S1: The First Heart Sound

S1 is your first landmark when listening to the heart. It marks the beginning of systole, and understanding what creates it—and what makes it louder or softer—will help you diagnose various cardiac conditions.

Before we dive in, consider this: if S1 is created by valve closure, why would it ever be loud in some conditions and soft in others? What determines the "force" of a closing valve?

What Creates S1?

S1 is primarily created by the closure of the mitral and tricuspid valves. As the ventricles start to contract, pressure rises rapidly, and these atrioventricular (AV) valves slam shut. The sound itself comes from multiple sources working together: the valve leaflets snapping closed, vibration transmitted through the valve apparatus (chordae tendineae and papillary muscles), and the sudden deceleration of blood as forward flow is abruptly halted.

Two Components: M1 and T1

Technically, S1 has two parts. M1 (mitral closure) comes first and is usually louder because left ventricular pressure rises faster than right ventricular pressure. T1 (tricuspid closure) follows almost immediately but is softer. In practice, you usually hear these as a single sound because they occur nearly simultaneously—the delay between them is only about 20-30 milliseconds. Unlike S2 splitting (which we'll explore in detail), splitting of S1 is rare and not clinically useful.

The Universal Principle

What Determines S1 Intensity?

Four key factors control how loud S1 sounds. Understanding these mechanisms makes everything else predictable.

The PR Interval Effect

The PR interval is one of the most important—and most testable—determinants of S1 intensity. It controls valve position at the moment of ventricular contraction.

Short PR → Loud S1

When the PR interval is short, the mitral valve is still wide open when ventricular contraction begins. The valve has to close from a wider position, creating a louder sound—think of slamming a door from fully open versus barely cracked. You see this in pre-excitation syndromes like WPW.

Long PR → Soft S1

When the PR interval is prolonged, the mitral valve starts drifting closed before ventricular contraction begins. When the ventricle finally contracts, the valve is already partially closed, producing less sound. If S1 seems unusually soft, check the ECG for PR prolongation (first-degree AV block).

Other Causes of Loud S1

Mitral Stenosis

The stenotic, stiff mitral valve closes forcefully. Additionally, elevated left atrial pressure keeps the valve in a wide-open position until ventricular contraction suddenly closes it. A loud S1 in the setting of a diastolic rumble at the apex should immediately suggest mitral stenosis.

Hyperdynamic States

More forceful ventricular contraction creates more forceful valve closure. You encounter this with exercise or anxiety, fever, thyrotoxicosis, and anemia—any condition that increases cardiac output through increased contractile force.

Causes of Soft S1

Mitral Regurgitation

An incompetent valve doesn't close properly. There's less forceful coaptation of the leaflets, producing a softer sound. You'll typically also hear a holosystolic murmur at the apex radiating to the axilla.

Poor Left Ventricular Function

Weak ventricular contraction means less forceful valve closure. In dilated cardiomyopathy, severe heart failure, or acute MI, you may also hear an S3 gallop and find a displaced PMI.

Anatomical Barriers

Obesity, COPD with hyperinflation, and pericardial effusion all increase the distance or add fluid between the heart and your stethoscope, muffling all heart sounds—not just S1. If all sounds are distant, think about these anatomical factors rather than valve pathology.

Variable S1: Arrhythmias

In some conditions, S1 varies in intensity from beat to beat. This finding is clinically important and often points to rhythm abnormalities where the relationship between atrial and ventricular contraction is disrupted.

Complete Heart Block (Third-Degree AV Block)

When the atria and ventricles beat independently, the relationship between atrial and ventricular contraction changes constantly. Sometimes the atrium contracts just before the ventricle (loud S1), sometimes while the mitral valve is already closing (soft S1), and sometimes during ventricular systole (producing cannon A waves in the JVP). Variable S1 with cannon A waves is a classic finding in complete heart block.

Atrial Fibrillation

The irregularly irregular ventricular rate means inconsistent diastolic filling times. Variable filling leads to variable contractile force and thus variable S1 intensity. You'll also note the absence of S4 (no organized atrial contraction).

Ventricular Tachycardia

AV dissociation creates changing relationships between atrial and ventricular contraction, similar to complete heart block.

Explore S1 Intensity

Use this simulator to see how different factors affect S1 intensity. The guided walkthrough will help you understand the relationship between PR interval, contractility, valve condition, and the loudness of S1.

Clinical Scenarios

Let's put this knowledge to work. For each scenario, think about what you would suspect before revealing the answer.

Scenario 1

You hear: Loud S1 + diastolic rumble at apex + opening snap

Scenario 2

You hear: Soft S1 + holosystolic murmur at apex radiating to axilla

Scenario 3

You hear: Variable intensity S1 + irregular rhythm

Now that you understand S1, let's move on to S2—arguably the more interesting and diagnostically useful of the two normal heart sounds, particularly because of its splitting patterns.