Cardiovascular | Cardiac Cycle

DEFINITION OF CARDIAC CYCLE
• Period between start of one beat to start of next. ⚫ It consists of one complete heart beat.It consists of one systole & one diastole.
INITIATION OF CARDIAC CYCLE
:• Initiated by Cardiac Impulse, which originates from SA node
.• EVENTS THAT OCCUR IN THE CARDIAC CHAMBERS DURING CARDIAC CYCLE
Pressure Changes
.Volume Changes
.Production of Heart Sounds
 Closure & Opening of Cardiac Valves
.Electric Changes /ECG recording

VENTRICULAR SYSTOLE 0.31 sec(Peak of R wave of QRS complex to the end of T wave) : 
consists of following:
 ISO-VOLUMETRIC CONTRACTION 0.06 sec
MAXIMUM EJECTION (2/3) 0.11 sec
REDUCED EJECTION (1/3) 0.14 sec
VENTRICULAR DIASTOLE  0.52sec (End of T wave to the peak of R wave of QRS complex):
consists of following:
PROTODIASTOLE 0.04 sec
ISO-VOLUMETRIC RELAXATION 0.06 sec
RAPID INFLOW 0.11 sec
SLOW INFLOW / DIASTASIS 0.2 sec
ATRIAL SYSTOLE (after P wave) 0.11 sec
So,
8 Phases of CARDIAC CYCLE= 0.8 sec.
PRESSURE CHANGES:
Heart has 4 chambers:
A) Pressure changes in left ventricle during cardiac cycle.
B) Pressure changes in right ventricleduring cardiac cycle.
C) Pressure changes in atria
.Pressure changes in Left Ventricle during cardiac cycle:
'Phase 1' of cardiac cycle / Iso-volumetric contraction of ventricle
:Isovolumetric contraction 
At the start of ventricular systole L.V is full of blood (received from left atrium during previous diastole).
Pressure in L.V at this stage = 1-3 mm Hg.
 Now L.V begins to contract → I.V.P (Intra- ventricular pressure) begins to rise closure of mitral valve / Left AV Valve 1st phase starts:
 ISOMETRIC CONTRACTION PHASE OR ISOVOLUMETRIC CONTRACTION PHASE.
• With closure of mitral valve ventricle is a closed chamber no change in blood volume → ISO-VOLUMETRIC,
 as both valves are closed → ISO- METRIC CONTRACTION (no change in length of muscle but rapid increase in I.V.P).
When IVP rises just above 80 mm Hg opening of Aortic or Semi-lunar valve.
⚫ Duration of I.V.C of Ventricle = 0.06 sec.•
With opening of Aortic valve, 2nd phase starts:
 MAXIMAL EJECTION PHASE.
Ventricular ejection
'Phase 2' of cardiac cycle / Maximal Ejection Phase (M.E.P)/Rapid Ejection Phase (R.E.P):
Ventricle muscle is contracting powerfully with opening of Aortic valve.
Blood is ejected from ventricle (2/3 of stroke volume) →Aorta (at maximum rate). 70% EMPTYING occurs in first1/3 of ejection phase.
In this phase: I.V.P maximum = 120 mm Hg.
Duration of M.E.P = 0.11 sec.
 'Phase 3' of cardiac cycle / Reduced Ejection Phase (R.E.P):Blood ejection (remaining 1/3 of stroke volume, 30% EMPTYING occurs in last 2/3 of ejection phase) from L.V Aorta, continues but at a reduced rate
.• I.V.P falls from maximum.This phase ends when I.V.P becomes equal to OR slightly less than AORTIC PRESSURE.
⚫ Duration of R.E.P = 0.14 sec.
Duration of ventricular systole (3 phases):
Isovolumetric contraction = 0.06 sec
Maximum Ejection Phase = 0.11 sec
Reduced Ejection Phase = 0.14 sec
Ventricular Systole = 0.31 sec
'Phase 4' of cardiac cycle / Protodiastole:
A short phase = 0.04 sec.At the junction of systole & diastole, but included in diastole.
⚫ At this stage, I.V.P = Aortic Pressure or I.V.P is slightly less than Aortic pressure, BUT SMALL AMOUNT OF BLOOD CONTINUES TO OOZE, because of momentum.
⚫ In protodiastole: THIS MOMENTUM IS OVERCOME due to further fall in I.V.P & there is some retrograde flow of Aortic blood in 1st part of Aorta → closure of Aortic valve end of Protodiastole.
Isovolumetric relaxation :'Phase 5' of cardiac cycle / Isovolumetric Relaxation Phase (I.V.R):
Starts with closure of Aortic valve.
Why it is called ISOVOLUMETRIC RELAXATION?
ANSWER:
Ventricle relaxation without change in volume, because: BOTH VALVES ARE CLOSED → no change in blood volume → Rapid fall in I.V.P because of relaxation.
When(left ventricular pressure) < (left atrial pressure) Opening of left AV valve/ mitral valve end of I.V.R phase.Duration of I.V.R= 0.06 sec.
'Phase 6' of cardiac cycle / Rapid InflowPhase (R.I.P) / Rapid filling phase (R.F.P). 
Starts with opening of mitral valve
.• Blood from Left Atrium → rapidly flows into Left Ventricle.2/3 of ventricular filling occurs in this phase(during first 1/3 of ejection phase)
⚫ Duration of R.I.P = 0.11 sec.
Phase 7' of cardiac cycle / Slow Inflow Phase / Diastasis: 
It appears that: No blood is flowing from Lt. Atrium →→ Lt.Ventricle because: During last phase (R.I.P), most of blood Lt. Vent.Mitral valve is open→ Lt. atrium & Lt. Ventricle = common chamber whatever blood that returns in small amount from pulmonary veins Lt. Atrium Lt. Ventricle (through open valve) so it appears that no blood is flowing.Only slight (1/3) filling of Lt. Ventricle in this phase.
⚫ Duration of diastasis / Slow Inflow Phase = 0.2 sec.
 ⚫ THE LONGEST PHASE OF CARDIAC CYCLE.
'Phase 8' of cardiac cycle / Atrial Systole:
• Last phase of cardiac cycleAtrial systole
⚫Lt. Atrium contracts → pushes the blood from its cavity → Lt. Ventricle → 20% ventricular filling by atrial contraction.
• Atria contract towards the end of ventricular diastole.
• With atrial contraction, ventricular filling is complete.
⚫ Duration: 0.11 sec.
⚫ Mechanism: AV nodal delay.
It allows atria to contract before ventricles begin to contract, at the end of ventricular filling.
Duration of ventricular diastole (5 phases):
Protodiastole = 0.04 sec
Isovolumetric Relaxation= 0.06sec
 Rapid Inflow Phase = 0.11 sec 
Slow Inflow Phase = 0.20sec
Atrial Systole = 0.11sec
So total of all of above is :
Ventricular Diastole = 0.52 sec
Duration of Cardiac Cycle (8 phases):
 Duration of systole + diastole = (0.31) + (0.52) = [0.8 sec]
Pressure Changes in Right Ventricle:
Same phases as for Lt. ventricle.Same duration as for Lt. ventricle.
Only change in pressure levels & in names of valves.
• Aortic valve is replaced by pulmonary valve.
• Mitral valve is replaced by Tricuspid valve.
At the beginning of Rt. Vent. Systole:
 Pressure 0-1 mm Hg.During I.V.C Pressure increases on the right side just exceeds 8 mmHg → opening of pulmonary valve (above 80 mmHg, there was opening of Aortic valve).
Maximum increase in pressure in Rt. Vent systole = 25 mmHg (it was 120 mm Hg in Lt. ventricle).
Right ventricle & Pulmonary artery is a low pressure system.
Pulmonary artery pressure variation:
 8-25 mm Hg.
Aortic pressure variation: 80 -120 mm Hg.