How Algorithms for Heart Rate Detection Are Designed in Hospital-Based Electrocardiographic Monitors

Scenario: Electrocardiogram (ECG) and other waveforms from a bedside monitor showing 1 of 10 asystole alarms generated during a 12-hour shift in a 72-year-old patient admitted to the medical/surgical intensive care unit (ICU) for abdominal pain. The bedside nurse has responded to the multiple alarms and is puzzled about why there are asystole alarms when an R wave is clearly visible in lead II and the arterial blood pressure (AR1) and pulse oximetry oxygen saturation (Spo2) waveforms are both normal. The patient has a history of heart failure, hypertension, and diverticulitis.Sinus tachycardia with low-amplitude QRS complexes in the 2 recorded ECG leads.The ECG algorithm used in this particular bedside ICU monitor requires the following in order to identify heart rate: (1) QRS amplitude > 5 mm; (2) the QRS amplitude requirement applies only to unidirectional (only positive or negative) Q, R, or S waveforms—in other words a waveform with a Q, R, and S wave is not added together; and (3) criteria 1 and 2 must be present in at least 2 of the following ECG leads: I, II, III or V (V1 at our hospital) but not aVR, aVL, or aVF. In this example, only lead II is available for heart rate detection; hence, an asystole alarm was generated (note that the heart rate = 0, indicated in upper left corner). Of note, the R wave in lead II is only 4 mm in height (positive portion of R wave), so the QRS amplitudes in leads I, III, or V1 would need to meet the above criteria or asystole alarms would persist. Although the arterial blood pressure and Spo2 (plethysmogram) waveforms are present and indicate sufficient blood pressure and oxygenation in this patient, these waveforms are not incorporated into the heart rate algorithm. Note that the American National Standard (ANS) for cardiac monitors requires that ECG devices not label QRS complexes if the waveform is less than 1.5 mm in amplitude to avoid mislabeling P waves as QRS complexes when critical arrhythmias such as complete heart block or asystole are present. However, it is not uncommon for device manufacturers to use higher QRS amplitude thresholds than those required by the ANS for cardiac monitors.Ensure that all of the ECG skin electrodes and lead wires are attached to the patient and remain in place during continuous ECG monitoring. Patients with low QRS amplitude are at higher risk of tachyarrhythmias and bradyarrhythmias. Underlying cardiac conditions (heart failure in this case), medications, and electrolyte imbalance, especially during acute illness, can lead to low QRS amplitude and subsequent arrhythmia risk and mortality. Given that the ECG shown in this example exhibits low-amplitude QRS complexes, the nurse should assess whether this remains after all of the skin electrodes and leads wires are replaced and discuss this finding with the physician/advanced practice provider to review potential underlying reversable causes. Examining a prior ECG (emergency department and before admission) could also be useful to determine if this ECG pattern is chronic or new.Although it is tedious to read ECG device manuals, nurses (bedside nurses, nurse educators, and clinical nurse specialists) should be aware of how device manufacturers calculate parameters such as heart rate and determine the presence of arrhythmias. This knowledge can be useful for understanding how and why alarms are generated, which might be helpful for developing alarm management strategies. Nurse scientists working in this area of research are also key allies in knowledge of ECG algorithms and are a resource for hospital-based nurses. Device manufacturers, in turn, have an obligation to collaborate with nurses to understand and develop strategies to overcome issues like that shown in this example. Possible solutions specific to false asystole alarms where heart rate algorithms have shortcomings might include: (1) QRS amplitude that meets ANS standards (>1.5 mm); (2) use the entire QRS complex (not only unidirectional) to determine QRS amplitude; (3) include QRS complexes from all available ECG leads, rather than only select leads; and (4) pair QRS complexes with other parameters such as arterial blood pressure and/or Spo2 to calculate heart rate. A limitation of the fourth solution is that invasive blood pressure monitoring is used only in ICU patients and even in the ICU is used in less than 50% of patients. However, Spo2 is typically used in all ICU patients and is commonly used in non-ICU units where ECG telemetry monitoring is used. This patient was treated for diverticulitis, was transferred to the step-down telemetry unit 24 hours after ICU admission, and was later transferred home.