Hypertension in pregnancy is defined as a diastolic blood pressure reading >90mm Hg or a systolic blood pressure reading >140 mmHg on two consecutive readings 4 hours apart. Devices to measure blood pressure are needed to predict the onset of eclampsia, a condition which causes tens of thousands of annual global deaths. Eclampsia is a dangerous seizure condition which is predicted by preeclampsia, the occurrence of hypertension and protein in the urine in pregnancy. Experts estimate that approximately half of all hypertension in pregnancy is associated with preeclampsia. Devices to measure blood pressure are also necessary in cases of hemorrhage to determine if fluid replacement or transfusion are necessary to save the life of a woman experiencing postpartum hemorrhage.
Mechanism of Action
Blood pressure measurement typically involves the occlusion of the brachial artery at approximately the height of the heart using an inflatable cuff. The cuff is slowly deflated, and the device detects the pressure at which blood flow resumes (systolic pressure) and the pressure at which it is fully unencumbered by the cuff (diastolic pressure). In the ausculatory method, the caregiver uses a stethoscope to listen to the Korotkoff sounds as the cuff deflates, noting the pressure at each sound. In the oscillometric method, a pressure transducer in the cuff measures oscillations in the cuff pressure (caused by blood flow) as it deflates. The device then calculates blood pressure and displays it on a digital read out. While these automatic devices do not require the skill and related accuracy challenges of manual devices, some models systematically under report blood pressure.
Current Use in High-Resource settings
Several authors have noted the inaccuracy of blood pressure measurement around the world, citing relatively low levels of agreement between sequential observers using manual equipment. Automated, oscillometric equipment is gradually replacing manual equipment, although it must be validated for use with pregnant women and calibrated at regular intervals. There is also movement to phase out mercury sphygmomanometers due to mercury concerns.
Application in Low-Resource settings
Manual, ausculatory cuffs are most commonly used in low-resource settings. Given the high level of user error associated with the auscultatory technique, however, the WHO recommends validated, low cost oscillometric blood pressure measurement devices replace older equipment. The WHO further recommends all devices be semi-automatic and solar powered due to concerns over batteries and battery maintenance. These devices are not yet common on the market.
Related Technologies in Development
Solar Blood Pressure Monitor (Conway Institute), Solar Blood Pressure Monitor (King's College)