Quick Revision

Cardiac Cycle — Last Night Revision

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Definition

• Sequence of mechanical and electrical events occurring during one heartbeat
• Duration: 0.8 seconds at 75 bpm

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Phases at a Glance

• Systole total: ~0.27 s
• Diastole total: ~0.53 s

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Critical Values

• EDV (End Diastolic Volume): 120–130 mL
• ESV (End Systolic Volume): 50–60 mL
• Stroke Volume: 70 mL (EDV − ESV)
• Ejection Fraction: ~65% (SV/EDV × 100)
• Cardiac Output: 5 L/min (SV × HR)
• Heart Rate: 72–75 bpm
• Aortic pressure: 120/80 mmHg
• LV peak systolic pressure: ~120 mmHg
• LV end diastolic pressure: 5–12 mmHg
• LA pressure: 5–10 mmHg

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Heart Sounds

• S3 = normal in children; pathological in adults (CCF)
• S4 = always pathological

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Valve Events

• Mitral closes → S1 (start of IVC)
• Aortic opens when LV pressure > aortic pressure (~80 mmHg)
• Aortic closes → S2 (start of IVR); dicrotic notch on aortic pressure trace
• Mitral opens when LV pressure < LA pressure

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Viva Pearls

• Longest phase: diastasis (reduced filling)
• Shortest phase: isovolumetric contraction
• Only phase where all valves are closed: IVC and IVR
• EF < 40% → significant systolic dysfunction
• S3 in adults = hallmark of left ventricular failure
• Dicrotic notch = aortic valve closure artifact on aortic pressure waveform
• During IVC: volume = constant, pressure rises sharply
• “Lub-dub” = S1 (AV valve closure) + S2 (semilunar valve closure)

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Frequently Tested Facts

• At HR > 100 (tachycardia): diastole shortens more than systole
• Atrial systole contributes ~20–30% of ventricular filling
• Frank-Starling law: ↑ EDV → ↑ SV (up to a limit)
• During exercise: ↑ HR + ↑ SV = ↑↑ CO

Notes

Cardiac Cycle — Detailed Revision Notes

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1. Introduction

The cardiac cycle is the complete sequence of events occurring in the heart during a single heartbeat. At a normal resting heart rate of 75 bpm, one complete cycle lasts 0.8 seconds.

The cycle has two broad phases:

• Systole — ventricular contraction (~0.27 s)
• Diastole — ventricular relaxation (~0.53 s)

At higher heart rates, diastole shortens proportionally more than systole, reducing ventricular filling time — this is a common exam point.

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2. Phases of the Cardiac Cycle

Phase 1 — Atrial Systole (0.1 s)

• Atria contract; AV valves (mitral + tricuspid) are open
• Active “top-up” filling of ventricles
• Contributes ~20–30% of total ventricular filling
• Produces the a wave in JVP
• At end of atrial systole → ventricles reach EDV (~120–130 mL)

Phase 2 — Isovolumetric Contraction (0.05 s)

• Ventricles begin contracting; AV valves snap shut → S1
• Semilunar valves still closed (LV pressure < aortic pressure)
• All four valves closed
• Volume = constant; pressure rises sharply
• Shortest phase of the cycle

Phase 3 — Rapid Ejection (0.09 s)

• LV pressure exceeds aortic pressure (~80 mmHg) → aortic valve opens
• ~70% of stroke volume ejected in this phase
• LV and aortic pressure rise together to ~120 mmHg peak

Phase 4 — Reduced Ejection (0.13 s)

• Remaining ~30% of SV ejected
• LV pressure begins to fall
• Aortic pressure starts to fall slowly
• Flow decelerating

Phase 5 — Isovolumetric Relaxation (0.08 s)

• Aortic valve closes → S2 + dicrotic notch on aortic trace
• Mitral valve still closed (LV pressure still > LA pressure)
• All four valves closed
• Volume = constant at ESV (~50–60 mL); pressure falls sharply

Phase 6 — Rapid Ventricular Filling (0.11 s)

• LV pressure falls below LA pressure → mitral valve opens
• Blood rushes passively from LA into LV
• ~70% of ventricular filling occurs here
• Produces S3 (audible in children; pathological in adults)
• Produces y descent in JVP

Phase 7 — Diastasis / Reduced Filling (0.19 s)

• Slow, passive filling continues
• Longest phase of the cardiac cycle
• Pressures in LA and LV equilibrate

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3. Atrial Events

• a wave: atrial contraction
• c wave: tricuspid valve bulging into atrium at IVC onset
• x descent: atrial relaxation
• v wave: venous return filling the atrium against a closed tricuspid valve
• y descent: rapid emptying of atrium when tricuspid opens

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4. Ventricular Events

Pressure Changes

Volume Changes

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5. Heart Sounds

S1 — First Heart Sound

• Caused by closure of mitral and tricuspid valves
• Marks the beginning of systole
• Best heard: apex (mitral area)
• Loud S1: mitral stenosis, tachycardia, short PR interval
• Soft S1: mitral regurgitation, long PR interval, poor LV function

S2 — Second Heart Sound

• Caused by closure of aortic and pulmonary valves
• Marks the end of systole
• Normal physiological splitting in inspiration (A2 before P2)
• Wide splitting: RBBB, pulmonary stenosis
• Reversed splitting: LBBB, aortic stenosis
• Fixed splitting: ASD

S3 — Third Heart Sound

• Caused by rapid ventricular filling in early diastole
• Normal in children and young adults
• Pathological in adults → left ventricular failure (CCF)
• Heard at apex with patient in left lateral position

S4 — Fourth Heart Sound

• Caused by atrial contraction into a stiff/non-compliant ventricle
• Always pathological
• Seen in LVH, hypertension, aortic stenosis, hypertrophic cardiomyopathy
• Pre-systolic gallop

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6. JVP Waveform Correlation

The jugular venous pulse reflects right heart pressure events.

a  c    x    v    y
↑  ↑    ↓    ↑    ↓

• a wave = right atrial contraction (just before S1)
• c wave = tricuspid bulging into RA at IVC start
• x descent = RA relaxation + tricuspid pulled down with RV contraction
• v wave = RA filling against closed tricuspid
• y descent = tricuspid opens; RA empties rapidly

Absent a wave → atrial fibrillation (no coordinated atrial contraction) Cannon a wave → atrial contraction against closed tricuspid (e.g., complete heart block, VT)

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7. Pressure-Volume Loop

The PV loop graphically represents one cardiac cycle for the left ventricle.

Four Corners of the Loop

Key Loop Segments

• Right vertical limb (diastole): filling phase — mitral open, volume increases
• Bottom horizontal segment: IVC — volume constant, pressure rises
• Left vertical limb (systole): ejection — volume decreases
• Top horizontal segment: IVR — volume constant, pressure falls

Derived Parameters from PV Loop

• Width of loop = stroke volume
• Area of loop = stroke work
• Preload = EDV (x-axis, rightward shift = ↑ preload)
• Afterload = end-systolic pressure (↑ afterload = taller, narrower loop)
• Contractility = slope of end-systolic pressure-volume relationship (ESPVR)

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8. Ejection Fraction

EF = (SV / EDV) × 100

• Normal EF: 55–70%
• EF with normal SV: (70 / 120) × 100 ≈ 58%

• EF preserved despite diastolic dysfunction = HFpEF
• EF is afterload-dependent — high afterload → lower EF even with normal contractility

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9. Frank-Starling Mechanism

• ↑ preload (EDV) → ↑ stretch of myofibrils → ↑ force of contraction → ↑ SV
• Works up to an optimal sarcomere length (~2.2 µm)
• Explains why the heart automatically adjusts output to match venous return
• Failure of this mechanism → cardiac failure

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10. Clinical Importance

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11. Exam Pearls

• At 75 bpm: cycle = 0.8 s (systole 0.27 + diastole 0.53)
• Only phases with all valves closed: IVC and IVR
• Longest phase: diastasis; Shortest phase: IVC
• Dicrotic notch = aortic valve closure, not aortic valve opening
• S3 in adults is abnormal; S4 is always abnormal
• During tachycardia: diastole shortens more → reduced coronary perfusion time
• Atrial systole contributes more in disease states (hypertrophy, poor compliance)

Diagrams

Cardiac Cycle — Diagrams

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Must Know Diagrams

The following diagrams are considered essential for physiology theory exams and viva. Each is described below with labelling guidance and common exam mistakes.

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1. Wiggers Diagram

The Wiggers diagram plots multiple cardiac cycle events against time on a single graph. It is the most tested diagram in cardiovascular physiology.

What to Draw

Plot the following curves on a shared time axis (x-axis = time in seconds):

1. Left ventricular pressure — rises steeply during IVC, peaks ~120 mmHg, falls during IVR
2. Aortic pressure — rises slightly after LV, shows dicrotic notch at aortic valve closure, baseline ~80 mmHg
3. Left atrial pressure — low throughout; shows a, c, v waves; drops at y descent
4. Ventricular volume — flat during IVC, falls steeply during ejection, flat during IVR, rises during filling
5. ECG trace — P wave before atrial systole; QRS before IVC; T wave at end of ejection
6. Heart sounds — S1 at IVC start; S2 at IVR start; S3 at rapid filling; S4 before IVC

Important Labels

• Mark S1 at mitral valve closure
• Mark S2 at aortic valve closure + dicrotic notch
• Mark isovolumetric contraction (between mitral closure and aortic opening)
• Mark isovolumetric relaxation (between aortic closure and mitral opening)
• Mark EDV and ESV on the volume curve
• Mark a wave, c wave, v wave on the LA pressure curve
• Mark dicrotic notch on the aortic pressure curve

Common Exam Mistakes

• Drawing the dicrotic notch at aortic opening instead of closure
• Forgetting that aortic pressure remains higher than LV pressure after S2
• Drawing S1 at aortic valve closure (it is mitral + tricuspid closure)
• Missing the c wave on the atrial trace
• Placing the ECG P wave too late — P wave precedes atrial contraction, which is before IVC

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2. Pressure-Volume Loop

The PV loop plots left ventricular pressure (y-axis) against volume (x-axis) over one complete cardiac cycle.

Labels to Include

• A — Mitral valve opens; rapid filling begins (bottom right)
• B — Mitral valve closes; IVC begins (bottom right, after filling)
• C — Aortic valve opens; ejection begins (bottom left)
• D — Aortic valve closes; IVR begins (top left)
• EDV on x-axis (rightmost point)
• ESV on x-axis (leftmost point)
• SV = EDV − ESV (width of loop)
• ESPVR line (end-systolic pressure-volume relationship — slope = contractility)
• EDPVR curve (end-diastolic — right margin of loop, curved)

Interpretation

Viva Points from PV Loop

• Shifting ESPVR rightward = reduced contractility (heart failure)
• In cardiac tamponade: loop becomes narrow and small (↓ filling → ↓ SV)
• In aortic regurgitation: EDV increases → wider loop, regurgitant volume adds to filling
• In mitral regurgitation: loop shifts left; LV empties into low-pressure LA → apparent EF increase
• Preload and afterload can both be read from a single PV loop

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3. Cardiac Cycle Flowchart

Sinoatrial node fires
        ↓
Atrial depolarisation (P wave)
        ↓
Atrial systole → active filling → EDV reached
        ↓
AV node delay → PR interval
        ↓
Ventricular depolarisation (QRS)
        ↓
Isovolumetric Contraction
(all valves closed; pressure ↑↑)
        ↓
LV pressure > Aortic pressure
→ Aortic valve opens
        ↓
Rapid Ejection (~70% SV)
        ↓
Reduced Ejection (~30% SV)
        ↓
LV pressure < Aortic pressure
→ Aortic valve closes → S2 + Dicrotic notch
        ↓
Isovolumetric Relaxation
(all valves closed; pressure ↓↓)
        ↓
LV pressure < LA pressure
→ Mitral valve opens
        ↓
Rapid Ventricular Filling (S3)
        ↓
Diastasis (slow filling)
        ↓
Next atrial systole → cycle repeats

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4. JVP Waveform Diagram

The JVP trace reflects right atrial pressure events and is visible in the jugular veins with the patient at 45°.

Waveform Components

     a   c          v
     ↑   ↑          ↑
_____|   |____x_____|____y___
                ↓        ↓

Abnormal JVP Patterns

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Diagram Checklist

Use this to confirm you can reproduce each diagram from memory.

• Wiggers Diagram with all five traces (LV pressure, aortic pressure, LA pressure, ventricular volume, ECG)
• Heart sounds (S1, S2, S3, S4) marked on Wiggers Diagram
• Valve events marked on Wiggers Diagram
• Pressure-Volume Loop with all four corners labelled (A, B, C, D)
• ESPVR and EDPVR lines on PV Loop
• Stroke volume and stroke work identified on PV Loop
• Cardiac Cycle Flowchart from SA node firing to diastasis
• JVP waveform with a, c, x, v, y labelled and causes listed
• Dicrotic notch correctly placed on aortic pressure trace
• EDV and ESV marked on volume curve

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Diagram-Based Viva Questions

1. On the Wiggers Diagram, where is S1 and what causes it?
2. What does the dicrotic notch represent and on which curve is it seen?
3. What is the width of the PV loop equivalent to physiologically?
4. What does a rightward shift of the ESPVR line indicate?
5. On the JVP trace, what causes the a wave?
6. What happens to the JVP a wave in atrial fibrillation?
7. During which phases are all four cardiac valves closed simultaneously?
8. On the Wiggers Diagram, which phase shows constant ventricular volume with rising pressure?
9. What does the area enclosed by the PV loop represent?
10. On the JVP trace, what produces the cannon a wave and in which condition is it seen?
11. Where on the Wiggers Diagram does the ECG QRS complex fall in relation to the cardiac cycle?
12. What change in the PV loop would you expect in a patient with cardiac tamponade?
13. On the Wiggers Diagram, mark the point where the mitral valve opens and state the pressure condition required.
14. What does a steeper ESPVR slope on the PV loop indicate?
15. Describe the JVP changes expected in constrictive pericarditis.

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Spotters

Identify the abnormality described in each of the following:

1. A JVP trace with an absent a wave and irregular rhythm — what is the diagnosis?
2. A PV loop that is narrow, tall, and shifted leftward — what haemodynamic state does this suggest?
3. A Wiggers Diagram showing S4 just before S1 — what does this indicate about the ventricle?
4. A JVP trace showing a giant v wave with a steep y descent — which valve lesion is most likely?
5. A PV loop with greatly increased EDV and a wide loop but reduced peak pressure — what cardiac condition could explain this pattern?

Other Questions (50)