AuscultifyReading Phonocardiograms: Visual Waveform Interpretation Guide
A phonocardiogram (PCG) is a graphical recording of heart sounds and murmurs plotted as amplitude over time. Where auscultation gives you audio, a phonocardiogram adds a visual dimension — you can see the timing, duration, shape and relative intensity of each sound. Learning to read PCGs makes it far easier to understand why sounds sound the way they do.
The basics: what you're looking at
A standard phonocardiogram shows:
- X-axis: time (one cardiac cycle = one heartbeat).
- Y-axis: amplitude (loudness of the sound).
- Systole: the interval between S1 and S2 (shorter interval at normal heart rates).
- Diastole: the interval between S2 and the next S1 (longer interval at normal heart rates).
S1 and S2 appear as brief, high-amplitude deflections. Everything between and around them tells the clinical story.
Identifying S1 and S2
Step one of reading any PCG: find S1 and S2 reliably.
- S1 is the first major sound — it coincides with the QRS on a simultaneous ECG. It has a slightly lower frequency than S2.
- S2 is the second major sound — it coincides with the end of the T wave. It's typically shorter and higher-pitched than S1.
- Timing trick: systole (S1 to S2) is shorter than diastole (S2 to next S1) at normal heart rates. The short gap is systole.
Waveform patterns of common murmurs
| Finding | Waveform shape | Timing |
|---|---|---|
| Aortic stenosis | Diamond-shaped (crescendo-decrescendo) envelope between S1 and S2 | Mid-systolic; peak shifts later as severity increases |
| Mitral regurgitation | Uniform rectangular band from S1 to S2 | Pansystolic (holosystolic) — fills the entire systolic interval |
| Aortic regurgitation | Decrescendo slope starting immediately after S2 | Early diastolic, tapering toward mid-diastole |
| Mitral stenosis | Low-amplitude rumble after an opening snap, with pre-systolic crescendo | Mid-to-late diastolic |
| MVP | Sharp click spike in mid-systole, followed by a crescendo murmur to S2 | Mid-to-late systolic |
| PDA | Continuous oscillation peaking at S2, extending through systole and diastole | Continuous |
Extra sounds on the PCG
- S3: a low-amplitude deflection in early diastole, shortly after S2. On a PCG it looks like a small bump — easy to miss visually just as it's easy to miss aurally.
- S4: a low-amplitude deflection in late diastole, just before S1. Often slightly larger than S3 on the tracing.
- Ejection click: a brief sharp spike immediately after S1, before the murmur envelope begins. Seen in bicuspid aortic valve and pulmonic stenosis.
- Opening snap: a brief spike in early diastole, before the mitral stenosis rumble. The closer the OS is to S2, the more severe the stenosis (higher LA pressure opens the valve earlier).
Spectrograms: the frequency dimension
A spectrogram (also called a sonogram) adds a third dimension — frequency — displayed as color intensity on a time-frequency plot:
- High-pitched murmurs (aortic regurgitation, mitral regurgitation) appear as bright bands in the high-frequency range.
- Low-pitched sounds (S3, S4, mitral stenosis rumble) appear as activity in the low-frequency range.
- Spectrograms make mixed sounds visible. When two findings overlap in time (e.g., a murmur and a gallop), the spectrogram separates them by frequency where the waveform alone cannot.
How to practice PCG reading
- Start with normal: get comfortable identifying S1, S2, systole and diastole on a normal tracing before adding pathology.
- Pair audio with visual: listen to a sound while looking at its waveform. Your brain builds a two-channel memory — sound + shape — that's far stronger than either alone.
- Focus on the envelope: is it diamond-shaped, rectangular, decrescendo or crescendo? The envelope shape narrows the differential immediately.
- Use timing as your anchor: systolic or diastolic? This single question eliminates half the possibilities before you even assess shape or pitch.
See and hear sounds together in Auscultify
Auscultify's Spectrogram mode (Pro) displays the real-time frequency spectrogram alongside the audio for every recording — so you can watch the waveform while you listen. This audio-visual pairing is the fastest way to build the pattern recognition that makes murmurs click. Pair it with quiz modes and you'll start seeing the diamond shape of AS or the flat band of MR before you even consciously process the sound.
Try spectrogram training free →