ECG basics
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Transcript of ECG basics
ECG BASICSDONE BY :
MOHAMMED A QAZZAZ
• Review of the conduction system• ECG waveforms and intervals• ECG leads• Determining heart rhythm / rate• Determining QRS axis• Normal waves / intervals
Outline
The Normal Conduction System
The electrocardiogram (ECG) is a representation of the electrical events of the cardiac cycle.
Each event has a distinctive waveform, the study of which can lead to greater insight into a patient’s cardiac pathophysiology.
What is an ECG?
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 which translates into 0.20 s (200 msec)
• Hence, there are 5 large blocks per second
• Voltage: 1 mm = 0.1 mV between each individual block vertically
ECG Graph Paper
Normal ECG
What types of pathology can we identify and study from ECGs?
• 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)
Waveforms and Intervals
Waveforms and Intervals
• 3 distinct waves are produced during cardiac cycle
• P wave caused by atrial depolarization
13-63
ECG
• QRS complex caused by ventricular depolarization
• T wave results from ventricular repolarization
13-63
Leads are electrodes which measure the difference in electrical potential between 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)
ECG Leads
ECG
The standard ECG has 12 leads:
3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from which it looks at the heart.
ECG Leads
ECG Leads
Standard Limb Leads
ECG
Augmented Limb Leads
All Limb Leads
Precordial Leads
ECGPrecordial Leads
Summary of Leads
Limb Leads Precordial Leads
Bipolar I, II, III(standard limb leads)
-
Unipolar aVR, aVL, aVF (augmented limb leads)
V1-V6
Arrangement of Leads on the ECG
Anatomic Groups(Septum)
ECG(Anterior Wall)
Anatomic Groups
ECG(Lateral Wall)
Anatomic Groups
ECG
(Inferior Wall)
Anatomic Groups
summary
Determining the Heart Rhythm
Normal Sinus Rhythm
Each P wave is followed by a QRS
oP wave rate 60 - 100 bpm with <10% variation
o rate <60 = sinus bradycardia o rate >100 = sinus tachycardia o variation >10% = sinus arrhythmia
Determining the Heart Rate
• Rule of 300
• 10 Second Rule
Rule of 300
Take the number of “big boxes” between neighboring QRS complexes, and divide 300 into this number. The result will be approximately equal to the rate
Although fast, this method only works for regular rhythms.
ECG
(300 / 6) = 50 bpm
ECG
(300 / ~ 4) = ~ 75 bpm
ECG
(300 / 1.5) = 200 bpm
The Rule of 300
It may be easiest to memorize the following table:
# of big boxes
Rate
1 300
2 150
3 100
4 75
5 60
6 50
10 Second Rule
As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the EKG and multiply by 6 to get the number of beats per 60 seconds.
This method works well for irregular rhythms.
What is the heart rate?
33 x 6 = 198 bpm
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
The QRS Axis
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)
The QRS Axis
By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°.
-30° to -90° is referred to as a left axis deviation (LAD)
+90° to +180° is referred to as a right axis deviation (RAD)
Determining the Axis
• The Quadrant Approach
• The Equiphasic Approach
Determining the Axis
Predominantly Positive
Predominantly Negative
Equiphasic
The Quadrant Approach2. 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.
Quadrant Approach: Example
Negative in I, positive in aVF RAD
The Alan E. Lindsay ECG Learning Center http://medstat.med.utah.edu/kw/ecg/
Quadrant Approach: Example 2
Positive in I, negative in aVF Predominantly positive in II
Normal Axis (non-pathologic LAD)
The Alan E. Lindsay ECG Learning Center http://medstat.med.utah.edu/kw/ecg/
The Equiphasic Approach
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.
ECG
Equiphasic Approach: Example 1
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 Approach: Example 2
Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
Normal waves / intervals
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)
ECG
Normal
P - pulmonale
ECG
Long P-R
P - mitrale
Normal waves / intervals
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 ventricular hypertrophy
ECG
LVH
Normal
ECG
Wide Complex
Pathologic “Q” wave
Normal waves / intervalsNormal 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)
ECG
Normal waves / intervals
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
ECG
Normal ST
ST elevation
ST depression
Normal waves / intervals
Normal T wave: variable morphology & amplitude / usually same direction 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 direction in the same lead / prominent at slow heart rates.
o Origin of the U wave is thought to be related to after depolarizations which interrupt or follow repolarization
ECG
The U Wave