Measuring The Cardiac Output Biology Essay

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Measuring cardiac output is an important part of hemodynamic monitoring in the perioperative period as it gives an indication of the therapeutic interventions needed to obtain adequate tissue perfusion. A pulmonary artery catheter (PAC) using thermodilution has been used as a clinical standard to measure cardiac output during cardiac surgery. However, insertion of PAC has the risk of various complications including pneumothorax, arrhythmia, air embolism, and pulmonary artery rupture1.

The Vigileo-FloTrac system (Edwards Lifesciences, Irvine, CA, USA) is a less invasive method to obtain continuous cardiac output using pulse contour analysis. It bases its calculations on arterial waveform characteristics after adjustment for vascular compliance and does not require calibration2. The Vigileo-FloTrac system has been used for intraoperative goal-directed therapy and fluid management3-10. Previous studies measuring cardiac output using the first and second version Vigileo-FloTrac system showed poor agreement in low systemic vascular resistance states11-16. The new third-generation Vigileo-FloTrac system showed an improvement over previous versions in measuring cardiac output17. However, several studies have suggested that acute changes in peripheral vascular resistance may decrease the reliability in measuring accurate cardiac output with both previous and new versions18 19. Meng et al. 19 showed a significant discrepancy between changes in cardiac output measured by the third-generation Vigileo-FloTrac system and that by PAC after phenylephrine administration. However, the ability of the Vigileo-FloTrac system to measure cardiac output and compensate for acute arterial vasomotor tone changes in low and high systemic vascular resistance states has not been fully evaluated. To assess the ability of the FloTrac system in various vascular tones is an issue of great importance as haemodynamics change continuously in cardiac surgery patients.

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The first aim of this study was to compare cardiac output measured by the third version Vigileo-FloTrac system with that measured by PAC in various peripheral vascular resistance states. The second aim of this study was to examine the ability of the Vigileo-FloTrac system to track cardiac output changes induced by increased vasomotor tone under different systemic vascular resistance states.

Subjects and Methods

Patients and Anaesthesia

After obtaining approval from the Ethics Committee of our hospital and informed consent from all patients, 40 patients scheduled for cardiac surgery were enrolled in this study. Exclusion criteria were cardiac arrhythmias (atrial fibrillation or frequent premature beats) and severe obesity (body mass index > 35 kg m-2). No pre-medication was given. Anaesthesia was induced with propofol (1 to 2 mg kg-1), midazolam (0.05 to 0.1 g kg-1), and fentanyl (2 to 3 g kg-1). Rocuronium was administered for muscle relaxation. After tracheal intubation, anaesthesia was maintained with sevoflurane (1.5 to 2.0%) and fentanyl (total dose 20 to 40 g kg-1 per case). Patients were ventilated with a tidal volume 8 to 10 ml/kg of body weight at a rate of 12 breaths per minute. The frequency of ventilation was controlled to keep end-tidal carbon dioxide between 4.7 to 5.3 kPa. Intraoperative inspired oxygen concentration was from 40 to 60%. After securing the airway, an arterial pressure line was inserted into the radial artery, and it was connected to the Vigileo-FloTrac system (Version 3.02, Edwards Lifesciences, Irvine, CA, USA).

After the induction of general anaesthesia, a central venous catheter (Presep catheter, Edwards Lifesciences, Irvine, CA, USA) and thermodilution PAC (Edwards Lifesciences, Irvine, CA, USA) were inserted into the right internal jugular vein. PAC was connected to the VigilanceTM monitor (Edwards Lifesciences, Irvine, CA, USA). The position of the catheter was confirmed by its pressure wave and transoesophageal echocardiography.

Cardiac output monitoring with the Vigileo-FloTrac system (APCO)

The Vigileo-FloTrac system does not require external calibration and measures cardiac output with arterial pressure waveform analysis. The Vigileo-FloTrac system performs its calculations using the most recent 20 seconds of hemodynamic data. To calculate cardiac output, the software uses an algorithm based on the relationship between arterial pressure and stroke volume and adjusts the K value every 60 seconds. The K value is a constant quantifying vessel compliance and peripheral resistance. Vessel compliance is estimated with nomograms based on age, gender, height, and weight, while arterial resistance is decided from arterial waveform characteristics11.

Thermodilution cardiac output monitoring (ICO)

A conventional PAC thermodilution method for measuring cardiac output was performed with a 10 ml bolus injection of ice-cold saline. Systemic vascular resistance (SVR) and the systemic vascular resistance index (SVRI) were calculated as follows:

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SVR = (mean arterial blood pressure - central venous pressure) - 80 / cardiac output

SVRI = (mean arterial blood pressure - central venous pressure) - 80 / cardiac index

Study protocol

The study protocol was performed under stable haemodynamic conditions before and after cardiopulmonary bypass (CPB). Haemodynamic variables including heart rate (HR), mean arterial pressure (MAP), central venous pressure (CVP), mean pulmonary arterial pressure (mPAP), APCO, ICO, and SVRI were recorded before (T1) and 2 min after (T2) phenylephrine administration (100 g). Simultaneous cardiac output measurements were performed by single bolus thermodilution and the Vigileo-FloTrac system. For measuring ICO, 3 consecutive measurements were made at random times. The average value of these measurements was used for analysis. APCO was automatically calculated by the Vigileo-FloTrac system.

Statistical analysis

All results were expressed as mean and standard deviation (SD) unless otherwise indicated. Statistical analysis was performed with JMP 9 (SAS Institute Inc., Cary, NC, USA) and SigmaPlot 11.2 (Systat Software Inc., San Jose, CA, USA). Demographic data were compared using the Student's t-test and Mann-Whitney U test. We used Bland and Altman analysis to compare ICO and APCO. Bland and Altman analysis is the conventional method to compare two methods measuring cardiac output, which was shown as the bias, 95% limits of agreement20. In this study, we used repeated measurements for each subject. The standard Bland and Altman analysis should not be applied to estimate agreement between two measurements methods done on repeated occasions. Therefore, we adjusted for repeated measurements using the method suggested by Myles and Cui21. Percentage error was calculated as 2SD of the bias divided by mean cardiac output of the reference method. The tested method is considered interchangeable with the reference method when the percentage error is less than 30% assuming that the precision of the thermodilution reference method is +20% as suggested by Critchley et al.22.

We defined percentage changes in ICO and APCO by administration of phenylephrine (between T1 and T2) as ΔICO and ΔAPCO, respectively. We used 4 quadrant plots shown by Critchley et al.23 to compare the concordance rate of ΔICO and ΔAPCO. The concordance rate was defined as the percentage of the number of data points that are in 2 of the 4 quadrants of agreement (upper right and lower left). The concordance rate was good when it was over 92%, as described by Critchley et al.23. We also performed a polar plot analysis shown by Critchley et al.24 to compare the trending abilities between ICO and APCO. The polar plot analysis gives an assessment of whether the new method (APCO) is calibrated compared with the reference method (ICO). As the size of cardiac output change is ignored in the 4 quadrant analysis, we cannot easily perform a similar quantitative assessment using the 4 quadrant plot analysis. The polar plot analysis describes the vector of cardiac output change as an angle to the line of identify (X = Y). In the polar plot analysis, an agreement between two methods is shown by the angle from the axis (0o). The transformation from 4 quadrant plot to polar plot needs a data rotation in 45o clockwise direction and changing the dimensions of radius to ICO and APCO. The following statistical data were calculated from the polar plot analysis: (1) the mean angular bias which is the average angle (positive in a clockwise direction) between all polar axes and polar data points: (2) radial limits of agreement which is the radial sector containing 95% of the data points and (3) angular concordance rate which is the percentage of points in the 30o radial zone. The acceptance limits in the polar plot analysis were (1) an angular bias of less than +5o, (2) radial limits of agreement of less than +30o, and (3) angular concordance rate of more than 95%, as Critchley et al.24 suggested. We excluded data in the 4 quadrant plot and polar plot analysis when the percentage increase in cardiac output was below 10%, as used in a previous study25 (exclusion zone). The exclusion zone was set because data at the center of the plot corresponding to small changes in cardiac output did not reflect trending ability23. The use of exclusion zone is an issue of great importance in the analysis of cardiac output changes. As Critchley et al.24 indicated, if the change of cardiac output is small, random measurement error will make difficult to detect any true change of cardiac output.

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We divided patients into 3 groups according to the SVRI value at point T1 (before phenylephrine administration). We referred to recent studies17,26 to produce the range of SVRI states. Akiyoshi et al.26 indicated that differences between ICO and APCO had become apparent at low systemic vascular resistance (especially below 700 dyne s cm-5) with the third version Vigileo-FloTrac system. A study by Biancofiore et al.17 also showed that the bias between ICO and APCO had become greater at high SVRI state (> 2500 dyne s cm-5 m2). Therefore, we set SVRI ranges as follows: low (below 1200 dyne s cm-5 m2), normal (1200 to 2500 dyne s cm-5 m2), and high (over 2500 dyne s cm-5 m2) SVRI states. We compared the ability of the Vigileo-FloTrac system using the Bland and Altman method, 4 quadrant plots, and polar plots in low, normal, and high SVRI states. For all analyses, a P value <0.05 was considered as significant.

Results

Table 1 shows patient characteristics. The number of enrolled patients was 40, and 155 paired data (131 before CPB, and 24 after CPB) were collected. We showed haemodynamic variables at points T1 and T2 in Table 2. The number of data sets was 44 in the low SVRI state, 63 in the normal SVRI state, and 48 in the high SVRI state. In each group, APCO at point T2 (after phenylephrine administration) was significantly higher than that at point T1 (before phenylephrine administration) (p < 0.05). However, as for ICO, there was no significant difference between T1 and T2 in low and normal SVRI states (p = 0.186 and 0.469, respectively). In the H group, ICO at point T2 was significantly lower than that at point T1 (p < 0.05).

We assessed the ability of the Vigileo-FloTrac system to measure cardiac output at low, normal, and high peripheral resistances. We used the Bland and Altman method to compare APCO and ICO in low, normal, and high SVRI states, which were shown in Figure 1 and Table 3. The adjusted percentage error was 46.3%, 26.4%, and 61.4%, in low, normal, and high SVRI states, respectively (Figure 1a, 1b, 1c, and Table 3).

We also examined the trending ability of the Vigileo-FloTrac system after phenylephrine administration at low, normal, and high peripheral resistances. We used the 4 quadrant plots and polar plots to examine the trending abilities of APCO against ICO in low, normal, and high SVRI states, which were shown in Figure 2 and 3. In the 4 quadrant plot analysis, we found that changes in cardiac output induced by phenylephrine administration were 67.5%, concordant in the low SVRI state, 28.8% in the normal SVRI state, and 7.7% in the high SVRI state (Figure 2a, 2b, 2c, and Table 3). The polar plot analysis showed that the mean angular bias was -22.3o, -46.0o, and -3.51o, and the radial limits of agreement was 70o, 85o, and 87o, and the concordance rate was 42.1%, 25.0%, and 5.3%, in the low, normal, and high SVRI state, respectively (Figure 3a, 3b, 3c, and Table 3).

Discussion

This is the first published study to evaluate the ability of the Vigileo-FloTrac system to measure cardiac output and track cardiac output changes induced by increased vasomotor tone in a wide range of systemic vascular resistance states. As shown in Figure 1, the mean bias between APCO and ICO in the normal SVRI state was lower than that in low and high SVRI states (0.52 versus 1.85 and -1.34 L min-1). The percentage error from Bland and Altman analysis was 26.4% in the normal SVRI state, which was lower than that in low and high SVRI states. The percentage error in the normal SVRI state was clinically acceptable (<30%)14, but was not in the other states. The agreement between the two methods APCO and thermodilution ICO as shown by the percentage error less than 30% showed that the Vigileo-FloTrac system has the ability to measure cardiac output accurately only in a normal peripheral resistance state, and not in low or high systemic vascular resistance states. As Figure 2 showed, the concordance rate in 4 quadrant plot analysis was poor in each SVRI state. The mean angular bias, radial limits of agreement, and angular concordance rate in the polar analysis were not clinically acceptable in 3 SVRI states either (Figure 3a, 3b, and 3c). This means that the trending ability of the Vigileo-FloTrac system against cardiac output measured by PAC was not clinically acceptable in 3 peripheral vascular resistance states. These results may suggest the limit of the third-generation Vigileo-FloTrac system in measuring cardiac output and tracking cardiac output changes induced by increased vasomotor tone. Systemic vascular resistance had an impact on the accuracy of the Vigileo-FloTrac system in measuring cardiac output and tracking cardiac output changes.

Many studies have been reported to evaluate the accuracy of the Vigileo-FloTrac system in measuring cardiac output17 26-30. Recent studies reported clinically unacceptable results by the first and second version of the Vigileo-FloTrac system in patients undergoing cardiac surgery and liver transplantation27-30. Edwards Lifesciences has introduced a new third-generation Vigileo-FloTrac system, in which the algorithm is improved over previous versions. Akiyoshi et al.26 studied the third-generation (version 3.02) Vigileo-FloTrac system and concluded that it had provided improvements over previous versions in measuring cardiac output in patients undergoing liver transplantation (bias: 0.89 L min-1, adjusted percentage error: 37.5%). They indicated that differences between ICO and APCO had become apparent at low systemic vascular resistance (especially below 700 dyne s cm-5) with the new version Vigileo-FloTrac system26. Biancofiore et al.17 also showed that the third version Vigileo-FloTrac system had been influenced by SVRI states in measuring cardiac output, but the effect of SVRI on the bias between ICO and APCO had been shown to be less than the previous version of the Vigileo-FloTrac system. These data were consistent with our results, where the percentage error and bias were higher in the low SVRI state than those in the normal SVRI state (percentage error: 46.3% versus 26.4%, bias: 1.85 L min-1 versus 0.52 L min-1). In the present study, patients with high peripheral resistance also had a higher percentage and bias than those in the normal SVRI state (percentage error: 61.4% versus 26.4%, bias: -1.34 L min-1 versus 0.52 L min-1). This result has not been reported in previous studies. Our results suggested that the ability of the Vigileo-FloTrac system to measure cardiac output was not clinically acceptable, not only in patients with low vascular resistance, but also in those with high peripheral resistance.

In this study, phenylephrine induced opposite changes between APCO and ICO, especially in normal and high SVRI states. As shown in Table 2, APCO consistently increased after phenylephrine administration, while ICO in low and normal SVRI states did not significantly change, and that in the high SVRI state decreased after phenylephrine administration. In a previous study by Meng et al. 19, a poor concordance rate of 23% was reported between changes in APCO and oesophageal Doppler cardiac output measurements after phenylephrine treatment. This result was consistent with our result in patients with normal and high SVRI. However, they did not consider patients with low SVRI and, as shown in our study, SVRI before phenylephrine administration influenced the concordance rate between ΔAPCO and ΔICO after phenylephrine administration. Phenylephrine increased not only left ventricular afterload, but also venous return, especially under a low SVR state. The former leads to decreased cardiac output, and the latter results in increased cardiac output. In the low SVRI state, the effect of phenylephrine increasing venous return may exceed its effect in decreasing cardiac output by increasing afterload, thus the concordance rate became greater than that of other SVRI states (67.5% versus 28.8%, 7.7%). Whereas the polar plot analysis in this study showed considerably poor agreement between ICO and APCO in trending ability after phenylephrine administration at 3 SVRI states, and this indicates the limitation of the Vigileo-FloTrac system for clinical use in cardiac surgery patients. Monnet et al.31 reported that the concordance rate between changes in APCO and ICO induced by noradrenaline was 60%. This result was comparable to our result in patients with low vascular resistance. They studied patients with relatively low SVR (677-1153 dyne s cm-5), which may have affected the results. Noradrenaline has an inotropic effect that may lead to an increase in cardiac output. Phenylephrine is a pure alpha1-agonist, and does not have an inotropic effect. In this study, we used phenylephrine for this reason.

We are aware of some limitations of our study. First, we did not consider cardiac function. Our patients had almost normal ranges of ejection fraction (mean 57.2%), but patients with low cardiac function may have been more influenced by phenylephrine administration. Second, we used PAC as a reference method to measure cardiac output. Since it is not a beat-to-beat monitor like oesophageal Doppler, it may not be suitable for tracking changes in cardiac output. Third, in patients with aortic and mitral regurgitation, phenylephrine administration may increase the degree of regurgitation, leading to decreases in cardiac output. Even with those methodological limitations, our data suggest that the ability of the Vigileo-FloTrac system to measure cardiac output and track changes in cardiac output was greatly influenced by systemic vascular resistance.

Conclusions

The ability of the new third-generation Vigileo-FloTrac system to measure cardiac output and track changes in cardiac output induced by phenylephrine administration was not clinically acceptable. It was greatly influenced by systemic vascular resistance. The Vigileo-FloTrac system has the reliable ability to measure cardiac output only in a normal peripheral resistance state, and not in low and high systemic vascular resistance states. Additionally, the trending ability of the Vigileo-FloTrac system against cardiac output measured by PAC was not clinically acceptable. Consequently, we should be aware that changes in systemic vascular resistance may affect the accuracy of the Vigileo-FloTrac system for measuring cardiac output when conducting goal-directed therapy.