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From the Departments of *Cardiothoracic Surgery, and Anesthesiology at the David Geffen School of Medicine, University of California, Los Angeles (UCLA), California.
Address correspondence and reprint requests to Aman Mahajan, MD, PhD, Department of Anesthesiology, David Geffen School of Medicine at UCLA, Los Angeles, CA 90095. Address e-mail to amahajan{at}mednet.ucla.edu.
Abstract
BACKGROUND: Central venous oxygen saturation (ScvO2) accurately reflects cardiocirculatory function, but is not always feasible in pediatric patients. Using an experimental and clinical approach, we determined the accuracy of a novel pediatric central venous catheter with integrated fiberoptic oximetry, correlated ScvO2 to periprocedural vital variables, and tested its feasibility in pediatric cardiac surgery patients.
METHODS: In five anesthetized pigs, hemodynamics (cardiac index [CI], heart rate; mean arterial blood [MAP]; mean pulmonary artery [MPAP], central venous pressure [CVP]), fiberoptic ScvO2 (ScvO2-cath), and blood gas oximetry (ScvO2-blood) were measured during stable baseline conditions, preload reduction (caval occlusion), and dopamine infusion (5 mcg · kg–1 · min–1). In 16 pediatric patients undergoing cardiac surgery (median age 8.4 mo; weight 8.0 kg), central venous oximetry catheters were placed percutaneously, and ScvO2-cath and hemodynamics recorded at several time-points during and until 24 h after surgery. Oximetry and hemodynamic data were compared by correlation (Pr) and the Bland–Altman analysis.
RESULTS: There were no catheter-related complications. ScvO2-cath and ScvO2-blood measurements correlated significantly (P < 0.001) in both the experimental (Pr = 0.96) and clinical protocol (Pr = 0.94). A similar bias and precision over all time-points was detected in both protocols (Exp-bias: +0.03% ± 4.11%; Clinical-bias: –0.03% ± 4.41%). ScvO2-cath correlated (P < 0.001) with CI (Pr = 0.87), MAP (Pr = 0.59), MPAP (Pr = 0.44), and CVP (Pr = 0.38) and estimated CI better than MAP (Pr = 0.61), MPAP (Pr = 0.38), CVP (Pr = 0.35), or heart rate (Pr = 0.25).
CONCLUSION: Integrated central venous oximetry catheters provide accurate continuous ScvO2 monitoring in pediatric patients undergoing cardiac surgery. ScvO2 fiberoptic oximetry correlates better with changes in CI as compared to routine hemodynamic variables.
Early recognition and prevention of cardiocirculatory dysfunction with inadequate tissue oxygenation is essential in perioperative management of pediatric patients undergoing surgery for congenital heart disease (1–3). However, measurement of cardiac output and mixed venous oxygen saturation using a pulmonary artery catheter (PAC) is often limited in pediatric patients given the small size and complex cardiac anatomy involving intracardiac shunting (4,5).
Central venous oxygen saturation (ScvO2) in the superior vena cava (SVC) accurately reflects oxygen extraction from the upper part of the body and can be used as a surrogate for mixed venous oxygen saturation (6,7). ScvO2 remains unaffected by intracardiac shunts, and is a better variable for oxygen delivery (DO2) estimation compared with mean arterial blood pressure (MAP) and heart rate (HR) (8). Several trials for adults emphasize the value of continuous ScvO2 monitoring in improving clinical outcomes in septic patients and patients undergoing major surgery (9,10). Data on whether continuous ScvO2 monitoring might be of clinical value in patients undergoing congenital cardiac surgery are limited (2,3), in part, due to the lack of availability of a pediatric catheter with integrated fiberoptic oximetry.
We report our initial experimental and clinical experience with continuous ScvO2 monitoring using a novel Food and Drug Administration-approved fiberoptic oximetry catheter, which is integrated into a pediatric multilumen central venous catheter. First, an experimental approach was used in a porcine model to (a) compare the accuracy of a new ScvO2 catheter’s oximetry to ScvO2 blood co-oximetry during profound hemodynamic changes and (b) to determine the correlation of continuous ScvO2 oximetry to cardiac index (CI) and systemic hemodynamic variables. Second, clinical application of the pediatric ScvO2 catheter was evaluated and its accuracy and reliability measured at different time-points during and after congenital cardiac surgery.
METHODS
Experimental Protocol
The experimental protocol was approved by the IRBs for animal research, and all animals received humane treatment in compliance with the 1996 National Research Council Guide for the Care and Use of Laboratory Animals.
Five pigs (34.5 ± 1.7 kg) were anesthetized IM with ketamine hydrochloride (15 mg/kg), diazepam (0.5 mg/kg). Anesthesia was maintained with isoflurane 1%–2%. Bolus injections of fentanyl (5–10 mcg/kg) were used for analgesia and pancuronium (0.1–0.2 mg/kg) given for muscular relaxation. Respiratory support was started after endotracheal intubation with a volume-controlled ventilator (Servo 900C, Siemens-Elema, Sweden). Tidal volume (10–15 mL/kg), breathing rate (10–12/min), inspired oxygen fraction (Fio2) (30%–35%) were kept constant to maintain Pao2 (100–250 mm Hg), Paco2 (35–45 mm Hg), and pH (7.36–7.44) within normal range. Body temperature was maintained at 
37°C (heating blanket) and saline solution was infused into the jugular vein at 3–5 mL/kg per hour (h). Arterial oxygen saturation (Spo2) (Radical Set®, Masimo, Irvine, CA) and electrocardiogram were continuously monitored. The carotid artery was used for MAP and blood gas measurements. A 7.5F balloon-tipped PAC (Swan-Ganz catheter, Edwards Lifesciences, Irvine, CA) was inserted via the right external jugular vein to measure central venous pressure (CVP) and mean pulmonary artery pressure (MPAP). CI was calculated after triplicate injections of 4°C cold saline solution.
After systemic heparinization (100 U/kg IV) a midline sternotomy and longitudinal pericardial incision was performed to expose the SVC. A double-lumen, 4.5F (5 cm length) central venous oximetry catheter (PediaSat TM oximetry catheter, Edwards Lifesciences) was placed directly into the SVC ensuring proper positioning of the oximeter tip by digital palpation at the lower part of the SVC directly above the caval–atrial junction. The ScvO2 catheter was connected to the optical module (reflection spectrophotometry) and monitoring platform (VigileoTM Monitor, Edwards Lifesciences). In vivo calibration was performed by adjusting for hemoglobin (Hb), hematocrit (Hct), and oxygen saturation values (ScvO2-blood) as measured by venous blood gas co-oximetry (IRMA Trupoint® Blood Gas Analysis System, Edison, NJ) withdrawn from the distal lumen of the catheter.
Experimental Data Acquisition
Data were acquired for 30 min under stable hemodynamic conditions (baseline). Performance of the ScvO2-cath was tested during a wide range of CI achieved by a 15 min period of inferior vena cava (IVC) occlusion (CI<2.5 L · min–1 · m–2) and followed by a 20 min period of dopamine infusion (5 mcg · kg–1 · min–1; CI>4.5 L · min–1 · µm–1). After the dopamine effects subsided, data were recorded for another 30 min. Animals were euthanized by 30 mg/kg pentobarbital and a subsequent bolus of potassium chloride (40 mEq).
Venous oximetry (ScvO2-cath), systemic hemodynamics (HR, MAP, MPAP, CVP), CI, and arterial O2 saturation (Sao2) were determined every 5 min throughout the protocol (25 data sets per experiment). ScvO2-blood was sampled every 15 min throughout the protocol (8 data sets per experiment). Arterial blood gas analysis was performed at the start (baseline) and end of the protocol to exclude significant changes of Pao2, Hb, and Hct during the experiments.
Clinical Protocol
After approval of the study by the IRB, data from 16 pediatric patients scheduled to undergo cardiac surgery with or without cardiopulmonary bypass (CPB) were prospectively collected. Patients requiring the placement of a central venous catheter were included in this study. Informed consent of patients was waived by the IRB since routine surgical or anesthetic care was not altered because of study enrolment.
Anesthesia management included an inhaled induction with sevoflurane and nitrous oxide, supplemented with fentanyl (3–5 mcg/kg) and with pancuronium (0.1 mg/kg) for muscle relaxation. Anesthesia was maintained with isoflurane (1%–1.5%) and supplemented IV fentanyl (10–15 mcg/kg) for the duration of the surgery. After endotracheal intubation, a central venous oximetry catheter was inserted in all patients through the internal jugular vein. Catheter size (4.5F/5 cm (double-lumen, 20G/23G), 4.5F/8 cm (double-lumen, 20G/23G), or 5.5F/8 cm (triple-lumen, 18G/23G/23G) was determined based on patient age and size. Proper placement of the catheter tip at the lower portion of the SVC and above the caval–atrial junction was confirmed by transesophageal echocardiography. In each patient, calibration of the oximetry catheter was achieved in vivo by adjusting to Hb, Hct, and Sao2 values from venous blood samples withdrawn from the distal lumen of the catheter. Patients were operated either on hypothermic (22°C–24°C) CPB (13 of 16 patients; 81%) or off CPB (3 of 16 patients; 19%) depending on the surgical procedure. Weaning of CPB and inotropic support was facilitated using dopamine, dobutamine, and/or milrinone. Recalibration of the system was performed after completion of surgery and transfer of the patients to the Intensive Care Unit (ICU).
Clinical Data Acquisition
Continuous ScvO2-cath data were recorded after catheter placement until 24 h postoperatively in the ICU. ScvO2-cath readings were compared to co-oximetry values of simultaneously withdrawn blood samples (1 mL blood/per sample) from the distal lumen of the catheter (ScvO2-blood). Measurements were performed before surgical incision (before surgery) or at skin closure (after surgery) and at 1, 3, 6, 12, and 24 h at ICU (seven data sets per patient). Corresponding to these time points HR, MAP, and CVP were recorded. Further data acquisition included Fio2%, pulse oximetry (Spo2), and arterial blood gas analysis (Pao2, Paco2, Hb, Hct).
Statistical Analysis
Only stable ScvO2-cath readings with signal quality index 1 to 2, as indicated by the oximetry monitor, were included into the data analysis (95% of total readings).
Experimental and clinical data sets of simultaneously recorded ScvO2-cath and ScvO2-blood saturations were compared using a paired t-test and by the Bland–Altman method for determination of systematic error (bias). Precision between ScvO2 measurement methods was determined (±1 standard deviation [sd]) and 95% limits of agreement were calculated as bias ±1.96 sd. Correlation of ScvO2-cath recordings with simultaneously recorded variables (CI, MAP, HR, Hb, Hct, ScvO2-blood) was calculated using the Pearson’s correlation coefficient (Pr) and regression analysis was performed. Time-point differences of determined hemodynamic variables and blood gas analysis were assessed by one-way analysis of variance (ANOVA) with the Fisher’s LSD procedure for post hoc repeated measurements. All data are reported as mean ± sd and P values <0.05 were considered significant.
RESULTS
Experimental Protocol
In all animals, profound hemodynamic changes were achieved by either IVC-occlusion or dopamine infusion, resulting in significant decrease of MAP, CI, CVP, and MPAP after IVC-occlusion (P < 0.05) and increase of MAP, HR, CI with dopamine support (P < 0.05) when compared to baseline (Table 1). Hemodynamics returned to baseline values at the end of the protocol. Confounding variables of Sao2 and venous oxygen saturation including Spo2, Pao2, Hb (Table 1), and Hct (baseline: 31.7% ± 1.2% vs end: 31.6% ± 4.5%; n.s.) remained unchanged throughout the protocol.
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Analysis of 34 paired data sets of ScvO2-cath and ScvO2-blood measurements showed a significant correlation (Pr = 0.96, P < 0.001) and close linear relationship as determined by regression analysis (y = 0.92x + 6.73, r2 = 0.91, P < 0.001; Fig. 1a). ScvO2-cath and ScvO2-blood measurements were not different and Bland–Altman analysis revealed a +0.03% difference of means (bias) with ±4.11% precision (Fig. 1b). ScvO2-cath correlated with CI (Pr = 0.87, P < 0.001) showing a polynomial regression (y = 20.4 + 24.5x – 2.1x2, r2 = 0.84, P < 0.001; Fig. 1c) in 119 paired data sets. Only marginal changes of ScvO2-cath were seen with very high CIs (CI>4.5 L · min–1 · m–2). Correlations of ScvO2-cath were found for MAP (Pr = 0.59, P < 0.001), MPAP (Pr = 0.44, P < 0.001), and CVP (Pr = 0.38, P < 0.001), but not for HR (Pr = 0.10, P = 0.29), Spo2 (Pr = 0.13, P = 0.20), Hb (Pr = –0.16, P = 0.34), or Hct (Pr = –0.16, P = 0.33). When compared to ScvO2-cath, correlations of hemodynamic variables to CI were weaker, including MAP (Pr = 0.61, P < 0.001), MPAP (Pr = 0.38, P < 0.001), CVP (Pr = 0.35, P < 0.001), and HR (Pr = 0.25, P < 0.05).
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Clinical Protocol
Placement of the ScvO2 oximetry catheter was uneventful in all patients. No catheter-related complications were reported during the 24 h observation period. Median patient age and weight was 8.4 mo (range 2.5 mo–11 yr) and 8.0 kg respectively (range 4.2–37.1 kg). In patients operated on CPB, mean bypass time was 103 ± 38 min with an average aortic clamping time of 77 ± 33 min. Pre- and intraoperative characteristics of pediatric patients are summarized in Table 2. Postoperative Hb and Hct values remained constant and changes in perioperative hemodynamics, Spo2, Pao2, and Fio2 are shown in Table 3.
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There were no significant differences between simultaneously recorded ScvO2-cath and ScvO2-blood measurements, with both being significantly decreased at 3 h after surgery (P < 0.01). Similar to our experimental results, analysis of 99 paired data sets of ScvO2-cath and ScvO2-blood measurements showed a significant correlation (Pr = 0.94, P < 0.001) and close linear relationship as determined by regression analysis (y = 0.95x+3.59, r2 = 0.88, P < 0.001; Fig. 2a). Difference of means (bias) was –0.03% with a ±4.41% precision (Fig. 2b). Correlations of ScvO2-cath were found for Pao2 (Pr = 0.46, P < 0.001) and Spo2 (Pr = 0.35; P < 0.001), but not for Fio2 (Pr = 0.18), HR (Pr = –0.30), MAP (Pr = 0.15), CVP (Pr = –0.09), Hb (Pr = –0.17), or Hct (Pr = –0.08). In the intraoperative period, continuous ScvO2-cath readings rapidly responded to acute hemodynamic changes occurring during the surgery (representative traces are depicted in Fig. 2c). The figure demonstrates the clinical utility of monitoring ScvO2 under a few clinical scenarios observed in three separate pediatric patients undergoing cardiac surgery.
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DISCUSSIONS
This study describes a combined experimental and clinical approach to evaluate the accuracy and clinical applicability of a novel, pediatric, multilumen central venous (ScvO2) catheter with integrated, continuous fiberoptic oximetry. Our experimental results demonstrate a close correlation between saturations obtained from the fiberoptic ScvO2 catheter and blood gas co-oximetry during controlled changes in CI and hemodynamics, with a low bias (+0.03%) indicating the absence of any systematic errors between the two different methods of ScvO2 measurement. Further, a close ScvO2 relationship to CI was found using the oximetry catheter; ScvO2 correlated better to CI than the more commonly used hemodynamic variables in children (HR, MAP, MPAP, and CVP). Consistent with our experimental observations, clinical evaluation of the ScvO2 oximetry catheter demonstrated the accuracy of this method with a linear relationship to blood sample co-oximetry and negligible differences between methods (bias –0.03%). No correlations were found between hemodynamic variables and ScvO2, likely due to stable hemodynamics of patients at time-points of data acquisition. However, as observed in Figure 2c, changes in ScvO2 reflected acute alterations of cardiocirculatory status secondary to the surgical or anesthetic interventions during the case. Therefore, accurate determination of the relationship between ScvO2 and hemodynamic variables in the clinical setting (ICU) may require continuous assessment and comparison of these variables, which was not part of this report. Further, percutaneous placement of the integrated central venous catheter was safely performed in small-sized patients with no catheter-related complications throughout our 24 h observation period, and the feasibility of insertion was comparable to a standard pediatric central venous catheter.
The results of the present study are in good agreement with in vivo and in vitro experimental studies using stand-alone fiberoptic probes for ScvO2 monitoring. Evaluation of fiberoptic ScvO2 oximetry in a blood-primed extracorporeal circuit during a wide range of different oxygen saturations resulted in an excellent accuracy (bias +0.96%) when compared to blood gas co-oximetry (11). In piglets, measurement of ScvO2 during various hemodynamic states was a better variable for indirect estimation of DO2 and CI when compared to routine hemodynamic variables such as MAP and HR (8), a finding that is also confirmed by our results using the novel integrated ScvO2 catheter.
The clinical application of ScvO2 as a surrogate for tissue oxygenation has generated interest in recent years. Continuous monitoring and maintenance of ScvO2 >70% has been successfully used as a early goal-directed therapy in the management of early sepsis, resulting in a 15% reduction of mortality and leading to an adoption in the guidelines of the Surviving Sepsis Campaign (12,13). The lowest ScvO2 was independently associated with postoperative complications after major surgery in adults (9). Thus, continuous measurement of ScvO2, in combination with other surrogates of organ perfusion (vital signs, lactate concentrations, urine output), can be used as a reliable monitor of cardiocirculatory function (6,14).
Monitoring of ScvO2 in patients undergoing congenital heart surgery can be particularly useful because invasive and noninvasive methods for assessment of perioperative cardiac output, including PACs and pulse contour analysis, are usually not an option. In pediatric patients with aberrant anatomy and intracardiac shunting, measurements of ScvO2 from the SVC reflect tissue oxygenation changes more accurately when compared to right atrial or mixed venous blood. ScvO2-guided therapy resulted in an improvement of outcome after palliation for hypoplastic left heart syndrome (2,3). In this high-risk pediatric patient population with single ventricle physiology and parallel circulation, perioperative ScvO2 monitoring may be exceptionally valuable for estimating systemic oxygenation and perfusion (2,3,15–17). Further, ScvO2 measured in the SVC after Stage 1 Norwood palliation was a good predictor for childhood neurodevelopmental outcomes, a finding that emphasizes that continuous ScvO2 oximetry of the SVC can be a trend indicator for detecting alterations of cerebral perfusion and oxygenation during and after congenital heart surgery (18,19). However, clinical application of current stand-alone pediatric oximetry probes is limited to patient size (>10 kg) and require additional invasive procedures, either through direct surgical intracardiac placement or through a separate sheath placed in the SVC (2,3,17).
Integration of continuous ScvO2 fiberoptic oximetry catheters into small-sized, multilumen pediatric central venous catheters overcomes these limitations and provides a less invasive, safe, and accurate method for perioperative continuous ScvO2 monitoring in this high-risk patient population. Incorporation of the fiberoptic probe into the ScvO2 catheter is not accompanied by significant changes in lumen size or ability to infuse volume; nor is there any significant increase in catheter stiffness and the ease of insertion of these catheters in pediatric patients. Although ScvO2-blood values have always been obtainable by blood gas analysis, the clinical utility of such measurements is severely limited by the rapid hemodynamic changes in the perioperative period. In this first clinical experience, the continuous ScvO2 measurements were responsive to alterations and corrections in hemodynamic status (sampling rate: every 2 s; display rate: every 20 s), but there remains a need for experimental quantification of temporal responsiveness to changes in DO2. Although our experimental and clinical results provide some initial information about the reliability and accuracy of ScvO2 oximetry, future studies are necessary to define its performance for early detection of compromised hemodynamic states.
In conclusion, our first clinical experience using a novel central venous oximetry catheter showed its safe application in small pediatric patients and its reliable continuous ScvO2 perioperative monitoring. We demonstrate the accuracy of fiberoptic oximetry during a wide range of experimentally controlled hemodynamic changes and in patients undergoing surgery for congenital heart disease. Additionally, ScvO2 was found to correlate better with changes of CI when compared to routine hemodynamic variables. Future studies should determine whether continuous ScvO2 monitoring and ScvO2-guided therapies are useful for optimizing perioperative management and improving clinical outcomes in pediatric patients.
Footnotes
Accepted for publication August 17, 2007.
This study was funded by intramural departmental funds. There was no external financial support for this study. A few (3/21) oximetry catheters used in the present study were provided by Edwards Lifesciences, Irvine, CA. Dr. Mahajan is supported by grants from NIH/NHLBI P01 HL078931 and NIH RO1-HL084261.
Authors Oliver J. Liakopoulos and Jonathan K. Ho contributed equally to this work.
Conflict of Interest: No financial relationships or commercial interest in anyways existed between the authors and the products investigated in the present study.
REFERENCES
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