BIPN 100 - N1 Prelecture 25

Cardiac Cycle Overview

The cardiac cycle involves pressure, volume, and electrical activity in the heart.
Understanding the cardiac cycle requires knowing when valves open and close, when blood is ejected, and how blood flows through the heart.
Key equations:
- Stroke Volume (SV): Blood ejected with each beat.
- Heart Rate (HR): Beats per minute.
- Cardiac Output (CO): CO = SV \times HR

Wiggers Diagram

The Wiggers diagram integrates EKG, pressure changes, and volume changes during the cardiac cycle.
The x-axis represents time in milliseconds; the y-axis represents pressure.
One complete cycle (R to R interval) is approximately 800 milliseconds, corresponding to a heart rate of about 75 beats per minute.
Resting heart rate is influenced by parasympathetic activity and the SA node.

EKG and Pressure Relationship

P Wave:
- Represents atrial depolarization and occurs before left atrial pressure increases.
- SA node activation precedes the P wave.
- Indicates the firing of atrial contractile cells.
Atrial Contraction:
- Follows the P wave, increasing atrial pressure.
- Completes the final 20% of ventricular filling after passive filling has already contributed 80%.
QRS Complex:
- Represents ventricular depolarization, masking atrial repolarization (TP wave).
- Indicates the activation of ventricular contractile cells.
Ventricular Contraction:
- Ventricular pressure increases sharply after the QRS complex.
- When ventricular pressure exceeds atrial pressure, the atrioventricular (AV) valves close, producing the first heart sound (S1 or "lub").
- The AV valves are the bicuspid (mitral) and tricuspid valves.

Isovolumetric Contraction

The period where the ventricles contract without changing volume because all valves are closed.
Ventricular pressure increases to exceed aortic pressure to open the semilunar valves.

Ejection Phase

Semilunar valves (aortic and pulmonic) open, and blood is ejected into the aorta and pulmonary artery.
Stroke volume is the amount of blood ejected during this phase.
End-diastolic volume (EDV) is the volume at the end of diastole (point e), and end-systolic volume (ESV) is the volume at the end of systole (point f). Stroke volume is calculated as EDV - ESV.
The contraction is neither purely isovolumetric nor isotonic; it's a mixed contraction with changing volume and increasing pressure.
Aortic and ventricular pressures are closely matched during ejection.

Ventricular Repolarization and Relaxation

T Wave:
- Occurs just before ventricular relaxation starts.
- The ventricular contractile cells are in the calcium plateau phase during the T wave, lasting 100-200 milliseconds.
Relaxation Phase:
- Aortic or semilunar valves close, creating the second heart sound (S2 or "dub").
- Aortic pressure shows a notch as blood rebounds against the closed semilunar valve, causing a temporary pressure increase.

Isovolumetric Relaxation

All valves are closed, and ventricular pressure decreases without volume change.
This phase continues until ventricular pressure drops below atrial pressure, allowing the AV valves to open.

Ventricular Filling

AV valves open, and blood flows passively from the veins to the atria and then into the ventricles.
This passive filling phase occurs during ventricular diastole.

Cardiac Cycle Phases

Atrial Systole: Atrial contraction.
Ventricular Systole: Ventricular contraction, including:
- Isovolumetric contraction.
- Ejection phase.
Isovolumetric Relaxation: All valves are closed, and the ventricles relax.
Ventricular Diastole: Includes:
- True diastole when all heart parts are relaxed.
- Critical for ventricular refilling.

Heart Rate and Filling Time

Shortened diastole (e.g., during increased sympathetic activity) reduces ventricular filling and cardiomyocyte relaxation.
Increased heart rate shortens the R-R interval, affecting filling time.
The heart's contraction strength may compensate for reduced filling.

Key Points on the Wiggers Diagram

End-diastolic volume (EDV).
Systolic blood pressure (maximum arterial pressure).
Ability to draw and label the Wiggers diagram from memory.

Cardiac Output (CO)

Cardiac output is measured in milliliters per minute.
CO = HR \times SV
Blood flow is dependent on velocity and cross-sectional area.
Analogous to electrical current, where:
- Q = \frac{P}{R}, where Q is flow, P is pressure, and R is resistance.
- Cardiac output (CO) is equivalent to current.
- Pressure is the difference between aortic and vena cava pressure (mean arterial pressure).
- Resistance relates to blood viscosity, vessel length, and radius.

Resistance Factors

Blood Viscosity:
- Depends on plasma and hemoglobin concentration (hematocrit percentage).
- Higher hematocrit increases viscosity.
Vessel Tone:
- Vasoconstriction increases resistance.
- Vasodilation decreases resistance.
- Primarily controlled by the sympathetic nervous system.
Total Peripheral Resistance (TPR) / Systemic Vascular Resistance (SVR):
- Determined by arterial tone and number of open capillary beds.
- CO = \frac{MAP}{TPR}, where MAP is mean arterial pressure.

Blood Pressure

Blood pressure measurements include systolic and diastolic pressures.
Systolic Blood Pressure: Maximum pressure during ventricular contraction.
Diastolic Blood Pressure: Minimum pressure during ventricular relaxation.

Determining Systolic and Diastolic Pressures on Wiggers Diagram

Diastolic blood pressure occurs just before the opening of the semilunar valves during isovolumetric contraction.
Systolic blood pressure is the maximum pressure during blood ejection into the aorta.

Clinical Indicators

Diastolic Blood Pressure:
- Indicates changes in resistance.
- Higher diastolic pressure suggests increased constriction of arterioles and veins.
Pulse Pressure:
- Difference between systolic and diastolic blood pressure.
- Indicates contractility.
Mean Arterial Pressure (MAP):
- Average pressure in the aorta and large arteries.
- MAP = \text{Diastolic BP} + \frac{1}{3} \times \text{Pulse Pressure}
- Reflects the voltage equivalent, dependent on diastolic blood pressure due to the longer duration of diastole.

Cardiac Muscle Tension

Cardiac muscle is stiffer than skeletal muscle.
Cardiac muscle operates on the ascending part of the active tension curve, ensuring optimal overlap of myosin heads and actin.
- Inotropic state (contractility) affects the total tension curve without changing the passive tension curve.
More contractility results in higher pressure.

Preload and Afterload

Preload:
- End-diastolic volume (EDV).
- Determined by venous return, influenced by respiratory rate, skeletal muscle activity, and sympathetic control of veins.
- Frank-Starling Law of the Heart: The heart pumps all the blood that returns to it within physiological limits.
Afterload:
- Tension or pressure required to open the semilunar valves.
- Inversely proportional to cardiac output.
- Affected by arterial blood resistance and diastolic volume.
- Increased diastolic blood pressure increases afterload.

Cardiac Cycle Loop

Filling Phase (1-2): Passive filling following the passive curve; atrial contraction completes the last 20% of filling.
Isovolumetric Contraction (2-3): QRS complex fires; pressure increases without volume change; afterload is reached at point 3 (diastolic blood pressure).
Ejection (3-4): Semilunar valves open; blood is ejected; peak pressure is systolic blood pressure; stroke volume is ejected. Blood is ejected out as volume is decreasing.
Relaxation (4-1): T wave fires; relaxation occurs until atrial pressure is surpassed.

Key Points in the Cardiac Cycle Loop

Point 1: Mitral (AV) valve opens.
Point 2: End-diastolic volume (preload); mitral valve closes, initiating isovolumetric contraction.
Point 3: Afterload (diastolic blood pressure); semilunar valve opens.
Point 4: Aortic valve closes.
Stroke volume is the distance between points 3 and 4.
Ejection fraction can be calculated from stroke volume and end-diastolic volume.