Cardiac cycle Lecture by Dr.Mohammed Sharique Ahmed QuadriAssistant professor ,Physiology

ObjectivesTo know about different phases of cardiac cycle To understand the volume, pressure and electrical changes during the cardiac cycleTo emphasize the inter-relationship between all these changes

Functional anatomy of heart

Cardiac cycle Cardiac events that occur from beginning of one heart beat to beginning of next are called the cardiac cycle .

Cardiac cycle consist of alternate period of systole( contraction/emptying) & diastole(relaxation /filling )

Contraction results from spread of excitation across the heart & relaxation follows subsequent repolarisation of cardiac musculature.

CARDIAC CYCLE

Cardiac cycle time : 0.8 second when heart rate is 75 beats per minute.

Atria and Ventricle go through separate cycles of systole and diastole.

When we use the term Systole and Diastole, we refer to ventricles, until unless specified for atria.

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We will discuss mechanical & electrical changes during Cardiac CycleMechanical changes:Phases of cardiac cyclePressure changes during cardiac cycleVolume changes during cardiac cycleHeart soundsElectrical ChangesElectrocardiogram (ECG)*

CARDIAC CYCLEAtrial Cycle (0.8 Sec.)Ventricular Cycle (0.8 Sec.)

ADAS

VSVD

0.7 Sec.0.1 sec.

0.3Sec.0.5 sec.

AS Atrial Systole; AD Atrial Diastole ; VS Ventricular systole; VD Ventricular diastole

*ECGECG ( Electrocardiogram) These electrical events can be recorded by putting electrodes on the surface of bodyP wave Atrial depolarizationQRS - Ventricular DepolarizationT wave Ventricular Repolarization

We will discuss ECG in detail in coming lectures.

Atrial systole 0.1 secondAtrial diastole 0.7 secondVentricular systole 0.3 secondIsovolumic contraction 0.01 secondsRapid ejection periodSlow ejection periodVentricular diastole 0.5 secondsIsovoumic relaxation 0.02 secondsRapid fillingSlow filling (Diastasis)Atrial contraction

Events during cardiac cycle

Events during cardiac cycle (continued)Ventricular filling:When ventricular pressure falls below atrial pressure AV valves opens & ventricular filling begins The first part of this period is known as first ventricular filling phase (3rd heart sound)Large part of ventricular filling takes place during this period

Events during cardiac cycle (continued)Ventricular filling:In the next phase of diastole ventricular filling slows down .As the ventricles are already full to large a large extent & ventricular pressure slowly rises This phase is known as diastasis or slow inflow phase Amount of filling during this period is minimum

Events during cardiac cycle (continued)Ventricular filling:Than comes the last phase of ventricular filling known as last rapid filling This phase corresponds with atrial systole (contributes to 20-30% of ventricular filling)

Events during cardiac cycle Atrial systole:Caused by atrial depolarizationLeft ventricle is relaxed As AV valves are open ,ventricles are filling with blood even prior to atrial systoleAtrial systole causes further increase in ventricular volume Through out atrial contraction atrial pressure exceeds ventricular pressure so AV valves remain open

Events during cardiac cycle (continued)Ventricular systole :After the atrial contraction impulse travels through AV node & specialized conduction system to excite the ventricle.As the ventricular contraction begins pressure in the ventricles exceeds that of atria and this backwards pressure forces the AV valves to close

Events during cardiac cycle (continued)Isovolumetric ventricular contraction:To open the semilunar valves ventricular pressure must exceeds that of aortic & pulmonary pressure After closing of AV valves & before opening of Semlunar valves ,a brief period of time when ventricles contracts as a closed chamber Ventricular pressure continues to increase

Events during cardiac cycle (continued)

Events during cardiac cycle (continued)Ventricular diastole (repolarization):As the ventricles starts to relax on repolarisation ,ventricular pressure falls below aortic & pulmonary pressure & semilunar valves closes .

Events during cardiac cycle (continued)Isovolumetric relaxation:when semilunar valves closes AV valves is not yet open because pressure in vetricles still exceeeds that of atria.Therefore all valves are once again closed for short period of time when ventricles are relaxing

Tachycardia, Effect on Cardiac CycleDuring Tachycardia, when heart rate is increased, both systole and diastole time decreases.Diastole length (time) is shortened much more than systole length. Therefore it reduces the time for ventricular relaxation and filling.*

Summary of events during cardiac cycle

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2. PRESSURE CHANGES DURING CARDIAC CYCLE*

Events of cardiac cycle

Pressure changes during cardiac cycleRt- Atrium = 0-7mm Hg.Lt- Atrium = 2-10 mm Hg.Rt Ventricle = 25 / 6(2-8) mm Hg.Lt ventricle = 120 / 8(2-10) mm Hg.Aorta = 120 / 80 mm Hg.Pul. Artery = 25 / 10 mm Hg.

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3. VOLUME CHANGES DURING CARDIAC CYCLE*

Define Stroke Volume (SV)

Define End Diastolic Volume (EDV)

Define End Systolic Volume (ESV)

Define Ejection Fraction (EF)*

Stroke Volume: It is a volume of blood pumped out by each ventricle per beat. It is about 70 - 80 ml.Stroke volume (SV) = EDV ESV

End Diastolic Volume: Volume of blood in each ventricle at the end of diastole. It is about 120 130 ml.

End Systolic Volume: Volume of blood in each ventricle at the end of Systole. It is about 50 to 60 ml*

Volume changes during cardiac cycle*

Events of cardiac cycle showing left ventricular pressure, volume changes, & phonocardiogram*

Ejection fraction (EF) is the percentage of ventricular end diastolic volume (EDV) which is ejected with each stroke.

EF = 75120X 100 = 62.5%Normal ejection fraction is about 60 65 %.Ejection fraction is good index of ventricular function.*

Relation Of Heart Sounds With Phases Of Cardiac CycleHEART SOUNDS1 st Heart Sound2 nd Heart Sound3 rd Heart Sound4 th Heart Sound*

ADAS

VSVD

0.7 Sec.0.1 sec.

0.3Sec.0.5 sec.

AS Atrial Systole; AD Atrial Diastole ; VS Ventricular systole; VD Ventricular diastole

Relation ship of heart sounds with events of cardiac cycle 1st heart sound = closure of AV valves at start of ventricular systole

2nd heart sound= closure of semi lunar valves at end of ventricular systole 3rd heart sound = first rapid filling phase 4th heart sound = last rapid filling phase

Heart SoundsWe will discuss Heart Sounds in next lecture.*

References Text book of physiology by Linda .S .Costanzo third edition

Text book of physiology by Guyton & Hall

review of medical physiology by william F.Ganang

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