Thank you for your inquiry regarding CMS996v4/OP-40 (Appropriate Treatment for ST-Segment Elevation Myocardial Infarction (STEMI) Patients in the Emergency Department (ED)). The measure only tracks the start time and end time of the ED encounter. Therefore, until the patient is discharged from the ED, the ED encounter remains active, regardless of other diagnostic procedures or services that may take place concurrently. The Denominator Exclusions state that cases in which cardiac arrest occurs 24 hours or less before the start of the ED encounter or during the ED encounter are excluded from the measure calculation. The exclusion applies to cases where a cardiac arrest occurs in the Cath lab, as long as it occurs within the designated timeframe. Please also note that discharging the patient to an acute setting within 45 minutes of ED arrival (either for alternative treatments or further diagnostics) counts toward the Numerator, assuming that the discharge disposition is documented as “Discharge To Acute Care Facility”. Thus, patients discharged to an acute care facility within 45 minutes of ED arrival, who then experience cardiac arrest in a post-discharge laboratory setting such as a Cath lab, would not be excluded from the denominator.