Ross-McGill and colleagues have demonstrated that in a low risk antenatal population routine antenatal visits could be replaced by a system of home blood pressure measurements without an increase in visits to hospital for ‘other reasons’ (Vol 107, February 2000)1. These results are very encouraging. Further studies should investigate both low and high risk antenatal populations to test fully this novel management strategy. The study is also timely as there is a need to evaluate alternatives to mercury sphygmomanometry which is being removed from clinical areas for reasons of health and safety.
There are a number of other benefits of using automated blood pressure monitors, including removing observer error and bias, and improving sampling. This may be why home measurements have a better relationship to outcome2. It is generally accepted that automated devices must be assessed for accuracy according to recognised protocols such as that of the British Hypertensive Society (BHS) and American Association of Medical Instrumentation (AAMI) before they are put into clinical use, as most devices available on the market are inaccurate. We have previously validated the blood pressure monitor used in this study (the Omron HEM 705 CP) in pregnancy. This achieved an acceptable BHS grade (B for both systolic and diastolic pressure) but failed to maintain accuracy in pre-eclampsia where it underestimated diastolic pressure by an average of 8 mmHg3. We recommend caution in interpreting blood pressures derived by oscillometric blood pressure devices. Even the ubiquitous DINAMAP (Critikon) significantly under-reads blood pressure in pre-eclampsia3. Other oscillometric devices are less inaccurate in pre-eclampsia, such as the portable Omron RX which maintains its B/B grading from normotensive pregnancy in pre-eclampsia4.
When conducting home monitoring studies it should be remembered that blood pressures recorded at home are significantly lower than those recorded in a hospital environment. There is as yet no data to quantify this difference regarding measurements in pregnancy, although the phenomenon of white coat hypertension is well recognised. Studies on ambulatory monitoring in pregnancy which provide sequential readings in a home environment suggest that the threshold with the best predictive value for adverse outcome is 135/85 mmHg3. Ross-McGill and colleagues have applied a traditional hospital threshold of 140/90 mmHg to a community setting and this may identify a higher risk population rather than an equivalent cohort of women (in terms of risk) who have a clinic blood pressure of 140/90 mmHg. For home monitoring to be a viable alternative to conventional antenatal care it will be essential to convince the obstetric community that any move away from ‘supervised care’ is both safe and effective. To achieve this threshold for referral to hospital when using home monitoring may be lower, and further studies should take this into account. There is however, the potential for home monitoring to improve surveillance while reducing antenatal visits.