Study Notes: Normal Electrocardiogram (ECG)

1. Basics of Recording Electrical Activity

• An electrical meter records potential differences using two electrodes: positive and negative.

• The meter displays the potential at the positive electrode relative to the negative electrode.

• If both electrodes are at the same potential → no reading (flat line).

• Greater potential difference → higher reading.

Example:

• Positive electrode faces a positive potential → upward deflection.

• Positive electrode faces a negative potential → downward deflection.

2. Cellular Electrical Activity

• At rest: inside of the cell is negative, outside is positive.

• During depolarization, positive ions enter the cell → outside becomes negative.

• During repolarization, positive ions exit the cell → outside becomes positive again.

• Depolarization and repolarization create potential differences detected as waves on ECG.

3. Direction of Current Flow

• The meter detects only the component of current parallel to electrode placement.

• Perpendicular currents produce no deflection because both electrodes sense the same potential.

• To record all directions, multiple leads are used, each oriented differently.

4. From One to Multiple Dimensions

• In 1D: current flows either left-to-right or right-to-left.

• In 2D: current spreads at different angles; requires at least two meters to record.

• In 3D (heart): electrical activity occurs in all directions, requiring multiple leads for full mapping.

5. Leads and Planes

• Total: 12 leads — each records from a unique angle around the heart.

Frontal (Coronal) Plane: 6 Leads

1. Standard Bipolar Limb Leads (Einthoven’s Triangle)

   • Lead I: RA (-) → LA (+) at 0°

   • Lead II: RA (-) → LL (+) at +60°

   • Lead III: LA (-) → LL (+) at +120°

   • Electrodes placed on both wrists and left ankle.

2. Augmented Unipolar Limb Leads

   • aVR: Positive on right arm (-150°)

   • aVL: Positive on left arm (-30°)

   • aVF: Positive on left foot (+90°)

Transverse (Horizontal) Plane: 6 Precordial Leads

• V1: 4th intercostal space, right of sternum

• V2: 4th intercostal space, left of sternum

• V3: Between V2 and V4

• V4: 5th intercostal space, midclavicular line

• V5: Between V4 and V6

• V6: 5th intercostal space, midaxillary line

All use a common negative reference (combined limb electrodes).

6. Lead Orientation and Heart Regions

• Inferior wall → Leads II, III, aVF (Right Coronary Artery)

• Lateral wall → Leads I, aVL, V5, V6 (Left Circumflex Artery)

• Anterior wall → V3, V4 (Left Anterior Descending Artery)

• Septum → V1, V2 (Left Anterior Descending Artery)

Pathology in a region affects its corresponding leads most prominently.

7. Production of ECG Waves

Atrial Depolarization

• Starts at the SA node, spreads across atria → P wave

  • Small amplitude, longer duration (slow conduction, no Purkinje fibers).

Ventricular Depolarization

• Begins at the septum (left to right) → Q wave (small negative).

• Main ventricular wall depolarization → R wave (tall positive).

• Final depolarization at base → S wave (small negative).

• Together form the QRS complex.

Atrial Repolarization

• Occurs simultaneously with QRS → obscured.

Ventricular Repolarization

• Occurs in the opposite direction of depolarization → T wave (positive).

• Takes longer, so the wave is broad and lower in amplitude.

8. ECG Intervals and Segments

Interval / Segment	Definition	Represents	Normal Duration
P–R interval	Onset of P to onset of Q	Atrial depolarization and AV node conduction	~0.16 s
Q–T interval	Onset of Q to end of T	Entire ventricular activity (depolarization + repolarization)	~0.35 s
S–T segment	End of S to start of T	Plateau phase of ventricular action potential	—
R–R interval	Between two R waves	Duration of one cardiac cycle	~0.83 s at 72 bpm

9. Summary

• ECG records electrical potential differences between electrodes.

• Only parallel currents are detected.

• 12 leads provide a 3D map of cardiac electrical activity.

• P wave = Atrial depolarization

• QRS complex = Ventricular depolarization

• T wave = Ventricular repolarization

• Intervals and segments help assess timing and conduction integrity.

• The right leg electrode serves as a ground (no recording function).

NORMAL ECG COMPONENTS & THEIR SIGNIFICANCE

Wave / Interval	Normal Duration	Represents	Significance (Normal Function)
P wave	< 0.12 sec (≤3 small boxes)	Atrial depolarization	Electrical activity from the SA node through atria → atrial contraction.
PR interval	0.12–0.20 sec	Atrial → AV node conduction	Time for impulse to travel from SA node to ventricles.
QRS complex	≤0.12 sec (usually 0.06–0.10)	Ventricular depolarization	Spread of impulse through ventricles → ventricular contraction.
ST segment	Flat, isoelectric	Early phase of ventricular repolarization	Should be level with baseline (no elevation or depression).
T wave	Upright, smooth	Ventricular repolarization	Recovery phase of ventricles.
QT interval	0.35–0.45 sec (varies with HR)	Total ventricular depolarization & repolarization	Represents total time for ventricles to contract and recover.

ABNORMALITIES AND THEIR CLINICAL SIGNIFICANCE

Abnormality	Meaning / Possible Cause	Clinical Significance
Absent P wave	Atrial fibrillation, SA node dysfunction	No organized atrial activity.
Peaked P wave	Right atrial enlargement (e.g., pulmonary HTN)	Atrial overload.
Prolonged PR (>0.20 sec)	1st-degree AV block	Delayed AV conduction; may progress if untreated.
Short PR (<0.12 sec)	Pre-excitation (WPW syndrome)	Early ventricular activation.
Wide QRS (>0.12 sec)	Bundle branch block, ventricular rhythm	Slow ventricular conduction.
ST elevation	Acute myocardial injury (STEMI)	Indicates infarction; immediate action required.
ST depression	Ischemia or digoxin effect	Indicates reduced oxygen supply to myocardium.
Inverted T wave	Ischemia or recent MI	Ventricular repolarization abnormal.
Peaked T wave	Hyperkalemia	High potassium; can cause cardiac arrest.
Prolonged QT	Electrolyte imbalance (↓Ca, ↓K, ↓Mg), meds (antiarrhythmics)	Risk of torsades de pointes (lethal).
Short QT	Hypercalcemia, digoxin toxicity	Faster repolarization.

INTERVENTIONS BY SPECIFIC ABNORMALITY / ARRHYTHMIA

1. Prolonged PR Interval (First-Degree AV Block)

• Assess: Usually benign if asymptomatic.

• Interventions:

  • Monitor ECG regularly.

  • Review medications (beta-blockers, digoxin, calcium channel blockers may prolong PR).

  • Treat underlying cause (electrolytes, ischemia).

  • If symptomatic → consider atropine (0.5 mg IV push).

2. Prolonged QRS Complex

• Possible causes: Bundle branch block, ventricular rhythm, hyperkalemia.

• Interventions:

  • Identify cause (electrolyte, ischemia).

  • Treat ventricular arrhythmia if present.

  • For hyperkalemia → administer calcium gluconate, insulin + dextrose, sodium bicarb, and loop diuretics.

3. Prolonged QT Interval

• Causes: Hypokalemia, hypocalcemia, hypomagnesemia, antiarrhythmic drugs (e.g., amiodarone, sotalol).

• Interventions:

  • Correct electrolytes (K+, Mg++, Ca++).

  • Stop QT-prolonging meds.

  • If torsades de pointes occurs → magnesium sulfate 1–2 g IV over 15 min, possible overdrive pacing or defibrillation if pulseless.

4. Abnormal ST Segment

• ST Elevation (STEMI):

  • Immediate interventions (MONA):

    • Morphine (pain relief and preload reduction)

    • Oxygen (if SpO₂ < 90%)

    • Nitroglycerin (vasodilation, unless hypotensive)

    • Aspirin (antiplatelet)

  • Activate cardiac cath lab within 90 min for PCI.

  • If PCI unavailable → thrombolytic therapy (within 30 min).

  • Continuous cardiac monitoring.

• ST Depression (NSTEMI or ischemia):

  • Same as above but no immediate PCI unless ongoing pain or instability.

  • Give antiplatelets (Aspirin + Clopidogrel), anticoagulant (Heparin), beta-blocker, statin.

5. Abnormal T Wave

• Peaked T wave (hyperkalemia):

  • Administer calcium gluconate IV, insulin + D50, albuterol, sodium bicarbonate, and furosemide or dialysis.

• Inverted or flattened T wave:

  • Assess for ischemia, hypokalemia, or digoxin toxicity.

  • Treat cause accordingly.

ARRHYTHMIA INTERVENTIONS

Rhythm	Meaning	Immediate Interventions
Atrial Fibrillation (Afib)	Chaotic atrial activity, irregularly irregular rhythm	Control rate (beta-blocker, CCB), anticoagulate (warfarin, DOAC), cardioversion if unstable.
Ventricular Tachycardia (VTach)	Rapid ventricular rate (>100 bpm)	With pulse: synchronized cardioversion, antiarrhythmic (amiodarone). Pulseless: CPR + defibrillation + epinephrine + amiodarone.
Ventricular Fibrillation (VFib)	Disorganized ventricular activity → no output	CPR + defibrillate immediately, epinephrine q3–5 min, amiodarone 300 mg IV push.
PVC (Premature Ventricular Contraction)	Early ventricular beat	Correct electrolytes (K+, Mg++), monitor if frequent, give amiodarone if symptomatic.
PJC (Premature Junctional Contraction)	Early beat from AV node	Usually benign; reduce stimulants (caffeine), monitor if frequent.
PEA (Pulseless Electrical Activity)	Electrical rhythm but no pulse	Start CPR immediately, give epinephrine, identify and treat H’s & T’s (hypoxia, hypovolemia, H+, hypo/hyperkalemia, tamponade, tension pneumo, toxins, thrombosis).
Asystole	Flatline, no electrical activity	CPR + epinephrine q3–5 min, do NOT defibrillate, check for reversible causes.
STEMI	Complete coronary occlusion	MONA, cardiac cath (PCI), or thrombolytics if PCI unavailable.
NSTEMI	Partial occlusion / subendocardial ischemia	MONA, heparin, beta-blockers, statin, cardiology consult.

Quick Ref.

Emergency Rhythm	Key Action
VFib / Pulseless VTach	Defibrillate → CPR → Epi → Amiodarone
Asystole / PEA	CPR → Epi → Correct cause (no shock)
Unstable Afib / SVT	Cardioversion
Stable VTach	Amiodarone or lidocaine
STEMI	Cath lab within 90 min
NSTEMI	Medical management + monitoring