Measured differences between fourth and fifth phase diastolic blood pressures in 4885 adults: implications for blood pressure surveys.

To examine in detail the relationship between fourth (D4) and fifth (D5) phase diastolic blood pressures, we used data from risk factor surveys of 4885 adults. Observers were carefully trained using audio tapes and double-stethoscope exams to record the onset of the first, fourth (muffling), and fifth (disappearance) phases of Korotkoff sounds with random-zero sphygmomanometers. The expected size of the difference between D4 and D5 pressures (D4-D5) was not discussed with observers. D4-D5 was greater (mean +/- SEM) in men (2.4 +/- 0.1 mmHg) than in women (1.9 +/- 0.1 mmHg), but two thirds of D4-D5s were not measurable, that is, 0 mmHg. Larger D4-D5s were associated with higher systolic and D4 levels, lower D5 levels, and younger ages. D4-D5 varied markedly among observers. Approximately 58% of D4-D5s exceeding 10 mmHg occurred among six observers who measured 16% of the blood pressures. However, generally the difference between D4 and D5 measured by our observers and methods was smaller than commonly believed. The possible reasons for this are discussed. We recommend that epidemiological studies measure and report both diastolic readings (D4 and D5) so that their interrelationship may be studied further in other populations.