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Cardiac Output Measurement Using a Modified Carbon Dioxide Fick Method

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Cardiac Output Measurement Using a Modified Carbon Dioxide Fick Method 0031-3998/07/6103-0279 PEDIATRIC RESEARCH Vol. 61, No. 3, 2007 Copyright © 2007 International Pediatric Research Foundation, Inc. Printed in U.S.A. Cardiac Output Measurement Using a Modified Carbon Dioxide Fick Method: A Validation Study in Ventilated Lambs WILLEM P. DE BOODE, JEROEN C.W. HOPMAN, OTTO DANIE¨ LS, HANS G. VAN DER HOEVEN, AND K. DJIEN LIEM Departments of Neonatology [W.P.B., K.D.L.], Clinical Physics [J.C.W.H.], Pediatric Cardiology [O.D.], and Intensive Care [H.G.H.], Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands ABSTRACT: Cardiac output can be measured using a modified reliable, (semi)continuous, preferably noninvasive, accurate carbon dioxide Fick (mCO2F) method. A validation study was method of cardiac output monitoring in critically ill newborns. performed comparing mCO2F method–derived cardiac output Only in this way can objective hemodynamic parameters be mCO2F (Q ) with invasively measured pulmonary blood flow. In seven obtained on which a rational therapeutic regimen can be randomly bred ventilated newborn lambs, cardiac output was manip- based. ulated by creating hemorrhagic hypotension. When steady state was Theoretically, cardiac output can be measured in ventilated reached, QmCO2F was measured. Gas analysis was performed in simultaneously obtained arterial and venous blood samples (right patients using a mCO2F method. We performed a validation atrium [RA], superior vena cava [SVC], and inferior vena cava study of this technique in a lamb model. [IVC]). Carbon dioxide exchange and pulmonary blood flow was measured continuously using a CO SMO Plus monitor and a pulmo- 2 METHODS nary ultrasonic flow probe (Qufp), respectively. Mean bias, defined as mCO2F ufp Ϫ1 Q Ϫ Q , was small (respectively, Ϫ0.082 L·min , Ϫ0.085 We compared the QmCO2F with measured pulmonary blood flow using a Ϫ Ϫ L·min 1 and Ϫ0.183 L·min 1 for venous sampling from RA, SVC, perivascular ultrasonic flow probe (ufp) positioned around the common and IVC). The limits of agreement were Ϫ0.328 to 0.164 L·minϪ1 pulmonary artery (Qufp). Ϫ1 Ϫ1 mCO F method. The mCO F method is based on the principle that in (RA), Ϫ0.335 to 0.165 L·min (SVC), and 0.415 to 0.049 L·min 2 2 steady state, carbon dioxide production in tissue (CO2P) equals pulmonary (IVC). In conclusion, measurement of cardiac output with the ˙ ˙ carbon dioxide exchange (VCO2.VCO2 can be measured in a ventilated patient mCO2F method is reliable and easily applicable in ventilated new- using a computer-aided analysis of expiratory airflow (Q ) and carbon born lambs. For clinical use, the site of venous blood sampling is of exp dioxide fraction in expiratory air (FeCO2). minor importance. (Pediatr Res 61: 279–283, 2007) V˙CO ϭ ͭ͐T Q (t)·FeCO (t)·dtͮ · TϪ1 (1) 2 exp 2 o ˙ ϭ ϭ onitoring cardiac output is essential in the treatment of where VCO2 carbon dioxide exchange (L/min); Qexp expiratory airflow (L/min); FeCO ϭ carbon dioxide fraction in expiratory air (gradient); T ϭ critically ill patients. It may improve the indication and 2 M time (min). choice of treatment in a hemodynamically unstable patient. CO2P is the product of cardiac output (Q) and the venoarterial difference Furthermore, the response to the intervention can be moni- in carbon dioxide concentration (C(v-a)CO2). tored and evaluated. CO2P ϭ Q ·(CvCO2 Ϫ CaCO2) (2) Bedside cardiac output monitoring is feasible in adult and ϭ ϭ where CO2P carbon dioxide production (mL/min); Q cardiac output ϭ ϭ pediatric patients, but remains complicated in the newborn. (L/min); CvCO2 venous carbon dioxide concentration (mL/L); CaCO2 Several techniques for cardiac output monitoring in children arterial carbon dioxide concentration (mL/L). Carbon dioxide concentration in blood can be measured using different are available (1–3). Most techniques for cardiac output mea- methods, depending on the calculation method of carbon dioxide concen- surement are not feasible in newborns. In critically ill neo- tration in the erythrocyte (6–10). In this study, we used the Douglas nates, cardiac output is usually estimated from the interpreta- equation (10): ͑0.0289 · Hb͒ tion of several clinical variables, such as blood pressure, urine ϭ Ϫ cbCO2 cpCO2 · ͫ1 Ϫ Ϫ ͬ (3) output, blood gas analysis, and capillary refill. Tibby and ͑3.352 0.456 · sO2͒ · ͑8.142 pH͒ ϭ colleagues (4) showed that clinicians using these indirect where cpCO2 total carbon dioxide concentration in plasma (mL/100 mL); ϭ ϭ cbCO2 total carbon dioxide concentration in blood (mL/100 mL); Hb parameters of cardiac performance were unable to predict the ϭ hemoglobin concentration (g/dL); sO2 oxygen saturation (gradient). actual cardiac output in ventilated children and infants. The Total carbon dioxide concentration in plasma is calculated using the same was shown in ventilated adults (5). Nonetheless, this Henderson-Hasselbalch equation: clinical estimation of cardiac output is nowadays still the most pHϪpK= cpCO2 ϭ 2.226 · s · pCO2 ·(1ϩ 10 ) (4) frequently used method. This emphasizes the need for a ϭ where cpCO2 total carbon dioxide concentration in plasma (mL/100 mL); 2.226 ϭ conversion factor mEq to mL/100 mL; s ϭ solubility coefficient of Received June 8, 2006; accepted October 24, 2006. Correspondence: Willem P. de Boode, M.D., Radboud University Nijmegen Medical Centre, Department of Neonatology, P.O. Box 9101, Internal Postal Code 833, 6500 HB Abbreviations: IVC, inferior vena cava; mCO2F, modified carbon dioxide mCO2F ufp Nijmegen, The Netherlands; e-mail: [email protected] Fick; Q , mCO2F method–derived cardiac output; Q , cardiac output There was no external financial assistance for this study. determined by pulmonary ultrasonic flow probe; RA, right atrium; SVC, ˙ DOI: 10.1203/pdr.0b013e318030d0c6 superior vena cava; VCO2 , pulmonary carbon dioxide exchange 279 280 DE BOODE ET AL. ϭ ϭ carbon dioxide in plasma (mEq/mm Hg); pK= apparent pK; pCO2 partial catheter exactly in the RA. To be able to evaluate the effect of different venous carbon dioxide pressure (mm Hg). sample sites on the accuracy of the mCO2F method, we surgically inserted intravascular lines with the tip positioned in the RA, the IVC, and SVC, ϭ ϩ ͑ Ϫ ͒ ϩ ͑ Ϫ ͒2 s 0.0307 0.00057 37 T 0.00002 37 T (5) respectively. The position of the tip of the catheters was estimated using the where s ϭ solubility coefficient of carbon dioxide in plasma (mEq/mm Hg); registered pressure waves during insertion. The final position of the catheters T ϭ temperature (°C) was verified by postmortem evaluation. An intra-arterial catheter was inserted in the abdominal aorta through the femoral artery. A left-sided thoracotomy pK= ϭ 6.086 ϩ ͓0.042·͑7.4 Ϫ pH͔͒ ϩ ͕͑38 Ϫ T͒· ͓ 0.0047 was performed, and after cautious preparation and identification of major structures, the ductus arteriosus was ligated. This was done to exclude any ϩ Ϫ ͖͔ 0.00139 · ͑7.4 pH͒ (6) possible influence of a ductal shunt (left to right and/or right to left) on the ˙ measurement. An adequately sized ultrasonic flow probe (Transonic Systems For use in calculations, CO2P and VCO2 need to be converted to standard Inc., Ithaca, NY) was placed around the pulmonary trunk. temperature pressure, dry (STPD) conditions. Experimental protocol. Measurement of the pulmonary blood flow (Qufp) BTPS was considered the gold standard because it is equal to systemic flow, when T0 (P Ϫ pH2O) CO PSTPD ϭ CO PBTPS · ͭ ͮ · ͭ ͮ (7) any shunts are excluded. Measurement of aortic flow will underestimate the 2 2 TBTPS P 0 systemic flow because the coronary flow will be missed. STPD ϭ where CO2 P carbon dioxide production under STPD conditions; CO2 After a 30-min period of stabilization, cardiac output was reduced by P BTPS ϭ carbon dioxide production under BTPS conditions; BTPS ϭ body creating hemorrhagic hypotension. Blood was withdrawn in a stepwise man- ϭ temperature pressure, saturated; T0 standard temperature (273 K); ner to obtain a decrease of mean arterial blood pressure (MABP) of 10 mm T BTPS ϭ temperature under BTPS (K); P BTPS ϭ pressure under BTPS Hg. After each reduction in blood pressure, a 15-min stabilization period ϭ BTPS conditions (kPa); pH2O partial pressure of water vapor at T (kPa); P0 ensured steady state. Steady state was assumed when carbon dioxide ex- ϭ standard pressure (101.4 kPa). change was stable for at least 6 min. The mCO2F method–derived cardiac ATPS output was measured before manipulating cardiac output and after each T0 (P Ϫ pH2O) V˙COSTPD ϭ V˙COATPS · ͭ ͮ · ͭ ͮ (8) stabilization period. Blood samples were taken from the different sampling 2 2 ATPS T P0 sites for blood gas analysis, i.e. arterial blood from the abdominal aorta and venous blood from the RA, SVC, and IVC. Gas analysis of the blood samples ˙ STPD ϭ where VCO2 pulmonary carbon dioxide exchange under STPD was done immediately after sampling using a Synthesis 25 analyzer (Instru- ˙ ATPS conditions; VCO2 ϭ pulmonary carbon dioxide exchange under ATPS mentation Laboratory, Barcelona, Spain). The stated 90% confidence interval ϭ ϭ Ϯ Ϯ Ϯ conditions; ATPS ambient temperature pressure, saturated; T0 standard is 2mmHgforPCO2, 10 mpH for pH, 0.25 mmol/L for Hb concentra- temperature (273 K); T ATPS ϭ ambient temperature (K); P ATPS ϭ pressure tion and Ϯ1.5% for oxygen saturation. Carbon dioxide exchange was mea- ϭ under ATPS conditions (kPa); pH2O partial pressure of water vapor at sured using the CO2SMO Plus Respiratory Profile Monitor (Model 8100, ATPS ϭ T (kPa); P0 standard pressure (101.4 kPa). Novametrix Medical Systems Inc., Wallingford, CT). According to the man- Then, cardiac output can be calculated using the following equation.
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