Blood pressure is the force exerted by circulating blood against the artery walls. This measurement is recorded as two numbers, expressed in millimeters of mercury (mmHg). The higher number is the systolic pressure, reflecting the maximum force when the heart contracts. The lower number is the diastolic pressure, which measures the residual force within the arteries when the heart rests between beats. Auscultation is the traditional, manual technique used by healthcare professionals globally to obtain these two values.
The Physiology of Korotkoff Sounds
Manual blood pressure measurement relies on detecting Korotkoff sounds, which are generated by changes in blood flow under a compressed artery. When the cuff is inflated, it completely collapses the brachial artery, stopping blood flow and creating silence. As the cuff pressure is slowly released, blood begins to pulse through the narrowed artery, creating turbulence that produces audible vibrations.
The first distinct, repetitive sound heard is Phase 1, and this tapping noise correlates with the systolic blood pressure. As the cuff continues to deflate, the sounds proceed through four additional phases, changing in tone and intensity. Phase 2 features a softer, swishing quality, followed by the louder, crisper tapping of Phase 3 as the artery opens further.
The sounds then become muffled and soft in Phase 4, marking a change in blood flow dynamics. The point at which all sounds completely disappear is Phase 5, which indicates the diastolic blood pressure. The disappearance of sound signifies that the cuff pressure is no longer compressing the artery, allowing smooth, non-turbulent blood flow to resume.
Step-by-Step Manual Measurement
Accurate auscultation requires a sphygmomanometer (inflatable cuff and pressure gauge) and a stethoscope to listen for the Korotkoff sounds. Before the procedure, the patient must be seated quietly with their back supported and feet flat for at least five minutes. The arm selected should be bare, supported at heart level, and free of restrictive clothing.
Proper cuff selection is important, as a cuff that is too small can artificially elevate the reading. The cuff should be wrapped snugly around the upper arm, with the lower edge positioned about 2.5 centimeters above the antecubital crease. The artery marker must be aligned over the brachial artery. The practitioner first uses palpation, feeling for the radial pulse while inflating the cuff, to estimate the systolic pressure.
Inflating the cuff 20 to 30 mmHg above the point where the radial pulse disappears prevents the auscultatory gap. This gap is a common error where sounds briefly disappear, leading to an underestimated systolic pressure. The stethoscope’s diaphragm is then placed lightly over the brachial artery, just below the cuff. The cuff must be deflated slowly and steadily at a rate of 2 to 3 mmHg per second.
Maintaining this slow deflation rate ensures the practitioner does not miss the exact pressure points of the Korotkoff phases. The reading on the manometer at the first sound (Phase 1) is recorded as the systolic pressure. The reading at the moment the sounds completely vanish (Phase 5) is recorded as the diastolic pressure. The cuff is then rapidly and fully deflated.
When Auscultation is the Preferred Method
While automated oscillometric devices are convenient, the manual auscultatory technique remains the clinical reference standard in many situations. This method is superior because it relies directly on the physical sounds of blood flow. These sounds are less susceptible to physiological variables that can confuse automated algorithms. The auscultatory technique provides results that correlate more closely with direct, intra-arterial measurements.
In patients with irregular heart rhythms, such as atrial fibrillation, beat-to-beat pressure variation can cause automated devices to yield inaccurate readings. Similarly, in cases of extreme blood pressure, including severe hypertension or hypotension, oscillometric algorithms may lose reliability. Manual auscultation allows the practitioner to interpret the sounds directly, ensuring a more dependable measurement despite erratic pulse or pressure extremes.
The manual method is also recommended for monitoring women with hypertensive disorders of pregnancy, such as pre-eclampsia. Accurate and consistent diastolic pressure measurement is important for diagnosis and management in this context. The auscultatory method, using Korotkoff Phase 5 for the diastolic value, provides the reliability needed for clinical decisions in this population.