Phase 1 gives systolic pressure and phase 5 gives diastolic
Mnemonic
Five phases, of which only two are recorded:
- Phase I - the first tapping sound. Systolic pressure
- Phase II - softening, with a swishing quality
- Phase III - crisper, louder sounds return
- Phase IV - abrupt muffling
- Phase V - disappearance. Diastolic pressure
“First tap is systolic, silence is diastolic.”
Phase IV is used instead of phase V where sounds never disappear: in pregnancy, aortic regurgitation, thyrotoxicosis, anaemia and in children.
The auscultatory gap is the trap: sounds vanish between phases II and III in some patients, particularly with hypertension, so the systolic pressure is underestimated unless the cuff is first inflated 20 to 30 mmHg above the palpated radial pressure.
Expansion
As the cuff deflates:
- Phase 1 - first appearance of clear tapping sounds: systolic pressure
- Phase 2 - softer, swishing sounds
- Phase 3 - crisper, louder sounds return
- Phase 4 - abrupt muffling
- Phase 5 - complete disappearance of sound: diastolic pressure
The sounds are turbulence, generated only while the cuff pressure lies between systolic and diastolic values, so the artery opens and closes with each beat.
Two pitfalls. The auscultatory gap, a silent interval between phases 2 and 3 seen in hypertension and arterial stiffness, causes systolic pressure to be underestimated unless the cuff is first inflated above the palpated systolic pressure. And a cuff that is too small overestimates pressure, while one that is too large underestimates it.