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The Interrupted Run Chart for Monitoring Blood Pressure Measurements

Robert F. Hart, Ph.D.
Marilyn K. Hart, Ph.D.

This is a case study, using simple tools, that shows the usefulness of monitoring individual patient data over time. Here, the blood pressure of a male patient in his forties was followed intermittently over a five-and-a-half-year period. During that time period, two important things changed: the level of exercise activity and the stress level of the job.

An excellent initial method of analysis is the use of an interrupted run chart. The run chart is much simpler than the control chart because it requires no assumptions about the distribution or stability of the data. The run chart is simply a graphical record of the data over time. An interrupted run chart is interrupted at points where there is a known change in the process or a known interruption in the data acquisition.

The interrupted run chart for systolic blood pressure is shown in Figure 1 and for diastolic in Figure 2. The interruptions to the process divide the data into four time eras. In the first three time eras, the patient had only a moderate exercise level, approximately equivalent to 0.5 hours of walking per day. In the fourth era, the patient walked 1.5 hours per day. The patient had the same job and about the same stress levels in all except the second era (where there are only two pairs of measurements). In the second time era, the patient was temporarily on a job that he found to be very stressful.

Time eras 1 and 3 are for the "usual" process with moderate exercise and no particular job stress. The run charts show the process to be relatively stable through these two time eras. One would not be surprised to find higher than "usual" blood pressures during the higher stress levels of era 2 and lower than "usual" blood pressures due to the increased exercise of era 4.

The systolic run chart identifies the special nature of eras 2 and 4. The diastolic run chart shows clearly the special nature of era 2. Although the diastolic indication of special cause variation is less clear with era 4, the similarity of the systolic and diastolic results adds additional credibility to this interpretation.

The interrupted run charts lead to the conclusion that the data are stratified by era and the results cannot reasonably be attributed to chance -- a "textbook" example of the influences of stress and exercise on blood pressure.

All too often decisions are made only on the most recent data point. Instead, the retrospective analysis shown above would make a good point of departure for a prospective, or ongoing, activity to monitor the patient's blood pressure. Often, ongoing process control efforts use a run chart on the observations with action limits beyond which observations would be deemed clinically important. This approach is simple, straightforward, and effective.


Figure 1 Interrupted Run Chart on Systolic Blood Pressure for the Four Eras


Figure 2 Interrupted Run Chart on Diastolic Blood Pressures for the Four Eras

Acknowledgment: This case study was developed with the assistance of Richard Stanula of Pleasant Prairie, Wisconsin, an advocate of statistical process control and a friend for 20 years.

Reference: Excerpted from Statistical Process Control for Health Care, by Marilyn K. Hart, Ph.D. and Robert F. Hart, Ph.D., Duxbury Press, 2002.

For more information, contact Drs. Robert and Marilyn Hart at robthart@aol.com.

If you would like additional information, please send email to statit.support@conduent.com.