Under normal circumstances, the SA node has the most rapid rate of spontaneous depolarization 

This makes the SA node the dominant cardiac pacemaker

From the SA node, the impulse spreads through the right and left atria

On the ECG, the depolarization of the atria is represented by the P wave

Once the wave of depolarization has reached the AV node, a delay is seen 

The delay permits contraction of the atria and allows filling of the ventircles

This delay is PR interval: 0.12-0.20

Indicates accelerated conduction

Such as seen in WPW syndrome

Common bundle of His

The left and right bundle branches

The distal bundle branches

Purkinje fibers

Represents the progress of depolarization through the conduction system 

Ordinarily duration is < 0.12 seconds

The ECG normally returns to baseline and repolarization begins 

Repolarization consists of the ST segment and T wave

Although ventricular depolarization occurs along established pathways, repolarization occurs independently in every cell 

Rate dependent, shortening with faster rates

Normally 0.44 seconds or less

Have been associated with polymorphic ventricular tachycarida

Torsades de pointes 

Hypokalemia

Hypothermia

Hypomagnesemia 

Hypocalcemia

Sotalol use

Phenothiazine use

Associated with U waves

and

J waves aka Osborn Waves

Bipolar

Measuring differences between 2 electrodes 

Also recorded

aVR, aVL, and aVF

3 bipolar, 3 unipolar

The axis of depolarization of the ventricles can be determined 

Additional information is obtained when electrodes are placed closer to the heart 

The neurtral electrode is formed by the standard leads and the 'exploring' electrode is placed on the chest wall

Each lead is indicated by the letter V followed by a number 1-6

Modified Chest Leads

(MCL)

several exist to view the desired areas of the heart 

MCL₁ through MCL₆ view single precordial leads 

Central Subclavicular Lead

(CS₅)

Particularly well suited for the detection of anterior ischemia

Lead I is selected

Lead II can monitor inferior ischemia 

This is probably the best alternative to a true V5

Central Back Lead 

(CB5)

Offers a considerably larger P wave for arrythmia detection

Similar detection of ischemia to a true V5

Permits the recording of the 6 standard leads and one precordial lead

The precordial lead is usually placed in the V5 positon for ischemia detection

Simultaneously monitoring the leads II and V5 offer the best detection of both arrythmias and ischemic events 

Represent Voltage

Normally calibrated monitors/cardiograms display 1 cm/1 mV

The electrocautery is the most important source of interference

Radiofrequency energy (800,000- 2,000,000 Hz) accounts for most of the interference 

Somatosensory and motor evoked potentials have also been shown to stimulate pacemaker spikes