The Cornell University Ambulatory Blood Pressure Project has used several generations of Spacelabs Ambulatory Blood Pressure devices beginning with the DelMar pictured above. The first generation device utilized a microphone taped over the brachial artery to record Korotkoff sounds to determine blood pressure. Subsequent devices have utilized the oscillometric method of measuring blood pressure as described below.

This was first demonstrated by Marey in 1876, and it was subsequently shown that when the oscillations of pressure in a sphygmomanometer cuff are recorded during gradual deflation, the point of maximal oscillation corresponds to the mean intra-arterial pressure. The oscillations begin at approximately systolic pressure and continue below diastolic pressure, so that systolic and diastolic pressure can only be estimated indirectly according to some empirically derived algorithm. One advantage of this method is that no transducer needs to be placed over the brachial artery, so that placement of the cuff is not critical. Two studies using the Dinamap automatic recorder (Critikon, Tampa, Florida, USA), which works on the oscillometric principle, have shown excellent agreement between intra-arterial and oscillometric measurement, particularly when compared with the rather poor results using the auscultatory technique. Another study using a different oscillometric device (The Takeda 751; A and D Engineering, Tokyo, Japan) also found that the agreement with intra-arterial readings was as good as for auscultatory measurement. Yelderman and Ream compared mean arterial pressure measured directly from the radial artery with oscillometriclly determined mean pressure from the brachial artery during anesthesia, and found remarkably close agreement between the two, with an average difference of 1 ± 6 mmHg. Furthermore, the correlation coefficients for the within-subject changes were also close (r = 0.87 on average). The oscillometric technique has recently been used successfully in ambulatory blood pressure monitors (such as the Spacelabs recorders, Spacelabs, Redmond, Washington, USA). It should be pointed out that different brands of oscillometric recorders use different algorithms, and there is no generic oscillometric technique.

Other potential advantages of the oscillometric method for ambulatory monitoring are that it is less susceptible to external noise (but not to low-frequency mechanical vibration), and that the cuff can be removed and replaced by the patient, for example to take a shower. The main disadvantage is that such recorders do not work well during physical activity, when there may be considerable movement artefact.

Source: Pickering TG. Ambulatory Monitoring and Blood Pressure Variability 1991, Science Press Ltd., London, England (page 2.8).