RELIABILITY OF CARDIAC OUTPUT CALCULATION BY THE FICK PRINCIPLE AND CENTRAL VENOUS OXYGEN SATURATION IN EMERGENCY CONDITIONS

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1 DOI: /s y Ó Springer 2008 RELIABILITY OF CARDIAC OUTPUT CALCULATION BY THE FICK PRINCIPLE AND CENTRAL VENOUS OXYGEN SATURATION IN EMERGENCY CONDITIONS Avi A. Weinbroum, MD 1,2, Philippe Biderman 1, Dror Soffer 3, Joseph M. Klausner 3 and Oded Szold 3 Weinbroum AA, Biderman P, Soffer D, Klausner JM, Szold O. Reliability of cardiac output calculation by the Fick principle and central venous oxygen saturation in emergency conditions. J Clin Monit Comput 2008 ABSTRACT. Background. For many years thermodilution has been the gold standard for determining cardiac output in the critically ill patients. Less invasive methods have recently been introduced. This study aimed at evaluating the agreement between cardiac output (CO) measured by a new Fick method, using central venous saturation (Scvo 2 ), and that measured by the classic thermodilution technique, in patients requiring emergent CO evaluation. Settings. Prospective clinical study in a university-affiliated, tertiary hospital, at surgical and general intensive care units. Patients and methods. Fifteen mechanically ventilated patients arriving in the emergency department in hemodynamic shock, had immediately a pulmonary artery catheter introduced under fluoroscopy upon arrival into the ICU. Cardiac output (CO) was obtained in each patient via both thermodilution and the Fick method, using oxygen consumption, SpO 2 and Scvo 2. Results. COs ranged between 2 and 2.3 (in the Fick and thermodilution methods, respectively) and 19 or 19.5 l/min (respectively). Mean thermodilution-derived CO was 6.2 ± 4.2 l/min whereas the Fick s was 7.0 ± 4.3 l/min. There was statistical significant correlation between the two modalities of measurements, with an r 2 = 0.9 (P < 0.001). Conclusions. The new method of Fick assessed emergent CO as reliably as the thermodilution, regardless of whether it was low or high. The use of Scvo 2 allows for prompt bedside calculation for most emergency patients. KEY WORDS. cardiac output, Fick, thermodilution, Scvo 2, Svo 2. INTRODUCTION Avi A. Weinbroum and Philippe Biderman concurred equally to the present investigation. From the 1 Department of Anesthesiology and Critical Care, Tel-Aviv Sourasky Medical Center and Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel; 2 Post Anesthesia Care Unit and Animal Research Laboratory, Tel Aviv Sourasky Medical Center and Sackler Faculty of Medicine, Tel-Aviv University, 6 Weizman Street, Tel Aviv 64239, Israel; 3 Division of Surgery and Surgical Intensive Care Unit, Tel-Aviv Sourasky Medical Center and Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel. Received 22 May Accepted for publication 22 September Address correspondence to A. A. Weinbroum, Post Anesthesia Care Unit and Animal Research Laboratory, Tel Aviv Sourasky Medical Center and Sackler Faculty of Medicine, Tel-Aviv University, 6 Weizman Street, Tel Aviv 64239, Israel. draviw@tasmc.health.gov.il Quick but reliable determination of cardiac output (CO) and stroke volume is of essential importance for the management of patients arriving in the emergency department (ED) or intensive care unit (ICU), especially among patient who are of high risk. Optimization of hemodynamic conditions is also essential during the early phase of resuscitation of septic patients [1]. Since its introduction in the 1970s, the thermodilution technique, through a pulmonary artery catheter (PAC, e.g., Swan Ganz catheter), has been routinely and satisfactorily used [2]. Nevertheless, the difficulties and risks associated with its insertion and maintenance, may account for the emergence of new, less invasive, modalities to monitor the hemodynamic status in the critically ill patient [3, 4]. Several previous studies have reported contradictory results when assessing CO using Fick s principle in the critically ill patient. In a study by Bremer et al. [5], the

2 average CO obtained by this method was higher than that obtained using thermodilution; however, the difference was not statistically significant. In another study, involving fifteen critically ill patients, the bias between the two methods was 1.7 ± 3.8 l/min, the values obtained by Fick s method yielding the higher results [6]. In these and other studies, mixed venous O 2 saturation (Svo 2 ) measurements required the placement of a PAC, with risks that may surplus its benefits [4, 7, 8]. The central venous catheter is routinely introduced in ED patients and used in most if not all critically ill patients, both for the administration of medications and for right heart monitoring. In recently published guidelines, central venous oxygen saturation (Scvo 2 ) and Svo 2 were equally suggested for the management of the septic patient [9]; some authors, however, reported conflicting data [10 12]. The primary aim of this study was to assess whether CO diagnostic data obtained by Fick s method upon the arrival of patients in the ED or ICU were as reliable as the thermodilution-originated values. The secondary goal was to determine the usefulness of Scvo 2 as a clinically reliable tool during such fast measurements in critically ill patients. PATIENTS AND METHODS Fifteen mechanically ventilated patients were studied (Table 1). Criteria for inclusion were their recent (within 2 h) arrival in the ED or in ICU; the placement of PAC by the physician in charge; controlled mechanical ventilation without patient s attempting spontaneous respiration, the requirement for fractionated inspired oxygen partial pressure (FiO 2 ) 0.6, stable level of inspired oxygen fraction and 10 min of hemodynamic stability prior to measurements. Thermodilution cardiac output (TCO) Correct placement of a PAC (Arrowä, Arrow International Ò, Reading, PA, USA) was verified by pressure waveform during the insertion and by later chest radiography. Thermodilution-derived CO (TCO) was obtained by injecting 10 ml of 5% dextrose in water at roomtemperature into the proximal injection port of the PAC within a 5 s period, at random times throughout the respiratory cycle. The average of three satisfactory curves yielded mean cardiac output measurement value. A commercial CO monitor (S/5 Critical Care Monitorä, Datex-Ohmeda Ò, Madison, WI, USA) was used to calculate the end results. Fick s method of cardiac output measurement (FCO) These measurements were accomplished simultaneously with the TCO ones. Specifically, oxygen consumption was determined from the measurements of carbon dioxide and oxygen contents in the inspired and expired gases, using a standard metabolic monitor (S/5 M-COVXä Datex-Ohmeda Ò, Madison, WI, USA) calibrated prior to the beginning of each series of measurements in each patient. Ten min of metabolic steady state was required ( 5% changes in the respiratory quotient [R], oxygen consumption [Vo 2 ] and carbon dioxide production [Vco 2 ]). Blood gas analysis and hemoglobin concentrations were simultaneously retrieved from arterial, central venous and mixed venous blood, using the RapidLab 865 Analyzerä (Bayer Healthcare Diagnostics Ò, Munich, Table 1. Individual data of the reported patients Patient Age Gender M M F F M M F M F F M M F M F 1 ry Diagnosis a VO Svo Scvo TCO FCO a Hemodynamic shock due to 1 = sepsis; 2 = cardiogenic; 3 = hypothermia. Abbreviations: BMI: body mass index, TCO: thermodilution-derived cardiac output, CI: cardiac index, FCO: Fick-derived cardiac output, VO 2 : oxygen consumption, Svo 2 : mixed venous O 2 saturation, Scvo 2 : central venous oxygen saturation, SpO 2 : arterial saturation.

3 Weinbroum et al.: Emergency Fick Cardiac Output Measurement Germany). The mixed venous sample was obtained via the distal port of the PAC, the arterial blood was drown via radial or femoral arterial catheter and the central venous sample was taken through a subclavian or jugular line, which site of placement was verified previously by a chest X-ray. Thus, CO was determined using Fick metabolic equation (FCO), as expressed by the ratio of Vo 2 to the difference between the arterial oxygen content and the central venous oxygen content (Cao 2 - Cvo 2 ) 9 10, where the variables were defined as oxygen consumption [Vo 2 ], arterial and central oxygen contents [Cao 2 - Cvo 2 ]. Statistical analysis The statistical analyses were performed at the Statistical Laboratory of the School of Mathematics, Tel-Aviv University, using the SPSS Release for Windows, Version (Chicago, IL, 2001). A linear regression test was done to correlate CO data of the two modalities, using the equation of Y(=FCO) = A + B*X(X = TCO). In addition, looking at the same variable at the same time under identical conditions, and since previous data have shown that despite the good correlation, poor agreement could be demonstrated [13], we also analyzed the data by the Bland and Altman method. We also determined CO data obtained in the same patient by the two methods, which the difference between them reached a maximum of 20%. This was done based on previous reports indicating this difference as clinically acceptable. Values are expressed as mean ± standard deviation (SD), with significance defined as P RESULTS Patients average age was of 62 (range 24 73) years. Eleven of the patients suffered from septic shock, three from cardiogenic shock, and one patient from low cardiac output due to severe hypothermia (Tables 1, 2). CO measurements ranged between 2 and 19 l/min (by Fick method) and l/min (thermodilution method). The mean cardiac output measurements by the two modalities were similar as well (Table 2). Data correlation Correlation between the CO data obtained by the thermodilution and the Fick modalities are reported in Figure 1A. There was a statistical significant correlation between the two series of measurements, with an r 2 = (P = ). The correlation between the Table 2. Demographic, physical and measurements values (mean ± SD, or absolute values) Parameters Svo 2 and the Scvo 2 saturation values also reached a statistical significance (Figure 1B). Following the Bland and Altman analysis of data, the thermodilution and the Fick determinations of CO were found interchangeable, with (Figure 2) an excellent degree of agreement between the two series of data, both of the CO and of the venous saturations. In this study, 11 out of 15 (73%) CO measurements agreed within 20% of each other, while the other four agreed within 35% of their values. When comparing the saturation data, agreement within 20% of each other included all sets of values in either method. LIMITATIONS Values Age (years) 62 ± 14 Gender (M/F) 8/7 BSA 2.0 ± ry Diagnosis (1/2/3) a 11/3/1 FiO ± 0.1 TCO (l/min) 6.2 ± 4.2 CI (l/min/kg) 3.5 ± 0.6 FCO (l/min) 7.0 ± 4.3 VO 2 (ml/min) 248 ± 79 SvO 2 (%) 70 ± 6.4 ScvO 2 (%) 70.8 ± 6.8 SpO 2 (%) 97.0 ± 1.6 Hemoglobin (g %) 10.3 ± 1.4 DAV (mmhg) 27.7 ± 4.9 a Hemodynamic shock due to 1 = sepsis; 2 = cardiogenic; 3 = hypothermia. Abbreviations: BSA: body surface area, TCO: thermodilutionderived cardiac output, CI: cardiac index, FiO 2 : fractionated inspired oxygen partial pressure, FCO: Fick-derived cardiac output, VO 2 : oxygen consumption, Svo 2 : mixed venous O 2 saturation, Scvo 2 : central venous oxygen saturation, SpO 2 : arterial saturation, DAV: arterio-venous oxygen content. This report indicates the usefulness of Fick s method upon the arrival of patients in the ED or ICU. Further assessment of the reliability of the method as compared to the thermodilution during patients stay is currently under investigation. Also, the method could possibly result in inaccuracies in some critical patients such as septic patients and those with acute respiratory failure: Scvo 2 values may vary significantly from Svo 2, and there may exist a significant level of pulmonary shunting, respectively.

4 A Cofick R Sq Linear= CO B Scvo R Sq Linear = Svo2 Fig. 1. (A) Correlation between the Fick metabolic (COfick) and the thermodilution-derived (CO) cardiac output. (B) Comparison between the central venous oxygen saturation (Scvo 2 ) and the mixed venous oxygen saturation (Svo 2 ). Both regression equations are shown along with their relative confidence limits. DISCUSSION The present study demonstrates, apparently for the first time, that the thermodilutionally-measured CO and the reported Fick-based CO value that uses Scvo 2 correlated closely. Furthermore, this extreme similarity Fig. 2. The Bland and the Altman method s correlation for both the two CO measurements (thermodilution-derived CO [CO] and Fick-derived CO [COfick]) and blood saturations (central venous oxygen saturation [Scvo 2 ] and the mixed venous oxygen saturation [Svo 2 ], A, B, respectively). existed both in septic and in non-septic patients. It appears that the newly-introduced non-invasive methodology of FCO is as reliable as TCO and therefore clinically promising even in ED patients, even in patients with both extremely low (2 l/min) and very high (19 l/min)

5 Weinbroum et al.: Emergency Fick Cardiac Output Measurement CO. Furthermore, most (73%) of the cardiac output values showed good agreement within 20% difference from one modality value to another, while in the rest the agreement laid within a 35% difference. Given these side by side statistical agreements, we believe that for an initial, quick but reliable diagnostic assessment of the hypotensive patient in the ED or ICU, the FCO method does not differ from the TCO methods and is of clinical reliability as is the latter. Our results are in agreement with the report by Capderou et al. who assessed hemodynamic patterns by the Fick method during cardiac catheterization in children [14]. Similarly to us, they found that the Fick method correlated linearly with TCO values. In addition, and as demonstrated previously by Bremer et al. [5], and by Engoren and Barbee [6], we found that the two modalities are equivalent in all measurements, even though average Fick s CO measurements were slightly higher than the results obtained by the thermodilution method. Nevertheless, in view of the correlation data obtained by the two statistical methodologies, these differences are of no clinical relevance for the clinician during the initial emergent diagnostic bed-side measurements. Importance of the Fick s method The use of TCO has been considered a cornerstone in cardiovascular and hemodynamic evaluation. In an early publication, Nishikawa and Dohi [15], described 13 factors that can result in false results when using TCO, leading to the assertion that this may not be the Gold Standard for the measurement of cardiac output. In another report [16], variance of up to 13% between the results of three measurements of cardiac output was considered acceptable variability. Based on the prior reports by Sageman and Amundson [17] and by Baylor [13], we opted for 20% acceptable clinical variability between the series of data originating from the two techniques (i.e., thermodilution vs. Fick). Our results are, indeed, superior to those of Baylor [13] that found only 57% of the cardiac output values in agreement within 20% of each other, as compared to 73% in the present study. This difference in the cohorts is probably due to the fact that instead of using an average Fick s technique-related Vo 2 values obtained over 20 min before the TCO measurement, we used the values obtained simultaneously with the TCO measurement. Furthermore, the present close correlation coefficients between the two groups of CO data existed both when measuring both very high and extremely low values. Also, all Svo 2 and Scvo 2 data herein presented were within the mentioned 20% of agreement in all patients. An additional support to the accuracy of the Fick method herein proven originates from an animal study involving mechanically ventilated septic pigs [18]. In that study, Marx et al. found good correlation between cardiac output measurements by arterial trans-cardiopulmonary and arterial thermodilution with the Fick s method even during hemodynamic instability. The usefulness of simultaneously Svo 2 and Scvo 2 measurements Unlike previous reports that looked into the Fick s method that used an average of Vo 2 over 30 min, we measured simultaneously Svo 2 and Scvo 2 as well; these also correlated significantly and closely between themselves. As reported by Reinhart [19, 20] and others [21, 22] earlier, the habitual difference of 2 3% between Svo 2 and Scvo 2 may change under conditions of shock. In septic shock, for example, oxygen consumption of the gastrointestinal tract may increase despite the increased regional blood flow, while cerebral blood flow is constant over time. In those cases, Scvo 2 may exceed by as much as 8% the Svo 2 [19]. These data, however, must be considered cautiously, since several recent publications [13] suggested that Scvo 2 can not replace Svo 2 value in managing the septic patient. In retrospect, finally, importance must also be given to the changes data may undergo during the first stages of treatment. Such assessments could be of interest and warrant additional investigations. CONCLUSIONS In summary, to the best of our knowledge, we are the first to demonstrate that the Fick method determination of CO, using Scvo 2 measured via central venous catheter is as reliable and accurate as the thermodilutionally-obtained CO, both in low and high COs. Since central vein catheterization is placed in most critically ill patients upon their arrival in the ED, at least for the initial and emergent determination of the nature of the shock, determination of CO in such patients can be obtained rapidly and not invasively, which facilitates treatment and circumvents complications. Further studies are needed to determine whether this method is as useful beyond the initial diagnostic CO measurement, as for continuous management of the hemodynamically unstable patient in the ICU. REFERENCES 1. Rivers E, Nguyen B, Havstad S, Ressler J, Muzzin A, Knoblich B, Peterson E, Tomlanovich M, Early Goal-Directed Therapy

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