Validation testing of the spacelabs PC2 ST-segment analyzer.

OBJECTIVES

Recent studies have demonstrated that perioperative myocardial ischemia, detected by electrocardiography, is a risk factor for myocardial infarction. ST-segment analyzers and hemodynamic monitors may be useful for on-line detection in perioperative and critical care environments. However, independent performance and accuracy standards for these devices have not been established. Therefore, a testing protocol was developed using an electrocardiogram (ECG) simulator that allowed selectively altered ST-segment displacement, in a calibrated fashion over a wide range.

DESIGN

Laboratory bench study.

SETTING

Not applicable.

PARTICIPANTS

Not applicable.

INTERVENTIONS

Not applicable.

MEASUREMENTS AND MAIN RESULTS

Custom digital ECG waveform templates were programmed for use with a commercially available ECG simulator (M311 ECG simulator; Fogg Systems, Inc., Aurora, CO). For each template, ST-segment morphology (horizontal elevation or depression, downsloping depression), QRS duration, and the presence or absence of a P wave were manipulated, resulting in seven different QRS shapes. Within each shape, the degree of ST-segment deviation was altered over a wide range. A PC2 Bedside Monitor (SpaceLabs Inc., Redmond, WA) was tested. One hundred forty-eight measurements of ST-segment deviation input from the simulator were made at each of two testing sessions. The first ST-segment value displayed by the analyzer was recorded, and the two measurements averaged for comparison. Placement of the J-point, J + 60 msec, and isoelectric reference points by the analyzer were evaluated. Simulator output was validated for accuracy and stability. Subtle errors in placement of the J-point marker were observed in all seven QRS shapes. These errors usually did not alter placement of the isoelectric marker before, but not exactly at the beginning of, the R-wave upstroke. Thus, ST-segment values returned by the monitor (J + 60 msec - isoelectric reference value) were unaffected. However, in two QRS shapes, the isoelectric point was displaced onto the upstroke of the R wave, resulting in erroneous ST-segment values. In one, the error may have been caused by the difference in QRS duration of that template (120 msec) relative to the fixed 115-msec interval from the J point used by the analyzer and was present in all points tested. In the second (normal QRS duration), the error was present in some, but not all points tested (4/21, 24%). All QRS shapes with proper placement of the isoelectric point returned ST-segment values within +/- 0.5 mm of expected, and 98% were within +/- 0.25 mm of expected. The mean difference between observed and expected ST-segment values for 100 measurements with normal QRS duration and proper isoelectric point placement was 0.08 mm +/- 0.07 mm (SD).

CONCLUSIONS

The bench results suggest that visual confirmation of ST-segment analyzer values may be advisable in the clinical setting. Although most complexes with normal conduction and a P wave are likely to be accurately analyzed, those with prolonged QRS duration were problematic. The simulator protocol may be helpful in ensuring accuracy of ST-segment analyzers, especially in their early development stages.