At present, cardiac output is usually determined

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1 Original Article / Transplantation Continuous versus bolus cardiac output monitoring during orthotopic liver transplantation Fang-Ping Bao and Jian Wu Hangzhou, China BACKGROUND: Cardiac output monitoring is important for critical patients. This study aimed to determine the delayed response of continuous cardiac output (CCO) thermodilution measurement, whether CCO and bolus cardiac output (BCO) thermodilution agree sufficiently to be used interchangeably, and whether CCO monitoring is reliable for patients undergoing liver transplantation. METHODS: Thirteen patients undergoing liver transplantation without veno-venous bypass were studied (37-66 years old, weight kg). Continuous and bolus thermodilution measurements were performed at predefined time points using an "Opti-Q" SvO 2 /CCO monitor (Abbott Laboratories, North Chicago, IL, USA). Bias and 95% limits of agreement were calculated according to Bland and Altman analysis. The limits of agreement by which two methods are judged to be interchangeable were defined in advance as ±(13% BCO mean ) L/min. The repeatability and relative error of CCO, and the differences between CCO and the mean of the two measurements were calculated. RESULTS: Cardiac output measurements yielded 196 data pairs with ranges of 1.9 to 17.9 L/min for CCO and 2.1 to 18.3 L/min for BCO. The response time of CCO was delayed in the early phases after caval clamping and after reperfusion. At most of the measurement points, bias and 95% limits of agreement were -0.18±1.91 L/min. 95% limits of agreement did not fall within the predetermined limits of agreement of ±1.14 L/min. The repeatability coefficient of CCO was 0.36 L/min and the relative error was 4.6±4.7%. The mean difference between CCO and the average of the two methods was L/min (0.49 L/min). Author Affiliations: Department of Anesthesiology, First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou , China (Bao FP and Wu J) Corresponding Author: Jian Wu, MD, Department of Anesthesiology, First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou , China (Tel: ; wujian888888@ 163.com) 2008, Hepatobiliary Pancreat Dis Int. All rights reserved. CONCLUSIONS: In patients undergoing liver transplantation, the delayed response of CCO limits its application during the early phases after caval clamping and after reperfusion of the graft. The two methods are not interchangeable even in hemodynamic stability. Continuous thermodilution monitoring, however, is reliable or acceptable for clinical purposes. (Hepatobiliary Pancreat Dis Int 2008; 7: ) KEY WORDS: cardiac output; thermodilution; anesthesia; liver transplantation Introduction At present, cardiac output is usually determined by the intermittent bolus thermodilution technique. However, the accuracy of the bolus cardiac output (BCO) measurement can be influenced by various factors, and some shortcomings limit its clinical application. [1, 2] Acute hemodynamic instability may occur in critically ill patients, so continuous measurement of cardiac output is preferable to intermittent measurement. Among the methods of continuous measurement, thermodilution is userindependent and semi-continuous and is widely accepted. Two continuous cardiac output (CCO) monitors are on the market: the "Vigilance" CCO/ oximetry system (Baxter Edwards Critical-Care, Irvine, CA, USA) and the "Opti-Q" SvO 2 /CCO system (Abbott Laboratories, North Chicago, IL, USA). They measure cardiac output using the same basic thermodilution principles and use a filament to generate heat. However, they have different signal generation and signal processing technologies. Both systems have a response time of 3-6 minutes, [3] which is too long to monitor acute changes in cardiac output. [4-7] This delayed response of CCO measurement limits its clinical application during acute hemodynamic instability. [4-9] 138 Hepatobiliary Pancreat Dis Int,Vol 7,No 2 April 15,2008

2 Continuous versus bolus cardiac output monitoring during OLT Several studies have analyzed the agreement between CCO and BCO measurements under [3, 4, 10-16] stable conditions, and the performance of the "vigilance" monitor in liver transplantation patients has also been evaluated. [8, 16, 17] However, the performance and equivalence of CCO compared with BCO has not been tested at the level of scrutiny expected to qualify as "evidence-based" monitoring options. [1, 18] It may be that the two methods are not interchangeable [19, 20] according to Bland and Altman [21, 22] analysis. In this study, we used the "Opti-Q" SvO 2 /CCO system in patients undergoing orthotopic liver transplantation without veno-venous bypass to 1) measure the delayed response of CCO during the early periods after caval clamping and after reperfusion of the liver graft; 2) assess the agreement between CCO and BCO measurements, that is, to determine whether we can use the two interchangeably; and 3) evaluate whether CCO monitoring is reliable and acceptable for clinical purposes. Methods After approval by our clinical research committee and written informed consent had been obtained, 13 adult patients (11 male and 2 female, years old, weighing kg) were enrolled in the study. They were suffering from end-stage liver disease or hepatocellular carcinoma, and were about to undergo orthotopic liver transplantation, without veno-venous bypass. Patients with valvular heart disease and intracardiac shunts were excluded. All patients were not premedicated. On the patient's arrival at the operating room, 3 forearm venous and left radial arterial lines were established, and a pulmonary artery flotation catheter (Opti-Q SvO 2 /CCO, 8F) was inserted through a 8.5-French introducer via the right internal jugular vein under local anesthesia before induction of anesthesia. The catheter was connected to a Q-Vue computer operated in the "normal" mode (cardiac output values were averaged over a period of 5 minutes in this mode). The proper position of the catheter was confirmed by pressure monitoring from its proximal and distal ports. Anesthesia was induced with etomidate (0.3 mg/kg), midazolam (0.05 mg/kg), fentanyl (5 μg/kg), and vecuronium (0.1 mg/kg), and maintained with midazolam, fentanyl, isoflurane (0.5%-1.5%) and vecuronium. Ventilation was adjusted to maintain end-tidal pco 2 at mmhg. Heart rate, SpO 2, invasive blood pressure, central venous pressure and pulmonary artery mean pressure were monitored. The patients were placed on a warming blanket (Gaymar Meditherm, Orchard Park, NY, USA). Blood products and colloid solutions were given through a warming infusion device (HOT LINE, SIMS Medical System, Graseby Ltd., UK) and other room temperature crystalloid solutions infusions were maintained at a rate of about 500 ml/h via the peripheral veins. Through another peripheral vein, dopamine (3 μg/kg/min) was given throughout the operation, and epinephrine was administered to stabilize the circulation after upper inferior vena cava clamping and, if needed, was continued to the end of the operation. CCO and BCO measurements were obtained at the following predefined time during the study period: 20, 40, 60, and 90 minutes after induction of anesthesia, 5, 15, 25, 35, and 45 minutes after caval clamping, and 5, 15, 25, 60, 90, and 120 minutes after reperfusion of the liver graft. At each measurement point, CCO was measured immediately before and after BCO measurement, and the means of these CCO data pairs were recorded. BCO was measured by manual injection of 10 ml iced saline solution through the injectate port of the CCO catheter using a closed injectate system. The average of three measurements, measured within a 10% range distributed randomly over the respiratory cycle, was calculated. All bolus thermodilution measurements were performed by the same person to avoid inter-individual variations. Intravenous infusions through the central venous catheter were stopped and the setting of the ventilator remained the same during the measurement. The peripheral volume infusion rates and change of epinephrine administration were independent of the cardiac output measurements and were adjusted to maintain hemodynamic stability according to clinical considerations (i.e. blood loss, mean blood pressure, and operation stage). In addition, to measure the delayed response phenomenon of CCO, its values were recorded before caval clamping and at 1, 2, 3, and 5 minutes, then at 5-minute intervals after caval clamping until before reperfusion of the liver graft, and at 1, 2, 3, and 5 minutes, then at 5-minute intervals until 30 minutes after the reperfusion. Rectal temperature and blood temperature in the pulmonary artery were recorded continuously. Statistical analysis Statistical analysis was performed using linear regression for the data pairs, Pearson's correlation coefficient, and the method described by Bland and Hepatobiliary Pancreat Dis Int,Vol 7,No 2 April 15,

3 Hepatobiliary & Pancreatic Diseases International Table 1. Mean values of cardiac output and correlation, bias and 95% limits of agreement between continuous and bolus cardiac output measurements at predefined times Measurement time points (min) CCO (L/min) BCO (L/min) Correlation (r)/p value Bias (L/min) 95% limits of agreement (L/min) After induction of anesthesia (3.5) 9.0 (3.4) 0.93/< to (3.2) 9.8 (3.2) 0.93/< to (2.7) 9.9 (2.3) 0.95/< to (1.9) 9.1 (2.2) 0.88/ to (2.3) 8.6 (1.8) 0.88/ to 2.85 After caval clamping (2.4) 5.5 (1.8) 0.72/ to (2.1) 5.7 (2.1) 0.94/< to (1.8) 6.3 (1.9) 0.89/< to (2.1) 6.0 (2.2) 0.96/< to (2.7) 6.1 (2.6) 0.96/< to 1.26 After reperfusion of liver graft (2.8) 11.7 (3.8) 0.73/ to (3.5) 11.3 (2.9) 0.93/< to (3.5) 10.9 (3.5) 0.98/< to (3.6) 10.2 (3.7) 0.98/< to (3.6) 10.4 (4.1) 0.99/< to (1.8) 8.8 (1.8) 0.88/ to 1.84 Altman. [21, 22] Bias between the methods was calculated as the mean difference between CCO and BCO, and tests for normality of distribution were made. The upper and lower limits of agreement were calculated as bias ±1.96SD, and defined the range in which 95% of the differences between the methods were expected to lie. The differences between each pair of values were plotted over the average of each pair. The precision of the bias analysis and the limits of agreement were assessed using 95% confidence intervals (CI). The repeatability coefficient of each method was calculated using one-way analysis of variance. [22] We defined the limits within which the two methods were judged to be interchangeable as ±(13% BCO mean ) L/min. [23] In addition, the mean difference between CCO and the assumed true value (average value of two methods) was calculated, and these differences were also plotted over the assumed true values accordingly. The relative error of CCO measurement, defined as 100 [CCO- (BCO+CCO)/2]/[(BCO+CCO)/2], was calculated. P<0.05 was considered statistically significant. Results The mean time in the three phases (paleohepatic, anhepatic, and neohepatic) during the operation was 133.8±22.8, 53.9±12.3 and 148.8±34.7 minutes, respectively. Boluses of epinephrine (10-80 μg) were required after upper inferior vena cava clamping and after reperfusion of the graft; meanwhile, infusions of epinephrine were given to stabilize the circulation, and mean arterial pressure was maintained above 55 mmhg throughout most of the procedure in all patients. All patients had successful transplants and survived at the end of the operation. No adverse effects of the "Opti-Q" SvO 2 /CCO catheter were noted during the study period. Since it was impossible to make all measurement as scheduled without interfering with treatment, a total of 196 data pairs were collected from 13 patients. The ranges of measured cardiac output values using bolus and continuous cardiac output were and L/min, respectively. At 5 minutes after caval clamping and 5 minutes after reperfusion of the graft, the correlation markedly decreased and the bias increased (Table 1). CCO decreased slowly in the early phase after caval clamping and was close to BCO at 15 minutes of clamping. In contrast, CCO increased slowly in the early phase after reperfusion and was close to BCO at 15 minutes of reperfusion. In the early phases after caval clamping and reperfusion, the delayed response of CCO when acute changes in cardiac output occurred was evident. So, the data pairs of two time points (5 minutes after caval clamping and 5 minutes after reperfusion) were excluded from the ensuing data analysis. The results of the analysis of agreement, 140 Hepatobiliary Pancreat Dis Int,Vol 7,No 2 April 15,2008

4 Continuous versus bolus cardiac output monitoring during OLT Fig. 1. Bland and Altman diagram for CCO and BCO. This analysis included 170 data pairs in the stable phases during liver transplantation, and a bias of L/min with 95% limits of agreement of to 1.73 L/min was determined. Table 2. Mean values of pulmonary arterial blood and rectal temperatures and difference between these measurements at each predefined time Measurement time points (min) After induction of anesthesia Pulmonary arterial blood temperature ( ) Rectal temperature ( ) Temperature difference ( ) (1.1) 36.9 (1.0) 0.2 (0.1) (1.0) 36.8 (1.0) 0.2 (0.1) (0.9) 36.8 (1.0) 0.3 (0.1) (0.9) 36.8 (1.0) 0.2 (0.1) (1.0) 37.0 (1.1) 0.3 (0.2) After caval clamping (0.8) 36.9 (0.9) 0.3 (0.2) (0.9) 36.8 (1.0) 0.4 (0.2) (1.0) 36.7 (1.0) 0.5 (0.2) (0.7) 36.9 (0.8) 0.6 (0.2) (0.7) 36.9 (0.8) 0.8 (0.2) After reperfusion of liver graft (0.9) 36.3 (0.9) 0.6 (0.1) (0.9) 36.2 (0.9) 0.3 (0.2) (0.9) 36.3 (0.9) 0.3 (0.2) (0.8) 36.5 (0.9) 0.2 (0.1) (0.9) 36.7 (0.9) 0.2 (0.1) (0.8) 36.8 (0.9) 0.2 (0.1) Fig. 2. Difference between continuous cardiac output and the mean of two measurements plotted against the average of these two values (n=170). A mean difference of L/min (SD 0.49 L/min) was determined and 95% of these differences between CCO and the average were expected to lie in the range of to 0.87 L/min (±1.96SD). comprising 170 data pairs from the other time points in 13 patients, revealed that the bias was L/min (95% CI: to L/min), and lower and upper limits of agreement (bias ±1.96SD) were L/min (95% CI: to L/min) and 1.73 L/min (95% CI: 1.48 to 1.99 L/min), respectively. The ±1.96SD values of the bias did not fall within the predetermined limits of agreement of ±1.14 L/min [13% BCO mean (8.75 L/min)]. The differences between CCO and BCO are plotted against the mean of the two methods (Fig. 1). The repeatability coefficient of CCO was 0.36 L/min and that of BCO was 0.86 L/min. The mean difference between CCO and the average of the two methods was L/min (SD 0.49 L/min) and the differences of these data pairs are plotted against the assumed true value (Fig. 2). The relative error of CCO measurement was 4.6±4.7%. Pulmonary artery temperature and rectal temperature were maintained well in all patients after induction of anesthesia. Pulmonary artery temperature, however, decreased while rectal temperature remained constant after caval clamping. The temperature difference increased to a maximum at 45 minutes after clamping. After reperfusion of the graft, both pulmonary artery temperature and rectal temperature increased gradually to basal levels (Table 2). Discussion The standard method of statistical evaluation of [21, 22] comparison studies is that of Bland and Altman. The primary aim of a comparison study is to determine whether two methods agree sufficiently to be used interchangeably. In a review, Mantha et al [18] pointed out several inadequacies and inconsistencies in the statistical results of comparison studies with regard to interchangeability of measurement methods, although 95% of the studies used Bland and Altman methodology for analysis. In our study, we compared the "Opti-Q" CCO with the conventional intermittent Hepatobiliary Pancreat Dis Int,Vol 7,No 2 April 15,

5 Hepatobiliary & Pancreatic Diseases International bolus thermodilution technique by strictly applying Bland and Altman analysis, [21, 22] and found that the two methods were not interchangeable. Ideally, one should define satisfactory agreement in advance in the comparison of two methods of measurement because how far apart measurements can be performed without leading to problems is a matter of clinical judgement, and statistical methods cannot make such a judgement. [22] In addition, a literature review revealed that acceptable, clearly defined criteria as to whether a newer technique can replace an older and more established one are lacking. [18] In the present study, we used the approach recommended by LaMantia et al, [23] who suggested using the results of Stetz et al, [24] i.e., 13% based on triplicate measurements as the criterion for comparing a newer method with thermodilution. In our study, the mean value of BCO from 170 data pairs was 8.75 L/min. Hence, ±1.96SD of the bias did not fall within the predetermined limits of agreement of ±1.14 L/min, indicating a lack of agreement between CCO and BCO. Factors associated with the lack of agreement between CCO and BCO included the repeatability of each method, the errors of thermodilution cardiac output measurement, rapid changes in cardiac output, and the delayed response of the CCO catheter. Under ideal circumstances, intermittent thermodilution [24, 25] has a 10%-15% error. Potential errors of 15%-50% can be caused by respiratory variations in pulmonary arterial blood temperature >0.05. [26] Therefore, a variation of ±10% within quadruplicate measurements is generally defined to be acceptable during intermittent thermodilution cardiac output measurements in order to reduce the influence of within-technique variability. In this study, the 95% limits of agreement were considerably wider than the repeatability. Therefore, the lack of agreement between the two methods cannot be explained by lack of repeatability, implying that other factors lowered the agreement. [22] Intermittent BCO is not the true value of cardiac output, although it is currently used as the standard method for its determination. Several factors including the volume and temperature of the injectate, the injection technique, and the timing of the injection within the respiratory cycle can influence the accuracy of intermittent bolus thermodilution. Because of methodological differences, the CCO method is not influenced by these factors, and may be more accurate than intermittent BCO under conditions with stable hemodynamics. Better accuracy with CCO than with intermittent BCO has been demonstrated in both in vitro [13] and in vivo [19] studies. The present study also showed better repeatability with CCO method with intermittent BCO. Several previous studies demonstrated the delayed response of CCO catheters to acute changes in cardiac output. [4-6] Significant delays in the response of CCO are induced by clinically relevant maneuvers, including rapid fluid infusion, hemorrhage, and drug therapy. [5] In the "normal" mode of the "Q-Vue" monitor, cardiac output values are averaged over 5 minutes. So, the monitor may ensure a response time of about 5 minutes under ideal conditions according to the averaging algorithms. However, response time may be significantly longer in the perioperative period. Aranda and colleagues [6] demonstrated in vitro that the response time for an 80% change in the "Opti-Q" CCO catheter is 5-11 minutes after controlled flow changes. Marked changes in cardiac output often occur after caval clamping and reperfusion during orthotopic liver transplantation, and they result in more thermal noise. Therefore, the monitor needs a longer analysis period, and the response time of CCO is delayed. In the present study, BCO and CCO cardiac output determinations could not be made simultaneously because of signal interference. Therefore, we could not calculate exactly the response time of the CCO catheter when cardiac output abruptly decreased after caval clamping and increased after reperfusion. However, the CCO values at 15 minutes after caval clamping as well as at 15 minutes after reperfusion were close to the BCO values, indicating that the response time of the CCO catheter was similar to that reported by Aranda et al. [6] In addition, this delay in response time of the CCO catheter is important for appropriate clinical use and must be considered when rapid changes in cardiac output occur. The rapid infusion of cold fluids may interfere with thermodilution cardiac output measurements by increasing thermal noise. Mihaljevic et al [13] found that the various rates of room-temperature fluid infusions (100 to 1000 ml/h) influences bolus measurements at flow rates of both 2 and 4 L/min, and affects the values from continuous measurement only at 2 L/min. Greim and colleagues [17] compared the two methods during liver transplantation and found that the bias increased with rising intravenous infusion rates. A small bias between the methods was achieved at low rates (<1000 ml/h) and during high cardiac output, whereas a large bias was most evident at high rates (>2000 ml/h) and low cardiac output. [17] Although our results of bias and 142 Hepatobiliary Pancreat Dis Int,Vol 7,No 2 April 15,2008

6 Continuous versus bolus cardiac output monitoring during OLT 95% limits of agreement were similar to those reported [3, 4, 8-12, 14-17, 19, 20] by other researchers, fluid infusion may still have contributed to the lack of agreement between the two methods found in this study. Bottiger et al [8] reported a bias (SD) of L/min (1.789 L/min) between the two methods during liver transplantation. The bias increased while the differences between pulmonary artery blood temperature and rectal temperature showed dynamic changes in the early phases after caval clamping and after reperfusion. [8] These findings are in agreement with our results. Both reflected hemodynamic and thermal instability during surgery and may also have contributed to the lack of agreement between the two methods. In this study, however, the difference between pulmonary artery blood temperature and rectal temperature reached a maximum at 45 minutes after caval clamping, indicating that the significant differences between the two methods, at 5 minutes after caval clamping and 5 minutes after reperfusion, were mainly induced by marked cardiac output changes and deficiency of thermodilution measurement. Another question in the current literature concerns what degree of accuracy is needed in CCO determinations for adequate clinical care [27] and correspondingly, if its accuracy is acceptable for clinical purposes. That the two methods are not interchangeable does not mean that continuous measurement is unacceptable for clinical purposes. They involve two different concepts because the bolus measurement does not give the true value of cardiac output. We did not know the true values, so the mean of two measurements was the best estimate we had. [22] Therefore, we compared continuous measurement with the average to decide its acceptability. Both our results and previous [13, 19] studies showed that continuous measurement had excellent repeatability in stable conditions. During the stable conditions in our study, the mean difference between continuous measurement and the average and its standard deviation were small (-0.09 L/min and 0.49 L/min, respectively). They were not clinically important, giving a mean cardiac output of 8.66 L/min. Combined with the small relative error of the continuous measurement, we considered the continuous measurement reliable and acceptable for clinical purposes. In conclusion, the delayed response of continuous thermodilution measurement limits its application during the early phases after caval clamping and after reperfusion of the graft. The two methods are not interchangeable even during hemodynamic stability in patients undergoing liver transplantation. Continuous thermodilution monitoring, however, is reliable and acceptable for clinical purposes. Funding: This work was supported by a grant from the Health Bureau of Zhejiang Province (No. 2006QN011). Ethical approval: Not needed. Contributors: WJ proposed the study and wrote the first draft. BFP analyzed the data. All authors contributed to the design and interpretation of the study and to further drafts. WJ is the guarantor. Competing interest: No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. References 1 Salmenpera M, Aittomaki J. Cardiac output monitoring: need for improvement? Acta Anaesthesiol Scand 2003;47: Bottiger BW. Continuous cardiac output monitoring-- further applications of the thermodilution principle. Intensive Care Med 1999;25: Mihm FG, Gettinger A, Hanson CW 3rd, Gilbert HC, Stover EP, Vender JS, et al. 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