• Review of the conduction system• ECG waveforms and intervals• ECG leads• Determining heart rhythm / rate• Determining QRS axis• Normal waves / intervals
The electrocardiogram (ECG) is arepresentation of the electrical eventsof the cardiac cycle.Each event has a distinctive waveform,the study of which can lead to greaterinsight into a patient’s cardiacpathophysiology.
Runs at a paper speed of 25 mm/sec• Each small block of ECG paper is 1 mm2• At a paper speed of 25 mm/s, one small block equals 0.04 s• Five small blocks make up 1 large block whichtranslates into 0.20 s (200 msec)• Hence, there are 5 large blocks per second• Voltage: 1 mm = 0.1 mV between each individual blockvertically
• Arrhythmias• Myocardial ischemia and infarction• Pericarditis• Chamber hypertrophy• Electrolyte disturbances (i.e. hyperkalemia, hypokalemia)• Drug toxicity (i.e. digoxin and drugs which prolong the QT interval)
• 3 distinct waves are produced during cardiac cycle• P wave caused by atrial depolarization 13-63
• QRS complex caused by ventricular depolarization
• T wave results from ventricular repolarization 13-63
Leads are electrodes which measurethe difference in electrical potentialbetween either:1. Two different points on the body (bipolar leads)2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads)
The standard ECG has 12leads:3 Standard Limb Leads3 Augmented Limb Leads6 Precordial LeadsThe axis of a particular lead represents theviewpoint from which it looks at the heart.
Limb Leads Precordial LeadsBipolar I, II, III - (standard limb leads)Unipolar aVR, aVL, aVF V1-V6 (augmented limb leads)
Normal Sinus RhythmEach P wave is followed by a QRS o P wave rate 60 - 100 bpm with <10% variation o rate <60 = sinus bradycardia o rate >100 = sinus tachycardia o variation >10% = sinus arrhythmia
Take the number of “big boxes”between neighboring QRScomplexes, and divide 300 into thisnumber. The result will beapproximately equal to the rateAlthough fast, this method onlyworks for regular rhythms.
It may be easiest to memorize the following table: # of big Rate boxes 1 300 2 150 3 100 4 75 5 60 6 50
As most EKGs record 10 seconds ofrhythm per page, one can simply count thenumber of beats present on the EKG andmultiply by 6 to get the number of beatsper 60 seconds.This method works well for irregularrhythms.
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/ 33 x 6 = 198 bpm
The QRS axis represents the net overall direction of the heart’s electrical activity.Abnormalities of axis can hint at: Ventricular enlargement Conduction blocks (i.e. hemiblocks)
By near-consensus, thenormal QRS axis is definedas ranging from -30 to +90 .-30 to -90 is referred to as aleft axis deviation (LAD)+90 to +180 is referred to asa right axis deviation (RAD)
• The Quadrant Approach• The Equiphasic Approach
2. In the event that LAD is present, examine lead II to determine if this deviation is pathologic.If the QRS in II is predominantly positive, the LAD is non-pathologic (in other words, the axis is normal). If it is predominantly negative, it is pathologic.
The Alan E. Lindsay ECG Learning Center http://medstat.med.utah. edu/kw/ecg/Negative in I, positive in aVF RAD
The Alan E. Lindsay ECG Learning Center http://medstat.med.utah. edu/kw/ecg/Positive in I, negative in aVF Predominantly positive in II Normal Axis (non-pathologic LAD)
1. Determine which lead contains the most equiphasic QRS complex. The fact that the QRS complex in this lead is equally positive and negative indicates that the net electrical vector (i.e. overall QRS axis) is perpendicular to the axis of this particular lead.2. Examine the QRS complex in whichever lead lies 90° away from the lead identified in step 1. If the QRS complex in this second lead is predominantly positive, than the axis of this lead is approximately the same as the net QRS axis. If the QRS complex is predominantly negative, than the net QRS axis lies 180° from the axis of this lead.
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/Equiphasic in aVF Predominantly positive in I QRS axis ≈ 0
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°
Normal P Waves height < 2.5 mm in lead II (higher = ? P-pulmonale) width < 0.11 s in lead II (wider = ? P-mitrale)Normal PR interval 0.12 to 0.20 s (3 - 5 small squares) Short PR interval (Wolff-Parkinson-White syndrome / Lown-Ganong-Levine syndrome) Long PR interval (first degree heart block / trifasicular block)
Normal QRS complex < 0.12 s duration (3 small squares) No pathological Q waves Pathologic “Q”: - > 0.04 sec (small box) - > 25% of “R” amplitude Wide QRS (right or left bundle branch block, ventricular rhythm, hyperkalemia) o No evidence of left or right ventricularhypertrophy
Normal QT interval: – Males: < 450 ms. – Females: < 470 ms. o Calculate the corrected QT interval (QTc) by dividing the QT interval by the square root of the preceeding R - R interval. o Long QT interval is a risk factor for VT / Torsades de Pointes. o Long QT interval (MI, myocarditis, diffuse myocardial disease / hypocalcaemia / hypothyrodism / intracerebral haemorrhage / drugs (sotalol, amiodarone) / hereditary (Romano Ward syndrome (autosomal dominant) / Jervill Lange Nielson syndrome (autosomal recessive)
Normal ST segment no elevation or depression ST elevation: acute MI / left bundle branch block, normal variants (e.g. athletic heart) acute pericarditis ST depression: myocardial ischaemia, digoxin effect / ventricular hypertrophy / acute posterior MI / right bundle branch block
Normal T wave: variable morphology & amplitude / usually samedirection as the QRS except in V1-2 leads. In the normal ECG the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR. Tall T: hyperkalemia / hyperacute myocardial infarction. Small, flattened or inverted T waves: ischaemia / LVH / drugs (e.g. digoxin) / pericarditis / PE / RBBB / electrolyte disturbance.Normal U wave: usually < 1/3 T wave amplitude & same directionin the same lead / prominent at slow heart rates. o Origin of the U wave is thought to be related to afterdepolarizations which interrupt or follow repolarization