ECG Review: ECG Review: PED 596PED 596

Electrophysiology ReviewElectrophysiology Review

Myocardial Action PotentialMyocardial Action Potential

mV

-100

+40

0

4

0

1 2

3

4

ECG

AP

AP AP ECG ECGMeasured 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 PotentialMyocardial Action Potential

mV

-100

+40

0

4

0

1 2

3

4

ECG

AP

ECG BasicsECG BasicsECG graphs:

1 mm squares

5 mm squares

Paper Speed:25 mm/sec standard

Voltage Calibration: 10 mm/mV standard

ECG Paper: ECG Paper: Dimensions5 mm

1 mm

0.1 mV

0.04 sec

0.2 sec

Speed = rate

Voltage ~Mass

Cardiac CycleCardiac Cycle: ECG WAVES

Normal ECG formation / conduction:

P Wave: Atrial depolarization/contractionQRS Waves: Ventricular depolarization/contractionT Wave: Ventricular RepolarizationPR and QT Intervals…conduction problems

ECG: Cardiac CycleECG: Cardiac Cycle

Cardiac Cycle Basics: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 waveNSR: Within the intrinsic rate of the SA Node: 60-100 bpmTachycardia: >100 bpmBradycardia: < 60 bpm

Step Two: 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”?Why “1500 / X”?Paper Speed: 25 mm/ sec60 seconds / minute60 X 25 = 1500 mm / minute

Take 6 sec strip (30 large boxes)Count the P/R waves X 10

OR

Rhythm ID: AlgorithmRhythm ID: AlgorithmP-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 Rhythms Involving Errors in Formation: P and QRSFormation: P and QRS

Normal and Ectopic RhythmsSinus: “Normal”

Atrial: “Ectopic”

Junctional Rhythms: “Escape”Retrograde Atrial Depolarization

Ventricular Rhythms:

P-Wave: P-Wave: 1.SA Node “fires”

2. Right and Left Atria Depolarize

3. AV Node “fires”

Questions:

P waves present?

Regular rhythm?

1/QRS?

SA Node

LA/RADepol

AV Node

Atrial Fibrillation:

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

Fibrillation vs. Flutter?Fibrillation vs. Flutter?Multi-focal origins -chaotic

Rate: >400 bpm

IRREGULAR-R

Cardiac Output is Negligible:

One focus - organized

Rate: 200-400 bpm

REGULAR - R

Cardiac Output is compromised

Atria contribute ~20% of the totalCardiac output: A-Fib is non-lethal

P-Interpretation: IrregularP-Interpretation: Irregular

Premature Beats: Narrow P waves: PAC’s

Atrial Flutter: >1P/QRS, classic “saw tooth” morphology

Summarize: Atrial Rhythms / Summarize: Atrial Rhythms / Supraventricular RhythmsSupraventricular Rhythms

Sinus: Normal, Tachy, Brady

Absent P: V-tach, A-fib, Junctional Rhythm

Irregular P: A-Flut, PAC

Ventricles: QRS RhythmsVentricles: 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”Regular “Irregular”

Premature Beats: PVCWidened QRS, not associated with preceding P waveUsually does not disrupt P-wave regularity T wave is “inverted” after PVCFollowed by compensatory ventricular pause

Notice a Pattern in the PVC’s?

PVC Patterns: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 CONDUCTION ARRHYTHMIASARRHYTHMIAS

AV-Blocks

Ectopic Focus or Ectopic Focus or Conduction Disturbance?Conduction Disturbance?

Ectopic Beats: Premature and/or wide QRS complexesAbsent and / or abnormal P waves

AV Blocks:Prolonged P-R intervalsIrregular P:R ratios

Ventricular: Bundle Branch BlocksWide QRS / Normal P-R

Bottom Line: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: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: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: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: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 complexQRS is normal

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

P p p

Second Degree Block: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 followingArrow.

Third Degree (Complete) Third Degree (Complete) AV BlockAV 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: Identifying AV Blocks:

1°: P = R > .20 Regular

2°:Mobitz

I

P > R Progressive Irregular

2°:Mobitz

II

P > R Constant Regular

3°: P > R Grossly

Irregular

Regular(20-40 bpm)

Name Conduction PR-Int R-R Rhythm

Most Important Questions of Most Important Questions of ArrhythmiasArrhythmias

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:Rhythm Documentation:

Rate and Regularity

Identification of RhythmA-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 12-Lead ECG InterpretationsInterpretations

Terminology: 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:Frontal Plane Leads:Standard (bipolar) Leads:

I: RA- to LA+II: RA- to LL+III: LA- to LL+

Augmented Vector (Unipolar) LeadsaVR: to RA+aVL: to LA+aVF: to LL+

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

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 The heart is situated in the chest at an angle from right chest at an angle from right arm to left hip:arm to left hip:

Waves of Depolarization Travel from theRight shoulder To the left hip.

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

Like So:Like So:

- +

Depolarization wave

Lead I

ECG:

- +

- +

Normal QRS Deflections(ve = + / -)

Positive: Leads I-III, aVL, aVF, V4-V6Negative: avR, V1-V2Both Negative and Positive: V3

Check Leads: I and aVF

The Following Quadrant The Following Quadrant System Quickly Identifies System Quickly Identifies QRS Axis DeviationQRS Axis Deviation

Interpreting 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)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 ChestLeads

V2V1 V3

V4

V5

V6

--

+ +

+

+

ST Segment Analysis: ST Segment Analysis: Ischemia DiagnosisIschemia 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-DepressionST-Depression

>1.0 mm depression: Downsloping: Very predictiveHorizontal: Very predictiveUpsloping: Predictive if angina present

>2.0 mm depressionUsually indicative of ischemia

Positive Co-Conditions – Positive Co-Conditions – Signals More Severe CAD: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: Determining Regions of CAD: ST-changes in leads…ST-changes in leads…

RCA: Inferior myocardiumII, III, aVF

LCA: Lateral myocardiumI, aVL, V5, V6

LAD: Anterior/Septal myocardium

V1-V4

Regions of the Myocardium:Regions of the Myocardium:

InferiorII, III, aVF

LateralI, AVL, V5-V6

Anterior / SeptalV1-V4