Cardiac Cycle
CARDIAC CYCLE
- Cardiac activity follows a cyclic pattern.
- Atria contracts first →Followed by delay with ventricular contraction.
- Systole – Contraction
- Diastole – Relaxation.
- “Cardiac systole”/simply “systole” & “Cardiac diastole”/simply “diastole”:
- Refers to “Ventricular systole & diastole” usually.
- Unless specifically mentioned otherwise.
- Cardiac cycle is explained in term of “Ventricular Cycle”.
PHASES OF CARDIAC CYCLE
- Cardiac cycle – Refers interval between onset of one heartbeat to onset of next heartbeat.
2 main phases:
- Ventricular systole.
- Ventricular diastole.
Valves involved:
- Semi-lunar valves includes,
- Aortic valve.
- Pulmonary valve.
- Atrio-ventricular (AV valve) includes,
- Mitral valve.
- Tricuspid valve.
1. VENTRICULAR SYSTOLE (Simply systole):
- Divided into,
- Isovolumetric contraction.
- Ventricular ejection.
- Protodiastole.
1a. Isovolumetric contraction:
As ventricular contraction starts.
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Intraventricular pressure rises → Abrupt AV valve closure.
- Marks 1st Heartbeat – Closure of AV valves.
Pressure insufficient to open semilunar valves.
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Causes closed AV valve to bulge into atrium.
- Resulting in small, sharp rise in atrial pressure.
- Referred as “C” wave on Jugular Venous Pulse (JVP).
Because both AV & semilunar valves are closed.
- Thus, No volume change.
- Hence, “Isovolumetric” (“Isometric”) Contraction.
Isovolumetric contraction ends with opening of semilunar valves.
1b. Ventricular ejection:
Opening of semilunar valves – Marks beginning of ventricular ejection.
- Due to steeply rising ventricular pressure exceeding aortic & pulmonary artery pressure.
Rapid blood ejection at first – Rapid ejection phase.
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Slower at later part – Slow ejection phase.
- During rapid ejection phase –
- As ventricles contract → AV ring is pulled down.
- AV ring – Fibrous partition separating ventricles & atria.
- Causes atrial muscle stretching & atrial dilatation.
- Thus, sharp fall in atrial pressure.
- This represents “X-decent” in JVP.
- Very short phase.
Ventricles start relaxing.
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Ventricular pressure falls very sharply.
- yet semilunar valves are still open.
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Resulting in blood flow from aorta into ventricle.
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Causes “Closure of semilunar valves” → Produces “2nd heart sound”.
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Venous blood flow continues in atria from SVC & ICC.
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Ventricular relaxation causes fibrous AV ring relaxation.
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Hence, rising atrial pressure.
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Represents ‘V’ wave in JVP.
2. VENTRICULAR DIASTOLE:
Divided into –
- Iso-volumetric relaxation.
- Rapid ventricular filling.
- Diastasis.
- Last rapid filling phase.
2a. Iso-volumetric relaxation:
- Phase is period between semilunar valve closure & AV valve opening.
- Continued ventricular relaxation.
- Continued fall in pressure.
- Both AV valves & semilunar valves are closed.
- Hence, no volume change.
- I.e., Iso-volumetric relaxation.
- Relaxation phase ends with opening of AV valve.
2b. Rapid ventricular filling:
When ventricular pressure falls below atrial pressure → AV valves open.
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Accumulated blood in atria rushes into ventricle very rapidly.
- Contributing 70% ventricular filling.
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Resulting in sharp fall in atrial pressure.
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Represents Y-descent in JVP.
2c. Diastasis:
- After initial rapid ventricular filling → Slow & smooth blood flow from SVC & IVC.
- Blood from pulmonary veins flows into left ventricle without any turbulence.
- This phase of nonturbulent ventricular filling – “Diastasis”.
- Atrial pressure slightly greater than ventricular pressure.
- Becuase atrial inflow exceeds atrial outflow.
2d. Last rapid filling phase (atrial systole):
- Atria contract & rapidly blood pumped into ventricles.
- Atrial systole is associated with sharp rise in atrial pressure.
- Producing “a”-wave in JVP.
DURATION OF CARDIAC EVENTS
- In normal person with average heart rate of 75 per min:
- Each cardiac cycle lasts about for 0.8 seconds.
- Atrial systole – 0.1 sec.
- Atrial diastole – 0.7 sec.
| Cardiac Event | Duration |
|
Isovolumetric contraction |
0.05 sec |
| Rapid ventricular ejection | 0.11 sec |
| Slow (reduced) ventricular ejection | 0.14 sec |
| Hence, Total ejection phase | 0.25 sec |
| Protodiastole | 0.04 sec |
| Isolvoumetric relaxation | 0.06 sec |
| Rapid ventricular filling |
0.10 sec |
| Diastasis | 0.20 sec |
| Atrial systole | 0.10 sec |
- Hence, only during last phase of ventricular cycle (last rapid filling phase), atria are contracting (atrial systole).
- During other six phases, atria are relaxed (atrial diastole).
Heart rate:
- Duration of various phases of cardiac cycle depends on heart rate.
- Increased heart rate→ Decreased duration of cardiac phase.
- Specifically, decreased diastolic duration than systolic duration.
- Marked reduction in diastolic time during tachycardia has important clinical implication.
During diastole –
- Most ventricular filling & perfusion of cardiac muscle.
- Especially left ventricular subendocardial portion.
- Hence, at HR > 180 per/min→ Reduced ventricular filling → Reduce cardiac output & perfusion→ Resulting in myocardial ischemia & infarction.
SYSTOLIC TIME INTERVALS:
- Measured by indices obtained from,
- Simultaneous recording of ECG.
- Phonocardiogram (record of heart sound).
- Carotid pulse (indicating aortic pressure change).
Parameters –
- Electromechanical systole.
- Left ventricular ejection time.
- Pre-ejection systole.
1. Electromechanical systole (QS2):
- Time interval between onset of QRS complex (ventricular activation) to aortic valve closure (S2 heart sound).
- Calculated by ECG & phonocardiogram.
- Carotid pulse is not required.
2. Left ventricular ejection time (LVET):
- Time interval between beginning of carotid pressure rise to dicrotic notch (incisura).
- Hence, requires only carotid pulse recording.
3. Pre-ejection systole (PEP):
- Difference between QS2 & LVET.
- Also gives duration of electromechanical events preceding systolic ejection.
- All 3 recordings of ECG, phonocardiogram & carotid pulse required.
- Since it utilizes parameters like QS2 & LVET.
4. Normal PEP\LVET ratio is 0.35.
- On impaired left ventricular function→ Increases PEP/LVET ratio without change in QS2.
CARDIAC CYCLE
- Isovolumetric contraction:
- Closure of AV valves – Marks 1st Heartbeat.
- Causes bulging into atrium.
- Resulting in small, sharp rise in atrial pressure.
- Referred as “C” wave on Jugular Venous Pulse (JVP).
- Both AV & semilunar valves are closed.
- Thus no volume change.
- Hence, “Isovolumetric” contraction.
- Isovolumetric contraction ends with opening of semilunar (aortic & pulmonary) valves.
Ventricular ejection:
- Begins with semilunar valve opening.
- Sharp fall in atrial pressure.
- Represents “X-decent” in JVP.
Protodiastole:
- Closure of semilunar valves.
- Produces “2nd heart sound”.
Iso-volumetric relaxation:
- Phase is period between semilunar valve closure & AV valve opening.
- Ends with AV valve opening.
- Coronary blood flow is maximum during this period.
Rapid ventricular filling:
- Represents Y-descent in JVP.
Last rapid filling phase
- Associated with sharp rise atrial pressure rise.
- Producing “a”-wave in JVP.
Each cardiac cycle lasts about for 0.8 seconds.
- Atrial systole – 0.1 sec.
- Atrial diastole – 0.7 sec.
- Heart rate decreases diastolic duration than systolic duration.
Electromechanical systole (QS2):
- Time interval between onset of QRS complex (ventricular activation) to aortic valve closure (S2 heart sound).
- Calculated by ECG & phonocardiogram.
- Carotid pulse is not required.
Left ventricular ejection time (LVET):
- Requires only carotid pulse recording.
Pre-ejection systole (PEP):
- All 3 recordings of ECG, phonocardiogram & carotid pulse required.
