Mixed venous oxygen saturation (SvO2) assessed by a pulmonary artery catheter has been used for many years to estimate the DO2/VO2 balance. Recently, central venous oxygen saturation (ScvO2) has replaced SvO2 in many clinical settings1,2 and is one of the core targets for early goal-directed therapy in the treatment of septic shock in adults.3–5 The role of ScvO2 is even more important in neonates and paediatric patients, in whom the positioning of a pulmonary artery catheter is problematic. The detection of variation of ScvO2 values has become a point of interest, particularly in the perioperative setting of cardiac and major surgery6 in high-risk patients. Standard means of monitoring ScvO2 to assess global oxygen balance and cardiac output in paediatric patients consist of repeated blood sampling from a central venous catheter placed in the superior vena cava and analysis of the blood using cooximetry.7 This approach is intermittent, is attendant on increased workload and costs and results in contamination of stopcocks and iatrogenic blood loss. Additionally, haemodynamic and metabolic conditions may vary rapidly and are often not detected by intermittent blood gas analysis. Techniques providing continuous monitoring of ScvO2 may be a real-time answer to this substantial issue.
Spectrophotometry using near-infrared light7 is one of these technologies, and several devices have been commercially introduced. Various devices have been evaluated in few clinical and experimental investigations, but the results remain controversial.8–12 Recently, a new system for paediatric patients, the PediaSat system (Edwards Lifesciences, Irvine, California, USA), has become available, using central venous catheters with inbuilt fibre-optic transmission. The aim of this study was to validate its performance in an in-vitro bench-top model.
The PediaSat system consists of a central venous catheter with inbuilt spectrophotometric probe containing two fibre-optic lines connected proximally to the optical module of the PediaSat system. Three different central venous catheters, 4 Fr one-lumen, 4.5 Fr two-lumen and 5.5 Fr three-lumen, with fibre-optic probes are available.
For the purpose of monitoring oxygen saturation values measured by PediaSat (SPediaSatO2), light of two wavelengths (660 and 800 nm) is transmitted through one of the two fibre-optic lines. Tissue chromophores, such as haemoglobin (Hb), absorb near-infrared light, depending on their oxygenation state. Changes in chromophore concentrations and oxygenation states, revealed by comparing emitted and detected near-infrared light, can therefore be quantified, using the modified Lambert–Beer law.8,13,14 Light reflected from the Hb in the red blood cells is detected by the second fibre-optic line of the probe and transmitted back to the sensor in the optical module. The sensor in the optical module of the PediaSat device determines light extinction, and SPediaSatO2 is calculated and displayed on the screen of the Vigileo monitor (Vigileo Monitor MHM1E; Edwards Lifesciences).
Before starting continuous measurements, a calibrating manoeuvre using a blood sample analysed by cooximetry is required according to the manufacturer's instructional recommendations. During continuous recording of oxygen saturation, the Vigileo monitor displays a ‘signal quality index (SQI)’ in a 4° range from 1 (best quality) to 4 (worst quality). The SQI is a composite signal that expresses the signal intensity, the blood pulsatility and the presence of outlying spikes in the re/infrared signal.
Two 6.0-Fr introducer sheaths (Avanti Catheter Sheath Introducer; Cordis Cooperation, East Bridgewater, New Jersey, USA) with a length of 7.5 cm and a distal lumen of 2.2 mm (Pulsion Medical Systems, Munich, Germany) were inbuilt into two separated chambers of a black test box of 100 cm3. The two chambers were connected to provide blood flow through both of them. Two three-lumen PediSat oximetry catheters (XT3515HS, 5.5 Fr, 1.83 mm, 15 cm; Edwards Lifesciences) were introduced into the distal lumen of the introducer sheets until about 8 cm of the length of the catheter, and then secured by the integrated luer-lock system. Each catheter was then connected to the optical module of one of the two Vigeleo monitors (Edwards Lifesciences). The black box was connected to a paediatric cardiopulmonary bypass (CPB) circuit. The configuration of the CPB circuit included a roller pump and a thermoregulating device (Paediatric Pump S3; Stöckert Instruments, Munich, Germany), a Safe Mini oxygenator (Polystan/Maquet-CP, Hirlingen, Germany), a Safe Micro Reservoir (Polystan/Maquet-CP) and a real-time blood gas analyser unit (CDI 500; Terumo, Eschborn, Germany) to monitor pCO2. Pressurized O2, N2 and CO2 were connected to a fresh gas mixer (full automatic gas blender; Stöckert Instruments) (Fig. 1) to provide air mixtures with various concentrations of oxygen and physiological pCO2. The CPB circuit was filled up with 300 ml of heparinized (10 000 ml heparin sodium; Hoffman la Roche AG, Grenzach-Wyhlen, Germany) human full blood provided by one of the investigators; the air was completely expelled. The flow rate provided by the roller pump was kept constant at 1000 ml min−1 throughout the experiments, and blood pressure in the test chamber was held at 15 mmHg. Blood was warmed up to 37°C, and alpha-stat blood pCO2 was held within physiological range (5–7 kPa) by external CO2 gas source, as continuously monitored by the real-time blood gas analyser unit (CDI 500; Terumo) of the extracorporeal circuit and confirmed by blood gas analysis.
After obtaining steady-state values for SPediStatO2 at 70%, both PediaSat systems were calibrated according to the instructions of the manufacturer. If SQI was higher than 2, the optical fibre was removed, cleaned and reintroduced into the introducer. Only SQI values of 2 or less were accepted. After initial de-saturation of the circulating blood by gradually changing the fresh gas mixture of the membrane oxygenator from oxygen to nitrogen, subsequent saturations were performed by increasing oxygenation of the fresh gas mixture.
With each change of the oxygen saturation level and under steady state, the SPediaSatO2 values were recorded. Simultaneously, blood samples were taken from the distal lumen of the introducer sheet, and the oxygen saturation and Hb concentration were measured by multiwavelength cooximetry (SCO-OXO2) (GEMOPL; Instrumentation Laboratory, Lexington, Massachusetts, USA) as well as pH, partial oxygen tension (paO2), partial carbon dioxide tension (paCO2) and bicarbonate base excess (GEM Premier 3000 with iQM; Instrumentation Laboratory). Two series of measurements were done on two occasions, using identical experimental set-ups, and for each set-up, two new fibre-optic central venous catheters were used.
Data were expressed as mean (±SD). Agreement between SPediaSatO2 and SCO-OXO2 was assessed by the analysis by Bland and Altman.15 Linear regression analysis was performed to compare SPediaSatO2 and SCO-OXO2 and the difference values of SPediaSatO2 and SCO-OXO2 with SCO-OXO2. Sensitivity/specificity of ΔSPediSatO2 between two consecutive readings to indicate fall or rise of SCO-OXO2 was calculated. Intraclass correlation was computed to quantify importance of differences between the experiments. Levene's test of equality of variances was performed to analyse the variation between the experiments. An analysis of covariance was performed to analyse the effects of the experiment and the dependence of the difference values of SPediaSatO2 and SCO-OXO2 on SCO-OXO2. SPSS version 16.1 (SPSS Inc., Chicago, Illinois, USA) was used from the hospital resources for this purpose.
A total of 50 SPediaSatO2 readings and simultaneous measurements of SCO-OXO2 were obtained and analysed. Metabolic parameters are listed in Table 1. SPediaSatO2 and SCO-OXO2 values ranged between 28–98 and 24.9–99.5%, respectively. Interprobe reliability of SPediaSatO2 values detected by the two simultaneously used PediaSat catheters showed a bias of 4% and a precision of 4% (r2 = 0.99, P < 0.0001; Fig. 2).
Linear regression analysis demonstrated a high correlation between SPediaSatO2 and SCO-OXO2 (r2 = 0.96, P < 0.0001; Fig. 3). Overall, SPediaSatO2 only slightly overestimated SCO-OXO2 (mean bias +2.9%); however, limits of agreement (bias ±2 SD of mean difference) were less acceptable with −6.8/+12.6% (Fig. 3). The differences between SPediaSatO2 and SCO-OXO2 significantly depended on SCO-OXO2.
SCO-OXO2 values above 70% resulted in a better agreement between SPediaSatO2 and SCO-OXO2, with a mean bias of 0.4% and limits of agreement of −6.0 and +6.8% (Fig. 4), whereas SCO-OXO2 values below 70% represented an overestimation of SPediaSatO2 with a mean bias of +5.2% and limits of agreement of −4.7 and +15.1% (Fig. 4).
Sensitivity and specificity of SPediaSatO2 to indicate a fall or rise of SCO-OXO2 between two subsequent measurements were 1.0 and 0.92, respectively.
This bench-top, in-vitro set-up investigated reliability of the PediaSat system for continuous monitoring of SvO2. The main findings were that PediaSat system considerably overestimated SvO2 values at SCO-OXO2 values of less than 70%; however, sensitivity and specificity of SPediaSatO2 to indicate deterioration or improvement of SCO-OXO2 were acceptable.
The PediaSat system, investigated in this study, is based on reflectance oximetry using two reference light wavelengths and is characterized by one transmitting and one detecting fibre-optic filaments. The PediaSat system is easy to handle, does not necessitate additional invasive venous access and allows in-vivo calibration. Calibration manoeuvre is recommended once a day by the manufacturer. Additionally, it is applicable in neonates, infants and children, as three different sizes of the oximetry catheters are available. Disadvantages are the increased stiffness of the catheter and the necessity of updating the Hb and haematocrit value if significant shifts of Hb (>1.8 g dl−1) or haematocrit (>6%) occur.
On the basis of our findings, SPediaSatO2 considerably overestimated SvO2 values at SCO-OXO2 values of less than 70%. The reliability of ScvO2 readings is of particular importance in this lower range (<60%), in which changes become of vital interest and therapeutic interventions become necessary. Even more, during and after palliative congenital cardiac surgery in neonates and small infants with cyanotic heart diseases, ScvO2 may range from 35 to 55%. Particularly in functional univentricular circulation and also following Norwood procedure or its modifications, ScvO2 is used to assess the ratio between pulmonary and systemic flow. Arterial oxygen saturation of 75% and ScvO2 of 50% may be adequate. In this patient population, it has been demonstrated that the risk of anaerobic metabolism increased from 4.8 to 29% when ScvO2 fell below 30%.16 Reliable detection of oxygen saturation values among 50% is therefore mandatory for systems measuring SvO2 continuously to support decision-making for adequate treatment options. In this setting, the PediaSat system did not reliably estimate SvO2 values and may contribute to miss necessary therapeutic interventions.
In the past, several related systems have been investigated, and the most recent studies8,10–13,17 show similar variations in the SvO2 range of interest. In a previous investigation,11 we reported on a three-light, reference, wavelength-based oximetry catheter system demonstrating only a poor agreement between fibre-optically measured SO2 and SCO-OXO2 values. Because of a nearly linear dependency of the mean difference between oxymetrically measured SO2 and SCO-OXO2 values in this study, a systematic error has been assumed. Huber et al.13 reported an excellent correlation in an in-vitro setting between fibre-optic measurements of two different fibre-optic catheters and SCO-OXO2. However, the Bland–Altman plots for both probes revealed high limits of agreement and demonstrated substantial overestimation of low SO2 and underestimation at high SO2. In a clinical investigation of a PediaSat system in neonates and paediatric patients undergoing cardiac surgery, Ranucci et al.17 reported a mean bias close to zero and an acceptable value of percentage error (17.3–23.2%) between SPediSatO2 and SCO-OXO2 for measurements before, during and after CPB. They concluded that the PediaSat system might be considered as an accurate tool for continuous measurement of the ScvO2 in paediatric patients undergoing cardiac surgery. Most recently, Spenceley et al.12 reported in critically ill children a mean bias of 1.1% and inadmissible high limits of agreement (−15.8 and +18%) between fibre-optic measured SO2 values of the PediaSat system and SCO-OXO2. Although the unacceptable high limits of agreement, the authors deduced that the PediaSat system provided accurate trending of continuous ScvO2 but only within physiologic range. Whereas the results of the subgroup analysis of SCO-OXO2 values above 70% in the current in-vitro set-up agreed well with those by Ranucci et al.,17 the SCO-OXO2 values from 70 to 25% were not sufficiently detected. Contrary to our investigation, the lowest SCO-OXO2 values achieved in the study by Ranucci et al.17 and Spenceley et al.12 were only close to 50%, which may explain the considerable disagreement with the presented findings.
Although the PediaSat system demonstrated a substantial overestimation of SCO-OXO2 values of less than 70% in the current investigation, sensitivity and specificity of SPediSatO2 to indicate deterioration or improvement of SvO2 were excellent. These findings may suggest that the PediaSat system might be a useful tool to indicate accurate trend of ScvO2.
Our bench-top model only used variations in oxygen saturation and tried to maintain other parameters constant. Consequently, there is no further information about the performance during changes of Hb, use of volume expanders, temperature shifts, metabolic disturbances and coagulation disorders. However, the used one-way bench-top model clearly demonstrated inaccuracy of the system in the clinically relevant range of ScvO2, even under otherwise constant conditions.
On the basis of our in-vitro findings, the new PediaSat system cannot be recommended as a reliable replacement of repeated invasive ScvO2 assessment in the clinically relevant range of ScvO2. Probably, rapid changes associated with haemodynamic deterioration and after resuscitation may be detected by the system.
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Keywords:© 2010 European Society of Anaesthesiology
central; continuous; monitoring; oxygenation; venous