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TECHNOLOGY, COMPUTING, AND STIMULATION

An Evaluation of a New Combined Spo₂/PtcCO₂ Sensor in Very Low Birth Weight Infants

Serafina Lacerenza, MD^*, Maria Pia De Carolis, MD^*, Francesca Paola Fusco, MD^*, Giuseppe La Torre, MD, Giacomina Chiaradia, MD, and Costantino Romagnoli, MD^*

From the *Division of Neonatology, Department of Pediatrics, Epidemiology and Biostatistics Unit, Institute of Hygiene, Catholic University of Sacred Heart, Rome, Italy.

Address correspondence and reprint requests to Maria Pia De Carolis, MD, Division of Neonatology, Department of Pediatrics, Catholic University of Sacred Heart, Largo A. Gemelli, 8, 00168, Rome, Italy. Address e-mail to mpia.decarolis{at}rm.unicatt.it.

Abstract

BACKGROUND: Recently, a new sensor for combined assessment of pulse oximetry oxygen saturation (Spo₂) and transcutaneous monitoring of carbon dioxide partial pressure (PtcCO₂) has been introduced (TOSCA 500, Radiometer America Inc.). We designed this study to evaluate the usability and reliability of TOSCA in neonates with birth weight 1500 g (very low birth weight).

METHODS: In a prospective study of 22 newborns, TOSCA was tested, positioning the sensor on the ear pinna with an adhesive attachment clip. Simultaneous monitoring with TOSCA, conventional pulse oximeter (HP; Datex Ohmeda 3740), and a transcutaneous device (TINA TCM3, Radiometer, Copenhagen) was performed for 60 min. PtcCO₂ measurement from TOSCA (PtcCO_2TOSCA) and TINA (PtcCO₂) were compared with Pco₂ from blood samples (PCO_2EAB) at 1 and 60 min. During the monitoring period, values of PtcCO_2TOSCA were compared with PtcCO₂, and SatO₂ values from TOSCA with those from a pulse oximeter. Corresponding data were compared using Bland–Altman analysis.

RESULTS: Bias (precision) at 1 min and at 60 min between PCO_2EAB and PtcCO₂ values were 3.5 (12.4) mm Hg and 2.8 (10.2), respectively, whereas between PCO_2EAB and PtcCO_2TOSCA values were 18.3 (30.4) mm Hg and 1.8 (25) mm Hg. Bland–Altman analysis shows a better correspondence PtcCO₂/PtcCO_2TOSCA between 7 and 15 min. No significant differences were found between Spo₂ and SpO_2TOSCA.

CONCLUSIONS: The TOSCA monitor is safe and easy to apply in very low birth weight newborns. The pulse oximeter measurements may be useful for titrating oxygen therapy. Pco₂ measurement with TOSCA is most useful as a trend and independent confirmation of arterial Pco₂ is required if an accurate value is needed.

Neonates admitted to the Neonatal Intensive Care Unit (NICU) frequently need assisted ventilation and oxygen supplementation. Continuous monitoring of oxyhemoglobin saturation (SatO₂) and repeated measurement of the arterial partial pressure of carbon dioxide (Paco₂) are important for patient management.

SatO₂ can be estimated by pulse oximetry, which uses the ratio of red/infrared light absorption of pulsating components to evaluate arterial blood oxygenation. Paco₂ can be assessed by sampling arterial blood, but noninvasive techniques, allowing the immediate detection of Paco₂ changes after modification of respiratory support, can reduce the need for frequent blood drawing and painful puncture.1

TOSCA is a new sensor that combines a pulse oximeter with the Stow-Severinghaus electrode for transcutaneous partial pressure of carbon dioxide (Pco₂) measurement. The device is attached to the earlobe with a clip, and has been tested in adults, children, and neonates with birth weight >1500 g and gestational age >28 wk.2–5 No data are available in very low birth weight infants (VLBW).

The aim of the present study was to evaluate the usability and the reliability of the TOSCA monitor in VLBW neonates when compared with other devices currently used.

METHODS

Neonates with a weight 1500 g during the first 2 wk of life were enrolled. Exclusion criteria included patients with anemia (hematocrit <40%), hypothermia (body temperature <35°C), diffuse edema, and hypotension requiring vasoactive drugs. Informed consent was obtained from the parents; the study protocol was approved by the Ethical Committee of our institution.

In addition to monitoring SatO₂ and Paco₂, birth weight, gestational age, sex, day of life, and weight at time of monitoring were recorded. Simultaneous monitoring with TOSCA, a conventional pulse oximeter (HP; Datex Ohmeda 3740) and another transcutaneous device (TINA TCM3, Radiometer, Copenhagen) was performed for 60 min. At 1 and 60 min, a blood gas analysis was performed using either an arterial sample from an umbilical arterial line, or alternatively by capillary sample.6,7

Because of the small size of the earlobe, the TOSCA sensor was placed at the ear pinna with an adhesive attachment clip and a drop of contact gel (Fig. 1) after cleaning the skin surface with ethyl alcohol. Sensor calibration was automatically done after every monitoring period, with an integrated calibration unit; the membrane was changed every 14 days. TOSCA was used in QUICKSTART mode (temperature 44°C during the first 20 min, and 42°C during other 40 min).

View larger version (86K): [in this window] [in a new window]   Figure 1. Application of TOSCA sensor at ear pinna.

The TINA sensor was applied on the abdomen or thigh skin with an adhesive ring and a drop of contact gel, after cleaning the surface. Sensor calibration was done before monitoring at 44°C–44.5°C.

The pulse oximeter probe was placed on the hand, forearm, or foot.

The TOSCA monitor was equipped with Masimo SET technology, a set of algorithms designed to improve arterial SatO₂ monitoring during motion and low perfusion.8

Data recorded during monitoring included Spo₂ and PtcCO₂ from TOSCA, Spo₂ from the pulse oximeter, and PtcCO₂ from TINA. Data were collected manually every minute during the first 15 min and every 5 min for the remaining 45.

At the end of the monitoring period, the transcutaneous sensors were removed and the underlying skin examined to reveal the presence of burns.

Data are presented as mean (±standard deviation, sd) and median (interquartile range).

The method of comparison analysis described by Bland and Altman9 was used to evaluate the agreement PCO_2EAB/PtcCO₂ and PCO_2EAB/PtcCO_2TOSCA at 1 and 60 min.

Bland–Altman analysis was also used to assess the agreement between PtcCO_2TOSCA and PtcCO₂, and to compare SatO₂ values from TOSCA (Spo_2TOSCA) and pulse oximeter (Spo₂) during the monitoring period at the following times: 1, 3, 7, 11, 15, 30, and 60 min. Precision was defined as 2 sd of the mean difference (bias).

RESULTS

Twenty-three neonates were enrolled in our NICU from December 2005 to March 2006; one was excluded due to repeated dislocations of sensors.

Neonatal characteristics are shown in Table 1. All patients required respiratory support consisting of either mechanical ventilation (n = 10) or nasal continuous positive pressure (n = 12). All infants but two received oxygen supplementation.

The TOSCA sensor was attached in all patients to the ear pinna, whereas the TINA device was applied in 13 patients to the abdomen skin and in 15 on the thigh medial surface. We performed arterial samples in 4 patients and capillary samples for the remaining 18 neonates.

The application of the TOSCA sensor was easy, and no signs of erythema or burn were found at the end of monitoring. The TINA device always caused persistent erythema, a sign of first-degree burn.

Pco₂ Data Analysis
Mean values of PCO_2EAB at the start and at the end of monitoring were 42.9 ± 9.1 and 42.3 ± 7.5 mm Hg.

At 1 min, bias (precision) between PCO_2EAB and PtcCO₂ values were 3.5 (12.4) mm Hg, whereas at 60 min, values were 2.8 (10.2) mm Hg. At 1 min, bias (precision) between PCO_2EAB and PtcCO_2TOSCA values were 18.3 (30.4) mm Hg, whereas at 60 min, values were 1.8 (25) mm Hg (Fig. 2).

View larger version (14K): [in this window] [in a new window]   Figure 2. Top: Bland and Altman plot for the comparison of PCO2EAB and PtcCO2 values at 1 and 60 min. Bottom: Bland and Altman plot for the comparison of PCO2EAB and PtcCO2TOSCA values at 1 and 60 min.

Table 2 presents the bias PtcCO₂–PtcCO_2TOSCA and the precision at every time considered. At the first minute there was a low agreement between TINA and TOSCA, as shown by the bias and the precision. The precision was greater between 7 and 15 min than at other times. After the 15th minute, the mean difference was low, but the agreement worsened. Figure 3 shows the median trend of PtcCO₂ and PtcCO_2TOSCA during monitoring.

View larger version (7K): [in this window] [in a new window]   Figure 3. Trend of median PtcCO2 values reported from TINA and TOSCA every minute during the monitoring period.

SatO₂ Data Analysis
Table 3 presents the bias Spo₂–Spo_2TOSCA and the precision at every time considered. Mean differences were not significant at all time points, although there were relatively large limits of agreement at different times. Figure 4 shows the median trend of Spo₂ and SpO_2TOSCA.

View larger version (6K): [in this window] [in a new window]   Figure 4. Trend of median Spo2 values reported from pulse oximeter and TOSCA during the monitoring period.

DISCUSSION

In this study, we evaluated the usability and the reliability of TOSCA sensor in VLBW infants, compared with other devices currently used in NICUs.

Our results confirm that TOSCA is easy to use without any significant side effect in VLBW infants. Sensor placement at the ear pinna is a practical location for the sensor and facilitated the use of the TOSCA in these babies for the monitoring period of the study. In our VLBW infants with a thin epidermal layer, no burns were found underneath the TOSCA sensor, whereas persistent erythema, as sign of first degree burn, was always observed with the TINA device.

To evaluate the reliability of TINA and TOSCA monitors, two points were analyzed for limits of agreement between the transcutaneous devices and blood gas measurements. At 1 min, the limits of agreement between PCO_2EAB and PtcCO₂ were better than between PCO_2EAB and PtcCO_2TOSCA, whereas at 60 min, the mean difference was similar, although the limits of agreement were larger for the TOSCA monitor compared with blood gas measurements.

Furthermore, Pco₂ trend analysis indicated three different phases during the monitoring period: the first phase when the PtcCO_2TOSCA values were lower than the ones from TINA; central phase (between 7 and 15 min) when the values from TOSCA and TINA were similar; final phase (after 20 min) when PtcCO_2TOSCA values were higher than values from TINA, with a low mean difference, but large limits of agreement.

We supposed that the first phase may depend on the placement site of the TOSCA sensor. In fact, the thin dermal layer and poor capillary bed of the ear pinna could determine a slower arterialization than the one obtained on the abdomen or thigh.

Instead, the differences in the final phase could be influenced by the temperature decrease of the TOSCA sensor from 44°C to 42°C in QUICKSTART mode.

Electrode temperature influences CO₂ diffusion through the skin layer and CO₂ production by epidermal cells. Previous TOSCA studies10,11 showed that the electrode warming to 42°C at the start of monitoring causes an overshoot of PtcCO_2TOSCA values for some minutes after sensor application, as a consequence of CO₂ accumulation caused by increased local CO₂ production without concomitant arterialization. This temperature increases cellular metabolism allowing a greater CO₂ local production, not compensated by an adequate grade of arterialization. To avoid this overshoot, the TOSCA manufacturer modified the initial temperature of the sensor to 44°C, promoting earlier arterialization.

Nevertheless, the present study highlights that an overshoot appears during monitoring exactly when the electrode temperature decreases. We think that 42°C could be insufficient in VLBW infants because of a poor capillary bed or other causes still unknown.

These results are confirmed by the limits of agreement obtained between PCO_2EAB and PtcCO₂, and between PCO_2EAB and PtcCO_2TOSCA.

Bland–Altman SatO₂ analysis shows a low bias between Spo₂ and SpO_2TOSCA at every time considered, although with relatively large limits of agreement.

Since the use of a pulse oximeter immediately after birth is desirable to titrate oxygen supplementation, as new guidelines about neonatal resuscitation recommend,12 the TOSCA sensor could be useful in the delivery room as a pulse oximeter. PtcCO₂ measurement from the TOSCA monitor could be useful as a trend monitor during neonatal transfer or longer term monitoring. TOSCA does not provide a reliable quantitative measure of Paco₂ and independent measurement of Pco₂ is required if an exact value is desirable.

ACKNOWLEDGMENTS

The TOSCA device and disposable attachment clips were provided for the study without charge by Linde Medical Sensor AG, Basel, Switzerland. Linde was the TOSCA manufacturer during the study period. Nevertheless, beginning January 2007 TOSCA was a product of Radiometer America Inc.

The authors do not hold any agreement on the device and did not receive any financial support from the manufacturer for the study.

Footnotes

Accepted for publication March 5, 2008.

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This Article Abstract Full Text (PDF) An erratum has been published Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Lacerenza, S. Articles by Romagnoli, C. Search for Related Content PubMed PubMed Citation Articles by Lacerenza, S. Articles by Romagnoli, C. Related Collections Obstetrics Monitoring (Non-cardiac) Pediatrics Technology