Defining the incidence of cardiorespiratory instability in patients in step-down units using an electronic integrated monitoring system

Abstract

Background: To our knowledge, detection of cardiorespiratory instability using noninvasive monitoring via electronic integrated monitoring systems (IMSs) in intermediate or step-down units (SDUs) has not been described. We undertook this study to characterize respiratory status in an SDU population, to define features of cardiorespiratory instability, and to evaluate an IMS index value that should trigger medical emergency team (MET) activation.

Methods: This descriptive, prospective, single-blinded, observational study evaluated all patients in a 24-bed SDU in a university medical center during 8 weeks from November 16, 2006, to January 11, 2007. An IMS (BioSign; OBS Medical, Carmel, Indiana) was inserted into the standard noninvasive hardwired monitoring system and used heart rate, blood pressure, respiratory rate, and peripheral oxygen saturation by pulse oximetry to develop a single neural networked signal, or BioSign Index (BSI). Data were analyzed for cardiorespiratory instability according to BSI trigger value and local MET activation criteria. Staff were blinded to BSI data collected in 326 patients (total census).

Results: Data for 18 248 hours of continuous monitoring were captured. Data for peripheral oxygen saturation by pulse oximetry were absent in 30% of monitored hours despite being a standard of care. Cardiorespiratory status in most patients (243 of 326 [74.5%]) was stable throughout their SDU stay, and instability in the remaining patients (83 of 326 [25%]) was exhibited infrequently. We recorded 111 MET activation criteria events caused by cardiorespiratory instability in 59 patients, but MET activation for this cause occurred in only 7 patients. All MET events were detected by BSI in advance (mean, 6.3 hours) in a bimodal distribution (>6 hours and < or =45 minutes).

Conclusions: Cardiorespiratory instability, while uncommon and often unrecognized, was preceded by elevation of the IMS index. Continuous noninvasive monitoring augmented by IMS provides sensitive detection of early instability in patients in SDUs.

Figure 1

Examples of charts of patients judged to have minimally fulfilled medical emergency team (MET) activation criteria (MET_min) (A) or who fulfilled MET activation criteria, which should have resulted in MET activation (MET_full) (B and C). A, Patient has baseline hypertension but heart rate (HR), respiratory rate (RR), and peripheral oxygen saturation by pulse oximetry (SpO₂) are in the normal range. The blood pressure (BP) was further elevated at 4:00 AM, with BioSign Index (BSI) alert threshold (dotted line), but then reverted to baseline. B, Note progressive and interactive increase in both HR and RR, and, finally, hypertension, resulting in recurrent BSI alerts. C, Progressive and interactive increase in both HR and RR and dips in SpO₂ result in persistent BSI elevation that intermittently crosses the alert threshold.

Figure 2

Time from initial deterioration as identified by a BioSign Index (BSI) of 3 or greater to medical emergency team (MET_actual) activation in 7 patients. Each of the patients’ data point is represented by patient number.

Figure 3

Single-day vital sign and BioSign Index chart for patient 1 over time leading to medical emergency team activation call. Cardiorespiratory status was stable until 6:00 AM (A), when the respiratory rate (RR) gradually increased. From 9:00 AM onward, the RR was high and peripheral oxygen saturation by pulse oximetry (SpO₂) gradually decreased, with occasional dips, and the systolic blood pressure (BP) remained high at about 180 mm Hg. BioSign alerts above the threshold value of 3 (dotted line) occurred from 12:30 PM until the medical emergency team activation was called at 1:29 PM (B). HR indicates heart rate.

Figure 4

BioSign Index charts for 6 patients over time leading to medical emergency team (MET) activation call (arrows). A, Patient 2 had low blood pressure and acute onset of bleeding from the arterial sheath site. MET activation was called at 11:05 PM. B, Patient 3 had heart failure and unsteady gait. The patient was off the monitor shortly before 1:00 PM and fell in the bathroom. MET activation was called at 3:16 PM. C, Patient 4 status after lung transplantation. Respiratory distress developed at 11:30 AM. MET activation was called at 3:19 PM. D, Patient 5 status after sustaining trauma and multiple fractures. Low peripheral oxygen saturation and compensatory tachycardia developed at 8:15 AM. MET activation was called at 8:40 AM. E, Patient 6 status after a fall and hip fracture. Acute respiratory deterioration developed with compensatory tachycardia. MET activation was called at 12:28 PM. F, Patient 7 status after a traumatic fall and delerium. Tachypnea and hypoxemia developed at about 10:00 AM, and hypotension developed shortly after 8:00 PM. MET activation was called at 8:27 PM.