Approach to ECG Interpretation

2004.9.22.

Interpretation

Heart rateP waveOrigin of the rhythmPR intervalQRS durationQT intervalQRS axisQRS voltage

Precordial R wave progressionAbnormal Q waveST segmentT waveU waveElectronic pacemaker

Heart Rate

Regular rhythmHR=300no. large boxes between R wave (assumes a standard paper speed of 25mm/s)It is easier to memorize the heart rate associated with each of the large boxes. (30015010075 60504337)

Heart Rate

Slow or Irregular Rhythm

Identify the 3-second markers at top or bottom of ECG tracingCount the number of QRS complexes that appear in 6 secondsMultiply by 10 to obtain rate in BPM

P Wave

P waveElectrical forces generated from atrialactivation.The first and second halves of the P wave roughly correspond to right and left atrialactivation, respectively.

What to measureDuration (seconds)Amplitude (mm)

P Wave

Normal P wave characteristics

Duration: 0.08-0.11 secondsAxis: 0-75Morphology:

Upright in I,II,aVFUpright or biphasic in III,aVL,V1,V2

Amplitude: Limb leads:

Origin of the Rhythm

Rhythm identificationOne of the most difficult and complex aspects of ECG interpretationProper rhythm interpretation requires

Integration of heart rateRR regularityP wave morphologyPR intervalQRS widthP:QRS relationship

Origin of the Rhythm- P:QRS Relationships

P:QRS

Origin of the Rhythm- Heart Rate

HR>100bpm

Narrow QRSComplex(3 morphologies : MATFine or coarse baseline oscillations : Atrial fibrillationFlutter waves: Atrial flutterAny regular rhythm with AV block or premature beats

Sinus P : sinus tachycardiaFlutter waves: Atrial flutterNo P : AVNRT, junctional tachycardiaShort R-P , R-P< 50% R-R interval:AVNRT , AVRT , Atrial tachycardia with 1AV block, junctional tachycardia with 1:1 retrograde atrial activationLong R-P , R-P> 50% R-R interval:Atrial bradycardia , sinus node reentrant tachycardia , atypical AVNRT , orthodromic SVT with prolonged V-Aconduction

Origin of the Rhythm-Heart Rate>100bpm

PR Interval & Segment

PR IntervalConduction time from the onset of atrial depolarization to the onset of ventricular depolarization

PR SegmentAtrial depolarization

How to Measure

PR Interval & Segment-Definitions

PR intervalNormal PR interval: 0.12-0.20 secondsProlonged PR interval: >0.20 secondsShort PR interval:

QRS Duration

QRS durationDuration of ventricular activation

How to Measure

DefinitionsNormal QRS duration: 0.12 seconds

QT Interval

QT intervalTotal duration of ventricular systoleVentricular depolarization (QRS complex) and repolarization (T wave)

How to Measure

QT Interval

Corrected QT interval (QTc)

QTc(sec)=QT/RRHR 50bpm, QT 0.4sec, RR 1.2secQTc=0.4/1.2 =0.4/1.1=0.36sec

HR 70, QTc 0.4sec, for every 10bpm change in heart rate above(or below), substract(or add) 0.02 sec.

DefinitionsNormal QTc:

0.35-0.43 seconds for HR 60-100 bpm.Should be

QRS Axis

QRS axisThe major vector of ventricular activation

QRS Voltage

DefinitionsNormal voltage

Amplitude of the QRS has a wide range of normal limits, depending on the leads, age of the individual, and others.

Low voltageTotal QRS amplitude (R+S)

R Wave ProgressionDefinitions

Normal R wave progression:

Transition zone= V2-V4Increasing R wave amplitude across theprecordial leads

Poor R wave progression:Transition zone= V5 or V6

Reverse R wave progression:

Decreasing R wave amplitude across the precordial leads

Q Wave

DefinitionsNormal Q waves:

Small Q wave (duration

ST Segment

ST segmentThe interval between the end of ventricular depolarization (QRS complex) and the beginning of repolarization (T wave).

ST segment morphology

ST Segment

DefinitionsNormal ST segment

Usually isoelectricBut may vary from 0.5mm below to 1mm above baseline in limb leads and up to 3mm concave upward elevation in the precordial leads (early repolarization)

It is especially important to consider the clinical presentation and compare it to previous ECGs(if available)

Nonspecific ST-T changeSlight (

T Wave

T waveVentricular repolarization

T wave morphology

T Wave

DefinitionsNormal T wave

Morphology: Upright in I, II, V3-6 ; inverted in aVR, V1; may be upright, flat, or biphasic in III, aVL, aVF, V1, V2. T wave inversion may be present in V1-3 in healthy young adults (juvenile T waves)Amplitude: usually 6mm in limb leads or >10mm in precordial leads

Nonspecific T waves:Flat or slightly inverted

U Wave

U waveControversial

Afterpotentials of ventricular muscle vs. repolarization of Purkinje fibers.

DefinitionsNormal U wave:

Not always presentMorphology: upright in all leads except aVRAmplitude: 5-25% the height of the T wave (usually 1.5mm

Pacemaker Overview

Pacemakers are described by 4 letter code:1st letter: chamber PACED(Atrial, Ventricular, Dual)2nd letter: chamber SENSED(A, V, D)3rd letter: pacemaker MODE(Inhibited, Triggered, Dual)4th letter:RATE RESPONSIVENESS or absence (rate-responsive or rate-adaptive pacemakers can vary their rate in response to sensed motion or physiologic alteration such as QT interval or temperature produced by exercise by increasing their rate of pacing)

For example: VVIR, VDD, DDD

Approach to Pacemaker Evaluation(1)

Assess underlying rhythm:

100% paced whether there is a non-paced intrinsic rhythm with a pacemaker functioning in demand mode

Approach to Pacemaker Evaluation(2)

Determine the chamber(s) PACED

Determine the relationship of pacing spikes to P waves and QRS complexes

Atrial(A) paced beats

Ventricular(V) paced beats

Atrial(A) andVentricular(V) paced beats

Approach to Pacemaker Evaluation(3)

Determine timing intervals

From 2 consecutively paced beatsAtrial pacing:

A-A intervalVentricular pacing:

V-V intervalDual chamber pacing:

A-V and V-A interval

Approach to Pacemaker Evaluation(4-1)

Determine the chamber(s) SENSED

Atrial pacemakerA native P wave that occurs at an interval less than A-A intervalAn atrial-paced beat that occurs after an interval equal to the A-A interval

Approach to Pacemaker Evaluation(4-2)

Determine the chamber(s) SENSED

Ventricular pacemaker

A native QRS complex that occurs at an interval less than V-V intervalAn ventricular-paced beat that occurs after an interval equal to the V-V interval

Approach to Pacemaker Evaluation(4-3)

Dual chamber pacemaker

Atrial sensingA native QRS complex that occurs at an interval less than A-V intervalAn ventricular-paced beat that occurs at an interval equal to the A-V interval

Approach to Pacemaker Evaluation(4-4)

Dual chamber pacemaker

Ventricular sensingA native P wave that occurs at an interval less than V-A intervalAn atrial-paced beat that occurs at an interval equal to the V-A interval

Approach to Pacemaker Evaluation(5)

Determine the sequence of complexes:Normal pacing function

Approach to Pacemaker Evaluation(6)

Look for pacemaker malfunction

Failure to captureAre any pacing spikes not followed by a depolarization ?

Sensing Abnormalities

UndersensingOversensing