European Journal of Cardiovascular Medicine

Cardiac output assessment plays a crucial role in perioperative hemodynamic management, particularly in patients undergoing OPCABG (Off-Pump Coronary Artery Bypass Grafting). Accurate and continuous monitoring of CO helps optimize fluid therapy, guide inotropic support, and maintain hemodynamic stability during surgery.^[1] Traditionally, the PAC CCO (Pulmonary Artery Catheter Continuous Cardiac Output) method has been considered the gold standard for CO measurement. However, its invasive nature and associated risks, including infections and vascular complications, have led to the exploration of less invasive alternatives.^[2]

The FloTrac system, a less invasive hemodynamic monitoring technique, estimates CO by analyzing arterial pulse waveforms without requiring external calibration.^[3] This method offers potential advantages in reducing procedural risks while providing real-time hemodynamic data. However, concerns have been raised regarding its accuracy, particularly in cases of altered vascular tone or significant hemodynamic fluctuations.^[4] Several studies have compared FloTrac™ with PAC CCO, with varying results regarding the level of agreement between the two methods.^[5]

Given the clinical significance of reliable CO monitoring, this study aims to compare the less invasive FloTrac™ method with the invasive PAC CCO method in patients undergoing OPCABG. The study evaluates the correlation between the two techniques, highlighting potential discrepancies and their implications for perioperative management.^[6]

AIM AND OBJECTIVES

This study aims to compare the accuracy of cardiac output measurements obtained using the less invasive FloTrac system with those obtained using the invasive CCO PAC in patients undergoing off-pump CABG. The objective is to determine whether FloTrac provides a reliable and clinically comparable assessment of cardiac output, potentially offering a less invasive alternative for hemodynamic monitoring in surgical patients. By evaluating the correlation and any significant differences between the two methods, this research seeks to assess the feasibility of FloTrac as an alternative to CCO PAC, ensuring patient safety and optimal clinical outcomes.

This prospective observational study was conducted in the Department of Anesthesia, Cardiac Surgery Unit, at a state government-run tertiary care hospital. Following approval from the institutional ethical committee, the study was carried out over a period of one year, from December 2016 to January 2018. A total of 33 patients scheduled for elective OPCABG were included.

Inclusion and Exclusion Criteria

The study included patients undergoing elective OPCABG with a stable sinus rhythm. Patients were excluded if they had preoperative intra-aortic balloon pump support, arrhythmias, peripheral vascular disease, lower limb arterial thrombosis, combined aortic regurgitation, or intracardiac shunts and regurgitant lesions. Additionally, patients initially included in the study were excluded if they required intra-aortic balloon pump support at any stage during surgery, required emergency CPB (Cardio-Pulmonary Bypass), or developed arrhythmias intraoperatively.

Sample Size Calculation

Sample size was calculated considering correlation between less invasive (pulse contour analysis based FloTrac) as described in Wacharasint et al,^[1] as the main outcome with power of 80% and confidence interval of 95%. Sample size was found to be 23. Therefore 33 subjects were included in this study.^[7]

Data Collection Procedure

The preoperative preparation was comprehensive and methodical. All patients underwent detailed pre-anaesthesia evaluations following the standard hospital protocol. Before the procedure, each patient was fully informed about the study and provided written informed consent. Importantly, all cardiac medications were continued up to the day of surgery, ensuring continuity of patient care.

Upon arrival in the operating room, standard non-invasive monitoring equipment was attached to the patient. A peripheral intravenous access was established using a 16G IV cannula in the right upper limb. For sedation, patients received carefully calculated doses of intravenous medications: fentanyl at 1 microgram per kilogram and midazolam at 0.05 milligrams per kilogram.

The invasive monitoring process was intricate and precise. Under local anesthesia, a 20G Jelco cannula was inserted into the right radial artery to initiate invasive blood pressure monitoring. Simultaneously, the right internal jugular vein was cannulated with an 8.5 FR PA introducer sheath. A Swan-Ganz catheter was carefully floated to the pulmonary artery, with pressure waveforms confirmed on the monitor. An Edward continuous cardiac output monitor was attached to the Swan-Ganz catheter to enable continuous cardiac output monitoring using the thermodilution technique.

An additional monitoring line was established by cannulating the left femoral artery with a 4 FR Vygon femoral arterial cannula. The FloTrac device was attached to this line, providing an alternative method of cardiac output monitoring through the Edward Vigileo monitor using pulse counter analysis. All patient demographic data was input into the monitoring systems, and cardiac output was recorded simultaneously at 10-minute intervals from both monitors.

Anesthesia was administered using a balanced technique. The induction involved a combination of fentanyl, midazolam, propofol, and vecuronium. Throughout the procedure, the patient was maintained on controlled mechanical ventilation with a 0.5% FiO2 air-oxygen mixture, using isoflurane for inhalation and intermittent boluses of fentanyl and atracurium.

The surgical procedure commenced with a midline sternotomy, followed by harvesting of the LIMA (Left Internal Mammary Artery). Patients were heparinized with 3-4 mg/kg to maintain an ACT (Activated Clotting Time) exceeding 300 seconds during grafting. The grafting sequence was systematic: first, LIMA was grafted to the LAD (Left Anterior Descending) artery. Subsequent distal anastomoses were performed on the OM (Obtuse Marginal), Diagonal, and PDA (Posterior Descending Artery). The proximal anastomosis was completed to the aorta. At the procedure's conclusion, heparin was neutralized using half the dose of protamine.

Hemodynamic stability was carefully maintained throughout the surgery. This was achieved through strategic interventions such as appropriate table tilting, fluid boluses, and medication administration including noradrenaline and dobutamine as required. A critical aspect of the study protocol was the exclusion of patients who developed arrhythmia, experienced hemodynamic instability, or required IABP (Intra-Aortic Balloon Pump) or CPB (Cardio-Pulmonary Bypass) support.

The entire intraoperative period was characterized by meticulous data collection. Invasive cardiac output was simultaneously monitored using both the PAC (Pulmonary Artery Catheter) CCO (Continuous Cardiac Output) and the less invasive FloTracVigileo monitor, with readings taken at consistent 10-minute intervals.

Statistical Analysis

Descriptive analysis included mean, standard deviation, and percentages. For inferential statistics, comparison of means across two groups was done using an unpaired t-test. Correlation between the methods was assessed by Pearson’s correlation coefficient (“r”). A p-value less than 0.05 was considered statistical significance. A Z-test for proportions was USED to compare the proportion of datasets with higher and lower estimations. Data was analyzed in statistical software SPSS VERSION 25.

Table 1 presents the study population, which consisted of 33 patients with a mean age of 57.54 years, predominantly male (81.81%). Patients had an average height of 172 cm and weight of 65.8 kg, with considerable variability in both physical characteristics.

Table 2 gives the invasive cardiac output measured by PAC-CCO, which showed a mean of 4.04 with a standard deviation of 1.15. The less-invasive FloTrac method demonstrated a higher mean cardiac output of 5.16 with a standard deviation of 1.30.

The dataset consists of a total of 3,620 entries, categorized based on the comparison between non-invasive and invasive estimations. Among them, 66 datasets (1.82%) show matching values, 586 datasets (16.19%) have lower non-invasive estimations, and 2,968 datasets (81.99%) have higher non-invasive estimations.

Figure 1 gives the data sets; the majority of lower cardiac output estimations were concentrated in the lower percentage ranges, with 252 data sets (43%) showing up to 10% lower estimation. The frequency of data sets progressively decreased as the percentage of lower estimation increased.

Figure 1: Less-invasive Lower Estimation of CO

Figure 2: Less-invasive Higher Estimation of CO

Figure 2 shows the collected 2,968 data sets showing variations in cardiac output estimation, with the highest frequency (456 data sets) in the 20-30% range. Interestingly, 232 data sets showed higher estimation beyond 100%, indicating significant variability in cardiac output measurements.

In Table 4 evaluated cardiac output changes across different acceptable ranges, with the number of clinically acceptable data sets increasing as the acceptable deviation range expanded. At ±20% deviation, 1,216 data sets (33.59%) were considered clinically acceptable, suggesting a broader tolerance for measurement variations.

Table 5 summarizes the clinical implications of using the FloTrac method, highlighting its tendency for higher estimation, moderate correlation strength, and statistical reliability. The method’s less invasive nature makes it advantageous for patient safety but requires cautious interpretation in clinical decision-making.

Figure 3: Scattered Diagram Showing Correlation in Non-Invasive and Invasive Method of CO Estimation

r= 0.446*, *statistically highly significant, p<0.001

The scatter plot demonstrates the relationship between invasive and less-invasive cardiac output measurements, revealing a generally positive correlation with significant variability across a range of 0 to 12 units. While most data points cluster between 2.0 and 6.0 on both axes, the plot shows numerous outliers and deviations, indicating measurement imprecision and the complexity of comparing different cardiac output measurement techniques.

In Given Dataset 81.99% Dataset Shows Higher Estimate by Less Invasive FloTrac.

The accurate measurement of cardiac output is crucial for hemodynamic management in patients undergoing (OPCAB). Our study compared the less invasive FloTrac/Vigileo™ system with the gold standard PAC-CCO monitoring, revealing important clinical insights that warrant detailed discussion.

Our findings demonstrated moderate correlation between FloTrac and PAC-CCO measurements, which aligns with previous validation studies. Sander et al.^[8] reported similar agreement levels in cardiac surgery patients, noting that while the FloTrac system provided clinically acceptable CO estimates, it showed greater variability in patients with rapidly changing hemodynamics. Saraceni et al. found that FloTrac/Vigileo did not provide reliable estimation of CO.^[5] In contrast, Canneson et al.^[9] reported that values from both methods correlated well with a statistically significant relationship (P<0.001), though a limitation of their study was the small sample size. This observation is particularly relevant to OPCAB procedures, where surgical manipulation of the heart frequently causes abrupt hemodynamic alterations.

The evolution of FloTrac algorithms has significantly improved its performance. Mayer et al.^[10] documented that the third-generation software (v3.02) reduced the percentage error from 46% to 38% compared to earlier versions.

Biancofiore et al. ^[11] reported better agreement in liver transplant patients, but OPCAB presents unique challenges due to cardiac manipulation and temporary coronary occlusion, which can cause abrupt CO fluctuations. This may explain why FloTrac struggled in our cohort compared to PAC-CCO, which uses thermodilution, a more direct CO measurement method.

The limitations of FloTrac become particularly apparent in unstable patients. Our data showed significant discrepancies during periods of rapid hemodynamic changes, a finding consistent with Compton et al.^[12] who reported poor performance of pulse contour analysis during periods of vasopressor use or severe vasodilation. Lorsomradee et al. found agreement between the CCO measured with the PAC and Vigileo™/FloTrac™ system using 2000 data sets from a heterogeneous population but noted that the variability of the waveform may produce inaccurate readings on the monitor. This is mechanistically understandable, as the FloTrac system relies on arterial waveform analysis and proprietary algorithms to estimate stroke volume, making it vulnerable to errors when vascular tone changes abruptly.

The minimally invasive nature of FloTrac offers distinct advantages in clinical practice. As Hadian et al.^[13] noted, avoidance of pulmonary artery catheterization reduces the risk of complications such as arrhythmias, pulmonary artery rupture, and infection. De Wilde found that the Vigileo™/FloTrac™ system produces CO values comparable to the PAC with a trend towards overestimation.

However, the comprehensive Cochrane review by Rajaram et al.^[14] provides crucial context, emphasizing that PAC remains the gold standard in high-risk cardiac surgeries. Their analysis of multiple studies concluded that while less invasive methods have improved, PAC provides more reliable data in complex cases. This is particularly relevant for OPCAB patients with poor ventricular function or significant comorbidities, where precise hemodynamic monitoring is paramount.

Several technical factors may influence the performance of FloTrac in OPCAB:

1. The quality of arterial waveform analysis depends on proper transducer positioning and damping characteristics.
2. Algorithmic assumptions about vascular compliance may not hold true in patients with severe atherosclerosis.
3. The system's calibration-free design, while convenient, may contribute to measurement variability.

LIMITATIONS

This study focused exclusively on patients undergoing OPCABG, limiting its generalizability to other cardiac surgeries or critically ill patients in different settings. Another limitation was the lack of postoperative follow-up to assess the impact of measurement discrepancies on clinical outcomes. The study was conducted exclusively during off-pump CABG; therefore, the results are applicable only to the intraoperative period and cannot be generalized to the postoperative phase. Future studies should consider a broader patient population and longer-term assessments to enhance the clinical applicability of these findings.

The comparison of cardiac output assessment using the less invasive FloTrac method and the invasive PAC CCO method in patients undergoing off-pump CABG revealed important findings. The FloTrac method demonstrated both higher and lower estimates of cardiac output compared to the invasive method, with a greater tendency towards overestimation. The moderate correlation observed between the two techniques suggests that while FloTrac provides a useful alternative for cardiac output monitoring, it should be interpreted cautiously in clinical settings. The potential for overestimation may lead to excessive interventions, necessitating careful validation before reliance on less invasive techniques. Overall, while FloTrac offers a less invasive approach, PAC-CCO remains the gold standard for precise cardiac output measurement in critically ill patients. Further research is required to refine the accuracy of less invasive methods for improved clinical decision-making.

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