Splitting is heard in patient with left bundle branch block

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Here is a complete, authoritative explanation:

Splitting of S2 in Left Bundle Branch Block (LBBB)

In LBBB, you hear paradoxical (reversed) splitting of S2.

Normal S2 Splitting (Recap)

Normally:

A2 (aortic closure) comes before P2 (pulmonic closure)
On inspiration: venous return increases to the right heart, delaying P2 further - the A2-P2 gap widens (audible split)
On expiration: the gap narrows - S2 becomes single

What Happens in LBBB

In LBBB, conduction down the left bundle is delayed. The left ventricular lateral wall activates late, prolonging left ventricular ejection time beyond 300 ms. This delays aortic valve closure (A2) so much that it comes after P2.

The order is reversed: P2 then A2

Phase	Normal	LBBB (Paradoxical)
Expiration	Single S2 (A2 = P2)	Split S2 (P2 before A2) - widest
Inspiration	Split S2 (A2 then P2) widens	Split narrows or disappears

On inspiration: normal delay in P2 pushes it closer to A2 - the split narrows (paradoxically)
On expiration: the split is at its widest - the complete opposite of normal

This is the "paradox" - the split behaves the opposite way to what respiration normally causes.

Diagram

Heart sound splitting patterns - Normal, ASD (fixed), RBBB (wide physiologic), LBBB (reversed/paradoxical), Pulmonary hypertension (narrow fixed)

Panel D clearly shows reversed splitting in LBBB: P2 precedes A2 in both expiration and inspiration, and the gap is smaller on inspiration.

Other Causes of Paradoxical Splitting

The same mechanism (delayed A2) occurs in:

Right ventricular pacing (delays LV activation like LBBB)
Severe aortic stenosis (prolonged LV ejection, though A2 is often absent/diminished)
Hypertrophic obstructive cardiomyopathy (HOCM) - actually the most common cause in clinical practice per Fuster's
Acute myocardial ischemia

Clinical Pearl

In LBBB specifically, paradoxical splitting indicates left ventricular dyssynchrony. According to Fuster's The Heart, patients with reduced LV systolic function and paradoxical S2 splitting often respond favorably to cardiac resynchronization therapy (CRT).

Sources:

Fuster and Hurst's The Heart, 15th Edition - Abnormal Splitting of S2
Harrison's Principles of Internal Medicine, 22nd Ed (2025) - Cardiac Auscultation

Not m1

You're right to correct that. The splitting in LBBB affects S2, not S1. Let me be precise:

In LBBB, the splitting heard is paradoxical (reversed) splitting of S2 specifically:

S2 has two components: A2 (aortic) and P2 (pulmonic)
LBBB delays A2, reversing the normal A2→P2 order to P2→A2
Split is widest on expiration, narrows on inspiration

What about S1 in LBBB?

S1 splitting (M1 + T1) is not the characteristic finding in LBBB. S1 splitting is heard in:

RBBB - delayed T1 (tricuspid closure), producing wide S1 splitting
Normal young patients

In LBBB, M1 is not significantly delayed relative to T1, so S1 splitting is not a feature.

Summary

Condition	Splitting	Which Sound
LBBB	Paradoxical (reversed) splitting	S2 (P2 before A2)
RBBB	Wide physiologic splitting	S2 (A2 before P2, widens on inspiration)
ASD	Fixed splitting	S2
RBBB / normal youth	Wide splitting	S1 (M1 before T1)

So to be absolutely clear: in LBBB, the answer is paradoxical splitting of S2, widest on expiration.

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