Objective: To compare three different software versions [read-only memory (ROM) versions 1.22, 1.24 and 1.28] of the SpaceLabs model 90202 ambulatory blood pressure monitor.
Design: Simultaneous measurements in a two-arm crossover design.
Methods: Ten measurements each in 14 normotensive persons were performed to compare each pair of monitors. The results were corrected according to the deviation from a mercury column. The systolic (SBP), diastolic (DBP) and mean arterial blood pressure (MAP) was noted and the factor K = (MAP - DBP)/(SBP - DBP) was calculated.
Results: The K factor in the two older ROM versions was close to 0.25, with no statistically significant difference. In contrast the K factor was higher in the most recent ROM version than in one of the previous versions. This could be explained by a 5.4 mmHg increase in MAP found by the new version. A significant difference in blood pressure was observed even between two monitors with the same ROM version.
Conclusion: Unnotified changes in the software version in the SpaceLabs 90202 monitor operating by an oscillometric technique seriously affect the reliability of MAP measurements. Whenever possible in clinical trials the same monitor should be used for the same patient on each occasion. The industry should inform clinicians of the consequences of updating apparently identical monitors.