In-Depth Notes on Cardiac Physiology lecture 2

Definition: A single cardiac cycle encompasses all events associated with one heartbeat, including the systole and diastole of both atria and ventricles. This cycle ensures efficient circulation of blood throughout the body, facilitating the delivery of oxygen and nutrients.

Phases:
1) Systole - contraction of heart muscle, where the heart pumps blood out to the arteries.
2) Diastole - relaxation of heart muscle, during which the heart chambers fill with blood, ensuring adequate volume before the next contraction.

Cardiac Action Potential

Phases of Cardiac Action Potential:

2. Rapid Depolarization: Triggered by the influx of Na+ ions through fast Na+ channels, causing a quick change in membrane potential.

3. Plateau Phase: Characterized by maintained depolarization due to a balance between Ca2+ inflow and K+ outflow; this phase is crucial for coordinating contraction and preventing tetany (sustained contraction).

4. Repolarization: Ca2+ channels close, leading to an increase in K+ outflow, restoring the resting membrane potential.

Duration: Approx. 0.3 seconds; includes a refractory period wherein the cardiac cells cannot be re-excited, allowing a time for the heart to fill.

Electrocardiogram (ECG)

P Wave: Represents depolarization of atrial fibers, indicating atrial contraction.

QRS Complex: Represents depolarization of ventricular fibers, which signifies ventricular contraction and is the most prominent part of the ECG tracing.

T Wave: Represents repolarization of ventricular fibers, indicating recovery and readiness for the next cardiac cycle.

Cardiac Cycle Phases

2. Atrial Systole:
   Triggered by depolarization of the SA node; the sinoatrial (SA) node serves as the heart's natural pacemaker.
   Atria contract, pushing approximately 130 mL of blood into the ventricles, effectively filling them before ventricular contraction. This phase is essential for maintaining optimal stroke volume.

3. Ventricular Systole:
   During this phase, increased pressure in ventricles occurs as they contract.
   Once pressures exceed 80 mmHg in the left ventricle and 20 mmHg in the right, the aortic and pulmonary valves open, leading to rapid ejection of blood into the aorta and pulmonary artery respectively.

4. Relaxation Period:
   Ventricles repolarize, and pressure decreases, causing all heart valves to close (isovolumetric relaxation).
   Atrioventricular (AV) valves reopen, allowing ventricles to fill with blood from the atria, thus initiating the next heart cycle.

Heart Sounds

First Sound (Lub): Occurs when AV valves close at the beginning of systole, marking the start of ventricular contraction.

Second Sound (Dub): Occurs when semilunar valves close at the beginning of diastole, indicating the end of ventricular contraction.

Audible heart sounds are critical for clinical assessment of heart health and function.

Cardiac Output (CO)

Definition: Amount of blood pumped by each ventricle per minute. It's a crucial parameter for assessing cardiac performance.

Calculation: CO = SV imes HR
where SV = Stroke Volume, the volume of blood ejected with each heartbeat, and HR = Heart Rate, the number of beats per minute.

Example: At a resting heart rate of 75 beats/min and an SV of 70 mL/beat, CO = 5250 mL/min (5.25 L/min), indicating adequate circulatory supply at rest.

Factors Affecting Stroke Volume (SV)

2. Preload: The degree of stretch of the ventricles; influenced by the volume of blood returning to the heart (venous return). Increased preload enhances stroke volume according to the Frank-Starling mechanism.

3. Contractility: The intrinsic strength of cardiac muscle contraction; influenced by sympathetic nervous stimulation and various hormones including epinephrine, which enhance myocardial contractility.

4. Afterload: The resistance the ventricles must overcome to eject blood, influenced by systemic vascular resistance and arterial pressure that dictate the workload on the heart.

Frank-Starling Law

Describes the relationship between ventricular filling (end-diastolic volume) and contraction strength; suggests that an increase in end-diastolic volume leads to a stronger contraction and thus increased stroke volume, optimizing cardiac efficiency.

Heart Rate Regulation

Autonomic Nervous System:
Sympathetic: Increases heart rate, often observed during stress responses or physical activity.
Parasympathetic: Decreases heart rate, promoting relaxation and recovery during rest.

Chronotropic Factors:
Positive factors (e.g., caffeine, certain medications) can increase HR, while negative factors (e.g., sedatives, some beta-blockers) decrease HR, illustrating the complexity of mechanisms influencing heart rate regulation.

Blood Pressure (BP)

Definition: The measure of the force exerted by circulating blood on the walls of the arteries, usually expressed in terms of systolic over diastolic pressure (e.g., 120/80 mmHg); critical for assessing cardiovascular health.

Normal BP: Approximately 120/80 mmHg; healthy blood pressure ranges support normal physiological functions.

Hypertension: Defined as blood pressure readings equal to or greater than 140/90 mmHg, posing risks for heart disease.

Hypotension: Defined as blood pressure levels equal to or lower than 90/60 mmHg, which can cause dizziness or fainting due to inadequate cerebral perfusion.

Mean Arterial Pressure (MAP)

Represents a more accurate indicator of blood flow to organs; calculated as: MAP = rac{(2 imes DBP) + SBP}{3}
Essential for assessing perfusion pressure during medical procedures and in critical care settings, ensuring adequate tissue oxygenation.

Factors Affecting Blood Pressure

2. Blood Volume: Any increase in blood volume elevates BP, while a decrease reduces it, stressing the importance of fluid balance in hemodynamic stability.

3. Neuronal Systems: Baroreceptors in the vascular system detect changes in blood pressure, sending signals to the central nervous system to regulate cardiovascular function.

4. Endocrine Factors: Hormones like renin-angiotensin-aldosterone system (RAAS) and adrenaline play significant roles in blood pressure regulation through fluid retention and vascular resistance.

5. Measurement Errors: Blood pressure readings can be influenced by observer bias, improper equipment calibration, and patient factors (position, activity). Accurate measurement is vital for diagnosis and management.

Summary of Key Concepts

A solid understanding of the cardiac cycle phases, the regulatory mechanisms of stroke volume and heart rate, and the factors influencing blood pressure is essential for comprehensive knowledge in cardiac physiology, paving the way for effective clinical assessment and intervention.

Contents

1. Definition

2. Phases

   • Systole

   • Diastole

3. Cardiac Action Potential

   • Phases of Cardiac Action Potential

   • Rapid Depolarization

   • Plateau Phase

   • Repolarization

   • Duration

4. Electrocardiogram (ECG)

   • P Wave

   • QRS Complex

   • T Wave

5. Cardiac Cycle Phases

   • Atrial Systole

   • Ventricular Systole

   • Relaxation Period

6. Heart Sounds

   • First Sound (Lub)

   • Second Sound (Dub)

7. Cardiac Output (CO)

   • Definition

   • Calculation

   • Example

8. Factors Affecting Stroke Volume (SV)

   • Preload

   • Contractility

   • Afterload

9. Frank-Starling Law

10. Heart Rate Regulation

    • Autonomic Nervous System

    • Chronotropic Factors

11. Blood Pressure (BP)

    • Definition

    • Normal BP

    • Hypertension

    • Hypotension

12. Mean Arterial Pressure (MAP)

13. Factors Affecting Blood Pressure

    • Blood Volume

    • Neuronal Systems

    • Endocrine Factors

    • Measurement Errors

14. Summary