Cardiac Cycle — Revision Notes

⚡ 30-Second Revision

Cardiac Cycle Duration: — $0.8,\text{s}$ (at $75,\text{bpm}$)
Phases: — Atrial Systole ($0.1,\text{s}$), Ventricular Systole ($0.3,\text{s}$), Joint Diastole ($0.4,\text{s}$)
Ventricular Systole Sub-phases: — Isovolumetric Contraction ($0.05,\text{s}$), Ventricular Ejection ($0.25,\text{s}$)
Ventricular Diastole Sub-phases: — Isovolumetric Relaxation ($0.08,\text{s}$), Rapid Filling ($0.11,\text{s}$), Diastasis ($0.21,\text{s}$)
Heart Sounds:

- S1 ('lub'): Closure of AV valves (Tricuspid & Bicuspid) - marks start of ventricular systole. - S2 ('dub'): Closure of Semilunar valves (Aortic & Pulmonary) - marks start of ventricular diastole.

ECG Correlation:

- P wave: Atrial depolarization (precedes atrial systole). - QRS complex: Ventricular depolarization (precedes ventricular systole). - T wave: Ventricular repolarization (during ventricular ejection, precedes ventricular diastole).

Volumes:

- EDV (End-Diastolic Volume): Max ventricular volume (approx. $120-130,\text{mL}$). - ESV (End-Systolic Volume): Min ventricular volume (approx. $50-60,\text{mL}$). - SV (Stroke Volume): $EDV - ESV$ (approx. $70,\text{mL}$).

Valve Actions: — Open/close passively due to pressure gradients.

2-Minute Revision

The cardiac cycle is the complete sequence of events in the heart during one heartbeat, typically lasting $0.8,\text{s}$ at $75,\text{bpm}$. It's divided into systole (contraction) and diastole (relaxation).

The cycle starts with joint diastole ($0.4,\text{s}$), where all chambers relax, and ventricles passively fill (70-80%). Then, atrial systole ($0.1,\text{s}$) occurs, where atria contract to 'top off' ventricles, reaching End-Diastolic Volume (EDV).

Next is ventricular systole ($0.3,\text{s}$), beginning with isovolumetric contraction ($0.05,\text{s}$), where AV valves close (S1 sound), and ventricular pressure rises with constant volume. This is followed by ventricular ejection ($0.

25, ext{s}$), where semilunar valves open, and blood is pumped out, leaving End-Systolic Volume (ESV). Finally, **ventricular diastole** begins with **isovolumetric relaxation** ($0.08, ext{s}$), where semilunar valves close (S2 sound), and ventricular pressure drops with constant volume, leading to rapid and reduced ventricular filling.

ECG waves correlate: P wave with atrial systole, QRS with ventricular systole, and T wave with ventricular repolarization (end of systole/start of diastole). Valve actions are passive, driven by pressure gradients.

5-Minute Revision

The cardiac cycle is the rhythmic sequence of events that constitutes one complete heartbeat, ensuring efficient blood circulation. At a resting heart rate of $75,\text{bpm}$, one cycle lasts approximately $0.8,\text{s}$. This cycle is meticulously orchestrated by pressure and volume changes within the heart chambers, which in turn dictate the opening and closing of heart valves.

1
Joint Diastole (0.4s): — This is the longest phase. Both atria and ventricles are relaxed. Blood flows passively from the great veins into the atria and then into the ventricles through open AV valves. This accounts for about 70-80% of ventricular filling.

1
Atrial Systole (0.1s): — The atria contract (triggered by the P wave on ECG), pushing the remaining 20-30% of blood into the ventricles. At the end of this phase, the ventricles contain their maximum volume, the End-Diastolic Volume (EDV, approx. $120-130,\text{mL}$). AV valves are open, semilunar valves are closed.

1
Ventricular Systole (0.3s): — This phase begins with the QRS complex on the ECG.

* Isovolumetric Contraction (0.05s): Ventricles begin to contract, rapidly increasing intraventricular pressure. This pressure rise exceeds atrial pressure, causing the AV valves (tricuspid and bicuspid) to snap shut, producing the first heart sound (S1, 'lub').

All four valves are closed, so ventricular volume remains constant. * Ventricular Ejection (0.25s): As ventricular pressure continues to rise, it eventually exceeds the pressure in the aorta and pulmonary artery, forcing the semilunar valves (aortic and pulmonary) open.

Blood is rapidly ejected into the great arteries. Ventricular volume decreases, leaving the End-Systolic Volume (ESV, approx. $50-60,\text{mL}$). The difference, Stroke Volume (SV = EDV - ESV, approx. $70,\text{mL}$), is ejected.

The T wave on ECG occurs during this phase.

1
Ventricular Diastole (0.4s): — This phase begins after the T wave on the ECG.

* Isovolumetric Relaxation (0.08s): Ventricles begin to relax, and their pressure drops rapidly. As ventricular pressure falls below arterial pressure, the semilunar valves snap shut, producing the second heart sound (S2, 'dub').

All four valves are again closed, and ventricular volume remains constant. * Rapid Ventricular Filling (0.11s) & Diastasis (0.21s): As ventricular pressure continues to fall, it drops below atrial pressure, causing the AV valves to open.

Blood rushes from the atria into the ventricles (rapid filling), and then the flow slows down (diastasis), completing the passive filling. The cycle then repeats.

Mini-Example: If EDV is $120,\text{mL}$ and ESV is $50,\text{mL}$, then the Stroke Volume is $120 - 50 = 70,\text{mL}$. If the heart rate is $70,\text{bpm}$, the Cardiac Output ($CO = HR \times SV$) would be $70 \times 70 = 4900,\text{mL/min}$ or $4.9,\text{L/min}$.

Prelims Revision Notes

The cardiac cycle is the sequence of events in one heartbeat, typically $0.8,\text{s}$ at $75,\text{bpm}$.

Phases and Durations:

Atrial Systole: — $0.1,\text{s}$ (Atria contract, push blood into ventricles. AV valves open, SL valves closed. P wave on ECG).
Ventricular Systole: — $0.3,\text{s}$

* Isovolumetric Contraction: $0.05,\text{s}$ (Ventricles contract, pressure rises. AV valves close (S1 sound). All valves closed. Volume constant. QRS complex on ECG). * Ventricular Ejection: $0.25,\text{s}$ (Ventricular pressure > arterial pressure. SL valves open. Blood ejected. Volume decreases. T wave on ECG).

Joint Diastole: — $0.4,\text{s}$ (All chambers relaxed. Ventricles fill passively).

* Isovolumetric Relaxation: $0.08,\text{s}$ (Ventricles relax, pressure drops. SL valves close (S2 sound). All valves closed. Volume constant. After T wave). * Rapid Ventricular Filling: $0.11,\text{s}$ (Ventricular pressure < atrial pressure. AV valves open. Rapid blood flow into ventricles). * Diastasis (Reduced Filling): $0.21,\text{s}$ (Slower filling rate).

Key Volumes:

EDV (End-Diastolic Volume): — Max volume in ventricles at end of diastole (approx. $120-130,\text{mL}$). Preload.
ESV (End-Systolic Volume): — Min volume in ventricles at end of systole (approx. $50-60,\text{mL}$). Residual volume.
SV (Stroke Volume): — Volume ejected per beat ($SV = EDV - ESV$, approx. $70,\text{mL}$).

Heart Sounds:

S1 ('lub'): — Closure of AV valves (Tricuspid & Bicuspid) at start of ventricular systole.
S2 ('dub'): — Closure of Semilunar valves (Aortic & Pulmonary) at start of ventricular diastole.

ECG Correlation:

P wave: — Atrial depolarization $ightarrow$ Atrial systole.
QRS complex: — Ventricular depolarization $ightarrow$ Ventricular systole (isovolumetric contraction).
T wave: — Ventricular repolarization $ightarrow$ Ventricular diastole (isovolumetric relaxation).

Valve Actions: Passive, driven by pressure gradients. AV valves prevent backflow to atria; SL valves prevent backflow to ventricles.

Important: Atrial systole contributes only $20-30$ of ventricular filling; $70-80$ is passive. Ventricles never completely empty.

Vyyuha Quick Recall

To remember the sequence of events in the cardiac cycle, think: All Valves In Every Room Really Dance.

Atrial Systole
Ventricular Isovolumetric Contraction
Ejection (Ventricular)
Relaxation (Ventricular Isovolumetric)
Rapid Filling (Ventricular)
Diastasis (Reduced Filling)