Cardiac output monitoring using a brachial arterial catheter during off-pump coronary artery bypass grafting

Cardiac Output Monitoring Using a Brachial Arterial Catheter During Off-Pump Coronary Artery Bypass Grafting Patrick F. Wouters, MD, PhD,* Bert Quaghebeur, MD,* Paul Sergeant, MD, PhD,† Jan Van Hemelrijck, MD, PhD,* and Eugène Vandermeersch, MD, PhD* Objective: To investigate the accuracy of cardiac output measurements by transpulmonary thermodilution and pulse contour analysis using a brachial arterial catheter. Study Design: Criterion standard study. Setting: University hospital, single institution. Population: Twenty-three adult patients undergoing offpump coronary artery bypass grafting. Measurements and Main Results: Cardiac output was measured with a thermistor-tipped brachial arterial catheter using pulse contour analysis (COpc) and transpulmonary thermodilution (COba), which serves to calibrate COpc in the system tested. Both methods were compared separately with standard pulmonary artery thermodilution (COpa). COba was closely correlated with COpa (r ⴝ 0.93, p < 0.001). Bland-Altman analysis showed a bias of 0.91 L/min with

limits of agreement of ⴞ0.98 L/min. COpc was also closely correlated (r ⴝ 0.80, p < 0.001) with COpa and was found to have a bias of 1.08 L/min with limits of agreement of ⴞ1.50 L/min. During the surgical procedure, changes in COpa from baseline were closely correlated with changes in COba (r ⴝ 0.90, p < 0.01) and COpc (r ⴝ 0.81, p < 0.01). Conclusions: The brachial arterial access allows a reliable assessment of cardiac output by transpulmonary thermodilution and pulse contour analysis in patients undergoing off-pump coronary artery bypass grafting. © 2005 Elsevier Inc. All rights reserved.

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ically designed 4F catheter may offer a reasonable alternative for CO monitoring, but this approach has not been validated yet in the setting of cardiac surgery. The goal of the present study was to assess the accuracy of CO measurements by transpulmonary thermodilution and pulse contour analysis in patients instrumented with a thermistor-tipped brachial arterial catheter undergoing OPCAB surgery.

FF-PUMP CORONARY ARTERY bypass grafting (OPCAB) is increasingly being performed as an alternative to the standard cardiopulmonary bypass–assisted procedure and aims at a reduction of postoperative morbidity and total costs.1 This new approach requires accurate hemodynamic monitoring because surgical manipulations, unprotected myocardial ischemia, and the use of stabilizers on the beating heart can provoke abrupt hemodynamic changes.2 For a long time, the pulmonary artery thermodilution technique has been the most popular method to measure cardiac output (CO) during cardiac surgery. However, the routine use of pulmonary artery catheters and its benefit/risk ratio have been challenged by several clinical studies.3,4 Furthermore, continuous CO monitoring by pulmonary thermodilution has a slow response time and for that reason may not be ideally suited to indicate abrupt hemodynamic changes during OPCAB surgery. Recent developments in the field of CO monitoring have focused on reducing invasiveness and/or shortening response time.5 Among these, the femoral arterial thermodilution-calibrated pulse contour analysis appears accurate both in measuring CO and in tracking its changes.6-11 However, in patients with significant lower-limb atherosclerosis, femoral arterial cannulation is relatively contraindicated. Many institutions also avoid femoral catheterization in cardiac surgical patients to keep unrestricted access to the groin for cardiopulmonary bypass cannulation or placement of an intra-aortic balloon pump when necessary. For patients in whom femoral arterial catheterization is considered suboptimal, a brachial arterial approach with a specif-

From the Departments of *Anesthesiology and †Cardiac Surgery, University Hospitals KULeuven, Leuven, Belgium. Address reprint requests to Patrick F. Wouters, MD, PhD, Department of Anesthesiology, University Hospitals Katholieke Universiteit Leuven, Herestraat 49, B-3000 Leuven, Belgium. E-mail: patrick.wouters@uz. kuleuven.ac.be © 2005 Elsevier Inc. All rights reserved. 1053-0770/05/1902-0006$30.00/0 doi:10.1053/j.jvca.2004.10.001 160

KEY WORDS: monitoring, cardiac output, transpulmonary thermodilution, pulse contour analysis, cardiac surgery, offpump coronary artery bypass grafting

MATERIALS AND METHODS Twenty-four patients (20 men, 4 women, mean age ⫽ 65 years) scheduled to undergo elective OPCAB surgery were enrolled. The preoperative echocardiographic left ventricular ejection fraction ranged from 51% to 84% (mean 71%). Patients with significant valvular regurgitation and/or atrial fibrillation were excluded. The study was approved by the ethics committee, and written informed consent was obtained the day before surgery from each patient. Patients were premedicated with sublingual lorazepam (0.05 mg/ kg). A 4F, 16-cm length brachial artery thermodilution catheter (PV2014L16; Pulsion Medical Systems AG, Munich, Germany) was inserted under local anesthesia with a Seldinger technique and connected to a cardiac output monitor (PiCCOplus, Pulsion). Anesthesia was induced with midazolam (0.05 mg/kg), propofol target control infusion at 1 ␮g/mL, pancuronium, 0.1 mg/kg, and sufentanil, 0.7 ␮g/kg. A pulmonary artery catheter (PV2057, Pulsion) was then inserted via the right internal jugular vein and connected to a second cardiac output computer (VoLEF, Pulsion). Simultaneous measurements of pulmonary and transpulmonary brachial arterial thermodilution were obtained from a single cold saline bolus injected through the atrial port of the pulmonary artery catheter. Each set of measurements consisted of three successive determinations and the mean values for pulmonary thermodilution (COpa) and transpulmonary brachial artery thermodilution (COba), respectively, were recorded for statistical analysis. To keep variations within one set of measurements as low as possible, the authors used iced injectate (3°C-8°C) at an amount of 15 mL to 20 mL according to each patient’s body weight and the PiCCO/VoLEF user instructions. The undamped arterial pressure curve was also continuously recorded and analyzed by the pulse contour monitor (PiCCOplus, V5.2.2 Pulsion) to display CO as a sliding average of 12 seconds. The principles of pulse contour analysis have been described in detail.10 The system requires a patient-specific calibration, which is automatically performed with every thermodilution COba measurement on the PiCCO system. For the purpose of this study, however, the automatic

Journal of Cardiothoracic and Vascular Anesthesia, Vol 19, No 2 (April), 2005: pp 160-164

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Fig 1. Bland-Altman plot of cardiac output measurements by pulmonary artery thermodilution (COpa) and transpulmonary thermodilution using a brachial arterial catheter (COba). (Color version of figure is available online.)

calibration mode of the device was disabled and calibration was performed only once for each patient during the first set of measurements. This allowed the authors to uncouple COpc data from COba measurements and to separately study the agreement of both systems with COpa in a prospective way during the procedure. For statistical analysis of COpc accuracy, the values preceeding every single thermodilution measurement within a set were averaged. COpc data obtained during initial calibration were obviously excluded from analysis. For each patient, at least 6 sets of CO measurements were made with the 3 systems operating simultaneously in the following steady-state conditions: (1) after induction of anesthesia and before surgery, (2) after sternotomy, (3) end of harvesting arterial grafts, (4) during revascularization of the anterior wall, (5) during hemostasis, and (6) at completion of surgery. Additional measurements were performed when the heart was tilted in position for additional distal coronary anastomoses. These data were included only when the condition lasted sufficiently long to allow a complete set of measurements and provided that hemodynamics remained stable throughout. A final assessment was performed at the end of the procedure after a 15-minute dobutamine infusion at a rate of 3 ␮g/kg/min. This step was omitted in patients with incomplete revascularization and in patients with documented left ventricular hypertrophy. Variability of COpa and COba measurements was assessed by the coefficient of variation for all sets of measurements. Comparison of methods was accomplished with regression analysis using Spearman test and Bland and Altman plot. A probability value below 0.05 was accepted for statistical significance. Limits of agreement were considered clinically acceptable if lower than 30% of the mean value.12 RESULTS

No complication was observed during or after insertion of the brachial arterial catheter. The data of 23 patients were used. One patient was excluded because of a technical problem with the thermistor of the arterial catheter, which produced fluctuating and

erroneous blood temperatures. Seven to 13 hemodynamic evaluations per patient (depending on the number of anastomoses) were performed, leading to a total of 247 COpa and COba thermodilution measurements and 224 COpc measurements (247 minus 23 exclusions of the first calibrating set) available for analysis. The duration of the surgical procedure ranged from 170 to 355 minutes (mean ⫽ 260 minutes). The variability of COpa and COba measurements was 4% and 6.5%, respectively. COba was closely correlated (r ⫽ 0.93, p ⬍ 0.001) with COpa, and the mean difference between COba and COpa was 0.91 ⫾ 0.98 L/min (mean ⫾ 2 SD) (Fig 1). COpc was also closely correlated (r ⫽ 0.80, p ⬍ 0.001) with COpa, and the mean difference between COpc and COpa was 1.08 ⫾ 1.50 L/min (mean ⫾ 2 SD) (Fig 2). Throughout the procedure, changes from baseline in COpa correlated with changes in COba (r ⫽ 0.90, p ⬍ 0.001) and with changes in COpc (r ⫽ 0.81, p ⬍ 0.001) (Figs 3 and 4). DISCUSSION

The present findings show that both transpulmonary thermodilution and pulse contour analysis using a brachial arterial access are reliable methods for measuring CO and tracking its changes in patients undergoing OPCAB. OPCAB has become increasingly popular as an option for patients who have traditionally undergone myocardial revascularization with the support of extracorporeal circulation.1 Surgical access to the left anterior descending coronary artery is relatively easy through a median sternotomy, but, to revascularize the posterior or lateral walls, the heart must be lifted and tilted out of the pericardial cradle.2 This displacement of the beating heart has important hemodynamic consequences such that CO monitoring becomes

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Fig 2. Bland-Altman plot of cardiac output measurements by pulmonary artery thermodilution (COpa) and pulse contour analysis using a brachial arterial catheter (COpc). (Color version of figure is available online.)

a helpful tool for optimal anesthetic care. The presence of air surrounding the heart, and the use of a posterior pericardial stitch and swab close to the esophagus, considerably restrict the performance of transesophageal echocardiography, particularly when the heart is in the vertical position.2 Many clinicians rely therefore on intermittent thermodilution measurements before and after mobilization of the heart to guide hemodynamic management. However, a reliable beat-to-beat stroke volume and hence continuous cardiac output monitoring system would be more appropriate to trace the sudden and sometimes dramatic hemodynamic changes that can occur during this procedure. Transpulmonary thermodilution has been shown to be an easy and reliable technique for assessing cardiac output and is increasingly being used for hemodynamic assessment in the operating room and intensive care unit.13 Indeed, in patients instrumented with a thermistor-tipped femoral arterial catheter, this technique has been shown to compare favorably with pulmonary artery thermodilution and the Fick method, in adults as well as in children.14-20 The present data show that the brachial arterial access is a valid alternative to the femoral approach for transpulmonary thermodilution measurements of CO and offers a solution to clinical conditions that prohibit femoral arterial catheterization. This is consistent with recent data on axillary artery-derived transpulmonary thermodilution.21 Limits of agreement in the present study are excellent and easily meet the criteria proposed by Critchley et al12 for a clinically acceptable agreement between methods. Assuming an accuracy of the reference method between 10% and 20%,22 calculations from the present data show that transpulmonary

thermodilution is at least as accurate (between 6% and 18%) as the standard pulmonary arterial thermodilution technique.12 In terms of absolute values, the authors found consistently higher CO with the transpulmonary approach than with pulmonary artery thermodilution over the entire range of measurements. This observation is reflected in a bias of 0.91 L/min and also agrees with most previous reports on transpulmonary thermodilution.14,23 Because pulmonary thermodilution is traditionally considered a gold standard, it is tempting to attribute the observed bias to an overestimation of CO by the transpulmonary technique. However, previous studies have indicated that the injection of iced saline can cause short-lived cardiodepressant effects,24 which would predominantly affect pulmonary artery measurements of CO. Indeed, the thermal effect is recorded immediately after bolus injection in the pulmonary artery, whereas it takes several heart beats to reach the distal arterial location with the transpulmonary method. In fact, the authors did use relatively large volumes of ice-cold saline because this is recommended by the manufacturer of the system to optimize signal-to-noise ratio at the distant location of the arterial thermistor. Although this practice guarantees high reproducibility with the transpulmonary technique, it may have affected the reference technique in the present study to an extent larger than seen in routine clinical practice. A recent study showed good agreement between 20 mL of room temperature saline and iced temperature saline for the transpulmonary technique in critically ill patients. The authors reported slightly lower absolute values with ice-cold saline and also attributed this to the direct effects on the heart.25 A comparison to an independent high-fidelity method such as transonic flow-

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Fig 3. Relationship between changes in pulmonary artery thermodilution cardiac output (⌬COpa) and changes in brachial artery thermodilution cardiac output (⌬COba) from baseline measurements during off-pump coronary artery bypass grafting. (Color version of figure is available online.)

metry would ultimately be required to solve this issue. Finally, for clinical purposes, changes in CO may be more relevant than absolute values to direct therapy. It has previously been shown that a monitoring system that clearly fails to accurately trace changes may still produce a seemingly acceptable agreement with the reference method.26 In the present study, changes in CO as observed with the pulmonary thermodilution method correlated closely with changes recorded with the transpulmonary brachial arterial technique. Although transpulmonary thermodilution is a noncontinuous method, accuracy is of great importance because its data are used to calibrate the continuous pulse contour system. The concept of arterial pulse contour analysis for the determination of continuous cardiac output has been the subject of investigation for a number of years. The basic algorithm for the determination of stroke volume from pulse contour was developed by Wesseling et al.27 According to this algorithm, stroke volume is computed by measuring the area under the systolic portion of the arterial pressure waveform and dividing this area by the aortic impedance. A subsequent multiplication by the heart rate yields COpc. To adjust for aortic impedance, which differs from patient to patient, a thermodilution measurement of cardiac output for the calibration of the system is required. This original Wesseling algorithm was implemented in the first PiCCO software generation. The second software generation used in the present study is based on a more elaborate formula that analyzes the actual shape of the entire pressure waveform in addition to the area under the systolic portion of the pressure wave.10 Indeed, the software takes into account individual aortic compliance and systemic vascular resistance based on the following considerations. During systole, more blood is ejected from the left ventricle than actually leaves the aorta. During the subsequent diastole, blood flows from the aorta into the arterial network at a rate depending on the aortic wall compliance, the systemic vascular resistance, and the blood pres-

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sure. The shape of the arterial pressure curve following the dicrotic notch is representative of this passive emptying of the aorta (Windkessel effect). For any system of continuous cardiac output monitoring to be valid in clinical practice, it is exactly in situations of hemodynamic instability that precise measurement of CO is essential. The reliability of the second-generation algorithm has been confirmed in cardiac surgery patients with hemodynamic instability.10 The good agreement and the close relationship that were observed between changes in COpc and changes in COpa show that the brachial access can also be used to accurately measure cardiac output and track changes using pulse contour analysis in OPCAB surgery. The bias between COpc and COpa is of the same magnitude as that obtained with the COba method because the latter is used to calibrate COpc. Limits of agreement were higher with COpc than with transpulmonary thermodilution and slightly exceed the criteria set at the start of the present study. It is important to note, however, that these data were obtained using only 1 initial calibration for an average study duration of 4.5 hours. It suggests that with the current algorithm, the brachial COpc should be calibrated more frequently to guarantee consistent reliability. In fact, a new algorithm for COpc is currently being developed to further optimize its accuracy. Extrapolation of the present results should be done with caution; atrial fibrillation and significant valvular disease were considered exclusion criteria for this study and all of the patients had ejection fractions higher than 50%. Prospective studies may be required to assess the effects of these specific disease states on accuracy of the system. It is also important to realize that, in contrast to pulmonary artery catheters, the transpulmonary technique provides no information on pulmonary artery pressures or mixed venous oxygen saturation, which are considered useful variables in the management of OPCAB surgery.2 Although the transpulmonary ther-

Fig 4. Relationship between changes in pulmonary artery thermodilution cardiac output (⌬COpa) and changes in brachial artery pulse contour cardiac output (⌬COpc) from baseline measurements during off-pump coronary artery bypass grafting. (Color version of figure is available online.)

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modilution technique enables the assessment of cardiac preload by calculating global end-diastolic volume and stroke volume variation and also enables quantification of extravascular lung water,28-30 it is not yet clear whether such variables can be used as substitutes for pulmonary artery catheter-derived information. However, the volumetric assessment of cardiac preload and the quantification of pulmonary edema may be of value in the postoperative management of cardiac surgery patients, but additional studies are required to determine if the brachial access is suitable to accurately assess these parameters. The authors conclude that in

patients undergoing off-pump coronary artery bypass grafting, the use of a thermistor-tipped brachial arterial catheter allows an accurate assessment of cardiac output by transpulmonary thermodilution and arterial pulse contour analysis. ACKNOWLEDGMENT Cardiac output computers and thermodilution catheters were generously supplied by Pulsion Medical Pulsion Systems AG, Munich, Germany and Belgium. The authors also thank the Pulsion Medical staff for providing technical support throughout the study.

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