NORMAL CARDIAC ACTIVATION AND RECOVERY

Cardiac conduction system anatomy, cellular action potentials, and surface ECG

• the neat, orderly, classic four-phased action potential is seen only in the Purkinje cells of the heart

• SA and atria fire first; this is the P wave on the surface ECG

• then, the ventricular depolarization leads to QRS complex

• repolarization is ventricular T complex

Key points of cellular APs

• SA and AV nodes exhibit upsloping phase 4 diastolic action potentials; this is because they exhibit automaticity

• phase 0 is due to Na+ channels in most cardiac cells except the AV node, where phase 0 involves Ca++

Normal ionic currents and cellular AP

• phase 0 depolarization is Na-dependent

• phase 2 is due to Ca and K

• phase 3 repolarization is due to K

• phase 4 is due to sodium pumps as well as Na/Ca exchanger

Category: Cardiology Notes