CARDIAC CYCLE - II -Dr. Chintan

ATRIAL PRESSURE CHANGESAtrial pressure waves (JVP)

a wave: Atrial contraction. Rt. Atria; 4 – 6 mmHg, Lt. Atria; 7 – 8 mmHg. A – Atrial systole – x wave

c wave: Bulging of A – V valves because of ↑ing pressure in the ventricles. C – Contraction of ventricle - x’ wave

v wave: At the end of ventricular contraction, slow flow of blood into the atria from the veins while A – V valves are closed. V – Venous filling – y wave

ECG RELATION WITH CARDIAC CYCLEThe P wave is caused by spread of depolarization through the atria,

and this is followed by atrial contraction, which causes a slight rise in the atrial pressure curve immediately after the electrocardiographic P wave.

About 0.16 second after the onset of the P wave, the QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the ventricles and causes the ventricular pressure to begin rising. Therefore, the QRS complex begins slightly before the onset of ventricular systole.

The ventricular T wave in the ECG represents the stage of repolarization of the ventricles when the ventricular muscle fibers begin to relax. Therefore, the T wave occurs slightly before the end of ventricular contraction.

TIMING Right atrial systole precedes left atrial systole, and contraction of the right

ventricle starts after that of the left.

Since pulmonary arterial pressure is lower than aortic pressure, right ventricular ejection begins before left ventricular ejection.

During expiration, the pulmonary and aortic valves close at the same time; but during inspiration, the aortic valve closes slightly before the pulmonary.

The slower closure of the pulmonary valve is due to lower impedance of the pulmonary vascular tree.

The outputs of the two ventricles are equal, but transient differences in output during the respiratory cycle occur in normal individuals.

LENGTH Cardiac muscle has the unique property of contracting and repolarizing faster when the

heart rate is high and the duration of systole decreases from 0.3 s at a heart rate of 65 to 0.16 s at a rate of 200 beats/min.

The duration of systole is much more fixed than that of diastole, and when the heart rate is increased, diastole is shortened to a much greater degree.

This fact has important physiologic and clinical implications. It is during diastole that the heart muscle rests, and coronary blood flow to the subendocardial portions of the left ventricle occurs only during diastole.

Most of the ventricular filling occurs in diastole. At heart rates up to about 180, filling is adequate as long as there is sufficient venous return, and cardiac output per minute is increased by an increase in rate.

However, at very high heart rates, filling may be compromised to such a degree that cardiac output per minute falls and symptoms of heart failure develop.

EDV, SV, ESV, EFEnd diastolic volume: 110 to 120ml

Stroke volume: 70ml

End systolic volume: 40 to 50ml

Ejection fraction: 60 %

When the heart contracts strongly, the end-systolic volume can be decreased to as little as 10 to 20 milliliters.

When large amounts of blood flow into the ventricles during diastole, the ventricular end diastolic volumes can become as great as 150 to 180 milliliters in the healthy heart.

By both increasing the end-diastolic volume and decreasing the end-systolic volume, the stroke volume output can be increased to more than double normal.

HEART VALVESThe A-V valves prevent backflow of blood from the ventricles to the atria during systole, and the semilunar Valves prevent backflow from the aorta and pulmonary arteries into the ventricles during diastole.

These valves open passively and close somewhat actively. That is, they close when a backward pressure gradient pushes blood backward, and they open when a forward pressure gradient forces blood in the forward direction.

Thin, filmy A-V valves require very low backflow to cause closure, whereas the much heavier semilunar valves require rather rapid backflow for a few milliseconds.

Difference

HEART VALVESFirst, The high pressures in the arteries at the end of systole cause the semilunar valves to snap to the closed position, in contrast to the much softer closure of the A-V valves.

Second, because of smaller openings, the velocity of blood ejection through the aortic and pulmonary valves is far greater than that through the much larger A-V valves.

Third, because of the rapid closure and rapid ejection, the edges of the aortic and pulmonary valves are subjected to much greater mechanical abrasion than are the A-V valves.

HEART SOUNDSWhen the valves close, the surrounding fluids vibrate under the influence of sudden pressure changes, giving off sound that travels in all directions through the chest.

When the ventricles contract, one first hears a sound caused by closure of the A-V valves. The vibration is low in pitch and relatively long-lasting and is known as the first heart sound.

When the aortic and pulmonary valves close at the end of systole, one hears a rapid snap because these valves close rapidly, and the surroundings vibrate for a short period. This sound is called the second heart sound.

HEART SOUNDS

The first heart sound identifies the onset of ventricular systole.The second heart sound identifies the onset of ventricular diastole.The third heart sound – rapid filling phase of ventricleThe fourth heart sound – atrial systolePhonocardiogram: The recording of the auscultatory cardiac activity, using a transducer placed on the thorax.MURMUR

AV valves*Semilunar valves†

Status of ventricles and atria

1.Early diastole open closed

• whole heart is relaxed

• ventricles are expanding and filling (passive filling, ~80% of volume)

2. Atrial systole open closed• atria contract and pump blood• additional 10–40% filling of ventricles due to active contraction of atria[3]

3.Isovolumic ventricular contraction

closed closed• ventricular myocytes begin to contract• ventricle volume unchanged

4.Ventricular ejection

closed open• ventricles fully contract• pump blood to rest of body

5.Isovolumic ventricular relaxation

closed closed• ventricles relax• ventricle volume unchanged• atria expand and are filling

* AV (atrioventricular) valves:

1) mitral valve – between the left atrium and the left ventricle2) tricuspid valve – between the right atrium and the right ventricle

† Semilunar valves:

1) aortic valve – between the left ventricle and the aorta2) pulmonic valve – between the right ventricle and the pulmonary artery

THANQ