Introduction to Electrophysiology;

What are they doing in there?

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

Refer to the discussion on the the heart’s conduction system.

From the ECG Learning Center
Dr. Alan Lindsay. Excellent site for learning to read ECGs.

P Wave:

• Normally upright in Lead I, II, V4-6 and AVF
• Normally inverted in AVR
• Variable in Lead III, AVL and other chest leads
• Amplitude should not exceed 2 -3 mm in any lead
• Normal conture gently rounded rather than pointed and notched

P Wave  Abnormalities

• Inversion when shouldbe upright (ectopic atrial or junctional)
• Increased amplitude: atrial hypertrophy or dilation, valve disease, HTN, cor  pulmonale
• Increased width: Left atrial enlargement P< 0.11
• Diphasic LAE (Lead III, or V1)
• Notching P. mitrale I.  III signifcant when peak distance > 0.04
• Peaking P-Pulmonale III>I
• Absence of P waves

Tp Wave (Ta)

• Represents atrial recovery
• Opposite direction of P wave

P – R Interval

• 0.12 – 0.20 seconds
• Varies with heart rate
• If conduction system is diseased or affected by digitalis, P-R may lengthen as if conducting system as the rate increases.

Prolonged PR Interval

• AV block due to coronary disease, rheumatic disease,
• In some cases of Hyperthyroidism
• As a normal varriation (1.3%)

Shortened PR Interval

• AV junctional and low atrial rhythms
• WPW
• Normal variation
• Glycogen storage issue
• Pheochromocytoma
• Some hypertensive patients
• Fabry’s disease

PR Segment

• Normally isoelectric
• May be displaced in atrial infarction and acute pericarditis 

For Self Test on the cardiac cycle Go Here 

Self Test Part 2 of the cardiac cycle Go Here

Simple Lessons In Cardiac Contracility, Cardiac Pressures,  The Cardiac Output And Starlings Curve Made Easy