Validation of a new arterial pulse contour-based cardiac output device

Eric E. C. de Waal, MD; Cor J. Kalkman, MD, PhD; Steffen Rex, MD; Wolfgang F. Buhre, MD

T he measurement of cardiacoutput (CO) is still an impor-tant technique in the hemody-namic management of periop-erative and critically ill patients. Untilnow, bolus pulmonary artery thermodi-lution using the pulmonary artery cath-eter has remained the clinical referencetechnique of CO monitoring (1, 2). How-ever, pulmonary artery catheterization ishighly invasive and time consuming andis associated with a considerable risk of

morbidity and mortality (3, 4). To avoidthe complications of the pulmonary arterycatheter, a number of efforts have beenmade to develop alternative, less invasivetechniques of intermittent or continuousCO monitoring. The ideal CO monitorshould be noninvasive, valid, and reliableunder various pathologic hemodynamicconditions; operator independent; easy touse; continuous; and cost-effective (5). ThePiCCO technique (Pulsion, Munich, Ger-many) uses a special arterial thermodilu-tion catheter in the femoral, axillary, orbrachial artery and measures continuousCO by analysis of the arterial pulse contour.An initial calibration and subsequent reca-libration using transpulmonary thermodi-lution (TPCO) is mandatory (6). The accu-racy of the PiCCO algorithm has beenproven both clinically and experimentally(6, 7). A good agreement between TPCO andpulmonary artery thermodilution CO hasbeen demonstrated (8, 9), even during off-pump coronary artery bypass surgery (10).

Recently, the Vigileo system (EdwardsLifesciences, Irvine, CA) has been intro-duced into clinical practice. This systemincludes a newly developed algorithm for

arterial pulse contour analysis using aspecial blood flow sensor (FloTrac Sen-sor, Edwards Lifesciences, Irvine, CA)that can be used with every arterial cath-eter. No thermodilution CO is needed forcalibration of this technique (11, 12).

Until now, no clinical data comparingthe results of the Vigileo and the PiCCOas the reference technique of known ac-curacy have been available. Therefore, weperformed a clinical trial to investigate theaccuracy and precision of this new arterialpulse contour device compared with a stan-dard technique of known accuracy in cor-onary artery bypass graft (CABG) patients.

MATERIALS AND METHODS

Patients. After approval by the institutionalreview board and written informed consent,22 patients undergoing elective CABG surgerywere included. Patients with severely reducedleft ventricular function (ejection fraction35%), intracardiac shunts, significant valvu-lar heart disease, or occlusive peripheral arte-rial disease or patients undergoing emergencysurgery were excluded. All cardiac medica-tions were continued until the day of surgery,except digitalis, angiotensin converting en-zyme inhibitors, and diuretics.

From the Division of Perioperative and EmergencyCare (EECdW, CJK, WFB) and Department of IntensiveCare (EECdW), University Medical Center, Utrecht, TheNetherlands; and Department of Anesthesiology (SR),University Hospital of Aachen, Aachen, Germany.

Supported solely by institutional grants.Dr. Buhre is member of the Medical Advisory

Board from Pulsion Medical Systems, the manufac-turer of the PiCCO System, and has received honorariafor lectures from Pulsion Medical Systems. The re-maining authors have not disclosed any potential con-flicts of interest.

For information regarding this article, E-mail:e.e.c.dewaal@azu.nl

Copyright 2007 by the Society of Critical CareMedicine and Lippincott Williams & Wilkins

DOI: 10.1097/01.CCM.0000275429.45312.8C

Objective: To evaluate the accuracy and precision of an arterialpulse contour-based continuous cardiac output device (Vigileo).Vigileo cardiac output (VigileoCO) was compared with intermittenttranspulmonary thermodilution cardiac output (TPCO) and an es-tablished arterial pulse contour-based cardiac output (PCCO).

Design: Prospective clinical study.Setting: University hospital.Patients: Twenty-two patients undergoing coronary artery by-

pass graft surgery.Interventions: Defined volume load during surgery and in the

postoperative period.Measurements and Main Results: We obtained 184 pairs of

VigileoCO and TPCO, 140 pairs of VigileoCO and PCCO, and 140pairs of PCCO and TPCO. Measurements were performed afterinduction of anesthesia (T1), after sternotomy (T2), immediatelyafter (T3) and 20 mins after volume challenge with 10 mLkg1

hydroxyethyl starch 6% (T4), 15 mins after coronary pulmonarybypass (T5), after retransfusion of autologous blood (T6), afterarrival at the intensive care unit (T7), and immediately after (T8)and 20 mins after (T9) a second volume load with 10 mLkg1

hydroxyethyl starch 6%. TPCO was used to calibrate PCCO. Forpooled data, including uncalibrated PCCO data immediately afterweaning from coronary pulmonary bypass (T5), the correlationcoefficient of TPCO vs. VigileoCO, PCCO vs. VigileoCO, and TPCOvs. PCCO was 0.75, 0.60, and 0.75 respectively. Bland-Altmananalysis showed a bias of 0.00, 0.01, and 0.02 Lmin1, theprecision (SD) was 0.87, 1.08, and 0.93 Lmin1, and the meanerror was 33%, 40%, and 35%. When we compared calibratedPCCO values (T2T4, T6, T79), the correlation coefficients ofPCCO-VigileoCO and TPCO-PCCO were 0.72 and 0.85, bias was0.16 and 0.19 Lmin1, and mean error was 33% and 27%,respectively. Best correlations and the least differences betweenTPCO and VigileoCO were observed in postbypass closed-chestconditions and in the intensive care unit period.

Conclusions: Our results showed that VigileoCO enables clin-ically acceptable assessment of cardiac output in postbypassclosed-chest conditions and during stable conditions in the in-tensive care unit. (Crit Care Med 2007; 35:)

KEY WORDS: method comparison; cardiac output; arterial pulsecontour analysis; transpulmonary thermodilution; cardiac surgery

1Crit Care Med 2007 Vol. 35, No. 8

Anesthesia. Patients were premedicatedwith midazolam 7.515 mg orally 1 hr beforeinduction of anesthesia. General anesthesiawas induced with sufentanil (2 gkg1) andmidazolam (0.05 mgkg1). Tracheal intuba-tion was facilitated by pancuronium bromide(0.1 mgkg1). After endotracheal intubation,anesthesia was maintained with a continuousinfusion of sufentanil (0.5 gkg1h1) and0.51.0 MAC Sevoflurane. All patients weremechanically ventilated with an inspired oxy-gen concentration of 0.4 and a positive end-expiratory pressure of 5 cm H2O. After induc-tion of anesthesia, a 5-Fr thermistor-tippedcatheter (Pulsiocath PV2015L20A, PulsionMedical systems, Munich, Germany) was in-serted into the femoral artery. A double-lumencentral venous catheter was inserted into theright internal jugular vein. Patients were inthe supine position throughout the entirestudy period.

Cardiopulmonary Bypass (CPB). The CPBcircuit was primed with a crystalloid-colloidmixture: 250 mL of Mannitol 10%, 500 mL ofhydroxyethyl starch 10%, 1500 mL of Ringerslactate, and 100 mL of sodium hydrogen car-bonate 8.4%. Before CPB, 300 IUkg1 heparinwas administered to achieve an activated clot-ting time 480 secs. After clamping of theaorta, cardiac arrest was induced using crys-talloid or blood cardioplegia at the discretionof the attending surgeon. CPB was managedaccording to the -stat principle with a min-imal nasopharyngeal temperature of 32C anda nonpulsatile flow of 2.0 to 2.4 Lmin1m2.After termination of CPB, protamine was ad-ministered to antagonize the heparin effect.

Transpulmonary Thermodilution andPulse Contour Cardiac Output. The arterialthermistor catheter was connected via a three-way stopcock to the PiCCO pressure transducer,positioned at the level of the midaxillary catheterfor monitoring of arterial pressure, TPCO, andpulse contour cardiac output (PCCO).

Four central venous bolus injections of 15mL of cold isotonic saline were injected within3 secs for the measurement of TPCO at everytime point. The first set of measurements wasused for initial calibration of PCCO (8). Inaddition, the Vigileo system was connected viathe FloTrac sensor to the femoral artery cath-eter. Calculation of CO via the Vigileo systemis based on the patients height, weight, gen-der, age, and arterial blood pressure (13). Theunderlying algorithm is the property of themanufacturer.

Study Protocol. Hemodynamic measure-ments included recordings of heart rate, meanarterial pressure (MAP), central venous pres-sure (CVP), TPCO, PCCO, and Vigileo cardiacoutput (VigileoCO), which were measured in-tra- and postoperatively at the following timepoints: after induction of anesthesia (T1), aftersternotomy (T2), immediately after a volumeload of 10 mLkg1 hydroxyethyl starch 6% (T3),20 mins after this volume load (T4), 15 minsafter weaning from CPB (T5), after retransfusionof autologous blood (from the extracorporeal

circulation) (T6), after arrival at the intensivecare unit (ICU) (T7), immediately after a secondvolume load of 10 mLkg1 hydroxyethyl starch6% (T8), and 20 mins later (T9). At each timepoint, PCCO and VigileoCO were recorded beforethe four transpulmonary thermodilution proce-dures, that is, before recalibration of the PiCCOdevice. After the four transpulmonary thermodi-lution procedures, VigileoCO was recorded againand averaged with the earlier obtained value. AtT1 and T5, no PCCO values were obtained be-cause of starting or restarting the system in theoperating room and the ICU. Systemic vascularresistance was calculated according to the fol-lowing formula: systemic vascular resistance (MAP CVP) 79.9/CO, in which TPCO wasused.

Data Analysis and Statistics. After gather-ing the data for this study, we performed apower analysis. The sample size calculationwas performed to limit the width of a 95%confidence interval for the mean error. Basedon a mean cardiac output of 5.0 Lmin1, amean error of 30% (14), and a desired totalwidth of the confidence interval of 20%, asample size of 22 was needed.

Statistical analysis was performed usingGraphPad PRISM (version 4.0., GraphPad soft-ware, San Diego, CA). All data are expressed asmean SD unless otherwise stated. Hemody-namic variables at each time point were com-pared with baseline by analysis of variance forrepeated measurements. If the analysis of vari-ance revealed a significant interaction, posthoc analysis was performed using paired-samples t-test. Pearsons correlation coeffi-cient was used to describe the correlation be-tween the three techniques. Bias, precision,and limits of agreement were calculated ac-cording to Bland and Altman (15). Mean errorwas calculated from 2precision divided bymean TPCO (or PCCO) and expressed as per-centage (14). Mean TPCO was used whenTPCO was the reference technique, whilemean PCCO was used when PCCO was thereference technique.

RESULTS

Patient characteristics, patient his-tory, and home medications are given inTable 1. The time course of heart rate,CVP, systemic vascular resistance, TPCO,PCCO, VigileoCO, and blood and rectaltemperatures are presented in Table 2.After induction of anesthesia and beforesurgery, a phenylephrine bolus was ad-ministered if MAP was 60 mm Hg.From the 22 patients studied, three pa-tients received low-dose dopamine (mean4 gkg1min1) during weaning fromCPB, and two patients received dopamine(2.7 and 5.7 gkg1min1) during post-operative measurements. Moreover, sixpatients were paced via atrial leads be-cause of sinus bradycardia at time points

T5 and T6, five patients of that six werepaced at T7 and T8, and four of that fivewere paced at T9. No significant differ-ences were observed when we comparedbias, precision, and mean error in pa-tients with atrial pacing compared withpatients with sinus rhythm.

CVP and TPCO increased significantlyafter volume load in the operating room(T3, T4) and in the ICU (T8, T9), whereasMAP and VigileoCO increased signifi-cantly only after volume load in the ICU.After weaning from CPB, MAP was de-creased compared with pre-CPB values.The decrease in MAP was accompanied bya significant decrease in TPCO (and adecrease in VigileoCO) compared withpre-CPB values. One patient died the sec-ond day after surgery due to pericardialtamponade and subsequent cardiac fail-ure.

A total of 184 sets of CO measurementswere available for comparison of TPCO andVigileoCO. The analysis of pooled data ac-cording to Bland-Altman is presented inTable 3. Ninety-five percent of the datawere within 2 SD of the bias. Bias betweenTPCO and VigileoCO was 0.00 Lmin1

with a precision of 0.87 Lmin1. The limits

Table 1. Patient characteristics, relevant history,and home medication

Mean SD Range

Patient characteristicsAge, yrs 66 8 5182Weight, kg 80 10 6699Height, cm 174 9 153190BSA, m2 1.95 0.14 1.692.20BMI, kgm2 26.3 3.3 19.432.9CPB time, mins 85 18 49114Cross-clamp time,

mins61 15 3188

Gender 18 M/4 F

No. of Patients

Relevant historyDiabetes 7Hypertension 15Myocardial infarction 11COPD 6

Medication-blocker 18Calcium blocker 9ACE inhibitor 10AR blocker 2Nitrates 11Diuretics 7

BSA, body surface area; BMI, body mass in-dex; CPB, cardiopulmonary bypass; COPD,chronic obstructive pulmonary disease; ACE, an-giotensin converting enzyme; AR, angiotensin re-ceptor.

n 22.

2 Crit Care Med 2007 Vol. 35, No. 8

of agreement were 1.74 Lmin1 1.74Lmin1 with a mean error of 33%. Thebias of PCCO and VigileoCO was 0.01Lmin1, the precision being 1.08 Lmin1,resulting in limits of agreement of 2.18Lmin1 2.16 Lmin1 and a mean errorof 40%. The bias between TPCO and PCCOwas 0.02 Lmin1, the precision 0.93Lmin1, the limits of agreement 1.83Lmin1 1.87 Lmin1, and the meanerror 35%.

Analyses of data for every defined mea-surement point are given in Table 3. Thecorrelation coefficient between TPCO andVigileoCO varied between 0.53 and 0.80.The mean error between both methodsvaried between 24% and 45%. The bestcorrelations and the least differences be-tween TPCO and VigileoCO were observedin patients in post-CPB closed-chest condi-tions and during the postoperative ICU pe-riod. The correlation coefficient between

PCCO and VigileoCO varied between 0.21and 0.78, with a mean error between 26%and 56%. As with TPCO and VigileoCO,the best correlation between PCCO andVigileoCO was observed in the postoper-ative period.

At T5, after weaning from CPB, weobserved a worse correlation (r .48)between TPCO and noncalibrated PCCOvalues with a mean error of 53%. In con-trast, the correlation between TPCO andVigileoCO at that time point was 0.58 andthe mean error was 33%, respectively.

If noncalibrated PCCO values obtainedat T5 were not included in the analysis (asrecommended by the manufacturer), therespective correlation coefficients of TPCO-VigileoCO, PCCO-VigileoCO, and TPCO-PCCO were 0.76, 0.72 and 0.85 with a biasof 0.01, 0.16, and 0.19 Lmin1 and amean error of 33%, 33%, and 27%, respec-tively (Table 3 and Figs. 1 and 2).

DISCUSSION

In this controlled clinical trial, we stud-ied a recently introduced, pulse-contourbased continuous CO monitor (Vigileo)during the perioperative time course in pa-tients undergoing CABG surgery. Our re-sults suggest an acceptable bias and preci-sion between TPCO and arterial pulsecontour-based VigileoCO during post-CPBclosed-chest conditions and in the ICU. Theaccuracy of the Vigileo device was found tobe clinically acceptable, except for pre-CPBvalues, when patients received bolus dosesof vasopressors resulting in a sudden in-crease in vascular tone.

The mean error of the established PCCOsystem (PiCCO, version 7.0, Pulsion Medi-cal Systems, Munich, Germany) in compar-ison to TPCO was 30% at all time pointsexcept T3, T5, and T6. When we comparedthe PCCO with VigileoCO, mean errors

Table 2. Hemodynamic data obtained at each time point

T1 T2 T3 T4 T5 T6 T7 T8 T9

HR, beats/min 59 9 62 10 61 8 61 9 72 11 71 12 71 12 72 12 72 12MAP, mm Hg 63 15 67 10 66 11 67 13 51 8a 60 11b 69 12c 78 13d 80 15d

CVP, mm Hg 6 2 7 3 10 3a 9 3 7 2 10 3b 8 3 13 3d 11 3d

TPCO, Lmin1 4.02 0.87 4.22 0.92 5.59 1.22a 5.71 1.08a 4.96 0.92a 5.60 1.23 5.25 1.01 6.15 1.28d 6.17 1.38d

PCCO, Lmin1 4.06 1.05 5.15 1.08 5.67 1.09a 5.79 1.58a 5.33 1.16 6.01 1.08 6.07 1.32VigileoCO, Lmin1 3.95 0.96 4.77 0.86 5.45 0.87 5.37 0.70 5.03 0.88 5.57 0.87 5.38 0.75 6.27 0.98d 6.03 1.00d

SVR, dyneseccm5 1194 395 1187 273 830 188a 837 205a 721 118a 754 165 952 259c 868 186 921 227Blood temp, C 35.9 0.4 35.5 0.4 35.3 0.6a 35.3 0.6a 36.4 0.4a 36.2 0.4b 35.8 0.5c 35.4 0.5d 35.5 0.5d

Rectal temp, C 36.2 0.4 36.0 0.4 35.9 0.5a 35.7 0.5a 36.4 0.5 36.4 0.4 36.0 0.5c 35.9 0.5d 35.7 0.5d

T1, after induction of anesthesia; T2, after sternotomy; T3, immediately after a volume load of 10 mLkg1 hydroxyethyl starch 6%; T4, 20 mins afterthis volume load; T5, 15 mins after weaning from cardiopulmonary bypass; T6, after retransfusion of autologous blood (from the extracorporeal circulation);T7, after arrival at the intensive care unit; T8, immediately after a second volume load of 10 mLkg1 hydroxyethyl starch 6%; T9, 20 mins later; HR, heartrate; MAP, mean arterial pressure; CVP, central venous pressure; TPCO, transpulmonary thermodilution cardiac output; PCCO, pulse contour-based cardiacoutput; SVR, systemic vascular resistance.

ap .06 (vs. T2); bp .05 (vs. T5); cp .05 (vs. T6); dp .05 (vs. T7).

Table 3. Bland-Altman analysis and Pearsons correlation coefficient for data per time moment and for pooled data

Time

TPCO vs. VigileoCO PCCO vs. VigileoCO TPCO vs. PCCO

rBias,

Lmin1Precision,Lmin1

MeanError, % r

Bias,Lmin1

Precision,Lmin1

MeanError, % r

Bias,Lmin1

Precision,Lmin1

MeanError, %

T1 .53 0.08 0.90 45T2 .65 0.57 0.74 35 .78 0.58 0.65 32 .87 0.07 0.52 25T3 .62 0.14 0.98 35 .51 0.23 0.98 38 .65 0.36 0.96 34T4 .56 0.42 0.93 33 .45 0.30 1.00 35 .87 0.12 0.58 20T5 .58 0.05 0.83 33 .21 0.81 1.63 56 .48 1.00 1.32 53T6 .72 0.09 0.85 30 .69 0.24 0.84 32 .62 0.33 1.04 37T7 .80 0.12 0.64 24T8 .74 0.11 0.86 28 .71 0.25 0.79 26 .88 0.14 0.60 20T9 .74 0.14 0.92 30 .72 0.04 0.93 31 .96 0.10 0.38 12Pooled data, T5 .76 0.01 0.88 33 .72 0.16 0.89 33 .85 0.19 0.72 27Pooled data .75 0.00 0.87 33 .60 0.01 1.08 40 .75 0.02 0.93 35

TPCO, transpulmonary thermodilution cardiac output; PCCO, pulse contour-based cardiac output; T1, after induction of anesthesia; T2, aftersternotomy; T3, immediately after a volume load of 10 mLkg1 hydroxyethyl starch 6%; T4, 20 mins after this volume load; T5, 15 mins after weaningfrom cardiopulmonary bypass; T6, after retransfusion of autologous blood (from the extracorporeal circulation); T7, after arrival at the intensive care unit;T8, immediately after a second volume load of 10 mLkg1 hydroxyethyl starch 6%; T9, 20 mins later.

3Crit Care Med 2007 Vol. 35, No. 8

were generally higher compared with thecomparisons of TPCO with VigileoCOalone. Immediately after weaning fromCPB, there was a worse correlation betweenPCCO and VigileoCO as well as betweenPCCO and TPCO.

Pulmonary artery thermodilution COis still the accepted reference techniquefor clinical assessment of CO (1, 2). How-ever, the pulmonary artery catheter is ahighly invasive monitoring technique,and subsequent efforts have been made todevelop alternative, minimally invasiveCO monitoring devices. One of the lessinvasive alternatives is TPCO. The accu-racy and precision of the TPCO techniqueare comparable to classic pulmonary ar-

tery thermodilution (68, 10). Moreover,when TPCO is used in conjunction withthe PiCCO system, measurement of con-tinuous PCCO is available (9). Both thePiCCO system and the Vigileo system arearterial pressure curve-based CO moni-toring systems. Therefore, we used TPCOas the reference technique in the presentstudy.

We compared both techniques with ar-terial pressure curves obtained in the fem-oral artery. CO determination via the fem-oral artery seems to be superior to COdetermination in the radial artery, particu-larly due to less damping of the femoralarterial pressure curve during the first fewhours after weaning from CPB. However,

the incidence of infection in femoral arterycannulation is higher (0.44% vs. 0.13% forradial artery cannulation), while the maincomplication rate of radial artery cannula-tion is temporary occlusion (19.7% vs.1.18% for femoral artery cannulation) (16).

Another major problem with studyingdifferent methods for CO monitoring isthe fact that usually no real referencetechnique (e.g., an electromagnetic flowmeter) is available in a clinical setting.Therefore, the true CO is unknown, asthe reference technique in the underlyingstudy (transpulmonary thermodilution)has an inherent bias of approximately10% to 20% (6, 7). In 1999, Critchley andCritchley (14) performed a meta-analysisof studies comparing different techniquesof CO monitoring. They analyzed the per-centage errors of different methodsagainst the clinical reference techniqueand used the data to determine clinicallyacceptable limits of agreements betweenmethods. According to this type of anal-ysis, mean errors up to 30% are accept-able for clinical purposes. Therefore, wecalculated not only bias, precision, andlimits of agreement according to Bland-Altman but also the mean error be-tween the three different techniques. Inthe present study, analysis betweenVigileoCO and TPCO as the referencetechnique showed a mean error of 33%,which is relatively close to the upperacceptable limit of 30%. The highestdifference between VigileoCO and TPCOwas found after induction of anesthesia,which may be attributable to the fact thatpatients received boluses of phenyleph-rine in that period due to a decrease inmean arterial pressure to 60 mm Hg.No patients received vasopressor supportafter weaning from CPB; therefore, it isnot likely that drug-induced changes invascular tone affected the results of thepresent study.

However, during situations with rapidlychanging cardiovascular conditions (e.g.,after induction of anesthesia and sternot-omy), the difference between methods wasincreased. Most likely, the increased differ-ence in CO may be caused by the algo-rithms for calculating of CO incorporatedin the Vigileo device. As far as we know, theVigileo system calculates stroke volume us-ing arterial pulsatility (SD of the pressurewave over a 20-sec interval), according tothe equation stroke volume Kpulsatility(11). K is a constant quantifying arterialcompliance and vascular resistance. K isderived from patient characteristics (gen-der, age, height, and weight) according to

Figure 1. Bias 2 SD according to Bland-Altman for pooled data excluding data obtained at T5 (15 minsafter weaning from cardiopulmonary bypass). TCPO, transpulmonary thermodilution cardiac output;PCCO, pulse contour continuous cardiac output.

4 Crit Care Med 2007 Vol. 35, No. 8

the method described by Langewouters etal. (13) and waveform characteristics (skew-ness and kurtosis of individual arterial pulsewaves) (11). The calibration constant K isautomatically recalculated (software ver-sion 01.01) every 10 mins. Therefore, underconditions of rapidly changing vascular re-sistance, the response of the Vigileo devicein the time domain will probably be de-layed. However, the manufacturer recentlyprovided a new software version (01.07)that recalibrates the system every minute.This may improve the accuracy and preci-sion during rapid changes in vasomotortone. Changes of CO by a defined volumechallenge were detected adequately underopen- (T3, T4) and closed-chest conditions(T8, T9).

Recently, Manecke et al. (11) comparedthe Vigileo monitor with intermittent COmeasurements by pulmonary artery ther-modilution. Over a wide range of CO (2.779.60 Lmin1), they found a close agree-ment (bias 0.04 Lmin1, precision 0.99Lmin1) between both techniques. Theseresults are in line with the present study.However, one major limitation of thestudy from Manecke et al. is that no stan-dardized changes in volume status weredone. Mayer and colleagues (17) com-pared the VigileoCO with pulmonary ar-tery thermodilution CO in patients un-dergoing CABG and/or valve repair. Intheir study, the overall mean error was46%, which was substantially higher thanthe overall mean error of 33% obtained inour study. However, the recent study ofMayer et al. differed from our study inseveral important points, which may ex-plain the diverging results. First, the pa-tient population differed considerably.Whereas Mayer et al. included a hetero-geneous patient population (patients un-dergoing CABG and/or valve surgery),our study population was homogeneous(CABG patients). Second, in our study theFloTrac sensor was connected to a femo-ral arterial catheter, whereas Mayer et al.connected the FloTrac sensor to the ra-dial arterial catheter, which may also in-fluence the results in particular afterCPB. Moreover, Mayer et al. conductedno definitive fluid challenge. It seemspossible that these differences in studydesign may have influenced the results ofthe study from Mayer et al. and at least inpart can explain the increased mean errorcomparing VigileoCO and pulmonary ar-tery thermodilution CO.

We used the PiCCO system as the ref-erence technique, which was alreadystudied under a variety of experimental

Figure 2. Pearsons correlation coefficients for pooled data excluding data obtained at T5 (15 mins afterweaning from cardiopulmonary bypass). TPCO, transpulmonary thermodilution cardiac output; PCCO,pulse contour continuous cardiac output; r, Pearsons correlation coefficient.

5Crit Care Med 2007 Vol. 35, No. 8

and clinical circumstances (610, 18).We obtained a good agreement betweenPCCO and TPCO, enabling continuousmeasurement of CO with the PCCO tech-nology. The mean error of pooled datawas 27% if uncalibrated data obtained atT5 were excluded from the analysis. Incontrast to Vigileo, measurement of arte-rial pulse contour-derived CO by thePiCCO is dependent on recalibration ifhemodynamic conditions change rapidly.However, some authors claimed thatPCCO can be measured with acceptableagreement without recalibration evenfor a longer period of time (9). In thepresent study, PCCO values obtainedimmediately after CPB (T5) withoutrecalibration showed a worse agree-ment with TPCO values obtained bythermodilution immediately thereafter.Therefore, we conclude that recalibra-tion after profound changes in hemody-namics is a prerequisite for adequatemeasurement of CO with the PCCOtechnique. The results of the presentstudy confirmed the findings fromSander et al. (18), who observed also agood agreement between TPCO andPCCO before CPB but found less goodagreement between TPCO and PCCOafter CPB. When comparing both arte-rial pulse contour-derived techniques(VigileoCO vs. PCCO), we found bias,precision, limits of agreement, andmean error to be increased comparedwith the results obtained for the com-parison between the Vigileo device andthe reference technique (TPCO). Themost pronounced changes were foundimmediately after weaning from CPB,which at least in part may be explainedby the fact that uncalibrated PCCO val-ues were obtained. Moreover, VigileoCOafter sternotomy was different fromPCCO and TPCO values. During the re-maining time points, both minor inva-sive techniques offer comparable accu-racy and precision. The lowest meanerror was observed during the ICU pe-riod; this holds true for both the Vigileoand the PCCO technique. A second vol-ume load maneuver resulted in a sig-nificant increase in CO.

CONCLUSIONS

The results of the present study sug-gest that the new arterial pulse contourdevice enables assessment of CO in pa-tients undergoing CABG surgery withclinically acceptable bias and precision inthe post-CPB period under closed-chestconditions and in the ICU. PiCCO andVigileo are interchangeable in the post-operative period. The latter technique isof particular interest as no calibrationis needed and the flow sensor can beused with any common arterial cathe-ter. VigileoCO was found to exceedTPCO in the pre-CPB period and open-chest condition, which makes the tech-niques not interchangeable under thesecircumstances. Thereby, during rapidlychanging hemodynamic conditions, theaccuracy and precision are not accept-able. Further refinement of the algo-rithm resulting in decreased responsetime may improve the accuracy undersuch hemodynamic conditions.

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