Quick review of cardiac function
The heart is just a hollow muscular pump that takes the blood it receives and pumps it out. The blood, depleted of oxygen, flows back to the heart through two main veins. The venous blood from the head and arms flows into the superior vena cava, and the venous blood from the legs and the trunk flows into the inferior vena cava. These large vena caval veins enter the right atrium. Blood returning from the heart muscle itself enters the coronary sinus, which also empties into the right atrium. The right atrium fills and then pumps the blood through the tricuspid valve into the right ventricle. The right ventricle then pumps the blood through the pulmonic valve into the pulmonary artery that takes the venous blood to the lungs where oxygen is added and CO2 removed. The oxygenated blood returns via four pulmonary veins to the left atrium. The left atrium then pumps it through the mitral valve to the left ventricle. The left ventricle then pumps the blood through the aortic valve into the aorta that then divides into smaller arteries going to every part of the body. The blood eventually goes into the capillaries where the tissues of the body absorb the oxygen they need and get rid of waste products like CO2, lactate etc…. The blood, again depleted of oxygen, finds its way to the small veins, which ultimately fill the larger veins completing the circuit.
Role of heart rhythm
Heart rhythm is vital to the function of the heart. If the heart fails to be excited, then no pulse is produced, and within 3-5 seconds when the systolic blood pressure falls to below 50 mm Hg, the person looses consciousness. If rhythm is not restored within minutes, brain death occurs.
The production of the heart beat therefore is critical. In normal sinus rhythm, the sinoatrial or sinus node, a pea-sized structure near the junction of the superior vena cava and the right atrium starts the heart beat. The sinus node has rich innervation from both the sympathetic and parasympathetic nervous systems making it the ideal sensor for the needs of the body.
After the sinus node depolarizes, the impulse causes excitation of the right atrium. The depolarization quickly spreads along special pathways to the left atrium and it contracts as well producing atrial systole. Thus both atria contract propelling blood across the tricuspid and mitral valves into the ventricles.
In the normal heart, there is only one pathway between the atria and ventricles: this is called the atrioventricular or AV node. The AV node acts as a filter and protects the ventricles against heart rhythms that are too fast or too slow. When the atrial rate is reasonable (50-200 depending on the conditions), the AV node, after a brief delay to optimize timing, passes each beat to the ventricles, and ventricular systole occurs so that the atria and ventricles are synchronized (AV synchrony). If the sinus node fails to fire, the AV node produces junctional escape rhythm at 30-50 beats to minute to sustain life. When the atria race, the AV node blocks some of the beats from making it to the left ventricle. For example, in atrial flutter, the atria are beating at 300 beats per minute. If the AV node conducted each of these beats to the left ventricle, the pulse would be 300 beats per minute, which could cause a cardiac arrest. So typically, the AV node blocks every other beat and the pulse therefore is 150 beats per minute.
The depolarization wave front that has excited the atria and conducted through the AV node now arrives at the bundle of His. This bundle of Purkinje fibers is capable of the most rapid conduction of any heart muscle: 3 meters per second. The bundle of His divides into the right and left bundle branches which in turn continue to divide producing a network of fibers coating the endocardial surfaces of both the right and left ventricles. Thus both ventricles are excited simultaneously, and ventricular systole occurs in a synchronous fashion. In the normal heart, with no way for the depolarization wave front to get back to the atria, that heat beat ends, and the heart relaxes (diastole).