Continuous monitoring of oxygen saturation via pulse oximetry has been standard practice in the postanesthesia care unit (PACU) since the early 1990s (1,2). A large, randomized multi-center trial demonstrated that routine use of perioperative pulse oximetry was associated with an improved ability to detect hypoxemia, decreased the rate of myocardial ischemia, and prompted a number of changes in patient care (3). Yet, in the postoperative patient, low perfusion (i.e., low signal strength) or, more commonly, patient motion (i.e., noise from nonarterial pulsations) often disrupts the measurement of Spo2 resulting in false alarms (FAs) (4–8). Wiklund et al. (8) studied pulse oximetry in adults in the PACU and demonstrated an average alarm frequency of once every 8 min, 77% of which were false, caused by motion or low perfusion.
The Masimo Signal Extraction Technology™ (Masimo SET™; Masimo Corporation, Irvine, CA) was designed to be minimally affected by artifact and provide continuous Spo2 data and, thereby, reduce FAs in the clinical setting. Masimo SET™ incorporates an algorithm that mathematically manipulates the pulse oximeter’s red and infrared light signals to subtract the noise components associated with motion and low perfusion. The Masimo SET™ paradigm begins with the assumption that venous pulsations during motion create a confounding signal that contributes to noise. Using the Discrete Saturation Transform, the venous blood absorbance signal is isolated and ignored by adaptive digital filtering (9). Its unique digital technology permits the Masimo SET™ to provide a near instantaneous and continuous measure of arterial hemoglobin saturation.
A study in healthy volunteers demonstrated that the Masimo SET™ pulse oximeter yielded fewer errors and dropouts during motion than either the Nellcor models N-200 or N-3000 instruments (Mallinckrodt, St. Louis, MO) (9). Dumas et al. (10) studied 50 adult patients in the PACU using a prereleased version of Masimo SET™ and observed an alarm frequency of once every 13 min with conventional pulse oximetry (CPO), 87% of which were considered false. In comparison, these investigators observed an alarm every 30 min using the Masimo SET™ device, of which 59% were considered false. Motion artifact may be particularly problematic in children during the early postanesthesia recovery period. There are no clinical data comparing Masimo SET™ with CPO in children in this setting. Therefore, this study was undertaken to compare the incidence and duration of FAs between Masimo SET™ and CPO in young children in the PACU. The study was designed to test the following hypothesis: There will be fewer FAs with continuous oxygen saturation monitoring by using the Masimo SET™ technology compared with conventional pulse oximetry.
This study was approved by our institutional review board, and before a subject’s inclusion, written consent was obtained from a parent or legal guardian. Healthy children aged 1–10 yr, with an ASA physical status of I–III who were scheduled to undergo general anesthesia for a surgical procedure were included. Children were excluded if they were having limb surgery that would interfere with placement of the pulse oximeter probes, if the anticipated recovery period or the duration of Spo2 monitoring requirements was expected to be brief, or if the child was not admitted to the PACU postoperatively.
After surgery, and on admission into the PACU, two disposable adhesive pulse oximeter sensors were placed on separate digits of one of the child’s hands. Although not always possible, attempts were made to avoid placing probes on the extremity used for noninvasive blood pressure monitoring. Sensors were optically shielded from ambient light and from each other to prevent sensor-to-sensor optical cross-talk. The probes were connected to either the CPO (Nellcor model N-200) or to the Masimo SET™ pulse oximeter. Spo2 and pulse rate data from these devices were recorded continuously via a bedside computer. Additionally, electrocardiograph (ECG) heart rate data obtained via the bedside Spacelabs monitor (Spacelabs, Andover, MA) were simultaneously and continuously recorded. Bedside nurses were blinded to Masimo SET™ readings and were instructed to make patient interventions based on their clinical observations and CPO data in accordance with standard practice. A trained research assistant concurrently documented events such as removal of an endotracheal tube or other airway device, patient motion, nursing interventions including placement of an artificial airway or oxygen administration, and blood pressure monitoring that could interfere with signal transmission. Pulse oximeter sensors were removed when the patient’s spontaneous activity indicated that Spo2 monitoring was no longer required.
All data were coded and analyzed for frequency and duration of relevant events defined below:
- Data Dropout: Any complete interruption of continuous Spo2 data associated with inflation of a noninvasive blood pressure cuff, patient movement, or other reasons. Regardless of the source, all dropouts were pooled to generate the total occurrence for each oximeter.
- False Negative (FN; True Alarm Missed): Failure of the device to detect a decrease in Spo2 to ≤ 90% as indicated by another device that is considered valid by correlation of the pulse rate with ECG heart rate, clinical observation, and/or response to intervention (e.g., administration of oxygen, airway manipulation).
- FA: A decrease in Spo2 to ≤ 90% that by expert clinical observation and opinion was considered to be artifactual, and as evidenced by lack of correlation between the pulse rate and ECG heart rate.
- True Alarm (TA): A decrease in Spo2 to ≤ 90% that is considered valid by correlation of the pulse rate with ECG heart rate, clinical observation and/or response to intervention.
Demographics and alarm data were presented as mean ± sds where applicable. χ2 with Fisher’s exact tests, as appropriate, were used to compare the incidence of TAs and FAs between Masimo SET™ and CPO.
Seventy-five children (4.9 ± 2.7 yr) were included in this study. The majority of the patients were Caucasian (87%), and the remaining patients were African-American (11%), Asian (1%), and Hispanic (1%). The total monitoring time was 42 h and ranged from 10 to 125 min per patient (35 ± 22 min per patient). Table 1 presents the data regarding significant events. The overall alarm frequency was once every 36 min for CPO, and once every 30 min for the Masimo SET™ device. Figure 1 demonstrates Spo2 and ECG output in a patient who experienced a TA situation with both monitors. There were 27 TAs, and all were identified by the Masimo SET™ pulse oximeter. Only 16 (59%) of the TAs were identified by CPO (P < 0.05). Sensitivity is defined as the proportion of times that an actual alarm condition is correctly detected (Sensitivity = TA/[TA + FN]) (9). Using this formula, we found a sensitivity of 100% for Masimo SET™ versus 59% for CPO. Spurious desaturations and pulse rate changes that triggered FAs most commonly occurred with significant gross movement (Figure 2). There was more than twice the number of FAs exhibited by CPO compared with the Masimo SET™ pulse oximeter (P ≤ 0.05). Based on these data, the positive predictive value (i.e., proportion of TAs/total alarms) of Masimo SET™ was 87% compared with 61% for CPO. The incidence and duration of complete data dropout were similar between the two monitors (Table 1).
The advantages of continuously monitoring Spo2 via pulse oximetry during the perioperative period have been well documented (3), yet erroneous values and interruptions in data because of low perfusion and, to a greater degree, motion, remain common. Motion artifact is particularly troublesome in children (6) and during early recovery from anesthesia (11) when patients are at greatest risk for hypoxemia (2). Therefore, a pulse oximeter that produces reliable data with minimal artifact would be a significant improvement in patient monitoring.
Our data demonstrated that FAs and data dropout occurred with both CPO and Masimo SET™ monitoring in our pediatric postoperative sample; however, there was a greater than 50% reduction in the incidence and duration of FAs with the Masimo SET™ compared with CPO. The incidence and duration of data dropout, however, was not significantly different between monitors. The lower frequency of FAs in our study compared wuth the previous PACU study by Dumas et al. (10) may be explained by their preselection of patients who were generating a high frequency of alarms.
Several methods have been recommended to reduce the incidence of FAs in pulse oximetry. Several studies have suggested that averaging signals over longer periods of time or incorporation of alarm delays may provide the optimal balance between sensitivity and specificity for detecting clinically relevant hypoxemia (11–13). Although such averaging may reduce the effect of intermittent artifact, it may do so at the expense of a delayed caregiver response to acute valid changes in Spo2. Furthermore, prolonged signal averaging may impart a false sense of security as a result of the display of past Spo2 data. The Masimo SET™ algorithm, however, mathematically manipulates the red and infrared light signals to subtract the noise components associated with motion and low perfusion. Furthermore, its digital technology permits the Masimo SET™ to provide an instantaneous measure of oxygen saturation.
The single most important characteristic of a pulse oximeter is to identify all episodes of hypoxemia (i.e., high sensitivity) to permit intervention before the development of clinically significant hypoxemia. Failure to capture these episodes may result in fewer alarms, but may also lead to a false sense of security, and thereby delay appropriate intervention. Indeed, Rheineck-Leyssius and Kalkman (13) found that lowering the pulse oximeter alarm limit from 90% to 85% decreased the frequency of alarms, however, resulted in a theoretical increase in the detection of clinically relevant hypoxemia. Our study demonstrated a higher sensitivity with the Masimo SET™ device (100%) compared with CPO (59%). These results concur with those reported by Barker et al. (14). In our study, each episode of device-detected hypoxemia responded appropriately to treatment (e.g., administration of oxygen or patient stimulation). Our data must be interpreted with caution, because TA situations were not confirmed with arterial blood co-oximetry. In a previous study using co-oximetry, however, Barker and Shah (9) demonstrated a bias and precision range of 0.8 ± 2.1 to 1.4 ± 3.5 with Masimo SET™ compared with a range of 4.5 ± 5.5 to 5.3 ± 5.3 with CPO.
A high incidence of FAs in pulse oximetry may lead to complacency of care providers who have become desensitized to alarms. This may result in delays in response to significant events (15–17). Indeed, Lawless (15) demonstrated a 7% positive predictive value for conventional pulse oximeter alarms in a pediatric intensive care unit and contended that the high incidence of FAs may cause health care personnel to accommodate to the alarm sound and delay appropriate intervention. Such accommodation has been reported in an neonatal intensive care unit setting, where it was found that almost 65% of all alarms were from CPO, and the nurses ignored nearly 70% of these alarms (16). Furthermore, Rheineck-Leyssius et al. (17) demonstrated a decrease in the incidence of severe hypoxemia when staff were motivated to respond to all alarms. Lastly, FAs may prompt unnecessary medical interventions.
The primary limitation of this study is the potential for misclassification of alarms, because arterial blood co-oximetry could not be used to confirm Spo2. To minimize this potential bias, bedside nurses were blinded to Masimo SET™ pulse oximetry data, and alarms were classified as true or false based on clinical assessment of the child, response of Spo2 data to clinical interventions, and correlation with ECG output. Another limitation of these data is our inability to calculate specificity, because the total number of true negatives is unknown.
In summary, this study demonstrated that Masimo SET™ significantly reduced the incidence and duration of FAs compared with CPO in the pediatric PACU. Furthermore, in this setting, Masimo SET™ pulse oximetry identified all of the TA events that responded to clinical intervention.
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