sinoatrial node and continues to the
atrioventricular node (AV node). There is a delay of 0.04 seconds
to allow the atria to contract. Conduction then spreads to both
ventricles simultaneously through the left and right bundle
branches. 
The atrioventricular (AV) node is located in the posterior center of the heart and is supplied with blood by either the right coronary artery (90% of the patients) or the circumflex artery (10% of the patients).
The electrical impulse that results in contraction of the
heart begins at the
sinoatrial node and continues to the
atrioventricular node (AV node). There is a delay of 0.04 seconds
to allow the atria to contract. Conduction then spreads to both
ventricles simultaneously through the left and right bundle
branches.
The 
P-wave is
associated with atrial depolarization. Normal
P-waves last from 0.06 to 0.11 seconds and should not exceed
2.0-2.5 mm in height. Dysfunctions of the sinoatrial node result
in the observance of abnormalities in the P-wave; (i.e., longer,
wider or absent), when viewed on an ECG (EKG).
The 
P-R interval
represents conduction of electricity from
the sinoatrial node to the atrioventricular node. It also
represents atrial depolarization through the AV node. The P-R
interval is measured from the beginning of a P-wave to the
beginning of the QRS complex. 
Normal lengths of time for the P-R
interval range from 0.12-0.20 seconds and should be consistent
throughout the ECG strip.
The 
QRS complex
is the result of ventricular depolarization
through the Bundle Branches and Purkinje fibers. The length of
the normal QRS complex should be no longer than 0.10 seconds.
The 
first negative
deflection is labeled "q" wave if small
and condensed and "Q" if large. A negative Q-wave is indicative
of necrotic heart tissue. The 
first positive deflection is
labeled "r" wave if small and condensed and "R" if tall and
large. The negative deflection that follows an "r" wave is
labeled an 
"s" wave
if small and condensed, and "S" if large. The
Bundle Branches and Purkinje fibers are the main conduits for
electrical activity in the heart.
The 
S-T segment
represents the ventricles in an active
contraction state but with no electrical activity occurring. The
S-T segment is measured from the end of the QRS complex to the
beginning of the T-wave. The 
T-wave represents ventricular
repolarization and is normally upright.
The 
U-wave's
representation is unknown but thought to
represent the repolarization of the His-Purkinje complex. Due to
the weakness of the signal, the U-wave is often not seen on the
ECG.