Normal ECG
A normal ECG is illustrated above. Note that the heart is beating in a regular sinus rhythm between 60 100 beats per minute (specifically 82 bpm). All the important intervals on this recording are within normal
ranges.
1. P wave:
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upright in leads I, aVF and V3 - V6
normal duration of less than or equal to 0.11 seconds
polarity is positive in leads I, II, aVF and V4 - V6; diphasic in leads V1 and V3; negative in aVR
shape is generally smooth, not notched or peaked
2. PR interval:
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Normally between 0.12 and 0.20 seconds.
3. QRS complex:
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Duration less than or equal to 0.12 seconds, amplitude greater than 0.5 mV in at least one
standard lead, and greater than 1.0 mV in at least one precordial lead. Upper limit of normal
amplitude is 2.5 - 3.0 mV.
small septal Q waves in I, aVL, V5 and V6 (duration less than or equal to 0.04 seconds; amplitude
less than 1/3 of the amplitude of the R wave in the same lead).
represented by a positive deflection with a large, upright R in leads I, II, V4 - V6 and a negative
deflection with a large, deep S in aVR, V1 and V2
in general, proceeding from V1 to V6, the R waves get taller while the S waves get smaller. At V3
or V4, these waves are usually equal. This is called the transitional zone.
4. ST segment:
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isoelectric, slanting upwards to the T wave in the normal ECG
can be slightly elevated (up to 2.0 mm in some precordial leads)
never normally depressed greater than 0.5 mm in any lead
5. T wave:
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T wave deflection should be in the same direction as the QRS complex in at least 5 of the 6 limb
leads
normally rounded and asymmetrical, with a more gradual ascent than descent
should be upright in leads V2 - V6, inverted in aVR
amplitude of at least 0.2 mV in leads V3 and V4 and at least 0.1 mV in leads V5 and V6
isolated T wave inversion in an asymptomatic adult is generally a normal variant
6. QT interval:
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Durations normally less than or equal to 0.40 seconds for males and 0.44 seconds for females
Acute anterolateral MI
Acute anterolateral MI is recongnized by ST segment elevation in leads I, aVL and the precordial leads
overlying the anterior and lateral surfaces of the heart (V3 - V6). Generally speaking, the more significant
the ST elevation , the more severe the infarction. There is also a loss of general R wave progression
across the precordial leads and there may be symmetric T wave inversion as well. Anterolateral myocardial
infarctions frequently are caused by occlusion of the proximal left anterior descending coronary artery, or
combined occlusions of the LAD together with the right coronary artery or left circumflex artery. Arrythmias
which commonly preclude the diagnosis of anterolateral MI on ECG and therefore possibly identify high risk
patients include right and left bundle branch blocks, hemiblocks and type II second degree atrioventricular
conduction blocks.
Acute inferior MI
Leads II, III and aVF reflect electrocardiogram changes associated with acute infarction of the inferior
aspect of the heart. ST elevation, developing Q waves and T wave inversion may all be present depending
on the timing of the ECG relative to the onset of myocardial infarction. Most frequently, inferior MI results
from occlusion of the right coronary artery. Conduction abnormalities which may alert the physician to
patients at risk include second degree AV block and complete heart block together with junctional escape
beats. Note that the patient below is also suffering from a concurrent posterior wall infarction as eveidenced
by ST depression in leads V1 and V2.
Acute right ventricular MI
In patients presenting with acute right ventricular MI, abnormalities in the standard 12 lead ECG are
restricted to ST elevation greater than or equal to 1 mm in lead aVR. Although isolated right ventricular MI
is usually seen in patients suffering from chronic lung disease together with right ventricular hypertrophy, it
can occur in patients suffering a transmural infarction of the inferior-posterior wall which extends to involve
the right ventricular wall as well. Right ventricular MI is most commonly caused by obstruction of the
proximal right coronary artery and is frequently associated with right bundle branch block. Furthermore,
only 5% - 10% of patients suffer from hemodynamic symptoms.
Atrial fibrillation
Atrial fibrillation represents disorganized atrial activity without contraction or ejection. The
electrocardiogram demonstrates an irregular baseline where the normal P waves are replaced with rapidly
quivering small deflection of variable amplitude (f waves - outlined below). An irregularly irregular
ventricular rate demonstrating narrow QRS complexes is established between 100 - 160 bpm. Atrial
fibrillation is common in patients with rheumatic heart disease, pulmonary emboli, cardiomyopathy,
pericarditis, ischemic heart disease and thyrotoxicosis. It causes minimal hemodynamic compromise and
often the patient presents complaining of palpitations as the only symptom. Although hemodynamic
compromise is minimal, atrial fibrillation is an important risk factor for the development of thromboembolic
complications, such as strokes and transient ischemic a ttacks.
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