The development of microelectronics first necessitated by the NASA space programme has resulted in new technologies for measuring blood pressure. Assessment of the waveform produced by blood flowing through arteries can be made oscillometrically, and devices now exist which can convert such waveforms into a single measurement of blood pressure equivalent to that obtained using a stethoscope listening to Korotkov sounds. This has led to two similar but different ways of obtaining multiple blood pressure measurements.
Ambulatory Blood Pressure Monitoring
While beyond the scope of this commentary, the use of devices that can be worn by patients over (usually) a 24-hour period with multiple measurements has demonstrated a number of points:
The average of multiple measurements at home tends to be lower that that measured in a surgery or clinic, although this difference is less if the blood pressure is measured by a nurse rather than by a doctor .
The reproducibility of the average produced is much greater than that of clinic measurements (see ref. ).
Observational data have demonstrated that such measurements are much better predictors of cardiovascular outcome than any clinic or surgery BP. Arguably, this is simply because the figure produced by the average of multiple measurements is a more robust measure of an individual's usual pressure.
Ambulatory monitors remain expensive, probably are best utilised in centres of expertise and, whilst cost-effective for the diagnosis of hypertension, are not practicable for the long-term management of individuals who require changes in drug therapy over months and years.
Self- or Home Blood Pressure Monitoring
There remains variability in the literature as to what to call this technology, and although the term 'home monitoring' is still used extensively, such devices can be used in any environment.
The science that led to the development of ambulatory monitors is available in small, simple-to-use self-monitors that use the same oscillometric technology and allow patients to measure blood pressure themselves semiautomatically as often as they wish or as required.
Home monitors are generally at least an order of magnitude cheaper than ambulatory devices and can be bought without reference to a health care professional in major pharmacies in the United Kingdom and many other countries.
As such electronic devices have developed; national specialist organisations have recommended that they are validated against the so-called 'gold standard' of the mercury manometer and a variety of validation protocols have been published to ensure that retailed machines are indeed accurate.
It is salutary to note that there is no obligation on a manufacturer to demonstrate accuracy before selling their devices, and there are examples of inaccurate machines being sold to the public. In Great Britain and Ireland, there are important agencies that have realised this problem and maintain a web-based list of validated devices for both patients and healthcare professionals [6, 7].
The average blood pressure obtained by multiple measurements with a self-monitor approximates to the daytime average of an ambulatory monitor , and there are some who have argued that self-monitoring can displace an ambulatory monitor in all aspects of hypertension diagnosis and management. What most home monitors will still not do is measure blood pressure through the night, and it is worth mentioning in passing that there is evidence that nocturnal pressure has independent predictability in terms of cardiovascular outcomes. As we do not yet have any clinical strategies as to how to treat nocturnal pressure, this remains an area for further research rather than a practical clinical reality.