How the Heart Works: Cardiac Cycle & EKGs

Welcome to our exploration of basic heart anatomy with Spark.E!The heart is divided into four main chambers, working together to pump blood throughout the body.The upper chambers are called atria. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs.The lower chambers are the ventricles. The right ventricle pumps blood to the lungs, while the left ventricle, the strongest chamber, pumps blood to the rest of the body.The septum is a thick muscular wall that completely separates the right and left sides of the heart, preventing mixing of oxygenated and deoxygenated blood.Four valves ensure one-way blood flow through the heart. The tricuspid valve lies between the right atrium and ventricle.The mitral valve, also called the bicuspid valve, connects the left atrium and ventricle.The pulmonary valve controls blood flow from the right ventricle to the pulmonary artery.And finally, the aortic valve regulates blood flow from the left ventricle into the aorta.Blood flows from the atria to the ventricles through the tricuspid and mitral valves.Each chamber has a specific role in the heart's pumping function. The atria serve as collecting chambers.While the ventricles are the main pumping chambers, with the left ventricle being particularly muscular to pump blood throughout the entire body.This basic understanding of heart anatomy sets the foundation for learning about the heart's electrical system and pumping cycle.The heart's electrical conduction system coordinates contractions through a specialized network of cells.The sinoatrial node, or SA node, is the heart's natural pacemaker. Located in the right atrium, it initiates each heartbeat.The electrical signal spreads through the atria, causing them to contract.The signal then reaches the atrioventricular node, or AV node, which delays the impulse briefly to allow the atria to finish contracting.From the AV node, the signal travels down the Bundle of His, which splits into left and right bundle branches.Finally, the Purkinje fibers distribute the electrical signal throughout the ventricles, causing them to contract in a coordinated manner.Let's examine how these electrical signals work at the cellular level through the cardiac action potential.The cardiac action potential has five distinct phases. Phase zero is rapid depolarization, where sodium channels open and positive charges rush into the cell.Phase one shows early repolarization as potassium channels briefly open.Phase two is the plateau phase, unique to cardiac cells, where calcium channels maintain the depolarized state.In phase three, repolarization occurs as potassium channels open and calcium channels close.Finally, phase four is the resting phase, where the cell maintains its negative membrane potential until the next stimulus.During early diastole, both the atria and ventricles are relaxed. The AV valves are open, allowing blood to flow passively from the atria into the ventricles.In atrial systole, the atria contract, pushing the remaining blood into the ventricles. This accounts for about 20 percent of ventricular filling.During ventricular systole, the ventricles contract forcefully. The AV valves close to prevent backward flow, and blood is ejected into the great vessels.The pressure changes during the cardiac cycle are crucial. Ventricular pressure rises dramatically during systole, reaching about 120 millimeters of mercury in the left ventricle.In late diastole, the ventricles relax and pressures fall. The cycle prepares to begin again as the chambers return to their resting state.Throughout the cycle, ventricular volume changes significantly. The difference between end-diastolic and end-systolic volumes gives us the stroke volume.Blood flow through the heart follows a specific pattern driven by pressure differences.Deoxygenated blood returns from the body through the superior and inferior vena cava into the right atrium.The tricuspid valve allows blood to flow from the right atrium to the right ventricle when pressure gradients are favorable.Blood is then pumped through the pulmonary valve into the pulmonary arteries, heading to the lungs.After becoming oxygenated in the lungs, blood returns through the pulmonary veins to the left atrium.The mitral valve controls flow from the left atrium to the left ventricle, where pressure builds significantly.Finally, the aortic valve opens to allow blood to be ejected into the aorta and systemic circulation.This continuous flow pattern is maintained by pressure gradients, with blood always moving from areas of higher pressure to lower pressure.The one-way valves ensure blood flows in the correct direction, preventing any backward flow.An electrocardiogram, or EKG, measures the electrical activity of the heart using electrodes placed on the skin.The standard 12-lead EKG uses ten electrodes: four limb leads and six precordial leads across the chest.These electrodes connect to an EKG machine that amplifies and records the electrical signals.The EKG machine converts these electrical signals into waveforms that are printed on special graph paper.EKG paper has a specific grid pattern. Each small square represents 0.04 seconds horizontally and 0.1 millivolts vertically.The vertical axis measures voltage, with each large square representing one millivolt.The horizontal axis measures time, with each large square representing 0.2 seconds.As the heart's electrical activity changes, the EKG machine records these changes as waveforms on the paper.Let's examine each component of a normal EKG waveform in detail.The P wave represents atrial depolarization. It's a smooth, upright deflection lasting about point one seconds.The QRS complex shows ventricular depolarization. It consists of three waves: the Q wave, R wave, and S wave.Following the QRS, we see the T wave, representing ventricular repolarization. It's typically asymmetric and concordant with the QRS complex.Several important intervals help us assess cardiac conduction. The PR interval measures conduction through the AV node.The QRS duration reflects how quickly the ventricles depolarize. Normal duration is less than point one two seconds.The QT interval represents the total time for ventricular depolarization and repolarization.Let's review some key characteristics of each waveform component.Heart sounds occur when heart valves close during the cardiac cycle.The first heart sound, S1, known as 'lub', occurs when the AV valves close.The second heart sound, S2, known as 'dub', occurs when the semilunar valves close.When using a stethoscope, there are four main areas where heart sounds are best heard.Heart sounds occur at specific points during the cardiac cycle.S1 occurs at the start of ventricular contraction.S2 occurs at the end of ventricular contraction.These heart sounds mark important transitions in the cardiac cycle.Normal sinus rhythm is characterized by regular beats between 60 and 100 times per minute.Let's examine the key characteristics of normal sinus rhythm.Sinus tachycardia occurs when the heart rate exceeds 100 beats per minute, often due to exercise, stress, or fever.Sinus bradycardia is defined as a heart rate below 60 beats per minute, which can be normal in athletes or concerning in others.Let's compare these different rhythms side by side to understand their key differences.Understanding these rhythm variations is crucial for clinical assessment. Each pattern can indicate different underlying conditions or physiological states.Cardiac output is determined by two key factors: stroke volume and heart rate.Stroke volume is the difference between the end-diastolic volume and end-systolic volume of the ventricle.Heart rate is regulated by the autonomic nervous system through sympathetic and parasympathetic influences.On an EKG strip, we can calculate heart rate by counting the number of large squares between R waves.For example, a typical resting heart rate of seventy beats per minute with a stroke volume of seventy milliliters gives us a cardiac output of four point nine liters per minute.Let's explore how EKG interpretation is used in real clinical scenarios.In acute chest pain, EKG changes can reveal a heart attack in progress, showing characteristic ST segment elevations.Patients with palpitations may show rapid heart rhythms that help diagnose the underlying condition.In cases of fainting, or syncope, the EKG might reveal dangerous rhythm disturbances or conduction problems.EKGs are commonly ordered in various clinical situations.The diagnostic process follows a systematic approach, starting from the patient's presentation to making clinical decisions.Certain EKG findings require immediate emergency intervention.Let's review the key points about clinical applications of EKG interpretation.Thank you for completing this comprehensive series on cardiac physiology and EKG interpretation!