Cardiogram Notes Sinoatrial node(SA node) · Located at right atrium near the entrance of the superior vena cava. · Natural pacemaker of the heart · Determines heart rate · Electrical impulses from SA node spread throughout both atria and stimulate them to contract. AV node(Atrioventricular node) · Located on other side of right atrium, near AV valve. · Serves as electrical gateway to ventricles; delays passage of electrical impulses to ventricles. . Delay ensures that atria have ejected all the blood into the ventricles before the ventricles contract. . The AV node receives the signals from the SA node and passes them onto the atrioventricular bundle(AV bundle or bundle of His) Bundle of His . Divided into left and right bundle branches which conduct the impulses toward the apex of the heart. · Signals are passed onto Purkinje fibers, turning upward and spreading throughout the ventricular myocardium. Electrocardiogram(ECG or EKG) · A composite recording of all action potentials produced by the nodes and the cells of the myocardium. . Each wave or segment of the ECG corresponds to an event of the cardiac electrical cycle. · Each cardiac electrical cycle repeats itself with every heartbeat. Cardiac Electrical Cycle(In Order from First to Last) 1. P-wave
2. P-Q segment 3. QRS complex 4. Q-wave 5. R-wave 6. S-wave 7. S-T segment 8. T-wave P-wave . When the atria are full of blood, the SA node fires, electrical signals spread throughout the atria and cause them to depolarize. · Atrial contraction(atrial systole) starts 100 milliseconds after the P-wave begins. P-Q segment . The P-Q segment represents the time the signals travel from the SA node to the Av node. QRS complex · Marks firing of the AV node · Represents ventricular depolarization Q-wave · Depolarization of the interventricular septum R-wave . Depolarization of the main mass of the ventricles S-wave . Represents last phase of ventricular depolarization at the base of the heart. · Atrial repolarization also occurs during this time but the signal is obscured by the large QRS complex.
5-I segment · Reflects plateau in the myocardial action potential(when the ventricles contract and pump blood). T-wave · Ventricular repolarization immediately before ventricular relaxation(or ventricular diastole).