Jun 7, 2012
Improving blood pressure measurement
A typical blood pressure measurement involves inflating a cuff around a patient's upper arm and taking a reading of their systolic and diastolic blood pressure (SBP and DSP) at two random instances in time. However, due to natural variability, SBP and DSP can shift up to 20 mmHg within a few heartbeats – a fluctuation that is notably greater than the maximum allowable device error of ±5 mmHg.
Writing in Physiological Measurement, researchers at the University of Ottawa in Canada have expressed concern that estimating blood pressure in this conventional manner does not provide information on these clinically significant fluctuations. Now, in order to get a better sense of the uncertainty associated with both blood pressure readings, the team has developed a technique that measures the SBP and DBP over a 90 s interval (Physiol. Meas. 33 881).
"Our technique provides the mean SBP and DSP and the associated confidence intervals over the measurement interval, which is arguably more representative than the conventional method using two random instants of time," Hilmi Dajani from Ottawa's School of Electrical Engineering and Computer Science told medicalphysicsweb. "Our method also provides the standard deviation, which gives information about the variability of blood pressure, and an indicator as to whether the estimate of the SBP and DSP is an outlier."
Two cuffs, simultaneous acquisition
The technique developed by Dajani, graduate student Karen Soueidan, and their colleagues uses two pressure cuffs, one for each arm. The first cuff is placed on the upper left arm and is held at a constant pressure close to the diastolic pressure of the patient in order to acquire the arterial pulse waveform. The second cuff, on the upper right arm, is inflated to just above the patient's systolic pressure and then slowly deflated in order to make an oscillometric measurement.
Simultaneous acquisition of data from the two cuffs is triggered by the team's LabVIEW-based control software and continues for a period of 90 s. "The oscillometric technique is popularly used in automated devices," commented Dajani. "We detect small oscillations in pressure that are superimposed on the cuff pressure. These oscillations are used to construct the oscillometric waveform from which systole and diastole are detected."
All data are then processed off-line in order to provide three quantities: the mean SBP and DBP over the 90 s measurement interval; the standard deviation (SD) of the SBP and DBP over the measurement interval; and an indication of any points exceeding the 2xSD margin. According to Dajani, the algorithm used to calculate this information can easily be adapted to report the results in real-time.
Tests on healthy volunteers
Fifteen adults with no known history of heart disease took part in a trial to assess the performance of the Ottawa team's approach. In each case, measurements were carried out six times to give 90 readings in total.
The team reports that, on average, over all subjects and trials, 15.7% of the SBP points and 14.6% of the DBP points were found to exceed ±5 mmHg from the mean values. In addition, 11.24% of the SBP and 4.49% of the DBP points exceeded the 2xSD margin and were classified as outliers.
"There was excellent agreement between the average of the interval mean values provided by our method and the average of the values obtained by the conventional method," commented Dajani. "This indicates that our method does not introduce additional measurement errors, which is important. The fact that we were able to identify outliers is also significant."
The next steps for the Ottawa group are to evaluate its approach using a cohort of healthy and hypertensive subjects, and also validate results against the gold standard invasive intra-arterial blood pressure measurement.
About the author
Jacqueline Hewett is a freelance science and technology journalist based in Bristol, UK.