The Cardiac Cycle

Definition

The cardiac cycle refers to all the events that occur from the beginning of one heartbeat to the beginning of the next. At a heart rate of 75 beats/min, one complete cycle lasts approximately 0.8 seconds (systole ~0.3 s + diastole ~0.5 s).

Each cycle is initiated by spontaneous depolarization of the SA node (sinoatrial node) in the right atrium. The impulse travels through both atria, is delayed at the AV node (>0.1 s), then spreads to the ventricles via the Bundle of His and Purkinje fibres. This delay allows the atria to contract before the ventricles, acting as "primer pumps."

The Cardiac Cycle Diagram (Wiggers Diagram)

The cardiac cycle - showing pressure, volume, venous pulse, heart sounds and ECG

Fig. 4.25 - Costanzo Physiology 7th Ed. Shows phases A-G with LV pressure, aortic pressure, atrial pressure, ventricular volume, venous pulse, heart sounds, and ECG.

Seven Phases of the Cardiac Cycle

Phase A - Atrial Systole (0-0.1 s)

ECG: P wave (atrial depolarization)
Atria contract → left atrial pressure rises → blood actively pumped into the already-filling left ventricle through the open mitral valve
LV volume increases to end-diastolic volume (EDV) ~120-130 mL
Venous pulse: 'a' wave (atrial contraction reflected into jugular veins)
Heart sound: S4 (not audible in normal adults; heard in ventricular hypertrophy/stiff ventricle)

Phase B - Isovolumetric Ventricular Contraction (IVC)

ECG: QRS complex (ventricular depolarization)
LV begins to contract → LV pressure rises rapidly
When LV pressure exceeds LA pressure → mitral valve closes → S1 (lub)
All valves are closed - LV pressure rises steeply but volume stays constant (isovolumetric)
LV pressure builds until it exceeds aortic pressure (~80 mmHg)
Duration: ~0.05 s
Venous pulse: 'c' wave (tricuspid valve bulges into atrium)

Phase C - Rapid Ventricular Ejection

ECG: ST segment
When LV pressure exceeds aortic pressure (~80 mmHg) → aortic valve opens
Blood rapidly ejected into aorta → LV volume drops sharply
LV pressure reaches peak systolic pressure (~120 mmHg)
Aortic pressure rises rapidly to its peak (~120 mmHg)
~70% of stroke volume is ejected in this phase
Atrial filling begins (atrial pressure slowly rises)
Venous pulse: 'x' descent

Phase D - Reduced Ventricular Ejection

ECG: T wave (ventricular repolarization begins)
Ventricles begin to repolarize → contracting force decreases
Blood continues to be ejected (aortic valve still open), but at a slower rate
LV volume reaches end-systolic volume (ESV) ~50 mL
Aortic pressure starts to fall (blood runs off into peripheral arteries faster than it is added)
Stroke volume = EDV - ESV = ~70 mL; Ejection fraction = SV/EDV = ~60%

Phase E - Isovolumetric Ventricular Relaxation (IVR)

ECG: End of T wave
LV relaxes → LV pressure drops rapidly
When LV pressure falls below aortic pressure → aortic valve closes → S2 (dub)
The dicrotic notch (incisura) appears on the aortic pressure curve at the moment of aortic valve closure
All valves are again closed - volume stays constant (isovolumetric)
Physiological splitting of S2 on inspiration: reduced intrathoracic pressure → increased RV venous return → prolonged RV ejection → pulmonic valve closes later than aortic valve
Venous pulse: 'v' wave (venous filling while tricuspid is closed)

Phase F - Rapid Ventricular Filling

ECG: Isoelectric (between T and next P)
When LV pressure falls below LA pressure → mitral valve opens
Blood flows rapidly from LA → LV (passive filling)
LV volume increases rapidly but LV pressure remains low (ventricle is relaxed and compliant)
Heart sound: S3 (rapid ventricular filling sound)
- Normal in children and young adults
- In adults >40 years: indicates volume overload (e.g., heart failure, MR, TR)
Aortic pressure continues to fall as blood runs off into the arterial tree

Phase G - Reduced Ventricular Filling (Diastasis)

The longest phase of the cardiac cycle
Filling continues at a slow rate
Ventricular volume approaches EDV
If heart rate increases, this phase is shortened first - explaining why tachycardia reduces stroke volume (less filling time)

Summary Table

Phase	Mitral Valve	Aortic Valve	ECG Event	Heart Sound
A - Atrial Systole	Open	Closed	P wave	S4
B - IVC	Closes	Closed	QRS	S1
C - Rapid Ejection	Closed	Opens	ST segment	-
D - Reduced Ejection	Closed	Open	T wave	-
E - IVR	Closed	Closes	End of T	S2 + Dicrotic notch
F - Rapid Filling	Opens	Closed	Isoelectric	S3
G - Diastasis	Open	Closed	Isoelectric	-

Key Values to Remember

Parameter	Value
Total cycle duration (75 bpm)	0.8 s
Systole	0.3 s
Diastole	0.5 s
End-diastolic volume (EDV)	120-130 mL
End-systolic volume (ESV)	~50 mL
Stroke volume (SV = EDV - ESV)	~70 mL
Ejection fraction	~60-65%
Peak LV systolic pressure	~120 mmHg
LV diastolic pressure	~0-5 mmHg

Important Clinical Correlates

S4 - Stiff/hypertrophied ventricle (aortic stenosis, hypertension, hypertrophic cardiomyopathy)
S3 - Volume overload: congestive heart failure, mitral regurgitation
Dicrotic notch - Marks aortic valve closure; used to define end-systole in clinical monitoring
Tachycardia - Reduces diastasis (Phase G) most, impairing ventricular filling and reducing stroke volume
Splitting of S2 - Physiological on inspiration; fixed splitting in ASD; paradoxical in LBBB/severe AS

Sources: Costanzo Physiology 7th Edition, p. 160-163; Guyton and Hall Textbook of Medical Physiology, p. 126-127

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