[Technical and methodologic aspects of ambulatory long-term blood pressure monitoring systems]

The development of noninvasive, portable blood pressure measuring units began in 1962 with semi-automatic devices and cassette recorders, followed by the first automatic unit in 1968 and the introduction of digital storage system in 1978. Systems in common use today consist of a portable, battery-driven blood pressure monitor and a print-out unit. In the following, the System 5200 from SpaceLabs, which has been in use for three years in our clinic, will be described.

In a monitor unit, amplication and filtering of analogue data measured and differentiation between signal and noise is carried out. An A/D converter digitalizes the analogue data. An integrated microprocessor analyses measured data, regulates inflation and deflation of the cuff pressure, out-put of measured and calculated values on an LCD display and storage of data. Data from 200 measurements is stored in a 2K byte RAM CMOS system. A personal computer serves for programming the monitor and evaluation of the stored data. Blood pressure measurement is carried out auscultatory with a microphone or oscillometrically if Korotkoff sounds are not detected. If the signal is disturbed, measurement is repeated within two minutes. Blood pressure measurements are performed at freely-programmable intervals from six to 60 minutes; varying time intervals can also be chosen. AUSCULTATORY BLOOD PRESSURE MEASUREMENT: A miniature pump integrated in the monitor inflates the cuff within a few seconds to a pressure of 160 mmHg or, on subsequent measurements, to 25 mmHg above the last recorded systolic value. On registration of Korotkoff sounds, the cuff pressure is increased in steps of 25 mmHg until the sounds disappear and then deflated in steps of 3 to 5 mmHg. On detection of the first Korotkoff sound, the instantaneous cuff pressure (which is converted to an electric signal by a transducer) is stored as the systolic value. Further deflation then occurs rapidly to 90 mmHg or 10 mmHg above the last measured diastolic value. With higher diastolic values, again, there is an increase in cuff pressure in steps of 25 mmHg until the onset of Korotkoff sounds and then renewed deflation in steps of 3 to 5 mmHg. On disappearance of the Korotkoff sounds, the prevailing cuff pressure is recorded and stored as the diastolic value (Figure 1). To register the Korotkoff sounds optimally, the microphone is positioned above the brachial artery.(ABSTRACT TRUNCATED AT 400 WORDS)