You're staring at a squiggly line on a monitor. Which means or you're listening through a stethoscope and hearing lub-dub, lub-dub but something sounds... Think about it: or maybe it's a pressure tracing from a cath lab. off.
The question is always the same: where exactly are we in the cardiac cycle right now?
It's the fundamental skill. On the flip side, everything else — interpreting hemodynamics, timing interventions, understanding murmurs, reading echoes — builds on this. But here's the thing: most people learn the phases as a list to memorize. They don't learn how to recognize them in real time, across different modalities Most people skip this — try not to..
Not obvious, but once you see it — you'll see it everywhere.
Let's fix that.
What Is the Cardiac Cycle (Really)
One heartbeat. That's all it is. But "one heartbeat" hides a carefully choreographed sequence of electrical, mechanical, and hemodynamic events that overlap in ways textbooks don't always make clear.
The cycle splits into two big phases: systole (contraction/ejection) and diastole (relaxation/filling). But each of those contains distinct stages — seven, by the classic Wiggers diagram count — and the boundaries between them aren't arbitrary. They're marked by specific events: valves opening, valves closing, pressures crossing, electrical signals firing.
If you can identify the stage from any single indicator — an ECG wave, a heart sound, a pressure notch, a valve motion — you can orient yourself instantly. That's the skill And that's really what it comes down to..
The Seven Stages (Quick Reference)
| Stage | Phase | Key Event |
|---|---|---|
| 1. Atrial systole | Late diastole | Atria contract, top off ventricles |
| 2. Isovolumetric contraction | Early systole | Ventricles squeeze, all valves closed |
| 3. Also, rapid ejection | Systole | Semilunar valves open, blood rockets out |
| 4. In practice, reduced ejection | Late systole | Flow slows, pressures start to fall |
| 5. In real terms, isovolumetric relaxation | Early diastole | Ventricles relax, all valves closed |
| 6. Rapid filling | Early diastole | AV valves open, blood rushes in |
| 7. |
Now let's learn to spot each one.
Why It Matters / Why People Care
You might be a med student cramming for boards. A cardiology fellow learning echo. On top of that, an ICU nurse titrating pressors. An anesthesiologist managing a crashing patient. The context changes — the skill doesn't Most people skip this — try not to..
Real talk: misidentifying the cycle stage leads to real errors.
- Timing a balloon pump inflation to the wrong phase? You just increased afterload instead of decreasing it.
- Measuring LVEDP at the wrong moment? Your preload assessment is garbage.
- Trying to time a cardioversion shock? If you don't know where the T wave sits relative to mechanical systole, you risk inducing VF.
- Interpreting a murmur? The timing within systole or diastole is literally how you classify it.
And here's what most people miss: **the stages don't align perfectly across modalities.Because of that, the dicrotic notch on an arterial line isn't exactly aortic valve closure. ** The ECG's R wave doesn't line up exactly with the start of ventricular systole. Close — but in critical care, "close" can matter.
How to Identify the Stage: By Modality
This is the meat. We'll walk through each major way the cardiac cycle reveals itself, and map every wave/sound/notch to its stage.
By ECG — The Electrical Roadmap
The ECG is the most common reference. But it's electrical, not mechanical. There's a delay — electromechanical coupling — that trips people up That alone is useful..
P Wave → Atrial Systole (Stage 1)
Atrial depolarization triggers atrial contraction. This is the "atrial kick" — the final 20-30% of ventricular filling. On a pressure tracing, you'll see the a wave in the RA or LA tracing, and a corresponding bump in LV/RV pressure And that's really what it comes down to. Nothing fancy..
Key point: P wave starts before atrial systole mechanically begins. The contraction follows depolarization by ~50ms Most people skip this — try not to..
PR Segment → AV Conduction Delay
Electrically quiet. Mechanically, atria are still contracting (early), then ventricles haven't started yet. This is the tail end of Stage 1.
QRS Complex → Start of Ventricular Systole
Here's where it gets tricky. The R wave marks the onset of ventricular depolarization. But mechanical contraction (isovolumetric contraction, Stage 2) begins ~20-50ms after the R wave peak.
So: R wave = electrical start. Mechanical systole starts after.
ST Segment → Ejection (Stages 3 & 4)
Ventricles are fully depolarized. Ejection is underway. The ST segment roughly corresponds to the ejection phase — but again, not perfectly aligned.
T Wave → Repolarization → Start of Relaxation
T wave peak roughly coincides with end of ejection / start of isovolumetric relaxation (Stage 5). The T wave end is closer to mitral valve opening (Stage 6).
Clinical pearl: The QT interval spans from ventricular depolarization to repolarization completion. It roughly covers mechanical systole + early diastole. That's why QT-prolonging drugs affect systolic duration.
By Heart Sounds — The Auditory Map
Two main sounds. Maybe a third, fourth. Each marks a valve event — which marks a stage boundary.
S1 (Lub) → Closure of AV Valves (Mitral & Tricuspid)
Marks the start of Stage 2: Isovolumetric Contraction.
Ventricles have just begun contracting. Pressure exceeds atrial pressure. Consider this: aV valves snap shut. S1 is not the start of ejection — it's the start of the "all valves closed" period.
Timing nuance: S1 occurs ~30-50ms after the R wave. The R wave → electromechanical delay → pressure rise → valve closure → sound transmission to chest wall And that's really what it comes down to..
S2 (Dub) → Closure of Semilunar Valves (Aortic & Pulmonic)
Marks the end of Stage 4 (Reduced Ejection) and start of Stage 5: Isovolumetric Relaxation.
Ventricular pressures fall below aortic/pulmonary pressures. Blood tries to flow back. Valves close. S2 splits with inspiration (A2 then P2) — a whole other topic That's the part that actually makes a difference..
Key: S2 is not the start of diastole filling. It's the start of "all valves closed again." Filling doesn't start until AV valves open — which happens after ventricular pressure drops below atrial pressure.
S3 → Rapid Filling (Stage 6)
Low-frequency vibration from rapid ventricular filling. Heard in early diastole, ~120-180ms after S2. Pathologic in adults over 40 (volume overload, systolic HF). Normal in kids, pregnant women, athletes.
S4 → Atrial Systole (Stage 1)
Atrial contraction into a stiff ventricle. Heard just before S1. **Always
Always pathologic. Indicates decreased ventricular compliance (hypertrophy, restriction, ischemia). Never heard in healthy adults.
By Murmurs — The Turbulence Map
Murmurs don’t mark stages per se — they expose pressure gradients across valves during specific stages. Location and timing are everything.
Systolic Murmurs (Between S1 and S2)
Stages 2, 3, 4.
| Timing | Mechanism | Classic Causes |
|---|---|---|
| Early systolic | Starts with S1, tapers before S2. Think about it: | |
| Mid-systolic (Crescendo-decrescendo) | Follows flow curve: rises during rapid ejection (Stage 3), falls during reduced ejection (Stage 4). Small VSD (large gradient early, equalizes fast). ** | |
| Late systolic | Starts mid-late systole. But | |
| Holosystolic (Pansystolic) | **All of Stages 2–4. But acute MR (large LA V-wave blunts gradient late). ** Constant gradient: VSD, chronic MR, TR. Think about it: **AS, PS, HOCM, flow murmurs. In practice, MVP (valve prolapses after pressure peaks), Papillary muscle dysfunction. Obscures S1/S2. |
Clinical pearl: In HOCM, the murmur intensifies with Valsalva (↓ preload → worse obstruction) and softens with squatting (↑ afterload/preload). Opposite of AS.
Diastolic Murmurs (Between S2 and S1)
Stages 5, 6, 1.
| Timing | Mechanism | Classic Causes |
|---|---|---|
| Early diastolic | Starts with S2 (valve closure), decrescendo. ** Low pitch. **MS, TS.Which means needs bell. Which means disappears with AFib. On top of that, high pressure → low pressure gradient highest at start of diastole. **MS, TS.On the flip side, | |
| Presystolic (Late diastolic) | Stage 1: Atrial kick across stenotic valve. ** | |
| Mid-diastolic (Rumble) | Stage 6: Rapid filling across stenotic AV valve. ** Crescendo into S1. Practically speaking, **AR, PR. Practically speaking, | |
| Holosystolic diastolic | Rare. Severe AR + MS (Austin Flint), or large VSD with aortic prolapse. |
Key distinction: Systolic murmurs = outflow or regurgitation into low-pressure chamber. Diastolic murmurs = inflow obstruction or regurgitation into high-pressure chamber. Diastolic murmurs are never innocent But it adds up..
The Pressure-Volume Loop — The Unifying Graph
If you understand the PV loop, you understand the cardiac cycle. X-axis: Volume. One loop = one beat. Y-axis: Pressure.
- Bottom-right corner (EDV): End of Stage 1. Mitral valve closes. S1.
- Vertical line up (Isovolumetric Contraction): Stage 2. Volume constant. Pressure rockets.
- Top horizontal line (Ejection): Stages 3 & 4. Aortic valve opens. Volume drops (SV). Pressure peaks then falls.
- Top-left corner (ESV): End of Stage 4. Aortic valve closes. S2.
- Vertical line down (Isovolumetric Relaxation): Stage 5. Volume constant. Pressure plummets.
- Diagonal filling line (Filling): Stages 6 & 7 & 1. Mitral valve opens. Volume rises. Pressure rises slightly (compliance curve).
- Shallow slope = stiff ventricle (↑ EDP).
- Steep slope = compliant ventricle.
- Back to start.
Area inside loop = Stroke Work. Width = SV. Height = Afterload. Position = Preload/Contractility Not complicated — just consistent..
Summary: The Stage Cheat Sheet
| Stage | Name | Valves | ECG | Sound | Key Event |
|---|---|---|---|---|---|
| 1 | Atrial Systole | AV: Open / SL: Closed | P wave → PR seg | S4 (if stiff) | "Atrial kick" (20-30% fill) |
| 2 | Isovolumetric Contraction | All Closed | PR seg → R wave | S1 | Pressure ↑, Vol constant |
| 3 | Rapid Ejection | AV: Closed / SL: Open | R wave → ST seg | — | Max flow, ↑ dP/dt |
| 4 | Reduced Ejection | AV: Closed / SL: Open |
| Stage | Name | Valves | ECG | Sound | Key Event |
|---|---|---|---|---|---|
| 1 | Atrial Systole | AV: Open / SL: Closed | P wave → PR segment | S4 (if stiff) | “Atrial kick” (20‑30 % of filling) |
| 2 | Isovolumetric Contraction | All Closed | PR segment → R wave | S1 | Pressure ↑, volume constant |
| 3 | Rapid Ejection | AV: Closed / SL: Open | R wave → ST segment | — | Max flow, ↑ dP/dt |
| 4 | Reduced Ejection | AV: Closed / SL: Open | ST segment → T wave | — | Pressure falling, flow tapering |
| 5 | Isovolumetric Relaxation | All Closed | T wave → QRS | S2 | Pressure ↓, volume constant |
| 6 | Rapid Filling | AV: Open / SL: Closed | QRS → end‑QRS | Early diastolic murmur (AR, PR) | Rapid passive fill |
| 7 | Diastasis | AV: Open / SL: Closed | — | — | Slow, low‑velocity fill |
| 8 | Atrial Filling | AV: Open / SL: Closed | — | Presystolic murmur (MS, TS) | Atrial kick, end‑diastolic pressure rises |
Tip: The heart’s “beat‑by‑beat” story is always the same. The PV loop is the single, continuous graphic that captures every change in pressure and volume; the auscultatory clues are the audible snapshots that echo the underlying mechanics Worth keeping that in mind..
Putting It All Together
-
Murmur Timing → Mechanism
Systolic murmurs arise when blood is forced through a narrowed or regurgitant outflow tract.
Diastolic murmurs arise when blood flows backward into a high‑pressure chamber or when a stiff valve resists inflow. -
Physiologic Modifiers
• Vasodilators (nitro, caffeine) lower afterload → ↑ ejection → louder AS, quieter VSD.
• Vasoconstrictors (phenylephrine) raise afterload → ↓ ejection → louder VSD, quieter AS.
• Inotropes (dobutamine) increase contractility → ↑ afterload → louder AS, quieter VSD.
• Volume shifts (standing) → ↓ preload → louder VSD, quieter AS.
• Position changes (squatting) → ↑ preload & afterload → louder VSD, quieter AS. -
Clinical Pearls
- AS: crescendo–decrescendo, harsh, louder with handgrip.
- MS: rumble, best heard at apex with left lateral decubitus; louder with Valsalva.
- VSD: harsh holosystolic, louder with handgrip.
- AR: early diastolic decrescendo, best at left sternal border; louder with squatting.
- PR: early diastolic, best at left sternal border; louder with handgrip.
-
Diagnostic Workflow
- Identify timing (systolic, mid‑diastolic, presystolic).
- Characterize quality (harsh, rumble, holosystolic).
- Apply modifiers (handgrip, Valsalva, position).
- Localize (apex, left sternal border, right sternal border).
- Correlate with ECG & hemodynamics (pressure gradients, Q‑wave changes).
- Confirm with imaging (echo, MRI).
Conclusion
The cardiac cycle is a choreographed sequence of pressure‑volume changes that produce the murmurs we hear. The pressure‑volume loop remains the ultimate unifying model, translating the audible clues into quantitative hemodynamics. By matching the timing of a murmur to the mechanism of blood flow, applying the physiologic modifiers, and listening at the right spot, clinicians can pinpoint the underlying lesion with remarkable precision. Mastery of this framework turns the stethoscope from a simple listening tool into a powerful diagnostic compass—guiding us from the first murmur to the appropriate imaging, therapy, and ultimately, patient care.
