Lecture #28: Physiology of Cardiac Cycle Part I- Pressure and Volume Changes

What is the cardiac cycle?

All electrical and mechanical events occurring within one heartbeat; it consists of diastole and systole.

What is the normal resting adult heart rate?

60–100 beats/min.

Why does atrial contraction occur before ventricular contraction?

Signal transmission is delayed at the AV node by >0.1 second, allowing the atria to pump blood into the ventricles before strong ventricular contraction begins.

At rest, what fraction of the cardiac cycle is spent in diastole versus systole?

Approximately 2/3 of the cardiac cycle is spent in diastole and 1/3 in systole.

How does increasing activity affect the duration of systole and diastole?

Increasing activity decreases the time spent in diastole and increases the relative time spent in systole.

What ECG event precedes atrial systole?

The P wave, representing atrial depolarization, always precedes atrial contraction.

What is the relationship between the P wave and the atrial pressure "a" wave?

The P wave precedes the "a" wave of the atrial pressure curve by approximately 0.1 second.

How much does atrial contraction contribute to ventricular filling?

Atrial contraction contributes approximately 10 mL out of about 140 mL of LV blood and can function as a "primer pump," contributing as much as 20%.

When does atrial diastole occur?

Atrial diastole occurs during ventricular systole and much of ventricular diastole.

What drives blood flow from the veins into the atria during atrial diastole?

A pressure gradient: pressure in the vena cavae and pulmonary veins is greater than pressure in the RA and LA, respectively.

What causes blood to flow from the atria into the ventricles?

When AV valves open, a small diastolic pressure difference in which atrial pressure exceeds ventricular pressure allows blood to flow into the ventricles.

When does most atrial blood enter the ventricles?

Most atrial blood enters the ventricles during atrial diastole rather than during active atrial contraction.

What causes the atrial pressure "a" wave?

Atrial contraction during atrial systole causes a small increase in atrial pressure.

What causes the atrial pressure "c" wave?

A slight increase in atrial pressure caused by atrial compression during ventricular contraction; it occurs just after the peak of the QRS complex.

What does the "x" descent of the atrial pressure curve represent?

Atrial relaxation.

What causes the atrial pressure "v" wave?

With the AV valves closed during ventricular systole, incoming blood gradually fills the atria and slowly increases atrial pressure, producing the "v" wave.

What does the "y" descent represent?

Ventricular filling; when the AV valves open, atrial blood flows into the ventricles and atrial pressure falls.

What ECG interval corresponds approximately to ventricular systole?

Ventricular systole occurs between the start of the QRS complex and approximately the beginning of the end of the T wave.

What is end-diastolic volume (EDV)?

The volume of blood in the ventricles at the start of ventricular contraction, or systole.

What are the three phases of ventricular systole?

Isovolumetric contraction, rapid ejection, and reduced ejection.

What happens to the valves during isovolumetric ventricular contraction?

Both AV and semilunar valves are closed, making the ventricles closed, blood-filled chambers.

What happens to ventricular pressure and volume during isovolumetric contraction?

Ventricular pressure rapidly increases while ventricular volume remains constant because all valves are closed and blood cannot be compressed.

What type of contraction occurs during isovolumetric contraction?

Isometric contraction; ventricular pressure increases without a change in ventricular volume.

What is the role of papillary muscles at the beginning of ventricular systole?

Papillary muscles contract and pull the AV valves inward toward the ventricles, helping prevent excessive backward bulging of the valves toward the atria.

What must the ventricles accomplish during isovolumetric contraction before ejection begins?

They must develop sufficient pressure to overcome their respective afterloads, such as aortic and pulmonary artery pressures.

What triggers the rapid ventricular ejection phase?

Rapid ejection begins when rising ventricular pressure exceeds arterial pressure, forcing the semilunar valves open.

What is the valve state during ventricular ejection?

The semilunar valves are open and the AV valves are closed.

What happens to ventricular pressure during rapid ejection?

Ventricular pressure continues to increase even after the semilunar valves open because the ventricles are still contracting.

What type of contraction occurs during rapid ventricular ejection?

Isotonic contraction.

How does aortic pressure change during rapid ventricular ejection?

Aortic pressure increases because blood is being forced from the LV into the aorta; during rapid ejection, LVP exceeds aortic pressure.

What happens to ventricular volume during the ejection phase?

Ventricular volume decreases as blood is ejected from the ventricles into the aorta and pulmonary artery.

What marks the transition from rapid to reduced ventricular ejection?

Left ventricular pressure falls slightly below aortic pressure; blood continues to be ejected because of the inertia imparted to the blood by ventricular contraction.

What drives blood ejection during reduced ventricular ejection?

The inertia previously imparted to the blood by ventricular contraction becomes the driving force rather than the pressure difference.

Which ECG wave coincides with reduced ventricular ejection?

The T wave.

What terminates the reduced ejection phase?

It ends when ventricular pressure falls below the level required to keep the aortic and pulmonary valves open.

What are the three phases of ventricular diastole?

Isovolumetric relaxation, rapid ventricular filling, and reduced ventricular filling or diastasis.

What event marks the beginning of ventricular diastole?

Closure of the semilunar valves marks the beginning of ventricular diastole; the AV valves are also closed initially.

What happens during isovolumetric ventricular relaxation?

The ventricles relax and ventricular pressure falls while ventricular volume remains constant because both the AV and semilunar valves are closed.

What ECG event precedes the fall in ventricular pressure during relaxation?

The fall in ventricular pressure follows T-wave repolarization.

What is end-systolic volume (ESV)?

The volume of blood remaining in the ventricles at the end of systole, or beginning of diastole; the ventricles do not completely empty during ejection.

What triggers rapid ventricular filling?

Ventricular pressure falls below atrial pressure, creating a pressure gradient that opens the AV valves and allows blood to rapidly enter the ventricles.

What is the valve state during rapid ventricular filling?

The AV valves are open and the semilunar valves are closed.

What is diastasis?

The reduced or slow ventricular filling phase following rapid filling; blood flows from the veins through the relaxed atria into the relaxed ventricles.

What ECG event ends diastasis?

The P wave of the next cardiac cycle ends diastasis.

What ECG event marks the end of ventricular diastole?

The appearance of the R wave marks the end of ventricular diastole.

What causes the first heart sound (S1)?

Closure of the AV valves at the beginning of ventricular systole.

What causes the second heart sound (S2)?

Closure of the semilunar valves at the beginning of ventricular diastole.

What is the S3 heart sound associated with?

S3 is associated with the rapid filling phase of ventricular diastole and is frequently heard in normal children.

What is the S4 heart sound associated with?

S4 is associated with atrial systole at end-diastole when the atria contract against a hypertrophied ventricle.

What causes the dicrotic notch (incisura) in the aortic pressure curve?

A slight backflow of aortic blood that fills the cusps of the semilunar valve as it closes, producing the dicrotic notch.