waves, no CO; emergency.
  • Agonal Rhythm: ventricular 20–40\,\text{bpm}, wide QRS; pre-terminal.
  • Asystole: flat line; clinical death.

Heart Blocks

First-Degree AV Block

  • Regular rhythm, PRI >0.20\,\text{s} constant; no dropped beats.

Second-Degree AV Block

  • Type I (Mobitz I / Wenckebach)
    • Progressive PRI lengthening until QRS dropped; irregular ventricular rate.
  • Type II (Mobitz II)
    • Constant PRI; random dropped QRS; atrial > ventricular rate.

Third-Degree (Complete) AV Block

  • Atria & ventricles beat independently (AV dissociation).
  • Atrial rate 60–100 bpm; ventricular escape 30–45 bpm; QRS often wide.

Bundle-Branch Block (BBB)

  • Either LBBB or RBBB; QRS >0.12\,\text{s} on 12-lead.

Causes of AV Blocks

  • Drugs: digoxin, beta-blockers, calcium-channel blockers, amiodarone.
  • Myocardial infarction (inferior wall → Type I/III; anterior wall → Type II).
  • Degenerative conduction disease (aging), rheumatic fever, myocarditis.
  • Post-surgery / catheterization, increased vagal tone, hypo-/hyper-kalemia.

Vocabulary Quick-Reference

  • NSR : Normal Sinus Rhythm (60–100\,\text{bpm}).
  • Bradycardia : <60\,\text{bpm}.
  • Tachycardia : >100\,\text{bpm}.
  • Arrhythmia : irregular rhythm.

Practical / Ethical Points

  • Always treat the patient, not just the monitor; correlate symptoms.
  • Emergencies (SVT, flutter, fib, V-tach, V-fib, high-grade blocks, asystole) require immediate physician notification & ACLS protocols.
  • Use disposable electrodes to minimize infection; clean skin to enhance signal & patient safety.