When confronted with an ECG, always use a systematicapproach, following all steps to come to the correctconclusion.In this course we use the 7+2 step plan
7+2 Step Plan1.Rhythm– Rate– Conduction– Heart axis– P wave morphology– QRS morphology– ST morphology1. Compare with previous ECG2. Conclusion
Step 1: What is the Rhythm? P waveNormal (conducted) sinus rhythm • A P wave (atrial contraction) precedes I every QRS complex. • A QRS complex follows every P wave. • The rhythm is regular, but varies slightly during respirations. II • The rate ranges between 60 and 100 beats per minute. • The P waves maximum height is 2.5 mm in II and/or III. AVF • The P wave is positive in I and II, and biphasic in V1. • The PQ time is between 0,12 and 0,2 seconds. V1
Not sinus rhythm?• If the rhythm is not sinus rhythm you proceed with the arrhythmia algorithm (which you will learn with the next topics) after determining the heart rate, heart axis and conduction intervals.• (A purist definition of sinus rhythm is that sinus rhythm is present in the atria and not by definition in the ventricles, e.g. there may be sinus rhythm in the atria, complete AV block and a nodal escape rhythm in the ventricles).
Step 2: What is the heart rate?Knowing the ECG paperspeed(usually 25 mm/sec) allows us tocalculate the heart rate from theECG using any of these methods: • Use the "square counting" method • Use a calculator • Use a separate ECG ruler
The square counting method The count method to determine the The square counting method is ideal The second QRS heart frequency. for regular heart rates. Use the between 75 and 60 beats complex is sequence: per minute. This heartbeat is between that, around 65 beats per minute. 300-150-100-75-60-50-43-37. Count from the first QRS complex (preferably occuring on a thick line). If the next QRS complex would occur on the next thick line, the heart rate would be 300, 150 if at the second thick line, 100 if at the third thick line and so on. When the second QRS complex is between two lines, take the mean of the two numbers from the sequence.
Step 3: ConductionThe speed of conduction ofthe signal through the heartresults in conductionintervals: • PQ interval • QRS duration • QT interval
PQ interval• Normal between 0.12 and 0.2 seconds.• Starts at the beginning of the atrial complex and ends at the beginning of the ventricular complex.May be shortened if there is pre-excitation of the ventriclesthrough abnormal conduction between the atria andventricles (accessory pathway).If the PQ interval is prolonged there is a degree of AVblock (which will be discussed later).
QRS duration• Normal < 0.10 - 0.12 seconds• Indicates how fast the ventricles depolarize• QRS longer than 120 milliseconds may result from: o Left bundle branch block o Right bundle branch block o Electrolyte disorders o Idioventricular rhythm or paced rhythm
QT interval• Indicates how fast the ventricles are repolarized, becoming ready for a new cycle.• The normal value for QTc is: below 450ms for men and below 460ms for women more about this topic on ECGpedia...
Correct assessment of the QTc intervalThe QT interval is comprised of the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation isthat the QT interval gets shorter as the heart rate increases. Thisproblem can be solved by correcting the QT time for heart rateusing the Bazett formula:Correct QT measurements are important because QT prolongationmay make the patient prone to arrhythmias, especially whencombined with QT-prolonging drugs.
Stepwise approach to correct measurement of the QT interval1. Use lead II. Use lead V5 alternatively if lead II cannot be read.– Draw a line through the baseline (PR segment, or TP alternatively)– Draw a tangent against the steepest part of the end of the T wave. If the T wave has two positive deflections, the taller deflection should be chosen. If the T wave is biphasic, the end of the taller deflection should be chosen.– The QT interval starts at the beginning of the QRS interval and ends where the tangent and baseline cross.– If the QRS duration exceeds 120ms, the amount surpassing 120ms should be deducted from the QT interval (i.e. QT=QT-(QRS width-120ms) )– Calculate QTc according to Bazett. You can use the QTc calculator for this.
Causes of QT prolongation• Congenital long QT syndrome.But QT prolongation can also occur as a consequence of(a.o.): • Medication (anti-arrhythmics, tricyclic antidepressants, phenothiazides). See torsades.org for a full list. • Electrolyte imbalances. • Ischemia.
Step 4: Heart axisThe heart axis points in the direction ofthe average electrical vector of all thedepolarizing heart cells.A change of the heart axis or an extremedeviation can be an indication ofpathology.A positive QRS complex (more abovethan below the baseline) in a certain leadmeans that the heart axis is going (atleast slightly) in that leads direction.The heart axis is normal between -30and +90 degrees.Therefore, if QRS is positive in bothleads I and II, the heart axis is normal.
InterpretationThere are four areas wherethe QRS vector can point:• Left axis deviation (between -30º and -90º)• Normal axis between -30º and 90º• Right lower quadrant --> right axis deviation (between 90º and -180º)• Right upper quadrant --> extreme right axis deviation(between -90º and -180º) more about this topic on ECGpedia...
Abnormal heart axisHeart axis deviation to the left in Heart axis deviation to the rightcase of an inferior infarct. Left can result from rightanterior hemiblock is another ventricular overload as in COPDcommon cause. A left axis is or pulmonary embolism. A rightpresent between -30 and -90 axis is between +90 and +180degrees. degrees. A left - right arm lead exchange is the most common cause of right axis deviation!
Step 5: P Wave morphologyThe P wave morphology can reveal Iright or left atrial dilatation or atrialarrhythmias and is best determinedin leads II and V1 during sinusrhythm. IINormal P wave morphology :• The maximal height of the P wave is 2.5 mm in leads II and / or III. AVF• The P wave is positive in II and AVF, and biphasic in V1.• The P wave duration is shorter than 0.12 seconds. V1
Left atrial dilatationTerminal part of V1 > 1mm2 and/or P >0.12 seconds in I and/ or II
Right atrial dilatationP > 2.5 mm in II and/ or III and/ or aVFand/ or P > 1.5 mm in V1
Condition P Wave MorphologyNormal Sinus RhythmRight atrial enlargement(= P Pulmonale)Left Atrial Enlargement(= P Mitrale)
Step 6: QRS MorphologyCheck presence or absence of any of the followingabnormalities: • Pathological Q waves • LVH / RVH • Microvoltages (QRS < 5 mm) • Conduction problems (normal or prolonged) • Abnormal R wave propagation more about this topic on ECGpedia...
Pathological Q wave• Q waves point at electrically silent areas and can be a sign of previous myocardial infarction• Definition of a pathologic Q wave: o Any Q wave in leads V2–V3 o Q wave ≥ 0.03 s and > 0.1 mV deep in other leads. o To be defined as pathologic, Q waves need to be present in two contiguous leads (e.g. II and AVF or I and AVL or V1 and V2)
Left ventricular hypertrophyHypertrophic myocardiumhas more electrical activity,resulting in larger peaks.Definition of LVH: R in V5 orV6 + S in VI > 35mm(Sokolow-Lyon criteria)Often a "strain pattern" isseen in V5 and V6.
Right ventricular hypertrophyRight ventricular hypertrophyis probably present when R is V1larger than S in VI
MicrovoltagesMicrovoltages:QRS < 5 mm in limb leadsQRS < 10 mm in chest leadsOccurs in infiltrative disease (e.g. amylodosis), and COPD
Conduction disordersIf the QRS duration is more than 0.12 seconds there maybe a block in the conductive tissue. Most often it will beeither right or left bundle branch block.Rule of thumb: when distinguishing leftand right bundle branch block--look atV1 only!Does the signal end negative (belowthe baseline) in V1? (away from V1) >> the ventricle farther from V1 isdepolarized later>> it must be a leftbundle branch blockDoes the signal end positive in V1?(towards V1) >> the ventricle closer toV1 is depolarized later >> it must be aright bundle branch block
Conduction disorders: right bundle branch block• QRS > 0.12 seconds V1• RSR-pattern in V1 where R > R• Slurred S wave in lead I and V6
Conduction disorders: left bundle branch block• QRS > 0.12 seconds• Broad monomorphic S waves in V1, may have a small initial R wave• Broad monomorphic R waves in I and V6 with no Q waves
Normal R wave progressionNormally R waves become larger from V1-V5. At V5 it should be maximal. Ifthe R wave in V2 is larger than in V3, this could be a sign of a (previous)posterior myocardial infarction.
Step 7 ST morphology• The ST segment represents ventricular repolarization. During repolarization the cardiomyocytes elongate and prepare for the next heartbeat. On the ECG, the repolarization phase starts at the junctional, or J point, and continues until the T wave. The ST segment is normally at or near the baseline.• The T wave is usually concordant with the QRS complex. Thus if the QRS complex is positive in a certain lead (the area under the curve above the baseline is greater than the area under the curve below the baseline) then the T wave usually is positive too in that lead. Accordingly the T wave is normally upright or positive in leads I, II, AVL, AVF and V3-V6. The T wave is negative in V1 and AVR. The T wave flips around V2, but there is likely some genetic influence in this as in Blacks the T wave usually flips around V3.• The T wave angle is the result of small differences in the duration of the repolarization between the endocardial and epicardial layers of the left ventricle. The endocardial myocytes need a little more time to repolarize (about 22 ms). This difference causes an electrical current from the endocardium to the epicardium, which reads as a positive signal on the ECG.
Step 7 ST morphology• ST segment elevation• Ischemia• Pericarditis• Aneurysma cordis• Normal variant• ST depression• Ischemia• LVH• Digitalis• Low potassium/ low magnesium• Neurologic• T wave changes• Ischemia• Pericarditis• Myocarditis• LVH / RVH• Electrolyte changes (especially potassium)
ST elevation normal90% of healthy (young) men and women to a lesser extenthave ST elevation in precordial leads.Normal variants ofST segment elevation are:• 1: normal• 2: ‘early repolarization’• 3: normal variant
Abnormal ST segment elevation1: LVH2: LBBB3: Pericarditis4: High pottasium5: Acute AS infarct6: Acute AS infarct +RBBB7: Brugada syndrome
Diffuse ST elevation in pericarditisST segment elevation (upper ECG) due to pericarditis. The lower ECG shows PTa depression, which is typically seen in pericarditis.
ST depressionThe most important cause of ST segment depression is ischemia. Causes ofST segment depression include: Reciprocal ST segment depression during ischemia. If one lead shows ST segment elevation then usually the lead "on the other side" shows ST segment depression. Left ventricular hypertophy with "strain" or depolarization abnormality Digoxin effect Low potassium/low magnesium Heart rate-induced changes (post tachycardia), cardiac memory During acute neurologic events more about this topic on ECGpedia...
ST segment elevation due to high potassium levels
T wave• The T wave is quite labile and long lists of possible causes of T wave changes exist. A changing T wave can be a sign that something is abnormal, but it doesnt say much about the severity. T waves can be peaked, normal, flat, or negative. Flat and negative T waves are defined as:• flat T wave: < 0.5 mm negative or positive T wave in leads I, II, V3, V4, V5 or V6• negative T wave: > 0.5 mm negative T wave in leads I, II, V3, V4, V5 or V6Possible causes of T wave changes o Ischemia and myocardial infarction o Pericarditis, myocarditis o Cardiac contusion o Acute neurologic events, such as subarachnoid bleeding (SAB) o Digoxin effect o Right and left ventricular hypertrophy with strain more about this topic on ECGpedia...
Prominent U waveSometimes a U wave is present: an extra wave following the T wave. Duringhypokalemia (and hypocalcemia) the U wave can become more prominent:
Step 7+1 Compare with previous ECG• New LBBB?• Change in heart axis?• New pathologic Q?• Decreased R wave height?All of these can exist as chronic abnormalities, but whenthe are new it can be a sign of acute ischemia or anothercondition. more about this topic on ECGpedia...
Step 7+2 ConclusionTry to formulate one sentence that summarizes yourfinding with a clinical useful conclusion.Examples: "Sinus tachycardia with ST elevation in V2-V5, likely caused by acute anterior myocardial infarction" "Supraventricular tachycardia of 200 beats per minute caused by an AV nodal re-entry" "Previous infarction combined with an acute lateral myocardial infarction with widening of the QRS complexes" "Normal ECG"
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