4. What alternatives measurement devices are there?
- 4.1 Technical aspects of the alternatives to Hg sphygmomanometers
- 4.2 Auscultatory mercury-free sphygmomanometers
- 4.3 Non-auscultatory mercury-free sphygmomanometers
The SCENIHR opinion states:
4.1 Technical aspects of the alternatives to Hg sphygmomanometers
The Korotkov sounds in the artery may be detected by auscultation which may be performed either manually (by the observer) or automatically (by electronic equipment). Since the Hg manometer is only the pressure sensing and displaying component in the occluding cuff technique, other manometers can be used instead. Although a lot of different pressure measuring techniques are conceivable, the following two are applied in sphygmomanometers:
- An aneroid manometer with an analogue display (circular scale with a pointer) and
- An electrical pressure transducer with analogue look, but digital display. In addition to the alternative devices using auscultation, there also exists the oscillometric technique which does not use auscultation, but instead uses the oscillation in the cuff pressure due to the pulsation in the artery.
4.2 Auscultatory mercury-free sphygmomanometers
4.2.1 Non-automated auscultatory devices
Sphygmomanometers using aneroid (or mechanical) gauges (based on an elastic pressure sensing element) are common alternatives to Hg sphygmomanometers. The aneroid machines do not use liquid to display the information about the estimated values for the blood pressure levels.
Aneroid sphygmomanometers have been available for probably as long as the mercury manometer. They are commonly used for handheld sphygmomanometers, but are also available for portable or wall-mounted sphygmomanometers. The reliability of the aneroid manometer is affected by the technical design of the device and the quality of its production to a much greater extent than the mercury manometer. As one example, the long-time stability (reproducibility) of the aneroid manometer requires a pre-aging of the elastic pressure sensing element.
Another important issue is the sensitivity to mechanical shock. A simple standard aneroid manometer will not usually withstand drops from the table or heavy strokes. Since this is not acceptable in daily life the ISO/IEC Joint Working Group was the first to introduce requirements on mechanical strength for portable and handheld aneroid manometers. With the exception of stationary non-automated sphygmomanometers, including the aneroid type, all devices must function normally following a free fall from 25 cm. Additional requirements exist for all non-automated sphygmomanometers, including the aneroid type when they are labelled "Shock Resistant”; these must withstand drops from 1 m without the loss of performance. Devices following the requirements of ISO 81060-1, especially those labelled "Shock Resistant”, will be robust enough for normal handling.
However, there are some reservations about the maintenance of the mechanical parts of the aneroid machine (Coleman et al. 2005). Other limitations with auscultation are similar to those with mercury manometer.
ELECTRONIC devices translate the pressure in the cuff into analogue-like or numerical display. The Hg column is simulated by a LCD (or LED), or there is a numerical display, or the pointer of the aneroid gauge is simulated by LEDs (Graves et al. 2004, Stergiou et al. 2008a).
These devices measure the pressure of the cuff with an electrical transducer similar to an automated sphygmomanometer. Regarding the pressure measurement, these devices follow the requirements for automated sphygmomanometers. A disadvantage of these devices is that electrical power is required.
4.2.2 Automated auscultatory devices
The first automated sphygmomanometers became available in the 1970s. These devices were designed to replace the observer and their stethoscope with a microphone and some analogue electronics. The microphone is placed in a small pocket in the cuff. The analogue electronics amplifies and filters the Korotkov sound detected by the microphone, and each detected Korotkov sound is displayed by a flashing light (LED). The user of the device has to place the cuff on the upper arm, place the microphone over the brachial artery on the upper arm, and inflate and deflate the cuff manually. They also have to read the displayed cuff pressure at the moment the LED starts to flash for systolic and at the moment it ceases to flash for diastolic blood pressure. There are still some of these devices available on the market (see Figure 2). The main applications for these devices are blood pressure measurements in subjects with an irregular heart beat, as oscillometric sphygmomanometers cannot give reliable readings in these situations.
Figure 2 Example of an auscultatory sphygmomanometer, which indicates Korotkov sounds by a flashing LED (red LED on the left). The cuff with the microphone is not shown3.
Another area of application of automated auscultatory sphygmomanometer is non- invasive blood pressure measurement during ergometric stress testing, because the oscillometric technique cannot be used here due to its sensitivity on arm movement. These devices are fully automated, i.e. they pressurise the cuff automatically and display numerical values of the blood pressure.
Figure 3 Ergometer with automated auscultatory sphygmomanometer3. [Source: http://testserver.vollewanne.de/../de/sana-bike_250f/sana- bike_250f.php]
3 Disclaimer: The devices shown on figures 2 and 3 are only for illustration as examples of the various existing applications irrespective of their validation status. The European Commission does not endorse their use or their manufacturers.)
The reliability of the blood pressure measurement of the automated auscultatory sphygmomanometer described above is highly dependent on the correct placement of the microphone over the brachial artery. Too much noise is another limitation of the application of such devices.
In recent years automated devices have been developed which measure the blood pressure using both the oscillometric and the auscultatory technique. These devices usually place the microphone not in the cuff but in the housing of the device. The Korotkov sound is transferred through the bladder and the hose to the microphone. Some devices give priority to the results determined by the oscillometric method, using the auscultatory signal for identifying artefacts due to arm movement or beats on the cuff. Other devices give priority to results determined by the auscultatory method and use the oscillometric measurement as a backup.
4.3 Non-auscultatory mercury-free sphygmomanometers
The non-auscultatory mercury-free sphygmomanometers use the oscillometric technique to measure the blood pressure based on changes in the artery pulsation during cuff inflation/deflation. These alternatives to the mercury sphygmomanometer are easy and uncomplicated to use. They do not use the auscultation technique, and it is easier to train users. Increasingly, they are used by patients for home blood pressure monitoring and also almost exclusively for 24-hour ambulatory blood pressure monitoring. They need very little maintenance, costs vary according to the additional capabilities of the machine, and calibration testing is needed regularly as per the manufacturer’s instructions, usually within two years. The inflation of the cuff may be performed manually (semi-automated) or automatically; however, the deflation is controlled by the device.