ECG Interpretation

Uses of ECG

Cardiac Conduction System

· Electrical activity initiated at SA node
- o Highest rate of spontaneous impulse generation
· Moves through cardiac tissue to ventricles at AV node
· Flows down bundle of His to bundle branches and Purkinje system
- o 2 bundle branches on left, 1 on right
· Impulse reaches refractory tissue & dies out
- o SA node recovers and fires again

Cardiac Myocytes Physiology

· Different types of excitable cells in heart
- o Conduction (electrical) + myocardial (mechanical)
- o Elicit action potential
· Electrical gradient between inside & outside of cardiac cell membranes
- o Xxxxx

Action Potential Curve: Non-Nodal Cells

· Phase 4: resting membrane potential (-90 mV); slows Na2+ channels allows influx Na+ ions
· Phase 0: rapid depolarization; rapid Na+ channels allow influx of Na+
· Phase 1: K+ efflux to correct overshoot
· Phase 2: plateau phase; Ca2+ influx and balanced by K+ efflux
· Phase 3: cellular repolarization; K+ efflux to restore resting membrane potential

Action Potential Curve: Nodal v Non-Nodal

· Atrial and Ventricular Tissue Activation (Panel A)
- o Rapid depolarizing current through Na+ channels & gates
· SA and AV Nodal Tissue Activation (Panel B)
- o Slow depolarizing current through Ca2+ channels & gates

Normal Conduction

· Electrical stimulation/depolarization results in cell membrane potential changes
- o Sudden depolarization affects adjacent cells  “Wave” effect
· Myocardial cells depolarize  Release calcium  Muscle contraction
· Voltage-gated channels re-open to facilitate repolarization back to baseline gradient
· Alternating waves of depolarization and repolarization generate electrical current captured on ECG

Action Potential to ECG Waves

Basics of ECG

· X-axis: Time
· Y-axis: Amplitude
· Standardized technique records all electrical activity over 10 seconds
· Recorded waves form intervals and segments that represent major electrical events
· No electrical activity  Baseline or isoelectric line
· Large 5 x 5 mm  0.2 seconds and 0.5 mV amplitude
· Small 1 x 1 mm  40 ms time and 0.1 mV amplitude
· Reference pulse  1 mV

Normal Sinus Rhythm on ECG: Single Heartbeat

· P wave: atrial depolarization
· PR interval: conduction delay through AV node
· QRS complex: ventricular depolarization
· ST segment: early ventricular repolarization
· T wave: ventricular repolarization
· QT interval: total ventricular activity
· In NSR  All waves and intervals are predictable  Same size, shape, distance apart
·

Lead Placement for 12 Lead EKG

· Heart is 3D  Need different viewpoints
· Each ECG lead is a different viewpoint
- o Records energy movement in relation to electrodes (leads)
· Rhythm strip  1 lead
· Combined “360” degree  12 leads
· Limb leads  Frontal/Vertical Plane
- o I, II, III, aVR, aVL, aVF
- o Top to bottom + Right to left
· Precordial (chest) leads  Horizontal plane on front/side chest
- o V1, V2, V3, V4, V5, V6
  o Anterior to posterior

ECG Wave Forms

12 Views of a Single Heart Beat

Grouping of Leads: Perspective on Heart Regions

Typical 12-Lead ECG

Steps for Analyzing ECG

· Assume electrical direction is normal
· HR
- o Bradycardia: < 60 bpm
- o Normal: 60 – 10 bpm
- o Tachycardia: > 100 bpm
· Rhythm – Pattern of QRS complexes
- o R-R distances, regular or irregular?
- o Shape of QRS, narrow or wide?
- o P waves present, yes or no?
- o Is there one P wave before each QRS, yes or no?
· Intervals and Segments
- o Evaluate duration of PR, QRS, QT intervals
- o ST segment, elevated or depressed?

Calculating HR from EKG

Diagnostic Testing in Chest Pain

· ECG
· Normal in ½ of patients with angina not experiencing an acute attack
· ST-T wave changes
- o Depression
- o T-wave inversion
- o ST-segment elevation
· Significant ischemia
- o ST-segment depression > 2 mm
- o Exertional hypotension
- o Reduced exercise tolerance

Electrocardiogram S-T Segment

· Ischemia exerts complex electrical properties of myocardial cells
· Ischemia changes voltage gradient between normal and ischemic zones
- o Severe, acute ischemia lowers resting membrane potential and shortens duration of AP
· Area of injury in LV are represented by current changes on ECG through deviation of ST segment
· Panel A: Ischemia confined to subendocardium, ST vector shifts towards subendocardium and ventricular cavity  ST-segment depression
· Panel B: Acute ischemia or infarction is transmural; ST vector is usually shifted in direction of outer layers  ST elevation
· ST changes in 1 lead are not clinically relevant; Changes seen in contiguous leads are important to localize injury and guide treatment

Localizing Injury

Supraventricular Arrhythmias

Ventricular Arrhythmias

Conduction Blocks

Differentiating Heart Blocks

Electroyltes and ECG Findings