Paradoxical S2 Split: Aortic Stenosis & LBBB (Medical Education)

You put your stethoscope on a patient's chest. On expiration, you hear a clear split in S2. On inspiration, the split disappears. A student next to you says "Oh, that's just normal splitting."

Is the student right?

Nailed it. Normal S2 splits on inspiration, closes on expiration. Paradoxical S2 does the exact opposite. The student just missed a big finding. Normal inspiration increases venous return, delays A2 (slightly), and you get a physiologic split. In paradoxical S2, A2 is SO delayed it flips to the other side of P2. On expiration, the gap reappears. On inspiration, it actually closes. This is the board's favorite trick.

Trace It

Why does A2 go rogue?

Watch it happen in real time.

Valve Closure Timing · Click a phase to see what changes

A2 (Aortic valve closes)

P2 (Pulmonary valve closes)

Normal S2: A2 closes FIRST, then P2. On inspiration, venous return pushes P2 a bit later. Small physiologic split.

The whole thing in one sentence

Normally: A2 then P2. The left side finishes ejecting first, aortic closes, then pulmonic closes. Easy.

In paradoxical S2: the left side is pathologically delayed. A2 keeps getting pushed back and back until it leapfrogs P2. Now the order is P2 then A2. The "split" you hear on expiration is P2...then a late A2. On inspiration, normal hemodynamics push P2 a bit later too, accidentally closing the gap.

Normal: S1 ............... A2 P2 ............

Paradoxical: S1 ............... P2 A2 ............ ← ORDER REVERSED

🔑 Paradoxical = the LEFT side is slow. Anything that delays left ventricular ejection can cause it. The order of valve closure flips: P2 comes first, late A2 comes second.

The Big Four

S2 Splitting Cheat Sheet

This table is a board favorite. Know every row cold.

Type	Inspiration	Expiration	Causes	One-liner
Normal (Physiologic)	Splits	Fuses	Healthy lung :)	More blood in, P2 delays
Paradoxical	Fuses (closes!)	Splits	LBBB, Severe AS, HCM/HOCM, RV pacing	A2 so late it flips past P2
Wide	Splits wider	Still splits	RBBB, Pulmonic stenosis, LPHB	P2 is slow but still behind A2
Fixed	Splits	Splits (same width)	ASD (ostium secundum)	RV volume is always high, breathing doesn't matter

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Board Trap #1: Wide vs Paradoxical

Both seem like "abnormal splitting" but they're opposites. Wide split = P2 is slow, but A2 still comes first, so it splits on inspiration AND expiration. Paradoxical = A2 has lapped P2, so splitting only happens on expiration, and inspiration CLOSES it. Wrong answer = wrong bundle branch.

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Board Trap #2: ASD gives FIXED, not Paradoxical

ASD (ostium secundum) causes fixed splitting, not paradoxical. The left-to-right shunt keeps RV volume constantly elevated, so breathing never changes the split width. If the vignette says "split doesn't change with breathing" + atrial septal defect, that's fixed. Paradoxical requires a problem on the LEFT side of the heart.

Cause Explorer

Tap each cause to unlock the mechanism

All four delay A2. Each has a different reason why. 💡Memory: "LAH" = LBBB + Aortic Stenosis + HCM. These are the Late A2 club. Four drunks who can't leave the bar on time. RV pacing is a sneaky fifth.

LBBB

Left Bundle Branch Block

EKG: wide QRS, M-shaped R in V5/V6, deep S in V1. No septal activation.

Mechanism: The left ventricle depolarizes late and SLOW because the fast conduction highway is out. LV takes forever to squeeze. A2 gets pushed way past P2. Classic LBBB causes paradoxical S2 in most cases.

Severe AS

Aortic Stenosis (critical)

Crescendo-decrescendo murmur at RUSB, radiates to carotids. Pulsus parvus et tardus.

Mechanism: The aortic valve is so calcified and stiff that the LV has to fight hard to push blood through it. Ejection takes longer, A2 is delayed mechanically. Mild/moderate AS does NOT do this. It needs to be severe.

HCM / HOCM

Hypertrophic Cardiomyopathy

Murmur increases with Valsalva or standing. SAM of mitral valve. Young athlete.

Mechanism: Systolic anterior motion (SAM) of the mitral valve creates dynamic outflow obstruction. The LV fights a moving wall to eject blood. Ejection is prolonged, A2 is delayed. Paradoxical S2 can occur at rest or worsen with provocation that increases obstruction.

RV Pacing

Right Ventricular Artificial Pacing

Pacemaker on EKG with wide QRS (artificial LBBB pattern). Seen in pacemaker patients.

Mechanism: When you pace the RV, activation spreads from right to left via slow myocyte-to-myocyte conduction instead of the fast bundle branch system. This mimics LBBB electrically. The LV activates late and A2 is delayed. Functional LBBB.

💡 WPW Type B (right-sided accessory pathway) can also cause paradoxical S2 by activating the RV early via delta wave, making P2 come early and A2 appear to come "after" it. Rare board appearance but worth knowing.

Clinical Pearl

LBBB on EKG: What to look for

Most common cause of paradoxical S2. You will see this EKG.

LBBB Criteria

QRS > 120 ms (3 small boxes wide)

Broad, notched (M-shaped) R wave in lateral leads: I, aVL, V5, V6

Deep QS or rS in V1 (no R wave territory)

No septal q waves in I, V5, V6 (septal activation direction reverses)

Why it matters here

LBBB disrupts the normal fast conductionNormal: SA node fires, signal hits His bundle, splits L+R, each ventricle activates nearly simultaneously via fast Purkinje fibers in ~80ms total. of the left ventricle.

LV activation slows to ~180ms instead of ~80ms. That extra 100ms is what pushes A2 past P2 and creates the paradoxical split you hear on expiration.

HCM vs AS murmur: how to tell them apart fast

Both cause LV outflow obstruction. Both can cause paradoxical S2. The murmur tells you which is which.

Aortic Stenosis: RUSB crescendo-decrescendo, radiates to carotids. Worse with standing (less volume). Better with squatting (more volume). Calcified valve on echo.

HCM: LLSB murmur, increases with Valsalva and standing (decreased preload unmasks obstruction), decreases with squatting or leg raise. SAMSystolic Anterior Motion: the mitral valve leaflet flaps forward into the LV outflow tract during systole, creating dynamic obstruction. visible on echo.

📋 Board trick: Valsalva makes HCM murmur LOUDER and AS murmur softer. If they tell you a murmur increases with Valsalva, HCM wins. ⚡Valsalva drains the tank. HCM loves an empty tank (obstruction worsens). AS hates an empty tank (less to squeeze through). Empty = Valsalva/Standing. Full = Squat/Leg raise.

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Board Trap #3: "Split on expiration = paradoxical" is NOT always true

You CAN hear a split on expiration in a normal patient if they have a very wide normal split (RBBB, PS). The key is what happens on inspiration. If it closes on inspiration, that's paradoxical. If it stays wide or gets wider on inspiration, that's RBBB-wide. Always reference both phases, not just one.

Decision Tree: Interpreting Abnormal S2 Splitting

Start at the root. Pick the branch. Work to the answer.

Does S2 splitting increase or decrease with inspiration?

Prove It

Clinical Questions

Five vignettes pulled from a bank of 25 and reshuffled every visit. Nail these and your attending will think you read extra chapters. (You did.) Cross out a choice with right-click or long-press, highlight with double-tap.

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