MMED2931 wk 3 and 4 l3

Speaker: Elke Shikoya
Topic: Cardiac Cycle
Learning Objectives: - Describe the phases of the cardiac cycle. - Relate heart sounds to pressure changes during systole and diastole. - Define cardiac output. - Describe factors regulating stroke volume and heart rate.

Definition: The cardiac cycle encompasses all mechanical events occurring during a single heartbeat.
Key Terms: - Systole: Describes contraction of the heart muscles. - Diastole: Describes relaxation of the heart muscles.

Typical duration: Approximately $0.8$ seconds.
Ventricular contraction duration (systole): Approximately $0.3$ seconds.
Ventricular relaxation duration (diastole): Approximately $0.5$ seconds.
Overall, the ventricles are in: - Systole: $rac{1}{3}$ of the cycle. - Diastole: $rac{2}{3}$ of the cycle.

Inflow Phase: - Inlet Valve: Open (blood moves into the ventricle). - Outlet Valve: Closed (no ejection of blood).
Isovolumetric Contraction Phase: - Inlet Valve: Closed. - Outlet Valve: Closed. - Condition: Blood volume remains constant (isovolumetric), while the ventricle contracts.
Ventricular Ejection Phase: - Inlet Valve: Closed. - Outlet Valve: Open (blood ejected from the ventricle).
Isovolumetric Relaxation Phase: - Inlet Valve: Closed. - Outlet Valve: Closed. - Condition: Blood volume constant (isovolumetric), while the ventricle relaxes.

Filling Phase: (End of Diastole) - Mitral valve open, allowing blood from the left atrium into the left ventricle. - Blood volume increases, leading to the highest volume at the end of diastole: - End Diastolic Volume (EDV): Maximum volume before contraction.

Outflow Phase: - Aortic valve opens, allowing blood to exit into the aorta. - Blood ejected from the ventricle is known as: - Stroke Volume (SV): Amount of blood ejected in one heartbeat. - Remaining blood post-contraction: - End Systolic Volume (ESV): Volume left in the ventricle after contraction.

Components: - Top Panel: Changes in pressures: - Aortic Pressure (red) - Left Atrium Pressure (brown/black) - Left Ventricle Pressure (red) - Middle Panel: Changes in left ventricular volume. - Bottom Panel: Electrocardiogram (ECG) and heart sounds.
Phases Illustrated in the Cardiac Cycle: - Starting at the P wave, autorhythmic cells in the SA Node fire action potentials, resulting in atrial depolarization. Atrial contraction leads to an increase in atrial pressure and blood flow into the ventricle. - End of ventricular diastole is marked by the peak in left ventricular volume (EDV). - QRS complex marks ventricular depolarization, initiating ventricular contraction (systole), causing rapid pressure increase. - Closure of the mitral valve generates the first heart sound (S1). - Isovolumetric contraction occurs until left ventricular pressure exceeds aortic pressure, leading to aortic valve opening and blood ejection. The second heart sound (S2) occurs when the aortic valve closes during isovolumetric relaxation. - Passive ventricular filling resumes as the mitral valve opens following ventricular relaxation.

S1 (Lub): Caused by closure of the mitral and tricuspid valves - louder and longer.
S2 (Dupp): Caused by closure of aortic and pulmonary valves - shorter and softer.
Possible third heart sound (S3): Occurs during passive filling, often normal in children, may indicate pathology in adults.
Possible fourth sound (S4): Associated with active filling, seen in ventricular hypertrophy.

Plotting left ventricular pressure (Y-axis) vs. left ventricular volume (X-axis) creates a pressure-volume loop.
Stages in the loop: 1. Mitral Valve Opens: Blood fills ventricle, increasing volume. 2. End Diastolic Volume: Marked closure of the mitral valve. 3. Isovolumetric Contraction: Both valves closed, pressure increases. 4. Aortic Valve Opens: Blood is ejected from the left ventricle. 5. End Systolic Volume: Aortic valve closure. 6. Isovolumetric Relaxation: Both valves closed and pressure decreases.

Definition: Volume of blood pumped by each ventricle in one minute.
Formula: $\text{Cardiac Output (CO)} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}$

Preload: - The stretch of cardiac muscle fibers before contraction (related to EDV). - Frank-Starling Law: Increased preload (higher venous return) leads to a stronger contraction and increase in stroke volume.
Contractility: - Forcefulness of contraction of cardiac muscle fibers. - Positive inotropic agents (e.g., sympathetic stimulation) increase contractility. - Negative inotropic agents (e.g., calcium channel blockers) decrease contractility.
Afterload: - The pressure that must be overcome for blood to be ejected. - Increased afterload leads to decreased stroke volume due to increased workload on the heart.

Body Position: Transitioning from lying to standing can reduce venous return due to gravity.
Skeletal Muscle Pump: Muscle contractions aid in blood return to the heart (e.g., during exercise).
Respiratory Pump: Inspiration decreases thoracic pressure, promoting venous return.

Chronotropic Effects: - Positive Chronotropic Agents: Sympathetic activation increases heart rate. - Negative Chronotropic Agents: Parasympathetic activation decreases heart rate.
Hormonal Effects: Adrenaline and noradrenaline increase heart rate by binding to beta receptors in the heart.

Phases of the cardiac cycle: Atrial contraction, active ventricular filling, isovolumetric contraction, ventricular ejection, isovolumetric relaxation, passive ventricular filling.
Cardiac output is the product of stroke volume and heart rate.
Key factors regulating stroke volume: preload, contractility, afterload.
Heart rate modulation occurs via autonomic nervous system and hormonal influences.