Large boxes are used to estimate heart rate. Measure from QRS to QRS. Rates are approximate:
1 large box = 300 bpm.
2 large boxes = 150 bpm.
3 large boxes = 100 bpm.
4 large boxes = 75 bpm.
5 large boxes = 60 bpm.

P Wave:
represents both right and left atrial depolarization
Q Wave:
is the first negative depolarization of the QRS complex, NOT preceded by an R wave. The QRS represents ventricular depolarization and buried atrial repolarization
R Wave:
is the first positive inflection in the QRS complex. Additional positive inflections are designated R' (R prime) and may be seen in conditions such as bundle branch blocks.
S Wave:
is the first negative deflection following the R wave in the QRS complex.
T Wave:
represents ventricular repolarization. It may be upright, inverted, elevated, or depressed depending on the lead sampled, or pathology.

PR Interval
This is the time from the initiation of SA nodal depolarization to the initiation of ventricular depolarization. It encompasses the time it takes for the action potential to pass through the AV node. The normal PRI is 120-200 msec. This demonstrates that the electrical impulses are originating from the atria and following the proper conduction pathways. It is normal for the PRI to shorten slightly during tachycardia, and lengthen during bradycardia within the limits stated above.

QRS Duration
The normal QRS duration is 60-100 msec. It is measured from the initiation of ventricular depolarization at the Q or R wave, to the last wave of the complex. This point where the R or S wave returns to baseline is called the J point. Prolonged QRS indicated conduction problems. This is often caused by bundle branch blocks.

QT Interval
This interval spans the onset of depolarization of to the completion of repolarization of the ventricules. Lengthened QT intervals indicated slowed ventricular repolarization. This may be due to hypokalemia, or other electrolyte imbalances. Shortened QTs are seen with hypercalcemia and digitalis toxicity.

The initial depolarization occurs in the interventricular septum. The directon of the current is left to right. This is represented by the red arrow on the diagram.

Current then follows the His-purkinji system toward the apex of the heart. The current flows from endocardium to epicardium and presents as a positive inflection on lead II. As the curent continues along the purkinji fibers the vectors change and the QRS is depressed.

The ekg inflection is dependent on the direction of current in relation to the specific lead being measured.

Rate is determined manually by pulses. An ECG machine will provide a heart rate by measuring the R-R interval. It continues to sample the inteval and provides an average heart rate. The rate provided by an ECG does not always correlate to actual myocardial contraction. Artifact can cause the machine to record a rate higher than it actually is. PVCs often do not perfuse and are recorded as beats. An extremely important arrythmia, pulseless electrical activity (PEA, EMD) often shows a normal heart rhythm on the ECG tracing, but the patient will not have any pulse or perfusion. This can be caused by: pulmonary embolism, acidosis, tension pneumothorax, cardiac tamponade, hypovolemia, hyperkalemia, hypokalemia, hypothermia, overdose, myocardial infarct.

Bradycardia is any heart rate less than 60.
Tachycardia is any heart rate greater than 100.

Regular Rhythm
This is the first tracing seen to the left. The R to R intervals are the same. The rhythm is maintained.
Regularly Irregular Rhythm
This rhythm represents a pattern of beats that repeats. This may be seen in third degree heart block, where there are two regular rhythms (P and QRS) superimposed on top of each other. The result is a tracing with variable PR intervals.
Irregularly Irregular Rhythm
This rhythm has no underlying regularlity. The R-R interval is not consistant, and there is no pattern to the beats. This is often seen in Atrial fibrillation