5. How accurate are the alternatives?
- 5.1 Clinical aspects of the alternatives to Hg sphygmomanometers
5.1 Clinical aspects of the alternatives to Hg sphygmomanometers
A wide variety of devices can be used to measure blood pressure and apart from the intensive care setting, the majority remain non-invasive and include non-automated auscultatory devices ( aneroid, non-mercury auscultatory), semi-automated and automated devices (that can be used either at the upper arm, wrist or finger). The alternatives to Hg sphygmomanometers have hugely different levels of reliability.
5.1.1 Auscultatory devices
Aneroid devices – These devices are mercury free, commonly used in clinical practice, and require auscultation to determine blood pressure. They consist of a system of bellows and gears that expand to display pressure using a gauge needle and a pressure display. These devices are easily susceptible to damage and drift of the cuff pressure measurement (Waugh et al. 2002) particularly if they are portable (Bailey et al. 1991) and this leads to inaccurate measurements. A recent study in a primary care setting (in the United Kingdom) has shown that more than 50 percent of aneroid devices had a cuff pressure measurement error >3mmHg compared to only 8 percent of mercury and automated devices combined (Coleman et al. 2005). This is consistent with previous literature. It is therefore recommended that these devices undergo a metrological check at least annually, although the implementation of this recommendation appears unlikely especially in primary care (Rouse and Marshall 2001). The number of erroneous readings obtained with aneroid devices is likely to be significant. Improvements in the technology to prevent measurement error may lead to a suitable and accurate alternative to the mercury sphygmomanometer. The use of harmonized ISO/CEN standards will promote further improvement of these devices.
ELECTRONIC non-mercury auscultatory devices: As an auscultatory alternative, electronic devices use a pressure sensor and a digital display (numerical, circular/linear bar graph). Models such as the Accoson Greenlight 300 (Graves et al. 2004), PMS Mandhaus (Wilton et al. 2006) and Nissei DM-3000 (British Hypertension Society, 2006) have been introduced, all of which have received clinical recommendation following an independent accuracy assessment. As the pressure transducers used within these systems are less prone to measurement error than the bellows in aneroid devices, these auscultatory devices can be assumed to be more reliable if used by a trained observer.
The cuff pressure is displayed as a simulated mercury column using an array of LCDs, and also as a digital LCD readout. The cuff is deflated in the normal way and, when the first and fifth Korotkov sounds indicating systolic and diastolic pressure are heard, a button next to the deflation knob is pressed, which freezes the digital display to show systolic and diastolic pressures, thus offering the potential of eliminating terminal digit preference, which is a major problem with the clinical use of any auscultatory monitor. With such devices, the physician is still able to measure blood pressure using the traditional auscultatory technique, without having necessarily to rely on automated readings, and this is achieved without the problems associated with mercury columns or aneroid devices.
These devices are suitable for patients where clinical conditions such as arrhythmia and pre-eclampsia may preclude the use of automated oscillometric devices. However the reading of such devices cannot be assumed to be equivalent to the reading of a mercury column, where the interpretation of a falling column of mercury with its own inherent dynamics, with an intermittent signal of Korotkov sounds, may not be the same as an electronic alternative. For this reason formal validation is required for any new device being introduced on the market. In addition features that are added to assist with the blood pressure determination, e.g. a hold button, may introduce an error as it does not control for the recognition, and reaction time and may result in a device not reaching an acceptable standard (Stergiou et al. 2008a). However, studies on the physician’s reaction time and decision time during blood pressure measurements with this method are in progress to improve the reliability of this approach.
Some non-mercury professional devices allow for both automated electronic (oscillometric) as well as auscultatory blood pressure measurement by an observer using a digital manometer (El Assaad et al. 2002, Omboni et al. 2007, Stergiou et al. 2008b, Stergiou et al. 2008c).
5.1.2 Automated non-auscultatory (oscillometric) devices
There is an ever-increasing market for oscillometric blood pressure devices that have also increased home surveillance such as self-measurement and ambulatory/24hr monitoring. Home blood pressure measurement has been shown to be more reproducible than office blood pressure measurement (Stergiou et al. 2002) more predictive of cardiovascular events (Bobrie et al. 2004, Ohkubo et al. 2004) and reliable when used by non-clinicians (Nordmann et al. 1999). The out-of-office measurements are effective at removing the white-coat effect (Parati et al. 2003) particularly when using an averaging mode (Wilton et al. 2007). Telemonitoring enables the patient to transmit home measurements directly to the clinician’s computer for further analysis, potentially enhancing early identification, reducing hospital visits (Pare et al. 2007) and improving the degree of blood pressure control also in general practice (Parati et al. 2009a).
Automated devices are generally intended for use on the upper arm, but finger and wrist devices are also available. Few of these latter devices have been shown to be accurate according to independent accuracy assessments; only a small minority of wrist devices assessed achieved an acceptable accuracy (five in total) (O'Brien and Atkins 2007). Wrist devices are sensitive to errors related to positioning of the wrist at heart level, and some devices have position sensors. Very few of the wrist devices have passed clinical validation after independent assessment (Altunkan et al. 2006, Nolly et al. 2004). However, even the validated wrist devices with position sensors appear to give significantly different blood pressure values than arm devices in a large proportion of hypertensive patients (Stergiou et al. 2008d), while in an earlier study no such differences were observed (Cuckson et al. 2004). The European Society of Hypertension Guidelines state the preference of arm over wrist oscillometric devices (O’Brien et al. 2003, Parati et al. 2008b). No finger device has yet achieved the established validation standards (Elvan-Taspinar et al. 2003, Schutte et al. 2004).
The oscillometric technique is usually used by automated devices to determine blood pressure by analysing the pressures transmitted through arterial oscillations/vibrations that occur during cuff inflation and/or deflation. The point of maximum oscillation equates to the mean arterial pressure. The recording of pressure waves is dependent on the anatomical position, elasticity and size of the artery, as well as the distribution of the surrounding tissue which is particularly difficult in the wrist. A device specific algorithm equates these signals to the pressure obtained by the pressure transducer. The technique is not generic in any way, and each device must have its algorithm validated.
Automated blood pressure measurement will eliminate the observer errors associated with the use of the manual auscultatory technique such as terminal digit preference, threshold avoidance, observer prejudice, rapid deflation etc. (Beevers et al. 2001). However, clinically significant differences exist between measurements obtained through automation compared to auscultation in many devices. Automated device accuracy is not only device dependent, but also user dependent. As these devices are more likely to be used by untrained individuals, errors related to selecting correct cuff size and taking the recommended arm position, ensuring no movement or talking during device measurement, or allowing for sufficient rest before measurements may be more pronounced than mercury sphygmomanometers. Various guidelines have been published for the correct use of automated devices with specific methodologies advocated (Chobanian et al. 2003, O'Brien et al. 2003, Parati et al. 2008a), but are not as established as training for auscultatory blood pressure measurement. Automated devices have accuracy limitations in special groups such as those with vascular damage that influences the oscillometric signal: these include patients with diabetes, arrhythmias or pre-eclampsia, and the elderly. This is related to arterial/vascular changes in these patients, which are likely to influence the recording of pressure waves by the device. The British Hypertension Society and some websites list devices that have achieved clinical recommendation under these conditions. arrhythmias maybe detected by devices fitted with an ‘irregular pulse detection’ indicator; however, clinical validation for measuring blood pressure during arrhythmias has not yet been performed. This is confounded by not having a reliable reference value as the "gold standard” as mercury sphygmomanometer is itself an indirect measure of blood pressure and how blood pressure relates to this measure is unknown in arrhythmias. A limited number of devices have been validated and found accurate for use in pregnancy (Shennan and de Greeff 2007, Chung et al. 2009) and most of these are inaccurate in pre-eclampsia. There is one anecdotal report of a maternal death in pre-eclampsia when an oscillometric device (not validated for this condition) was used and underestimated the blood pressure level (Lewis and Drife 2001).
There are some "preliminary positive” data regarding the accuracy of oscillometric devices in "difficult” populations, such as in patients with end-stage renal disease (Thompson et al. 2007), atrial fibrillation (Watson and Lip 2006), the elderly (Omboni et al. 2007) and children (Stergiou et al. 2006). However, it should be realised that there are always some patients in which the oscillometric blood pressure measurement might differ significantly from that taken by a mercury sphygmomanometer without apparent reason, probably influenced by arterial wall properties and pulse pressure (Stergiou et al. 2009, Van Popele et al. 2000,).
An accurate automated sphygmomanometer capable of providing printouts of systolic, diastolic and mean blood pressure, together with heart rate and the time and date of measurement, should eliminate errors of interpretation and abolish observer bias and terminal digit preference. Moreover, the need for elaborate training of observers would no longer be necessary, although a period of instruction and assessment of proficiency in using the automated device will always be necessary. Another advantage of automated measurement is the ability of such devices to store data for later analysis (Parati G et al. 2008b). This development is in fact taking place, and a number of long-term outcome studies are using automated technology to measure blood pressure instead of the traditional mercury ‘gold standard’. For example, in the large Anglo–Scandinavian Cardiac Outcome Trial, the validated Omron HEM-705CP automated monitor was used including thousands of patients followed for about five years (Dahlöf et al. 2005, Hansson et al. 1998, Yusuf et al. 2008).
The mercury sphygmomanometer is disappearing from use and there are many alternative devices available to replace it. Blood pressure measurement with the auscultatory technique by a trained observer, using the mercury sphygmomanometer remains the most accurate and reliable form of indirect blood pressure measurement and is currently regarded as the gold standard.
The alternative devices using auscultation have similar limitations as the mercury sphygmomanometers regarding the observer bias associated with auscultation itself. Even though oscillometric instruments are not considered as true "alternatives" to Hg sphygmomanometers because they operate under a completely different principle, those instruments are currently replacing the Hg sphygmomanometers. The advent of accurate oscillometric devices, however welcome, is not without problems. First, oscillometric devices have been notorious for their inaccuracy in the past, although more accurate devices are now appearing on the market. Secondly, most of the available oscillometric devices were designed for self-measurement of blood pressure by patients, and it should not be assumed that they will be suitable for clinical use, or that they will remain accurate with use, although some are being used successfully in hospital practice. Thirdly, oscillometric techniques cannot measure blood pressure accurately in all situations, particularly in patients with pre-eclampsia, arrhythmias such as atrial fibrillation, and there are also individuals in whom these devices cannot measure blood pressure, for reasons that are not always apparent (Stergiou et al. 2009a, Van Popele et al. 2000). All alternative blood pressure measurement devices need to be clinically validated in clinical protocols against the current gold standard of the mercury sphygmomanometer, until an alternative device is developed and recognised as such. Several international protocols, such as the ISO protocol (in preparation), the British Hypertension Society (BHS) and the European Society of Hypertension (ESH) International Protocol are available for such a clinical validation. A list of validated oscillometric devices is available on dedicated websites, such as the British Hypertension Society as well as other national learned societies.