AuscultifyCan You Hear S3 in AFib? Gallops and Extra Sounds in Atrial Fibrillation
Yes, you can hear an S3 in atrial fibrillation. S3 depends on passive rapid ventricular filling in early diastole, which continues whether or not the atria contract. You cannot hear an S4 in AFib, because S4 is produced by atrial contraction and there is no organized atrial contraction in fibrillating atria. That pairing is a classic exam point, and it also has practical consequences at the bedside.
Why S3 survives and S4 does not
Diastole has two filling phases. The first is rapid passive filling: the mitral valve opens and blood pours from a full atrium into the relaxing ventricle down a pressure gradient. The second is atrial contraction, the atrial kick, which tops up the ventricle just before S1.
S3 is produced at the end of the first phase, when inflow decelerates abruptly in a volume-loaded or dilated ventricle. Nothing about that requires a coordinated atrium, so in AFib the mechanism is intact. S4 is produced by the second phase, when the atrium pushes blood into a stiff ventricle. In AFib the atria quiver at 400 to 600 impulses a minute and generate no effective contraction, so the second phase and its sound are gone. The same logic explains why the presystolic accentuation of a mitral stenosis murmur disappears in AFib. For the physiology of each gallop see S3 heart sound and S4 heart sound.
What AFib does to the S3 you hear
An S3 in AFib is rarely as tidy as the "Ken-TUCK-y" cadence in a recording taken in sinus rhythm. Three things change.
- It comes and goes. Each beat follows a different diastolic interval. After a long pause the ventricle has more time to fill and the inflow volume is larger, so an S3 is more likely and louder. After a short cycle there is barely a rapid-filling phase and the S3 may vanish.
- Its loudness varies. For the same reason, the S3 is not a fixed-intensity sound. Do not dismiss a gallop just because it is audible on some beats and not others.
- It hides behind the irregularity. With no regular lub-dub to compare against, a soft extra sound after S2 can be misread as another irregular beat. Anchoring S1 to the carotid pulse helps you keep the sounds in order.
Variable S1 in AFib
The other famous auscultatory feature of AFib is a first heart sound that varies in intensity from beat to beat. S1 loudness depends partly on where the mitral leaflets are when the ventricle starts to contract. After a short diastole the leaflets are still wide open and snap shut from a distance, giving a loud S1. After a long diastole they have drifted almost closed, giving a soft S1. Because every R-R interval in AFib is different, S1 changes with every beat. This variability is a useful confirmation of the rhythm when the irregularity alone leaves you unsure, and it is covered in more detail in What does atrial fibrillation sound like?.
If you hear an "S4" in an irregular rhythm
Treat it as a prompt to re-examine, not a finding to write down. There are three possibilities:
- The rhythm is not AFib. Frequent premature atrial or ventricular beats, atrial flutter with variable block, or sinus rhythm with Wenckebach can all sound irregular yet keep an organized atrial contraction on most beats. An S4 is possible in those rhythms. Check the ECG.
- The sound is not an S4. A split S1, an ejection click just after S1, or an S3 misplaced in your mind because of the irregular spacing are the usual culprits. Pitch and chest piece settle it: a split S1 and a click are high-pitched and clearer with the diaphragm; an S4 is low-pitched, apical and fades with firm bell pressure.
- Both rhythm and sound are what you think, which is not possible. AFib cannot generate an S4. If the examination and ECG genuinely conflict, the ECG wins.
Which extra sounds and murmurs persist in AFib?
| Sound | Present in AFib? | Why |
|---|---|---|
| S3 | Yes, intermittent and variable | Depends on passive rapid filling, not atrial contraction |
| S4 | No | Requires an organized atrial contraction |
| Summation gallop | No | Needs an S4 to merge with the S3 |
| Opening snap (mitral stenosis) | Yes | Produced by the stenotic valve opening under atrial pressure, which does not need contraction |
| Mid-diastolic rumble of MS | Yes, but variable in length | Passive flow across the narrowed valve continues; the murmur lengthens after long cycles |
| Presystolic accentuation of MS | No | Produced by atrial contraction pushing flow across the valve just before S1 |
| Systolic murmurs (AS, MR) | Yes, variable in loudness | Stroke volume changes with each cycle length; AS murmur is louder after a long pause, MR murmur changes less |
| Variable S1 | Yes, characteristic | Mitral leaflet position differs with each R-R interval |
The behaviour of the aortic stenosis murmur after a long pause is worth remembering. A longer diastole means a larger stroke volume through a fixed orifice and a louder ejection murmur on the next beat, whereas a mitral regurgitation murmur, which depends on the LV-to-LA gradient rather than stroke volume, changes much less. This is the same principle that makes a post-extrasystolic beat useful for separating AS from MR.
What an S3 in AFib suggests
In an adult, an S3 in AFib carries the same meaning as in sinus rhythm: a volume-loaded, dilated or failing ventricle. The usual causes are systolic heart failure, dilated cardiomyopathy and significant mitral regurgitation, all of which also predispose to AFib. Practically, an S3 tells you the arrhythmia is sitting on structural heart disease with raised filling pressures, which raises the priority of an echocardiogram, changes the rate-control drug you reach for, and shifts the anticoagulation and heart failure conversations. A young patient with lone AFib and a compliant ventricle can have a physiologic S3, so age and context still matter.
How to listen for an S3 in AFib
- Use the bell with light pressure over the apex, patient in the left lateral decubitus position. See bell vs diaphragm for why pressure matters.
- Palpate the carotid with your free hand so you always know which sound is S1.
- Listen through many cycles, at least 20 to 30, because the S3 is most likely after the longest pauses.
- Focus on the beats that follow a long gap and ask whether a dull thud trails the S2 of that beat.
- Then press firmly. If the extra sound disappears it was low-pitched and consistent with an S3; if it persists, think opening snap or split S2.
Hear AFib and gallops together in Auscultify
Auscultify's library includes real atrial fibrillation recordings captured at several precordial sites, plus S3 and S4 recordings you can compare against them. In the Heart Sounds and Source Check quiz modes, AFib is mixed in with regular rhythms and extra sounds so you learn to keep S1 and S2 straight when the spacing is chaotic. The Spectrogram view (Pro) makes the variable S1 and the intermittent S3 visible beat by beat.
Hear AFib recordings free →FAQ
Can you hear S3 in AFib?
Yes. S3 is generated by passive rapid ventricular filling in early diastole, which does not require atrial contraction, so it can be heard in atrial fibrillation. It tends to be intermittent and to vary in loudness because diastolic filling changes from beat to beat.
Can you hear S4 in AFib?
No. S4 is produced by atrial contraction into a stiff ventricle, and in atrial fibrillation there is no organized atrial contraction. An S4 in AFib is a classic impossibility on exams and at the bedside.
What are the heart sounds in AFib?
An irregularly irregular rhythm with S1 that varies in intensity from beat to beat, a normal S2, no S4, and sometimes an S3 that comes and goes. Murmurs that depend on atrial contraction, such as the presystolic accentuation of mitral stenosis, disappear.
What does an S3 in atrial fibrillation mean?
In an adult it means the same as an S3 in sinus rhythm: a volume-loaded or poorly compliant ventricle, most often systolic heart failure, mitral regurgitation or a dilated cardiomyopathy. It suggests the AFib is sitting on top of structural heart disease and raised filling pressures.
Why does S1 vary in AFib?
S1 loudness depends on how far open the mitral leaflets are when the ventricle contracts. In AFib each beat follows a different diastolic interval, so leaflet position and ventricular filling differ from beat to beat and S1 changes in intensity accordingly.