Blood pressure is in itself a highly variable parameter. We explain why.
Some degree of blood pressure fluctuations happens to all of us, so it is absolutely normal and desirable. Even walking, talking, singing or laughing can cause noticeable increases, but differences may also arise if blood pressure is measured in a clinical environment or at home or if it is recorded manually or with an ambulatory blood pressure monitor.
Oscillometry vs. auscultation
There can be individual differences between an automated oscillometric blood pressure reading and that obtained by a human observer using a pressure meter and a stethoscope by auscultatory observation of Korotkoff-sounds on the same patient.
The auscultatory technique has been the gold standard of blood pressure measurement for over 100 years. The method relies on the professional to detect the audible sounds (Korotkoff-sounds) that arise during constricted blood flow. The oscillometric method, however, analyzes pulse waves collected from the cuff during constricted blood flow. Today's ambulatory blood pressure monitors use the oscillometry principle. Oscillometric devices sense small oscillations in the cuff pressure accompanying each heart cycle and resulting arterial pulse wave, and they use special algorithms to derive systolic and diastolic blood pressure values from a complex analysis of detected waveforms. Therefore most ambulatory blood pressure monitors do not use Korotkoff-sound detection at all. This has a lot of advantages included the elimination of the observer's bias such as poor hearing or too fast deflation of pressure which can prevent accurate detection of K-sounds. These advantages have practically made oscillometry the most wide-spread solution in the ABPM field.
Other reasons for differing blood pressure (BP) results
1. Potential white coat or reverse white coat effect: BP may fluctuate if measured in a medical environment or on an outpatient basis.
2. BP can be different on left and right arm.
3. Human and device readings are for technical reasons (almost) never conducted as simultaneous same-arm measurements, so either time or between-arm difference may have an effect.
4. Posture: different body positions may also affect BP results, as BP tends to drop in standing position.
5. Any observable change in breathing patterns may also have a significant effect on BP in itself.
6. The static pressure measurement accuracy of typical pressure measuring devices is at best +/- 3 mmHg, which may be another source of bias.
7. A lot of pressure meters (mercury, aneroid, digital) are often known to have even less static pressure measurement accuracy.
8. Cuff size and tightness: undercuffing or overcuffing with a non-average arm size doen not provide accurate results.
9. Repeated BP measurements, especially with short time intervals: the longer the sequence, the more serious the effect.
10. Accuracy problems in the BP monitor.
Being aware of these differences helps medical professionals in better hypertension control and drug treatment.