Interval –  measures time from start of one wave to start of other (at least one wave).

Segment – measures time between waves (no waves).

Waveform –  is produced when electrical potential of cardiac cell membrane changes -> depolarization & repolarization.  

Baseline or isoelectric line is the absence of voltage change in cells. PR segment is most often used but TP segment is more accurate (end of T wave and beginning of P wave).


1. P wave                   4. QRS complex         7. ST segment

2. PR segment          5. Q wave                    8. T wave

3.PR interval             6. QT interval            9. U wave 

1. P wave

P wave is the depolarization of Rt and Lt atria. 
Begins with 1st deviation from baseline, finishes when meets baseline again. 
Gives atrial rate.
Normal: smooth, rounded
                  0.5-2.5 mm amplitude; < 0.11 sec duration. 
                  Upright in leads I and II
If the P wave doesn’t happen at same time then it’s not a P wave. If have to search for it then not there.

2. PR segment

PR segment is the line between the end of P wave and beginning of QRS complex
It is the time taken to conduct thru AV junction (slow), this allows for atrial kick
It is a benchmark for isoelectric line.

3. PR interval

PR interval is measured from start of P wave to start of QRS. 
It is the time for impulse to travel from SA node thru atria and AV junction to Purkinje network.                                    Provides the origin of impulse and integrity of conduction.

Normal: 0.12-0.20 sec (3-5mm).  Length is most important!
If longer then too slow thru AV junction -> 1st degree block
If shorter then junctional rhythm (comes from AV junction, atrial or some other node).
If the P wave is consistently followed by a QRS complex across a consistent PR interval, this is strong evidence that the originating impulse is supraventricular in origin.

4. QRS complex

Represents the depolarization of ventricles and also repolarization of atria which is buried in the QRS.
Ventricles are 3 x larger than atria so produce a larger wave.

3 waveforms:
1st depolarization of ventricular septum from Lt to Rt away from positive electrode in lead II -> small down (negative) deflection -> Q wave.
2nd progresses from endocardium to epicardium across both ventricles producing an R wave (1st positive deflection) and S wave (1st wave below isoelectric line). 

J point is the end of S wave is where begins to flatten out.

Normal QRS:  narrow, less than 0.11 sec (originates above ventricles). 
                              Can be upright, inverted or biphasic (part up, part down).  
                              Time is important!
                             The width of QRS indicates location of electrical impulse.

Junctional – normal QRS with embedded PR interval.
Atrial –  normal QRS with shorter PR interval.
Ventricular – normal QRS with wider PR interval.

5. Q wave

When present, a prominent Q wave represents an MI that has already occurred recently or some time ago.
Normal Q wave:  < 25% of R wave.
                                 no deeper than 2mm and < than 1 mm square width.                                        

6. QT interval

QT interval represents a complete ventricular cycle of depolarization and repolarization.  
Measured from beginning of QRS to end of T wave.  

Normal:than ½ the R-R interval (0.36-0.44 sec).
                     varies with HR, age, sex

Tip: take QT measurement and bring it up to R wave, if less than ½ = OK;  if more -> significant
Longer QT associated with v tach and other unstable dysrhythmias
R on T phenomenon is when a QRS comes at the tail end of a T wave -> dangerous dysrhythmias. Can be caused by long QT syndrome, antiarrythmics (quinidine, procainamide), tricyclic antidepressants and hypokalemia.
– short QT classic sign of hypercalcemia often with low phosphate.

7. ST segment

Represents early repolarization of ventricles.
Usually follows an isoelectric line (plateau phase where membrane potential doesn’t change).
Starts at J point (a notch/bump where QRS changes direction), 0.04 sec (1 mm) after J point. Draw an imaginary line from PR interval )or if can’t measure PR then draw draw T to T).

ST segment is evaluated for any deviation from baseline. 
Has to be > 1 box above or below to be significant. 
Measure 1.5 boxes from end of S wave (J point). 
If not regular then not increased or decreased; have to be in all complexes.

ST depression of 1 mm or more in 2 contiguous lead (neighboring leads) suggests ischemia, injury or MI. Often a horizontal and downward.
ST elevation of 1mm or more in 2 contiguous leads highly suggests MI or injury. 
The changes mean that acute coronary events are happening now!   

Digoxin dip is a concave shape ST depression in patients taking digoxin even at normal blood levels.
Ventricular hypertrophy is a depressed and upward sloping ST.
Not every ST change means ischemia but the bottom line is: most ST changes indicate cardiac ischemia!

8. T wave

Repolarization of ventricle (slower than depolarization). Normally opposite to depolarization, from outside in.

Normal:  0.10-0.25 sec wide with < 5mm amplitude
                    larger than P wave
                    upright and slightly assymetrical in lead II

As HR increases, P wave (atria) and T wave (ventricles) begin to share same space.

Peaked T wave can indicate early ischemia and infarction or hyperkalemia.  Flattened T wave can be caused by hypokalemia and digitalis.   Inverted T wave can mean ischemia.
Wide T wave can be caused by Quinidine.   T wave opposite to QRS indicates abnormal QRS.
Abnormal T waves can also be benign, therefore it is a weak sign of diagnosis.

9. U wave

Occasionally there is a wave after T wave and before P wave. Possibly a final stage of repolarization.

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