The Cardiac Cycle | Events | Pressure & Volume changes

Understanding the Cardiac Cycle

Introduction to the Cardiac Cycle

• The speaker introduces the topic of the cardiac cycle, highlighting its complexity for medical students who often memorize rather than understand it.

• Emphasizes that understanding the sequence of events in the cardiac cycle will simplify learning and retention for exams.

Overview of Content

• The video will cover definitions, stages, duration, and a graphical representation known as a "vigorous diagram."

• The vigorous diagram illustrates changes in pressure and volume within ventricles alongside electrocardiographic changes during the cardiac cycle.

Structure of Discussion

• The discussion is organized into subheadings: definition, systole vs. diastole, duration, atrial events, and ventricular events.

• Focus on pressure and volume changes primarily in the left ventricle throughout each cycle.

Defining Key Terms

What is the Cardiac Cycle?

• Defined as all cardiac events occurring from one heartbeat to the next; includes mechanical (pressure/volume changes and valve actions) and electrical (ECG changes) events.

Systole vs. Diastole

• Systole refers to heart contraction where blood is pumped from atria to ventricles or from ventricles to arteries.

• Diastole indicates relaxation when chambers fill with blood; atria receive blood from veins while ventricles fill from atria.

Duration of Cardiac Cycle

Timing of Events

• Atrial and ventricular events occur simultaneously; when one contracts, the other relaxes.

Understanding the Cardiac Cycle

Duration of Atrial and Ventricular Events

• The atrial systole lasts for 4.1 seconds, while atrial diastole takes up the remaining 0.7 seconds, indicating a longer duration for diastole compared to systole.

• Ventricular systole occurs over 0.3 seconds, followed by ventricular diastole lasting 0.5 seconds, contributing to the overall cardiac cycle duration.

Relationship Between Heart Rate and Cardiac Cycle

• The duration of the cardiac cycle is inversely proportional to heart rate; an increase in heart rate results in a decrease in cycle duration.

• As heart rate increases, both systolic and diastolic durations decrease, but diastolic reduction is more significant, leading to less time for heart relaxation.

Impact on Stroke Volume and Cardiac Output

• Reduced diastolic time affects the filling of the heart; insufficient relaxation leads to decreased stroke volume (blood pumped per beat) and ultimately lowers cardiac output (blood pumped per minute).

• The key takeaway is that increased heart rates lead to shorter cardiac cycles which adversely affect blood volume ejected from the ventricles.

Overview of Cardiac Cycle Stages

• Total duration of the cardiac cycle is approximately 0.8 seconds: with atrial events also taking this time frame into account.

• Atrial events consist of atrial systole (0.1 seconds) and atrial diastole (0.7 seconds), while ventricular events include ventricular systole (0.3 seconds) and ventricular diastole (0.5 seconds).

Atrial Events During Diastole

• Atrial events can be categorized into two main types: atrial systole and ongoing atrial diastole during which important ventricular events occur.

• During atrial diastole, blood fills the right atrium from superior/inferior vena cava and left atrium from pulmonary veins.

Mechanism of Atrial Systole

• In a simplified diagram of the heart, when the atrioventricular valves are open, about 80% of blood flows passively from the atria into ventricles without active contraction.

• Only 20% requires active contraction during atrial systole to push remaining blood into ventricles.

Changes in Ventricular Pressure and Volume

• As blood enters ventricles from the atria during contraction, there is a slight increase in ventricular pressure due to incoming volume.

Atrial and Ventricular Events in Cardiac Cycle

Atrial Contraction and Its Effects

• During atrial contraction, there is a slight increase in ventricular pressure as 20% of blood from the atria enters the ventricles, leading to a corresponding increase in ventricular volume.

• The contraction of the atria results in the production of the fourth heart sound, indicating active pumping into the ventricles.

• Atrial depolarization occurs during this phase, represented by the P wave on an ECG, marking significant electrical activity associated with atrial systole.

• The completion of atrial events signifies that 20% of blood has been pumped into the ventricles, setting up for subsequent ventricular changes.

Transition to Ventricular Systole

• Following atrial systole, ventricular pressure begins to rise as blood fills the ventricles; this triggers closure of the atrioventricular (AV) valves.

• Closure of AV valves produces the first heart sound (S1), indicating that ventricles are now closed chambers preparing for contraction.

• As ventricles contract without changing volume (isovolumic contraction), tension increases while volume remains constant due to closed valves.

• This phase coincides with QRS complex on ECG, representing ventricular depolarization.

Ejection Phase Dynamics

• As ventricular pressure exceeds that in major vessels (pulmonary artery and aorta), semilunar valves open allowing ejection of blood from ventricles.

• The ejection phase is divided into rapid and slow ejection phases; rapid ejection occurs immediately after valve opening where 70% of blood is expelled quickly.

• During slow ejection, remaining 30% is pushed out; however, both ventricular pressure and volume begin to decrease as blood exits.

• Key events during ventricular systole include S1 sound from AV valve closure, isovolumetric contraction phase, and overall ejection phase dynamics.

Understanding the Cardiac Cycle

Phases of Ventricular Diastole

• As pressure inside the ventricles decreases, high pressure in the pulmonary artery and aorta risks blood seeping back into the ventricles. This is prevented by the closure of valves.

• The closure of both semilunar valves produces the second heart sound (S2), marking an important transition in the cardiac cycle.

• During this phase, known as isovolumic relaxation, ventricular pressure falls rapidly while volume remains unchanged due to closed chambers.

• A significant drop in ventricular pressure compared to atrial pressure leads to the opening of atrioventricular (AV) valves, allowing blood flow from atria to ventricles.

Filling Phase of Ventricular Diastole

• The filling phase occurs when blood from fully filled atria gushes into relaxed ventricles. This phase is crucial for preparing for subsequent contractions.

• The filling phase consists of rapid filling (initial influx of blood), followed by slow filling; both are passive processes without active contraction from the atria.

• Approximately 80% of blood enters ventricles passively during rapid and slow filling phases; remaining 20% requires active contraction known as atrial systole.

Atrial Systole and Heart Sounds

• Atrial contraction pushes remaining blood into ventricles, contributing to what some texts refer to as rapid active filling. This process generates a fourth heart sound (S4).

• The T wave on an ECG indicates ventricular repolarization during isovolumetric relaxation, highlighting electrical activity associated with physical changes in the heart.

Understanding Pressure and Volume Changes

• The "vigor diagram" illustrates left ventricle pressure and volume changes throughout different stages of the cardiac cycle alongside ECG changes and heart sounds.

• Key stages include:

• Atrial Systole: Contraction results in slight increases in ventricular pressure and volume.

• Fourth Heart Sound: Produced during this stage due to atrial contraction coinciding with P wave on ECG indicating depolarization.

Understanding the Cardiac Cycle

Atrial Systole and ECG Correlation

• The fourth heart zone is produced during atrial systole, which coincides with the P wave of the ECG. This phase shows a slight increase in pressure and volume within the atria.

Ventricular Filling and First Heart Sound

• After atrial systole, the ventricles fill completely, leading to high ventricular pressure that causes closure of the atrioventricular valves. This closure produces the first heart sound (S1), marking the beginning of ventricular systole.

Isovolumic Contraction Phase

• Ventricular systole aligns with the QRS complex on the ECG. During this isovolumic contraction phase, ventricular pressure rises while blood volume remains static, indicating no change in blood volume despite increasing pressure.

Rapid Ejection Phase

• Following isovolumic contraction, semilunar valves open due to increased ventricular pressure, allowing rapid ejection of blood into arteries. Here, ventricular pressure increases while volume decreases as blood exits into the aorta or pulmonary artery.

Slow Ejection Phase and Transition to Diastole

• The rapid ejection phase accounts for 70% of stroke volume; subsequent slow ejection contributes to an additional 30%. As this occurs, ventricular pressure begins to fall while volume continues decreasing until diastole starts, corresponding with the T wave on ECG.

Closure of Semilunar Valves and Second Heart Sound

• After both phases of ejection, semilunar valves close producing the second heart sound (S2). At this point, ventricles are closed chambers experiencing continuous relaxation as internal pressure drops without a change in volume.

Isovolumic Relaxation Phase

• In this phase, known as isovolumic relaxation, ventricular pressure decreases but there’s no change in blood volume until atrioventricular valves reopen for filling.

Rapid and Slow Filling Phases

• Once atrioventricular valves open again, rapid filling occurs with minimal increase in pressure but significant rise in ventricle volume. This phase leads to approximately 80% of blood entering from atria into ventricles passively.

Return to Atrial Systole

• Following slow filling (the seventh phase), remaining 20% of blood enters ventricles during another round of atrial systole. Understanding these sequential events aids comprehension of cardiac function throughout each cycle.

Summary Points for Examination Preparation

• Key points include recognizing stages starting from atrial systole marked by P wave and fourth heart sound through various phases including isovolumetric contractions and both ejections leading back to diastolic filling.

Understanding the Cardiac Cycle

Phases of Ventricular Function

• During the slow ejection phase, ventricular volume decreases as ventricular pressure falls. High aortic pressure may cause blood to seep back into the ventricles, leading to the closure of semilunar valves.

• The closure of semilunar valves produces the second heart sound (S2). At this point, the ventricle enters diastole, where contraction occurs without a change in volume.

• This phase is known as isovolumic relaxation. Here, ventricular pressure decreases while volume remains constant until atrioventricular valves open.

Filling Phases and Heart Sounds

• After isovolumetric relaxation, rapid and slow filling phases occur passively. Rapid filling results in the third heart sound and corresponds with the T wave on an ECG.

• Approximately 80% of blood moves from atria to ventricles during passive filling; 20% comes from atrial contraction (atrial systole).

Summary of Cardiac Cycle Dynamics