ECG Complexes and Intervals As Related To The Ventricles:
QRS Complex Represents Ventricular Depolarization
Labeling:
- If the first deflection is negative it is the Q
- All upright deflections are R waves
- A negative deflection following an R is an S
- Capital letters are used to label tall waves, small letters for small waves
T Wave represents ventricular repolarization:
Direction:
- Upright in I,II, V3-6
- Variable in III, AVL, AVF, V1, V2
- Inverted in AVR may be related to one of the following; myocardial ischemia, ventricular hypertrophy, BBB, pericarditis, cor pulmonale, cardiomyopather, electrolyte imbalance, metabolic deficiency, digitalis intoxication
Shape:
- Normally slightly rounded
- Slightly asymmetrical
- Sharply pointed or notched may be normal and is common in children; also found in pericarditis. A sharply pointed T wave, either upright or inverted, is suspicious for an MI
Height:
- Normally not above 5 mm in any standard lead and not above 10 mm in any precordial lead
- Tall ususally indicative of MI or elevated potassium
- Tall T can also bbe seen in myocardial ischemia without an infarction, ventricular overload, psychotics and CVA
- Flat T wave can be seen in obesity with the normal amplitude returning after weight loss.
Q-T Interval is the time between onset of ventricular depolarization and end of ventricular repolarization.:
- Beginning of the QRS to the end of the T wave
- Affected by autonomic influences and catecholamines
- Fluctuates diurnally becoming longer during sleep
- Varies with heart rate, sex, age
- Ashman chart or nomogram for QTc – corrected QT interval
- QT should be less than half the preceding R-R interval except if the HR <65 or > 90
- Proportionate to the preceeding cycle
- U wave may merge with QT – presence of U wave is abnormal and may be related to hypokalemia, hypomagnesemia, ischemia
Prolonged QT:
- Lengthened by ischemic heart disease, rheumatic fever, MI, myocarditis, mitral valve prolapse
- Electrolyte disturbances, hypocalcemia, hypokalemia, hypomagnesemia, hypothyroidism
- Subarachnoid or intracerebral hemorrhage, stroke
- Antiarrhythmics – eg: sotolol Trade Name BETAPACE amiodarone Trade Name CORDARONE quinidine Trade Names CARDIOQUIN QUINAGLUTE - tricyclic antidepressants, phenothiazines, other drugs
- Hypothermia
- Stringent dieting
Shortened QT:
- Digitalis
- Hyperkalemia
- Hypercalcemia
- Hypermagnesemia
U Wave:
- Low voltage, small wave
- Polarity usually same as T wave
- Best seen in V3
- Taller with hypokalemia
- Inverted with myocardial ischemia aortic or mitral regurg, and left ventricular overload from HTN.
- Negative U in resting ECG may show significant stenosis of left main or LAD
- Affected by digitalis, quinidine, epinephrine, hypercalcemia, thyrotoxicosis, intracranial hemorrhage and exercise - all of which will increase the amplitude of the U wave. Size may vary with cycle length – the longer the cycle length, the taller the U wave
Want to know more about the U wave? Go Here
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