Hemodynamics of the Heart

Hemodynamics refers to the dynamics of blood flow, the forces and motion of blood through the cardiovascular system. It involves the study of cardiac function and the systemic circulation, providing insights into how the heart pumps blood and how blood is distributed throughout the body.

Key Concepts in Hemodynamics:

1. Cardiac Cycle:

   – Systole: The phase of the cardiac cycle where the heart contracts and pumps blood out of the chambers.

     – Isovolumetric Contraction: All heart valves are closed, and pressure builds up in the ventricles.

     – Ventricular Ejection: The semilunar valves open, and blood is ejected into the aorta and pulmonary artery.

   – Diastole: The phase where the heart relaxes and fills with blood.

     – Isovolumetric Relaxation: The ventricles relax, and all valves are closed.

     – Ventricular Filling: The atrioventricular valves open, allowing blood to flow into the ventricles from the atria.

2. Cardiac Output (CO):

   – Definition: The volume of blood the heart pumps per minute.

   – Equation: CO = Heart Rate (HR) x Stroke Volume (SV)

   – Stroke Volume (SV): The amount of blood ejected with each heartbeat.

     – Preload: The initial stretching of the cardiac myocytes prior to contraction, influenced by the venous return.

     – Afterload: The pressure the heart must work against to eject blood.

     – Contractility: The inherent ability of cardiac muscle fibers to contract.

3. Blood Pressure:

   – Systolic Pressure: The pressure in the arteries during the contraction of the heart muscles.

   – Diastolic Pressure: The pressure in the arteries when the heart muscles are relaxed.

   – Mean Arterial Pressure (MAP): An average pressure in the arteries during one cardiac cycle, crucial for tissue perfusion.

4. Vascular Resistance:

   – Total Peripheral Resistance (TPR): The resistance to blood flow offered by the systemic circulation.

   – Influences blood flow and blood pressure, regulated by vessel diameter, blood viscosity, and vessel length.

5. Venous Return:

   – The flow of blood back to the heart.

   – Affected by factors such as muscle contraction, venous valves, and thoracic pressure changes during respiration.

Electrophysiology of the Heart

Electrophysiology deals with the electrical properties and activities of the heart, which are critical for initiating and regulating heartbeats.

Key Concepts in Electrophysiology:

1. Cardiac Action Potential:

   – Phases of Action Potential:

     – Phase 0: Rapid depolarization due to the influx of Na+ ions.

     – Phase 1: Initial repolarization caused by the transient outward K+ current.

     – Phase 2: Plateau phase maintained by the balance between inward Ca2+ and outward K+ currents.

     – Phase 3: Rapid repolarization due to the efflux of K+ ions.

     – Phase 4: Resting membrane potential maintained by the Na+/K+ pump and K+ leak channels.

   – Resting Membrane Potential: The difference in voltage across the cell membrane in a resting state, typically around -90 mV in cardiac myocytes.

2. Conduction System of the Heart:

   – Sinoatrial (SA) Node: The natural pacemaker of the heart, located in the right atrium, initiates the electrical impulse.

   – Atrioventricular (AV) Node: Relays the impulse from the atria to the ventricles with a slight delay to ensure proper filling.

   – Bundle of His: Conducts the impulse from the AV node to the interventricular septum.

   – Purkinje Fibers: Distribute the impulse throughout the ventricles, ensuring coordinated contraction.

3. Electrocardiogram (ECG/EKG):

   – P Wave: Atrial depolarization.

   – QRS Complex: Ventricular depolarization.

   – T Wave: Ventricular repolarization.

   – PR Interval: Time from the onset of atrial depolarization to the onset of ventricular depolarization.

   – QT Interval: Total time for ventricular depolarization and repolarization.

4. Autonomic Regulation:

   – Sympathetic Nervous System: Increases heart rate and contractility through the release of norepinephrine.

   – Parasympathetic Nervous System: Decreases heart rate through the release of acetylcholine.

5. Ion Channels and Pumps:

   – Sodium (Na+) Channels: Involved in the rapid depolarization phase.

   – Calcium (Ca2+) Channels: Play a critical role in the plateau phase and excitation-contraction coupling.

   – Potassium (K+) Channels: Key in repolarization and setting the resting membrane potential.

   – Na+/K+ ATPase Pump: Maintains the ionic gradients essential for action potential generation.

Understanding the hemodynamics and electrophysiology of the heart is crucial for diagnosing and treating cardiovascular diseases, ensuring effective management of conditions such as arrhythmias, heart failure, and hypertension.