Which statement best describes how the upper threshold is used?

• A. It is used to cover up the T wave
• B. It is used to detect the P wave
• C. It is used to shape the QRS amplitude
• D. It is used to suppress noise

Answer: (A)

In ECG signal processing for detecting the sharp QRS complex, an upper threshold serves as a clipping or masking level that suppresses larger, smoother deflections like the T wave. By limiting use of the signal above a certain level, the detector avoids treating the T wave—which is broader and slower—as a potential R peak, and it stays focused on the brief, steep QRS spike. This helps reduce false detections caused by the T wave while preserving the ability to catch true R waves.

The other ideas don’t fit as well because the upper threshold is not about isolating the P wave, shaping the QRS amplitude through gain-like effects, or primarily suppressing noise. Those functions are handled by different aspects of filtering and processing; the upper threshold specifically helps keep T-wave activity from being mistaken for the QRS.