ECG Review: PED 596

Electrophysiology Review

Myocardial Action Potential 2 +40 1 AP 3 mV 4 4 -100 ECG

AP ECG Measured in the Cardiac Cell Resting Potential = -90mV Depolarization = Phase 0 Repolarization = Phase 3 Measured at the Skin Surface Resting Potential = Isoelectric Line Depolarization = +/- deflection Repolarization = “T-Wave”

Myocardial Action Potential 2 +40 1 AP 3 mV 4 4 -100 ECG

ECG Basics ECG graphs: Paper Speed: Voltage Calibration: 1 mm squares 25 mm/sec standard Voltage Calibration: 10 mm/mV standard

ECG Paper: Dimensions Voltage ~Mass Speed = rate 5 mm 1 mm 0.1 mV 0.04 sec 0.2 sec Speed = rate

Cardiac Cycle: ECG WAVES Normal ECG formation / conduction: P Wave: Atrial depolarization/contraction QRS Waves: Ventricular depolarization/contraction T Wave: Ventricular Repolarization PR and QT Intervals…conduction problems

ECG: Cardiac Cycle

Cardiac Cycle Basics: Begins with SA Node depolarization P – P = 1 Cycle Heart rate (pulse) is determined by ventricle depolarization/contraction R – R = 1 heart beat

Calculating Heart Rates from ECG’s: Step One Sinus Rhythm: Each QRS complex is preceded by P wave NSR: Within the intrinsic rate of the SA Node: 60-100 bpm Tachycardia: >100 bpm Bradycardia: < 60 bpm

Step Two: Count the number of small squares between R – R waves (X): Divide 1500 by X: Rate = 1500 / X Example: X = 20 1500 / 20 =75 Rate = 75 bpm

Why “1500 / X”? Paper Speed: 25 mm/ sec 60 seconds / minute 60 X 25 = 1500 mm / minute Take 6 sec strip (30 large boxes) Count the P/R waves X 10 OR

Rhythm ID: Algorithm P-Wave: What is the atrial rhythm? < 0.12 sec (3 mm) QRS: What is the ventricular rhythm? <0.10 sec (<3 mm) P-R Interval: Is AV conduction normal? 0.12-0.20 sec (3-5 mm) Any unusual complexes? IS IT DANGEROUS?

Rhythms Involving Errors in Formation: P and QRS Normal and Ectopic Rhythms Sinus: “Normal” Atrial: “Ectopic” Junctional Rhythms: “Escape” Retrograde Atrial Depolarization Ventricular Rhythms:

P-Wave: 1/QRS? 1.SA Node “fires” 2. Right and Left Atria Depolarize 3. AV Node “fires” Questions: P waves present? Regular rhythm? 1/QRS? AV Node SA Node LA/RA Depol

Atrial Fibrillation:

Atrial Flutter: 2:1 Ventricular “capture” Ventricles only respond to every other Atrial conduction

Fibrillation vs. Flutter? One focus - organized Rate: 200-400 bpm REGULAR - R Cardiac Output is compromised Multi-focal origins -chaotic Rate: >400 bpm IRREGULAR-R Cardiac Output is Negligible: Atria contribute ~20% of the total Cardiac output: A-Fib is non-lethal

P-Interpretation: Irregular Premature Beats: Narrow P waves: PAC’s Atrial Flutter: >1P/QRS, classic “saw tooth” morphology

Summarize: Atrial Rhythms / Supraventricular Rhythms Sinus: Normal, Tachy, Brady Absent P: V-tach, A-fib, Junctional Rhythm Irregular P: A-Flut, PAC

Ventricles: QRS Rhythms Regular rhythms? R-R intervals equivalent Regular “irregular” rhythms? R-R intervals equivalent with occasional irregularities Irregular rhythms? R-R intervals irregular

Regular “Irregular” Premature Beats: PVC Widened QRS, not associated with preceding P wave Usually does not disrupt P-wave regularity T wave is “inverted” after PVC Followed by compensatory ventricular pause

Notice a Pattern in the PVC’s?

PVC Patterns: PVC: 1 Isolated beat Couplet: 2 consecutive PVC’s Bigeminy: PVC every other beat Non-Sustained VT: >3 beats for less than 1 minute Sustained VT: > 1 minute of ventricular tachycardia

CONDUCTION ARRHYTHMIAS AV-Blocks

Ectopic Focus or Conduction Disturbance? Ectopic Beats: Premature and/or wide QRS complexes Absent and / or abnormal P waves AV Blocks: Prolonged P-R intervals Irregular P:R ratios Ventricular: Bundle Branch Blocks Wide QRS / Normal P-R

Bottom Line: The Speed of conduction in the Atria and ventricles is similar (Very Fast) The AV Node Necessarily slows down conduction to allow time for the ventricles to fill before contraction About 50% of the cardiac cycle is “held up” at the AV-Node

BLOCKS: Conduction is slowed or interrupted A-V Blocks occur in the conduction between the atria and ventricles Ventricular Blocks: Occur in the Bundle Branches

Atrio-Ventricular Blocks: SA Node fires, but conduction is impaired: Normal P-RI = 0.14-0.22 seconds (3-5 mm) Degrees of Block: 1°: Conduction delayed, but QRS captured 2°: Partial Block: Occasional ventricular “capture” 3°: Complete: Atria and ventricles completely dissociated

First Degree Block: Prolonged P- R interval In otherwise healthy middle-aged men, not related to CAD Regular P – P and R – R rhythms When T – P interval is very short, coronary artery filling is compromised

1° AV Block: PRI > 0.20

Second Degree Block: Type I: Wenckebach P-R Interval gets progressively longer until the AV conduction is completely blocked: When AV conduction blocked, there is not QRS complex QRS is normal

P>R, progressive PR- interval Wenckebach: 2° AV Block P>R, progressive PR- interval

Second Degree Block: Type II: Regular ventricular rate – slow 2:1, 3:1 or 4:1 P:R waves Only occassional but regular ventricular capture QRS is normal

2° AV Block: Note 2:1 P:R following Arrow.

Third Degree (Complete) AV Block AV conduction is completely dissociated Ventricles contract at intrinsic rate (30-40 bpm) Normal P waves, but more than QRS waves QRS complexes may be normal or widened

3° AV Block: P and R dissociation

Identifying AV Blocks: Name Conduction PR-Int R-R Rhythm 1°: P = R > .20 Regular 2°:Mobitz I P > R Progressive Irregular II Constant 3°: Grossly (20-40 bpm)

Most Important Questions of Arrhythmias What is the mechanism? Problems in impulse formation? (automaticity or ectopic foci) Problems in impulse conductivity? (block or re-entry) Where is the origin? Atria, Junction, Ventricles?

Rhythm Documentation: Rate and Regularity Identification of Rhythm A-V association but ectopic focus in either atria or ventricle A – V are independent: conduction block (rates may be similar or not)

12-Lead ECG Interpretations

Terminology: Lead: Recording of wave of depolarization between negative and positive electrodes Einthoven Triangle: An equilateral triangle depicting the leads of the frontal plane (I-III and aVR – aVL) Frontal Plane: Vertical plane of the body, separating the front from back Transverse Plane: Horizontal plane separating the top from the bottom

Frontal Plane Leads: Standard (bipolar) Leads: I: RA- to LA+ II: RA- to LL+ III: LA- to LL+ Augmented Vector (Unipolar) Leads aVR: to RA+ aVL: to LA+ aVF: to LL+

Blue Segment: -30° to +90° Is normal “QRS axis”

QRS Axis? Used to determine right or left heart hypertrophy or other anatomical anomalies But How do we Determine Axis?

The heart is situated in the chest at an angle from right arm to left hip: Waves of Depolarization Travel from the Right shoulder To the left hip.

The ECG deflection (-/+) is determined by the direction of the depolarization wave relative to the “reading” or POSITIVE electrode

Like So: ECG: Depolarization wave - + Lead I - + + -

Normal QRS Deflections (ve = + / -) Check Leads: I and aVF Positive: Leads I-III, aVL, aVF, V4-V6 Negative: avR, V1-V2 Both Negative and Positive: V3

The Following Quadrant System Quickly Identifies QRS Axis Deviation

Interpreting Axis Deviation: Normal Electrical Axis: (Lead I + / aVF +) Left Axis Deviation: Lead I + / aVF – Pregnancy, LV hypertrophy etc Right Axis Deviation: Lead I - / aVF + Emphysema, RV hypertrophy etc.

NW Axis (No Man’s Land) Both I and aVF are – Check to see if leads are transposed (- vs +) Indicates: Emphysema Hyperkalemia VTach

“Seeing” the heart in the Transverse plane: The Chest Leads + V5 + V4 V1 V3 + - + V2 -

ST Segment Analysis: Ischemia Diagnosis Key Reference Points: Isoelectric line: Use the PR segment as reference J-Point: Point at which QRS complex ends and ST segment begins Most Common Measurement: .06-.08 sec (>1-2 mm) past J-Point ST Slope: Downsloping > Horizontal > Upsloping (questionable/angina)

ST-Depression >1.0 mm depression: >2.0 mm depression Downsloping: Very predictive Horizontal: Very predictive Upsloping: Predictive if angina present >2.0 mm depression Usually indicative of ischemia

Positive Co-Conditions – Signals More Severe CAD: Exertional Hypotension Angina that limits exercise Exercise capacity < 6 METs ST changes at RPP < 15,000 ST changes persist into recovery

Determining Regions of CAD: ST-changes in leads… RCA: Inferior myocardium II, III, aVF LCA: Lateral myocardium I, aVL, V5, V6 LAD: Anterior/Septal myocardium V1-V4

Regions of the Myocardium: Lateral I, AVL, V5-V6 Anterior / Septal V1-V4 Inferior II, III, aVF