Cardiac output (CO) monitoring is in daily use in the cardiac anaesthesia setting. The most commonly used device for CO determination is the pulmonary artery catheter (PAC) with thermodilution capabilities. However, this highly invasive technology has potential adverse effects such as infections, thrombosis and mechanical complications . Moreover, its clinical utility is still doubtful and its use has decreased over the past 10 years [2,3]. Alternative solutions for CO monitoring are of major importance in the cardiac anaesthesia setting. Several new techniques have been developed in this regard: aortic transpulmonary thermodilution [4-6], oesophageal Doppler, thoracic bioimpedance, partial CO2 rebreathing (NICO) and echocardiography. Nevertheless, these new tools are either invasive (aortic transpulmonary thermodilution that requires a specific femoral arterial catheter), operator dependent (oesophageal Doppler), poorly accurate (NICO) or require a long training (echocardiography). Moreover, many of these indices are complex and require calibration [7-10].
Recently, arterial pulse waveform analysis has been proposed for CO determination and monitoring (FloTrac™/Vigileo™; Edwards Lifesciences, Irvine, CA, USA) [11-14]. The main advantage of this new device is that it can be used with any commercially available arterial catheter and that it only requires a specific bedside monitor (Vigileo™) connected to a pressure transducer (FloTrac™) attached to the arterial catheter with no calibration. However, the accuracy and clinical applicability of this new technology has not been fully evaluated.
We designed this prospective study to compare the accuracy of the FloTrac™ system vs. PAC standard bolus thermodilution in patients undergoing coronary artery bypass grafting (CABG).
Materials and methods
The protocol was approved by the institutional review board for human subjects of our institution (Comité Consultatif de Protection des Personnes dans la Recherche Biomédicale Lyon B). All patients gave informed and written consent. Between August 2006 and December 2006 we studied 11 consecutive patients undergoing CABG. This group consisted of seven males and four females aged between 58 and 83 yr (mean age 69 ± 7 yr). Induction of anaesthesia was performed with propofol (3-5 mg kg−1) and sufentanil (0.5-1.0 μg kg−1), and orotracheal intubation was facilitated with pancuronium (0.1-0.15 mg kg−1). After induction of anaesthesia, an 8 cm 5-Fr tipped catheter (Arrow International Inc., Reading, PA, USA) was inserted in the left or right radial artery, a triple lumen 16 cm 8.5 Fr central venous catheter (Arrow International Inc.) and a PAC (PAC, Swan-Ganz catheter, 7.5 Fr; Baxter Edwards Lifesciences LLC, Irvine, CA, USA) were inserted in the right internal jugular vein. Pressure transducers (Medex Medical Ltd, Rossendale, Lancashire, UK) were placed on the midaxillary line and fixed to the operation table in order to keep the transducer at the atrial level throughout the study protocol. All transducers were zeroed to atmospheric pressure. Correct position of the PAC in West's zone 3 was assessed using the method of Teboul and colleagues . Anaesthesia was maintained with continuous infusions of propofol (5-8 mg kg−1 h−1) and sufentanil (0.7-1.0 μg kg−1 h−1) in order to keep a bispectral index (BIS, Aspect 1000; Aspect Medical Systems Inc., Natick, MA, USA) between 40 and 50. All patients were ventilated in a volume-controlled mode with a tidal volume of 8-10 mL kg−1 of body weight at a frequency of 12-15 cycles min−1. Positive end-expiratory pressure was set between 0 and 2 cm H2O according to the attending physician.
Cardiac output determination
CO determination was performed at six time points in the operating room: (1) 5 min after induction of anaesthesia and placement of the catheters, (2) 5 min after a volume expansion (500 mL 6% hetastarch infused over 10 min), (3) 10 min after sternotomy and before cardiopulmonary bypass (CPB) initiation, (4) 10 min after CPB discontinuation and before chest closure, (5) 10 min after chest closure and (6) just before leaving the operating room. Measurements were performed at the arrival in the ICU and every 4 h until discharge from the ICU. At each step of the protocol, CO was determined using FloTrac™ system (COFT) and PAC (COPAC).
COPAC was measured by thermodilution, using the average of five successive measurements omitting the maximum and minimum values obtained by injection of 10 mL of dextrose at room temperature randomly during the respiratory cycle. Cardiac index and stroke volume index were calculated using the same formula: cardiac index = CO/body surface area.
The FloTrac™ device analyses the arterial waveform to determine COFT. This technique does not need prior calibration. The method has been described in details elsewhere. Briefly, the FloTrac™ is a specific pressure transducer attached to any commercially available arterial catheter and connected to a specific monitor (Vigileo™). The arterial waveform is assessed at 100 Hz. The SD of the pulse pressure is determined over a 20-s period. To calculate CO, the software uses an algorithm based on the relationship between arterial pulse pressure and stroke volume and takes into account vessel compliance and peripheral resistance. Vessel compliance is estimated from abacuses based on age, gender, height and weight and peripheral resistance is determined from arterial waveform characteristics .
Apart from CO determination, the FloTrac™ and Vigileo™ devices allow for the determination of the respiratory variations in stroke volume (SVV). This index is displayed continuously by the monitor. In order to test the ability of this system to predict fluid responsiveness, we recorded SVV variations before and after volume expansion (before and after step 2 of the study protocol). At the same time, we recorded central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP) and we calculated the respiratory variations in pulse pressure (ΔPP) from the arterial waveform. ΔPP was calculated from three consecutive respiratory cycle as described previously: ΔPP = (PPmax − PPmin)/[(PPmax + PPmin)/2], where PPmax and PPmin are maximum and minimum pulse pressure over a single respiratory cycle. According to previously published results, we classified patients according to the percent increase in COPAC following volume expansion: patients were classified as responders to volume expansion when COPAC increases >15% and as non-responders to volume expansion when COPAC increases <15%.
Data are expressed as mean ± SD. Correlation between COPAC and COFT was determined by linear regression. Bland-Altman analysis was used to compare the bias (mean difference) and precision (SD of the bias) of COFT vs. COPAC. The Kolmogorov-Smirnov test was used to determine the normality of the distributions. Changes in CO after volume expansion were analysed using paired t-test. A P<0.05 was considered as statistically significant.
We studied 11 patients. Mean ± SD age was 69 ± 7 yr, height 170 ± 8 cm, weight 80 ± 18 kg, body surface area 1.9 ± 0.2 m−2 and Acute Physiology and Chronic Health Evaluation (APACHE) score 14 ± 4. Haemodynamic data are shown in Table 1. Mean length of postoperative stay in the ICU was 32 h. No patients received vasoactive drugs or intra-aortic balloon pump for weaning from CPB. We were able to analyse a total of 166 pairs of data. Among them, 66 pairs of data were collected intraoperatively and 100 were collected postoperatively.
Comparison between absolute values of COPAC and COFT
Over all patients (n = 166 data points), COPAC ranged from 2.0 to 7.6 L min−1 and COFT ranged from 1.9 to 8.2 L min−1. There was a statistically significant difference in mean COPAC and mean COFT (4.47 ± 1.08 vs. 4.74 ± 0.94 L min−1; P =0.02). We observed a statistically significant relationship between COPAC and COFT (r = 0.662; P < 0.001) (Fig. 1). Agreement between COPAC and COFT was −0.26 ± 0.87 L min−1 (Fig. 1).
When analysis was limited to intraoperative data (n = 66) we found statistically significant difference in mean COPAC and mean COFT (4.27 ± 0.98 vs. 4.66 ± 1.04 L min−1; P = 0.03). We observed a statistically significant relationship between COPAC and COFT (r = 0.674; P < 0.001) (Fig. 2). Agreement between COPAC and COFT was −0.37 ±0.82 L min−1 (Fig. 2).
When analysis was limited to postoperative data (n = 100), we found no statistically significant difference in mean COPAC and mean COFT (4.61 ± 1.14 vs. 4.79 ± 0.87 L min−1; P = 0.22). We observed a statistically significant relationship between COPAC and COFT (r = 0.666; P < 0.001) (Fig. 3). Agreement between COPAC and COFT was −0.17 ± 0.85 L min−1 (Fig. 3).
Comparison between changes in COPAC and COFT
Volume expansions induced a significant increase in both COPAC and COFT (from 3.4 ± 0.8 L min−1 to 4.4 ± 1.0 L min−1; P < 0.001 and from 3.9 ± 1.2 to 5.0 ± 1.1 L min−1; P < 0.001, respectively). We observed no statistically significant difference between percent change in COPAC and COFT following volume expansion (for 30 ± 23% vs. 34 ± 33%; P = 0.68). Moreover, there was a statistically significant relationship between percent change in COPAC and COFT following volume expansion (r = 0.722; P = 0.01) (Fig. 4).
SVV and ΔPP decreased significantly (from 15 ± 6% to 7 ± 3%; P < 0.05 and from 14 ± 6% to 6 ± 4%; P < 0.05) and CVP and PCWP increased significantly following volume expansion (from 11 ± 5 to 15 ± 3 mmHg; P < 0.05 and from 14 ± 5 to 17 ± 4 mmHg; P < 0.05, respectively). Nine (81%) patients were responders to volume expansion. SVV and ΔPP were significantly higher in responders than in non-responders (18 ± 4% vs. 4 ± 1%; P < 0.001 and 16 ± 3% vs. 3 ± 0%; P < 0.001, respectively) whereas no difference was found in CVP or PCWP (12 ± 6 vs. 11 ± 4 mmHg; P = 0.90 and 13 ± 11 vs. 20 ± 6 mmHg; P =0.07). There was a statistically significant relationship between SVV before volume expansion and percent increase in COPAC following volume expansion (r = 0.61; P < 0.05) as well as between ΔPP before volume expansion and percent increase in COPAC following volume expansion (r = 0.63; P < 0.05) (Fig. 5). On the other hand, we did not find a significant relationship neither between CVP before volume expansion and percent increase in COPAC following volume expansion (r = 0.11; P = 0.75) or between PCWP before volume expansion and percent increase in COPAC following volume expansion (r = —0.44; P = 0.17) (Fig. 5).
There was no significant difference in percent change in COPAC and COFT between consecutive measurements over the 155 pairs of data (4 ± 23% vs. 3 ± 21%; P = 0.65). Moreover, we found a significant relationship between percent change in COPAC and COFT between two consecutive measurements (r = 0.54; P < 0.001) (Fig. 6).
FloTrac™ system and Vigileo™ monitor allow continuous CO determination and do not require any calibration. This system analyses the arterial waveform and derives CO determination from this waveform and from the patient's age, height and weight every 20 s. To the best of our knowledge, only two previously published studies have investigated the reliability of this new device in the anaesthesiology and intensive care settings [11,13]. Opdam and colleagues  studied six postoperative cardiac surgery patients equipped with both FloTrac™ system and PAC. They found weak correlation (r2 = 0.1218) between both methods for cardiac index determination. Agreement between both methods was 0.21 ± 0.51 L min−1 m−2. However, that study presented several limitations that preclude any definitive conclusion. Firstly, CO determination using PAC was intermittent in three patients and continuous in the other three patients. However, several studies have shown that there is a discrepancy between both methods for CO determination. Recently, Bendjelid and colleagues  found a 0.33 ± 0.6 L min−1 agreement between both methods in the postoperative period following cardiac surgery. Thus, mixing intermittent and continuous determination for the comparison with a new technique is hazardous. Secondly, arterial cannula site was femoral in three patients, brachial in two patients and radial in one patient. As the FloTrac™ system directly derive CO from the arterial waveform it is most likely that the cannulation site will affect this determination. Consequently, this two bias greatly affect the validity of the results. In our study, we used only intermittent thermodilution and the arterial cannula site was radial in all patients. More recently, Sander and colleagues  studied 30 patients in the perioperative cardiac surgery setting. CO was determined using PAC (intermittent thermodilution) and a FloTrac™ system at four different time points prior, during and after coronary artery bypass grafting. They found a mean bias and limit of agreement of 0.6 L min−1 and −2.2 to +3.4 L min−1 between COPAC and COFT and a significant relationship between both values (r = 0.53; P < 0.01). In this study, the authors did not perform any intervention aimed at inducing changes in CO (such as volume expansion or changes in body position) and they do not show the relationship between the evolution in COPAC and COFT. Interestingly, we found better agreement between COPAC and COFT than Sander and colleagues (−0.26 ± 0.87 vs. 0.6 ± 1.4). The FloTrac™ system allows to select CO determination every 20 s or every 5 min. In this study, Sander and colleagues do not detail the way CO was determined and we can postulate that such a low agreement was induced by selecting a 5 min determination instead of a 20 s one as we did.
Many invasive (PAC, aortic transpulmonary thermodilution) and non-invasive (oesophageal Doppler, thoracic bioimpedance, partial CO2 rebreathing (NICO), echocardiography) systems have been proposed for CO determination in the operating room and in the ICU. Most of them have already been evaluated and are now widely used in the clinical setting. However, apart from their intrinsic limitations, they all demonstrate imperfect agreement when compared to pulmonary artery determination [7,18]. In our study, agreement between COPAC and COFT was consistent with most of the previously published studies focusing on other devices [5,6,8,19-23]. Moreover, a recently published study showed that even the bias and limits of agreement between continuous and intermittent CO using PAC are significant .
On the other hand, most of the studies focusing on CO determination compare absolute CO values. We feel that this kind of analysis is not deep enough. Instead, attention should be paid to the evolution in CO between two time points. In our study, we performed volume expansion in order to induce changes in CO and we found a good relationship between percent changes in COPAC and COFT following volume expansion. However, relationship between both methods was significant but relatively weak in the overall sample of data.
Our study presents some limitations. No patients received vasoactive drugs or intra aortic pump balloon, and FloTrac™ system still has to be evaluated in these situations. We can postulate that since the FloTrac™ system relies on the arterial waveform analysis, it cannot be applicable using an intra aortic pump balloon. As mentioned above, COPAC determination using intermittent thermodilution does not provide the true CO since the physiologic gold standard is the dye dilution method. However, most of the studies focusing on other devices used this technique as the most commonly accepted system for CO determination in the clinical setting.
In conclusion, we found a significant relationship and a clinically acceptable agreement between COPAC and COFT in patients undergoing CABG surgery suggesting that both methods could be interchangeable in this setting. COFT was significantly related to COPAC and both techniques showed reasonable agreement as compared to other commonly accepted techniques for CO determination [4,5,8,10,18,19,23-27]. However, the relationship between percent change in COPAC and COFT between two measurements was weakly significant. Further studies will be needed to investigate the clinical utility of this new device.
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