Cardiovascular | Cardiac Cycle 2

 Heart Working As A Pump

Duration of cardiac cycle & heart rate:

Duration of cardiac cycle = 0.8 sec at heart rate = 70 beats/min. • When heart rate increases duration of cardiac cycle decreases. Diastole is more affected as compared to systole with rapid heart rate.

At heart rate = 180 / min, cardiac cycle duration 0.33 sec: (systole = 0.18 sec, diastole 0.15 sec).

QUESTION

At a very rapid heart rate: Cardiac output decreases, Why???

ANSWER

Because diastole becomes too short →→→ ventricular filling decreases → decrease stroke volume & decrease in cardiac output, in spite of increase in heart rate.

Pressure changes in Atria during the Cardiac Cycle:

Atrial systole duration = 0.11 sec

Atrial diastole duration = 0.7 sec

Atrial systole + Atrial diastole = 0.8 sec = cardiac cycle.

• Atrial diastole > Atrial systole, because basic function of atria is to receive blood from large veins & it can receive blood only when it is relaxed.

3 waves can be recorded from atria which represent atrial pressure changes:

⚫ a-wave, c-wave & v-wave (Seen as Jugular Venous Pulse, not a true pulse, but a reflection of pressure changes in right atrium..

• a-wave: Due to increase in atrial pressure during atrial systole.

• c-wave: Recorded at beginning of contraction of ventricle. During isovolumetric contraction, ventricular pressure increases Cusps of AV valves are pushed into atrial cavity → pressure rises in atria→ ascent of c-wave.

• The top of c-wave coincides with opening of semi-lunar valves (Aortic & Pulmonary).

• With opening of semi-lunar valves, 2nd phase starts, which is maximum ejection phase.

⚫ It is later on followed by iso-volumetric relaxation of ventricle muscle length increases → now AV valve is pulled to ventricular cavity → atrial cavity increases → pressure falls in the atria → descent of c-wave.

v-wave: Due to gradual increase in atrial pressure, resulting from venous filling of blood (from the venae cavae) into the atria, with closed AV valves → ascent of v- wave.

Top of v-wave coincides with opening of AV valves rapid inflow phase → decrease pressure in atria → descent of v- wave.

Right atrial pressure = Central Venous Pressure.

During most of cardiac cycle, this pressure remains almost zero.

During wave a, c & v→ pressure rises.

Otherwise remains almost zero.

4-6 mm Hg Rt. Atrium (during a, c, v) • 7-8 mm Hg → Lt. Atrium (during a, c, v)

JUGULAR VENOUS PULSE: (a, c, v waves)

Normally arteriolar pulse ends in arterioles & in veins no pulsation.

• But we can record pulsation in jugular vein, which is not a true pulse.

• It is just backward transmission of pressure changes in Rt. Atrium (a, c. v waves) transmitted in neck veins.

Significance of J.V.P:

⚫ ac interval coincides with PR interval of ECG.

ac interval increases in delayed AV conduction.

a waves are absent in: ATRIAL FIBRILLATION.

(a wave) > (c wave) in COMPLETE AV BLOCK.

'Giant a waves' in TRICUSPID & PULMONARY STENOSIS.

Pulsating Neck Veins in CCF (Congestive Cardiac Failure).

Volume changes in Ventricles during Cardiac Cycle: Beginning of ventricular systole:

Ejection Phases: (Maximum Ejection Phase & Reduced Ejection Phase)

Iso-volumic Relaxation Phase:

Rapid Inflow Phase:

Diastasis / Slow Inflow Phase:

Atrial Systole:

Beginning of ventricular systole:

• Ventricle is full of blood (collected during previous diastole, EDV = 120 ml) at the onset of ventricular systole.

• This much volume is there at the start of ventricular systole.

• With this volume onset of Isovolumic/ Isometric Contraction, with no change in blood volume.

Ejection Phases:

Ejection Phase &

(Maximum Reduced Ejection Phase)

Maximum Ejection Phase:

2/3 of Stroke Volume (total = 70 ml) is ejected out.

Reduced Ejection Phase:

Remaining 1/3 is ejected out.

After ejection phases, the volume of blood left behind is ESV = 50 ml.

Diastolic phases;

Iso-Volumic Relaxation Phase:

• No change in blood volume occurs.

Rapid Inflow Phase:

2/3 of ventricular filling.

Diastasis / Slow inflow phase:

Only slight filling occurs.

Atrial Systole:

Remaining 1/3 filling (30%). Now filling of ventricles is complete & EDV of 120 ml is left.

Sequence of systole in chambers of heart:

Right atrial systole begins earlier as compared to left atrial systole.

Left ventricular systole begins earlier as compared to right ventricular systole.

But blood ejection from right ventricle → pulmonary artery, starts earlier as compared to ejection from left ventricle aorta, BECAUSE pressure in pulmonary artery < Aortic pressure.

Closure & Opening of Heart Valves:

AV VALVES:

• Are closed at the beginning of Isovolumic contraction phase.

Are open at the beginning of Rapid Inflow phase.

AV valve closure is slow & soft & does not require backward flow of blood. Cusps of AV valves are soft & thin because they are not

subjected to increase in pressure & rapid blood flow.

SEMILUNAR VALVES:

Are closed at the beginning of Isovolumic relaxation phase.

Cusps of these valves are thick & heavier (as they are subjected to increased pressure & rapid blood flow).

Their closure is rapid & requires backward flow of blood (incisura in case of Aortic valve).

Closure & Opening of both AV & Semilunar Heart Valves:

Forward pressure gradient → opening. Backward pressure gradient → closure.

• AV valves prevent, leakage of blood from ventricle atria, during ventricular systole (when pressure rises in ventricle).

Semilunar valves prevent leakage of blood from large arteries → ventricles, during ventricular diastole (when pressure falls in ventricle)

Preload & Afterload

PRELOAD

Degree of tension on muscle when it begins to contract.

AFTERLOAD

• Load against which muscle exerts its contractile force.