OBJECTIVE: To compare pulse oximetry oxygen saturation (SpO2) measured by a novel reflectance method from core body to arterial oxygen saturation (SaO2) in neonates and infants. Transmission pulse oximetry (TPO) was measured for comparison.
STUDY DESIGN: We monitored 18 infants by the two pulse oximeters simultaneously. The reflectance pulse oximetry (RPO) (PRO2, ConMed, Utica, NY) was measured on the upper back or chest, while the TPO (N395-Nellcor, Pleasanton, CA) was measured from the finger of the infant on the left hand or feet. Data from the two methods were compared to functional SaO2 derived from blood sample drawn from arterial line for patient care and measured by a Co-oximeter (Ilex, Instrument Lab. Lexington, MA). The potential advantage of the RPO is demonstrated in a case of a premature infant with hypovolemic shock, where SaO2 or TPO could not be obtained but oximetry was available from the RPO.
RESULTS: We used for analysis 56 RPO and 32 TPO measurements. SpO2 obtained from the RPO was 88.3±9.8%, from the TPO 84.2±10.1%, and functional SaO2 was 88.2±11.7%, with correlation coefficient of 0.93 and 0.88, respectively (p<0.0001). The mean difference (bias) and standard deviation of the differences (precision) between the RPO and the TPO compared to functional SaO2 were −0.09±4.5% and 1.26±5.9% and the absolute errors were 3.2±3.1%, and 4.4±4.0%, respectively. The accuracy of both RPO and TPO was diminished when SaO2 was <85%, but only the RPO remained correlated with the functional SaO2.
CONCLUSIONS: Reflectance pulse oximetry measured from core body of neonates and infants is accurate and reliable and is comparable to the transmission SpO2 when compared to functional SaO2. We speculate that the reflectance method might be advantageous in cases of poor peripheral perfusion in neonates and infants.
Pulse oximetry arterial oxygen saturation (SpO2) has become the “fifth vital sign” in the examination of every newborn and infant with respiratory system presentation.1,2,3 Pulse oximetry is not invasive, easy to use, has no side effects, is accurate and allows continuous monitoring and is the preferred method of oxygen monitoring in neonates.4,5
However, the traditional method of transmission pulse oximetry (TPO) has several limitations. In conditions of poor peripheral perfusion or cardiovascular collapse, the measurements of this method are not accurate, and in case of arm or leg movements, motion artifacts may emerge.3,6,7 The reflectance pulse oximetry (RPO) is a potential alternative to the traditional TPO. Few investigators have tried to use this method in neonates.8,9,10 A novel RPO (with innovated sensor and internal algorithm) capable of measuring from core body of neonates is presented. Its major advantage would be in overcoming the limitations of TPO.
The aim of our study was to compare the RPO SpO2 from core body to arterial oxygen saturation in neonates and infants. TPO was measured simultaneously for comparison.
We monitored 18 infants by the two methods (RPO and TPO) simultaneously, and data were collected on a computer placed at the bedside. The reflectance oximetry (PRO2, ConMed Corporation, Utica, NY) was measured on the upper back or chest, while the transmission oximetry (N395-Nellcor, Pleasanton, CA) was measured from the finger of the infant on the left hand or the foot. Data from the two methods were compared to functional arterial oxygen saturation (SaO2) derived from blood samples drawn from an arterial line placed for patient care. SaO2 (%HbO2/[100%−(%HbCO+%MetHb)] × 100) was measured by a co-oximeter (Ilex 482, Instrument Lab., Lexington, MA). Informed consent was obtained from all parents whose infants participated in the study, which was approved by the Investigational Review Board in our center.
The PRO2 reflectance pulse oximeter consists of a sensor that emits and detects red and infrared light, holder and a microprocessor-controlled monitoring signal processing unit. The sensor is based on unique geometry, in which light source derives light from the center of the chip in three different wavelengths (one red, 660 nm, and two infrared, 850 and 940 nm). The effective detecting areas are defined two optic rings, which are arranged concentrically around the central light sources. Since the rings constitute an angular shape, the detection area is capable of acquiring signals from a larger tissue zone (multipath) than with TPO. The internal algorithm enables analysis of signals obtained from newborns and adults. We used for TPO the Nellcor model N395 that represents the group of modern pulse oximeters that was found to be reliable in combination of the most challenging situations of motion and reduced perfusion.6 Figure 1 illustrates the differences between the RPO and the TPO oximeters.
Linear regression analysis was used to compare the SpO2 determined by the two methods (reflectance and transmission) to functional SaO2 measured by a co-oximeter. We calculated bias and precision of the reflectance and transmission oximetry when compared to functional SaO2; p<0.05 was considered significant. Data are presented as mean±SD.
Our patients’ characteristics are shown in Table 1. They comprise of infants with a relatively low and wide range of oxygen saturation (mean±SD by RPO was 88.3±9.8%, by the TPO 84.2±10.1%, and the functional SaO2 was 88.2±11.7%) (Figure 2a, b). The infant's weight ranged from 1.2 to 14.7 kg (3.7±3.0 kg) and the study age ranged from day 1 in a premature infant (gestational age 29 weeks) to 618 days (median: 4.5 days). The mean blood pressure was 52±12 mmHg. The RPO was placed on core-body sites (back or chest). All infants had an indwelling arterial catheter for clinical care: eight umbilical, one left radial, and two femoral. No side effects were seen during the study.
Reflectance and Transmission Pulse Oximetryvs Functional SaO2
We used for analysis of the reflectance (PRO2) method 56 measurements (3.1±1.8 measurements per patient) and for the transmission (N395-Nellcor) method 32 measurements from the 18 patients (some transmission oximetry data collected by other instruments were omitted from analysis for consistency). Both, reflectance and transmission oximetry significantly correlated with functional SaO2 (p<0.0001); correlation coefficients were 0.93 (0.90 for n=32) and 0.88, respectively (Figure 2a, b).
Mean difference (bias) and SD of the differences (precision) for the RPO and the TPO compared to functional SaO2 were −0.09±4.5% (0.86±5.4% for n=32) and 1.26±5.9%, respectively (Figure 3a, b). The absolute errors were 3.2±3.1% (3.7±3.9% for n=32) and 4.4±4.0%, respectively.
Analysis of the data with SaO2 below 85% includes 23 RPO measurements and 21 TPO measurements. The regression analysis for RPO and TPO as compared with functional SaO2 had a correlation coefficient of 0.58 (p<0.005) and 0.35 (p=0.11, NS), respectively. The mean difference was 2.4±5.4% and 2.9±6.6%, and the SD of the absolute errors was 4.4 and 4.1%, respectively. Thus, the accuracy of both methods in the low range of oxygen saturation (<85%) was diminished, but remained significantly correlated to functional SaO2 only with the RPO method.
The following case report illustrates the potential significance of the RPO measured from core body in a premature newborn.
A preterm male infant, the third of a triplet, was born via cesarian section at 32 weeks of gestation with a birthweight of 1370 g. His course was uneventful until day 9 of life when the diagnosis of necrotizing enterocolitis was made, which required surgery. The infant returned from the operating room to the NICU in cardiovascular collapse due to hypovolemic or septic shock. His heart rate was 170 b/min, noninvasive blood pressure 32/17 mmHg and mean of 19 mmHg, temperature 36.6°C, with poor peripheral perfusion and oxygen saturation of 94% by TPO. The hematocrit was 19.3% and platelet count 37,000. It was impossible to obtain a blood gas by inserting an arterial line or by a peripheral or capillary stick. A transfusion of red blood cells, fresh frozen plasma and platelets as well as dopamine and dobutamine were administered. Within an hour, the TPO signal was lost and could not be obtained on any limb. The RPO was placed on his back and gave readings as presented in Figure 4. The capillary blood gas obtained after few hours of intensive treatment showed severe metabolic acidosis (pH 6.8, BE −23.7 meq, HCO3 10.6 meq, PO2 43 mmHg, PCO2 65 mmHg). He received bicarbonate and the ventilator settings were adjusted; however, the next venous blood gas was: pH 7.0, BE −22.5 meq, HCO3 9.1 meq, PO2 64 mmHg, and PCO2 37 mmHg. Despite the maximal support, his mean noninvasive blood pressure increased only temporarily to 32 mmHg for few hours but he remained anuric. The infant died 13 hours after the operation despite aggressive resuscitation efforts.
Our study showed that the RPO measured from core body of neonates and infants is accurate and reliable and is comparable to the transmission SpO2 when compared to functional SaO2. The reflectance method is safe to use in neonates and infants. The presented case, demonstrates the potential advantage of measuring reflectance pulse oximetry from core-body in extreme situation of cardiovascular collapse, when no arterial blood gas or transmission SpO2 are available because of poor peripheral perfusion.
There is a difference between different brands of instruments of TPO.11 In this study, it was found that the Nellcor SpO2 correlated best with functional SaO2, that SpO2 determined by different pulse oximeters in not interchangeable, and that this may be of clinical importance in predicting PaO2 on the basis of SpO2. Fetal hemoglobin (HbF) shifts the oxyhemoglobin dissociation curve to the left. Experimental data show that HbF has no clinically significant effects on pulse oximetry.11,12,13,14,15 The degree of HbF of course affects the correlation of SaO2 to PaO2. Clinicians concerned with PaO2 value must understand the effect of HbF when interpreting the SpO2 reading. Accordingly, we elected to compare the new reflectance oximeter to functional SaO2. Functional SaO2 is calculated from measurements derived from co-oximeter (HbO2, HbCO, MetHb) that determines the SaO2 by spectrophotometry, and this method is accepted as a valid standard.12 We also compared our results to the TPO that is currently the common pulse oximetry method used in critical care units of newborns and infants.
TPO can be used reliably for continuous monitoring in normotensive neonates with SaO2 of 80 to 100%.16 However, TPO has performance limitations because of motion artifacts, hypotension and vasoconstriction.3,6,7 While during control desaturation the SpO2 of the new generation of TPO was within ±3% of the reference reading >95% of the time, during motion and reduced perfusion the error increased by 20 and 10%, respectively.6 In order to minimize these limitations, new transmission pulse oximeters were recently introduced.17 A different approach to solve these problems was to develop the RPO.
The RPO is designed to measure SpO2 by reflectance of the signal from the tissue, and not by transmission of the signal through the tissue. Thus, it allows to obtain SpO2 from core body (chest, back and forehead), and might be less dependent and less affected by peripheral perfusion and movements of the extremities. Faisst et al.8 showed close agreement between the reflectance and transmission oximetry in newborns. They had unreliable signals from the back because of breathing artifacts, and both systems were equally sensitive to motion artifacts. Fanconi and Tschupp9 reported on accuracy of a new transmittance–reflectance sensor from hand, foot and calf of newborn infants. They compared SpO2 from transmittance–reflectance sensor to SaO2 derived from arterial blood SaO2. Comparison of femoral or umbilical SaO2 with lower limb transmittance–reflectance sensor had a mean difference of 1.44±3.51% and correlation of r2=0.96, and radial artery SaO2 with upper limb transmittance–reflectance sensor had a mean difference of 0.66±3.34% and r2=0.94. The mean error was slightly larger for arterial saturation values < 90%, a recognized limitation of several pulse oximetry devices.2,18 They did not report core-body measurements. Takatani et al10 reported in an animal study, the effect of temperature on TPO and RPO, and they also monitored 18 critical patients perioperatively with the reflectance sensor. To our knowledge, our study is the first to report accurate reflectance SpO2 from core body (chest and back) in newborns and infants compared to functional SaO2. Our patients were normotensive at the time of the study, but their characteristics (that included respiratory as well as cardiac diagnoses, Table 1) allowed us to evaluate the accuracy of the RPO along a wide range of oxygen saturations (including the low range <85%); SpO2 obtained from RPO was 88.3±9.8% (range 66 to 97%), from TPO 84.2±10.1% (range 65 to 100%), and functional SaO2 was 88.2±11.7 (range 59 to 100%); RPO and TPO had comparable accuracy, with correlation coefficient of 0.93 and 0.88, respectively (p<0.0001, Figure 2). The mean difference (bias) and the standard deviation of the differences (precision) between RPO and TPO compared to functional SaO2 was 0.09±4.5% and 1.26±5.9% (Figure 3), and the standard deviation of the absolute errors was 3.1 and 4.0%, respectively. Thus, we found good accuracy (according to published literature9) between RPO from core-body of neonates and infants and functional SaO2. The accuracy was comparable to TPO, which is now the prevailing method of pulse oximetry. Accuracy of pulse oximetry in the low range of oxygen saturation is problematic and several oximeters tend to overestimate the SaO2 at this range.2,18 Analysis of the subgroup of measurements with SaO2 below 85% includes 23 measurements with RPO and 21 measurements with TPO. The accuracy of both RPO and TPO in the low range of oxygen saturation (<85%) was diminished, but significant correlation with functional SaO2 was maintained only with the RPO method (p<0.005). Our improved results with reflectance oximetry from core body in neonates and infants were probably achieved because of our innovative sensor and the internal algorithm of the PRO2 instrument.
While our patients were relatively well with adequate perfusion at the time of the study, the presented case demonstrated the possible advantage of the RPO compared to TPO in a hypoperfused patient. The TPO probes needs to be placed on tissue that can be easily transilluminated, usually at the periphery of the circulation (finger, toe and ear lobe), where in shock conditions, the changes in pulsatile capillary blood volume are much smaller and more susceptible to errors. In contrast, RPO may be placed on core body areas like the chest and back. In this premature infant with cardiovascular collapse and poor peripheral perfusion, we could not insert an arterial line or obtain an arterial blood gas by a stick, nor could we have continuous monitoring by TPO. The only way that was possible to monitor the oxygenation status of this infant for several hours until his death was SpO2 measured by RPO (Figure 4).
This study is a preliminary report. Our case report is only an illustration of the potential advantage of the RPO in an extreme situation, and we should be cautious in coming to any conclusion from a single case. Our new RPO sensor needs further evaluation on large number of infants in different situations of hypotension, poor peripheral perfusion, hypothermia, phototherapy, high humidity and other possible physical limitations. It further needs an evaluation for prolonged continuous monitoring as compared to the commonly used TPO, and it needs to be compared to other modern transmission pulse oximeters like the Masimo SET that might be advantageous as compared to the Nellcor N395, according to few recent studies.17 One of the limitations of our study was the wide range of study weight and postnatal age, and in future studies, will evaluate the RPO in unique groups of infants like very low birth-weight infants (<1500 g).
We conclude that RPO measured from core-body sites of neonates and infants is accurate and reliable and is comparable to the transmission SpO2 when compared to functional SaO2. We speculate that the reflectance method might be advantageous in cases of poor peripheral perfusion in neonates and infants.
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Data were presented in part as a poster at the Pediatric Academic Society meeting in Baltimore 2002 (“late breaker” session) and in the ATS meeting in Seattle 2003. This work was supported by ConMed Corporation, Utica, NY, USA.
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Kugelman, A., Wasserman, Y., Mor, F. et al. Reflectance Pulse Oximetry from Core Body in Neonates and Infants: Comparison to Arterial Blood Oxygen Saturation and to Transmission Pulse Oximetry. J Perinatol 24, 366–371 (2004). https://doi.org/10.1038/sj.jp.7211102
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