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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