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Pulse Contour Analysis and Transesophageal Echocardiography: A Comparison of Measurements of Cardiac Output During Laparoscopic Colon Surgery

Concha, Mario R., MD; Mertz, Verónica F., MD; Cortínez, Luis I., MD; González, Katya A., MD; Butte, Jean M., MD

doi: 10.1213/ane.0b013e3181a491b8
Technology, Computing, and Simulation: Research Reports
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Chinese Language Editions

BACKGROUND: Pulse wave analysis (PWA) allows cardiac output (CO) measurement after calibration by transpulmonary thermodilution. A PWA system that does not require previous calibration, the FloTrac/Vigileo (FTV), has been recently developed. We compared determinations of CO made with the FTV to simultaneous measurements using transesophageal echocardiography (TEE).

METHOD: Ten ASA I-II patients scheduled for laparoscopic colorectal surgery were studied. A radial 20-gauge cannula was inserted and connected to a hemodynamic monitor and a FTV system for PWA and determination of CO (COPWA). TEE CO (COTEE) was determined as previously described. Measurements were made after intubation, 5 min after establishing the lithotomy position, 5 min after establishing pneumoperitoneum, every 30 min, or each time mean arterial blood pressure decreased below basal values. Statistical analysis was made with the Bland and Altman method.

RESULTS: Eighty-eight measurements were compared. The COTEE values ranged from 3.23 to 12 Lt/min (mean 6.21 ± 1.85). Values for COPWA ranged from 2.9 to 8.5 Lt/min (mean 4.84 ± 1.14). Bias was 1.17 and limits of agreement −2.02 and 4.37. The percentage error between all COTEE and COPWA measurements was 40% (27%-50%) mean (range).

CONCLUSION: During laparoscopic colon surgery, clinically important differences were observed between CO determinations made with TEE and FTV.

From the Departments of Anesthesiology and Digestive Surgery, Escuela de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile.

Accepted for publication January 18, 2009.

Address correspondence and reprint requests to Mario Concha, MD, Departamento de Anestesiología, Hospital Clínico Universidad Católica de Chile, Santiago, Chile. Address e-mail to mconcha@med.puc.cl.

The “gold standard” method for measuring cardiac output (CO) in the clinical setting is thermodilution using a pulmonary artery catheter (PAC) (COPACT).1–3 However, because the risk of PAC insertion cannot be justified in routine cases, a less invasive method for obtaining this information is desirable. Pulse wave analysis of CO (COPWA) has been established as a valid and cost-effective method for CO measurement after calibration by transpulmonary thermodilution or COPACT to compensate for interindividual differences in arterial compliance.4–7 A device that does not require a previous calibration, the FloTrac/Vigileo (FTV), has been introduced by Edwards Lifesciences LLC, Irvine, CA. The system uses the pressure sensor attached to arterial pressure tubing to derive continuous CO measurements from the arterial pressure wave. Conflicting results have been reported and its clinical utility and reliability must be tested in different clinical settings and under changing hemodynamic conditions.8–15 Considering that in most clinical situations COPACT has no indication, it would be useful to compare FTV to a less invasive measurement of CO, such as transesophageal echocardiography (TEE). Good agreement between determinations made with COPACT and TEE CO measurements (COTEE) has been demonstrated.16,17 This provides an alternative method for the intraoperative measurement of CO, allowing the comparison of measurements made with FTV with this validated method.

The objective of this study was to compare simultaneous determinations of CO made with COPWA using FTV and COTEE during laparoscopic colon surgery.

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METHODS

After institutional ethics committee approval (Facultad de Medicina, Pontificia Universidad Católica, Santiago, Chile), written informed consent was obtained from 10 patients scheduled for laparoscopic colorectal surgery. All patients were ASA physical status I-II and had no contraindication for the use of TEE. After standard monitoring (continuous electrocardiogram, noninvasive arterial blood pressure, and pulse oximetry), anesthesia was induced with fentanyl 2-4, thiopental 5-6, and vecuronium 0.1 mg/kg. The trachea was then intubated and ventilation was adjusted to an end-tidal carbon dioxide tension of 30-35 mm Hg. A 20-gauge cannula was inserted into the radial artery and connected to a hemodynamic monitor and a FTV system. All pressure transducers were zeroed to the midchest level and the pressure wave form was not dampened at the time of measurements. The software used was version 1.07. During surgery, anesthesia was maintained with isoflurane 1-1.5 minimum alveolar anesthetic concentration in combination with nitrous oxide (50%), in oxygen. Supplemental bolus doses of fentanyl 1-2 μg/kg were administered to maintain mean arterial blood pressure (MAP) and heart rate (HR) within 20% of basal values (mean value of three determinations made at rest the day before surgery). Muscle relaxation was maintained with supplemental 1 mg doses of vecuronium throughout surgery. Nasopharyngeal temperature and urine output were also monitored. Patients were actively warmed with a forced air warming system to maintain temperatures above 36°C. Before TEE probe insertion, a nasogastric tube was used to empty air from the stomach and then removed. All TEE determinations were performed by the same certified anesthesiologist (VM, American National Board of Echocardiography) with a Philips Envisor C and an Omniplane III (2-7 MHz) TEE probe. CO by TEE was determined according to the method described by Perrino et al.,16 the left ventricular outflow tract (LVOT) diameter was determined in a midesophageal aortic long-axis view to allow the automatic calculation of the cross-sectional area of the LVOT (CSALVOT) (CSALVOT = π (LVOT diameter/2)2). Then, to obtain measurements of aortic blood flow, the transesophageal ultrasound probe was positioned in a transgastric short-axis view of the left ventricle at the midpapillary level. By rotating the imaging array to approximately 120°, the LVOT and ascending aorta were imaged lying parallel to the ultrasound beam. Aortic blood flow velocities were measured by a continuous wave Doppler at the level of the aortic valve. At end expiration, two consecutive velocity wave forms were recorded and then the velocity time integral was traced. Doppler CO is calculated as the product of velocity time integral, CSALVOT, and HR. Simultaneously, CO was determined with the FTV system that bases its calculations on arterial wave form characteristics in conjunction with demographic data. The system calculates the arterial pressure using arterial pulsatility (standard deviation of the pressure wave over a 20-s interval), resistance, and compliance, according to the following general equation:

where k is a constant quantifying arterial compliance and vascular resistance, and pulsatility is proportional to the standard deviation of the arterial pressure wave over a 20-s interval. k is derived from patient characteristics (gender, age, weight, and height) according to the method described by Langewouters et al.18 and wave form characteristics (e.g., skewness and kurtosis of individual waves). The exact calculation method and the relative weight given each factor are proprietary information. k is recalculated every 10 min and CO every 20 s on the basis of the last 20-s interval of arterial wave form analysis.13,19,20

Measurements were performed by an investigator blinded to FTV data at the following times:

  • After tracheal intubation, in supine position (T1), and after a period of at least 5 min of MAP and HR within ±20% of basal values.
  • Five minutes after installing the patient in the lithotomy position and with no degree of inclination of the operating table (T2).
  • Five minutes after establishing pneumoperitoneum (T3).
  • Every 30 min or each time MAP decreased more than 20% of basal values. Some of these measurements were made in Trendelenburg (T4) and some in reverse Trendelenburg position (T5).
  • During colonic anastomosis by a Pfannenstil incision, in steep Trendelenburg position (T6).
  • At the end of surgery, without pneumoperitoneum, and with no inclination of the operating table (T7).

During surgery intraabdominal pressure was maintained at a maximum of 15 mm Hg.

Agreement between methods was analyzed with the method described by Bland and Altman21 for repeated measurements. Limits of agreement (LOA) were defined as the range in which 95% of the differences between both methods were expected to lie. Bias was calculated as the mean difference between COTEE measurements and COPWA. Because Bland and Altman’s method does not compensate for the relationship between the magnitude of CO measurements and the size of the error, the percentage error (±2 sd/μ) was calculated for each set of data according to Critchley and Critchley22 suggestions. A percentage of error of up to ±30% was considered acceptable for this new technique.22

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RESULTS

Demographic and surgical data are shown in Tables 1 and 2. Eighty-eight measurements were analyzed. Figure 1 shows the Bland Altman plot. The COTEE values ranged from 3.23 to 12 Lt/min (mean 6.21 ± 1.85), whereas the values for COPWA ranged from 2.9 to 8.5 (mean 4.84 ± 1.14). Bias was 1.17 and LOA −2.02 and 4.37. The percentage error between all COTEE and COPWA measurements was 40% (27%-50%) mean (range).

Table 1

Table 1

Table 2

Table 2

Figure 1.

Figure 1.

Comparisons of data points in each situation when CO determinations were made are shown in Table 3 and Figure 2.

Table 3

Table 3

Figure 2.

Figure 2.

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DISCUSSION

The most important finding of this study was that CO determined with the FTV system showed important differences with COTEE measurements and that these differences were independent of the surgical conditions at the time of CO determinations. Even though the clinical setting and comparison method of this study are very different to the ones described in other studies,8–10,14 results are similar in the sense of an important bias and a wide range of LOA when measurements were made with a PAC.

The observed differences may be explained by the characteristics of the algorithm used by the FTV system to determine the CO, the clinical situation in which comparison was performed, and the temporal relationship between the time in which k was calculated and COTEE determinations were made. As explained by Mayer et al.,8 algorithms of arterial wave form analysis are based on properties of the arterial system, such as impedance, peripheral vascular resistance, and compliance. Aortic impedance is required to calculate stroke volume, and it has important patient-to-patient variations. Former approaches to arterial wave analysis excluded this source of error by performing an initial invasive calibration.4,7 Interindividual differences in aortic impedance may contribute to inaccuracies in calculating the CO when calibration is performed based solely upon demographic data. Pneumoperitoneum and the important changes in patient position required by laparoscopic colon surgery may influence afterload and aortic diameter.23–25 Both of these factors are directly involved in the algorithm used by the FTV system to determine CO; however, these changes are not considered in the derivation of the k value. This fact can be a serious limitation for the use of FTV in this kind of surgery or in other situations that are associated with changes in peripheral vascular resistance. Finally, and as occurs in laparoscopic surgery, in situations of frequent hemodynamic changes, the k value may be determined at a time in which arterial compliance, vascular peripheral resistance, or impedance may not be the same as the time of measurement. Furthermore, potential weaknesses of the system include artifacts or alterations of the arterial pressure wave form, as in aortic valve disease, use of an intraaortic balloon pump, or important reductions of systemic vascular resistance.14,20,26 However, none of these conditions were present in the patients studied. All these factors may be clinically relevant in situations of hemodynamic instability because it can expose patients to incorrect therapeutic decisions. A new software (version 1.10, introduced in spring 2006), that updates variables that account for dynamic changes in vascular tone every 60 s as opposed to every 10 min with the previous version, has been developed and could help reduce this problem. However, conflicting results have been reported.12,15

A possible limitation of our study is that there was no comparison with COPACT and that echocardiographic measurement of CO is an operator-dependent technique. To reduce these problems, all determinations were performed by the same certified anesthesiologist (VM). However, when a good determination of aortic valve area and proper alignment of the ultrasound beam and the LVOT are obtained, there is agreement between TEECO and PACCO.16,17,26 Another possible limitation of this study is that, although not evident from the visual inspection of the Bland Altman plots, our results might have been influenced by outlier data because we studied 88 CO measurements coming from only 10 patients.

Even though our clinical setting was different from those used in other studies,8,10,14,15 we agree with their conclusions in the sense that determinations of COPWA were not comparable with those obtained using other validated methods. The FTV must be evaluated in different clinical situations and compared with a standard method of CO measurement to avoid exposing patients to ineffective or harmful therapeutic decisions.

In summary, this study shows that during laparoscopic colon surgery, there is an important difference between CO determinations made with TEE and FTV. Hemodynamic changes determined by laparoscopic surgery, that are not considered in the algorithm used by FTV, may explain these differences and suggest that more evaluation in other situations with frequent and fast hemodynamic changes is necessary.

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