Oscillometric methods of BP measurement
The auscultatory methods of BP measurement are gradually being replaced by oscillometric techniques that are better suited to automated methods of measurement. When oscillations of pressure in the gradually deflating bladder cuff are sensed and recorded, the point of maximal oscillation corresponds to the mean intra-arterial pressure.15 The oscillations sensed are vibrations in the arterial wall that are detected and transduced to an electric signal, producing a digital readout, and correspond approximately to the systolic pressure and continue below the diastolic pressure. The actual systolic and diastolic pressures are indirectly estimated according to a proprietary, empirically derived algorithm that differs from 1 manufacturer to another.
Validated oscillometric techniques have been successfully used in ambulatory BP monitors, which record pressure at regular intervals (typically 20 to 30 minutes) over a 24-hour period while patients perform normal daily activities, including sleep. The US Preventive Services Task Force16, the UK’s National Institute for Health and Clinical Excellence17, the European Society of Hypertension18, and the Canadian Hypertension Education Program19 collectively endorse ambulatory BP monitoring as the optimal method for BP measurement.
The oscillometric method has also been used for automated office BP measurement, which averages multiple BP readings recorded with a fully automated device while the patient rests alone in a quiet room in clinic. Compared with conventional auscultatory office BP measurement, this method has been promoted to provide a more standardized BP measurement by reducing observer error and the “white coat” effect.20-22
There are some limitations to oscillometric methods. The amplitude of oscillations is influenced by factors other than BP, notably, arterial wall stiffness. Therefore, in older patients13 or those with diabetes14 who have reduced arterial wall elasticity, oscillometric BP measurements overestimate systolic pressure and underestimate diastolic pressure. In contrast, acutely ill patients, particularly those with hypovolemia and more compliant arterial walls, may have significant underestimation of BP by oscillometric techniques.23 In patients with peripheral arterial disease, calcified leg vessels can affect the diagnostic accuracy of oscillometric measurement of the ankle-brachial index (ABI).24 A meta-analysis reported that in patients with atrial fibrillation, oscillometric measurement accurately assesses systolic BP but not diastolic BP, and therefore it may be inappropriate for office measurement of BP in these patients.25 Other studies have reported that atrial fibrillation does not significantly affect the accuracy of oscillometric BP measurement if 3 repeated measurements are performed.26,27
Moreover, the algorithms used in these devices are proprietary trade secrets that can be modified by the manufacturer at any time without notice. Therefore, different devices—and even different models from the same manufacturer—may function differently. Only devices calibrated using a validated protocol should be used.10,28 There are currently 4 unique protocols for validation of BP devices, although an international collaborative group recently published recommendations for a universal protocol for validation of BP measurement devices.29
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