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C10 Cardiology and Cardiac Surgery
Approach to ECGs Toronto Notes 2019
Significant ECG Changes
• Look for ST changes starting at 60 msec from J point
• J point = the junction between the QRS complex and the ST segment
• ST elevation: at least 1 mm in 2 adjacent limb leads, or at least 1-2 mm in adjacent precordial leads
• ST depression: downsloping or horizontal
• Q Wave: pathological if Q wave ≥1 small square (≥40 msec) or >33% of the total
QRS
P WAVE
• thePwaverepresentsatrialcontraction;bestseeninleads:II,VI
■ lead II: the P wave should be rounded, <120 msec and <2.5 mm in height
■ lead VI: the P wave is biphasic with a negative phase slightly greater than the positive phase
• atrial flutter: sawtooth P wave with continuous atrial activity at 300/min indicates the interval (Hint: flip the ECG upside-down to see it better if unclear)
• atrialfibrillation:absentPwav,mayhavefibrillatorywave,irregularrhythm
• rightatrialenlargement:tallPwave(>2.5mm)inIIorV1(Ppulmonale)
• leftatrialenlargement:negativedeflection>1mmdeepor>1mmwideinV1,wide(>100msec)
notched P wave in II may be present (P mitrale)
P-R INTERVAL
• theP-Rintervalindicatestheintervalbetweensinusnodeactivationandthestartofventricular depolarization; includes the impulse traveling through the atrium, the AV node, and the bundle of the magnitude of the conduction velocity is referred to as “dromotropy” ( faster= positive, slower= negative dromotropy)
• positive dromotropy associated with increased conduction velocity (e.g. sympathetic stimulation), negative dromotropy with decreased velocity (e.g. vagal stimulation)
• P-Rintervalshouldbe120-200msec • longP-Rinterval(>200msec)
■ heart block
◆ first degree: fixed, prolonged P-R interval (every p wave has a QRS following)
◆ second and third degree AV block: some P waves NOT followed by a QRS
◆ second degree Mobitz II/Hay: fixed P-R interval with ratio of beat to dropped beat (e.g. for every
3 beats, there is one dropped beat [3:1])
◆ third degree/complete: variable P-R intervals, P-P and R-R intervals individually constant but
not in sync
■ sinus bradycardia (normal to have long P-R if heart rate slow)
■ hypokalemia
■ “trifascicular” block -1st degree AV block with LAHF and complete RBBB
• shortP-Rinterval(<120msec)
■ pre-excitation syndrome (delta wave: upswooping of the P-R segment into the QRS complex
indicating pre-excitation) ◆ accessory pathways
■ low atrial rhythm, P waves inverted in 2,3,aVF
QRS COMPLEX
• theQRSiswhereventricularcontractionisvisualized
• rate:checktheR-RintervaltoseeifitmatchestheP-Pinterval
• amplitude:checkforhypertrophy(seeTable2,C7)
• narrowwidth(<120msec)QRSmeansthattheHis-Purkinjesystemisbeingused
• widewidth(>120msec)QRSmeansthattheHis-Purkinjesystemisbeingbypassedorisdiseased
■ BBB, VT, ventricular hypertrophy, cardiomyopathy, WPW, ectopic ventricular beat, hyperkalemia, drugs (e.g. TCAs, antiarrhythmics)
• Qwave:thefirstdownwarddeflectionoftheQRScomplex
■ significant Q wave: >40 msec or >33% of total QRS amplitude; indicate myocardial necrosis (new or
historical)
• RandSwaveabnormalitiestypicallyshowpathologyintermsofBBBorintraventricularabnormalities
ST SEGMENT
• locatedbetweenQRScomplexandthebeginningofTwave
■ corresponds to the completion of ventricular depolarization
• normallyatthesamelevelas“baseline/TPsegment”
• STelevation:pleaseseetheinfarctsectionabove
• STdepression:ischemia
■ ischemia which causes ST depression can result in myocardial damage (NSTEMI)
■ lateral ST depression (leads I, aVL, V5, V6) may actually indicate a STEMI in the right heart
■ ST depression may be nonspecific, or associated with remote myocardial infarction or ischemia
T WAVE
• thisistherepolarizationphaseoftheventricles(repolarizationoftheatriaareobscuredbytheQRS complex)
• typicallypositive(exceptinaVRandV1)onECGbutnormalisolatednegativeTwavesmaybepresent (especially in V1 and V2)
• pathologywhenTwavevariationoccurinconsecutiveleads
■ inversion: BBB, ischemia, hypertrophy, drugs (e.g. digitalis), pulmonary embolism (lead III as part
of S1Q3T3 sign)
■ elevation: infarction (STEMI, Prinzmetal, hyperacute), hyperkalemia (wider, peaked)
■ flattened: hypokalemia, pericarditis, drugs (e.g. digitalis), pericardial effusion; T waves may be flat as
a nonspecific finding without clinical significance (common)
■ variations: T wave alterans; beat-to-beat variations due to PVC overlap (R on T phenomenon which
may precipitate VT or VFib)
• appropriateTwavediscordance:inBBB,Twavedeflectionshouldbeoppositetothatoftheterminal
QRS deflection (i.e. T wave negative if ends with R or R’; positive if ends with S) ■ inappropriate T wave concordance suggests ischemia or infarction
Insignificant Q Wave
• Septal depolarization by the left bundle • Seen in leads I, II, III, aVL, V5, V6
• <40 msec