Chest
Volume 118, Issue 3, September 2000, Pages 631-640
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Clinical Investigations
Techniques
Comparison of Exercise Cardiac Output by the Fick Principle Using Oxygen and Carbon Dioxide*

https://doi.org/10.1378/chest.118.3.631Get rights and content

Background and study objective

Theoretically, cardiacoutput (CO) calculated by the Fick principle should be the same using, O2 (CO[o2]) or CO2(CO[co2]) as the test gas. However, agreementdepends on the accuracy of gas exchange and blood gas measurements and the validity of the equations to convert measured variables into bloodgas contents. Considering the widespread use of indirect estimates ofpulmonary artery blood Pco2 and CO2content to measure Fick principle CO during exercise, we wished todetermine whether CO[o2] and , CO[co2] were equal during exercise and whether CO[co2] could be accurately and precisely determined using direct measures of pulmonary arteryblood.

Preparation and methods

Five healthy youngnonsmoking volunteer men performed incremental exercise from rest topeak exercise on two separate occasions with intervening rest. Catheters were placed in brachial and pulmonary arteries to allowrepeated blood sampling every minute during concurrent breath-by-breathgas exchange measurements from rest to peak exercise. CO[o2] was compared with, CO[co2] at multiple levels of exercise. Usingstandard equations, arterial and mixed venous O2 contentswere calculated from hemoglobin concentration (Hb), oxyhemoglobinsaturation (So2), and , Po2, whereas CO2 contents werecalculated from Pco2, p, H, Hb, and , So2. Blood gas analyzers were used formeasurement of p, H, Pco2, and , Po2, and a co-oximeter was used for measurementof Hb and So2. Initial calculations suggestedthat exercise CO[co2] was 14% higher than CO[o2] and helped disclose smallsystematic measurement errors in Pco2 forvalues > 45 mm Hg detected by proficiency testing surveys and documented with blood tonometry in the blood gas analyzer.

Results

After correcting Pco2 forthe small systematic measurement error found, the measures and equations used to calculate arterial and mixed venous O2and CO2 contents were adequate to provide mean CO valuesthat are reasonably similar. However CO[co2]values were more than twice as variable as, CO[o2].

Conclusions

Theincreased variability of Fick principle, CO[co2] compared with, CO[o2] is attributable to the much lowerextraction ratio for CO2 and the greater complexity incalculation of blood CO2 than O2 contents. These results raise concerns about the accuracy and precision ofestimating CO and stroke volume using CO2 as a test gas, even with direct measurement of blood CO2 contents innormal subjects.

Section snippets

Subjects and Preliminary Ramp Exercise Testing

The Human Subjects Committee at Harbor-UCLA Medical Center approved the researchprotocol. After informed consent, five healthy nonsmoking male subjectsperformed a preliminary noninvasive increasing work rate exercise teston an electromagnetically braked cycle ergometer (type 18070;Gould-Godart; Bilthoven, Netherlands) to determine their maximum workrate, anaerobic threshold (AT), and maximal, V˙o2( V˙o2max). During each test, apedal frequency of 60 revolutions/min was maintained

Blood Analysis

The blood samples were agitated and immediately chilled in ice slurry. Blood gas analysis was performedwith an Instrumentation Laboratory (IL) 1306 blood gas analyzer(Instrumentation Laboratory; Lexington, MA) for p, H,Pco2, and , Po2, and with an IL 482 co-oximeter(Instrumentation Laboratory) for total Hb and , So2. The measured values obtainedwere corrected for heparin dilution. The analyzer precision wasverified with quality control materials every 20 to 30 min.

Respired Gas Analysis

The subjects respired througha

Calculations of Blood O2 and CO2Contents

Blood O2 contents(Cbo2) in milliliters per deciliter werecalculated from the following equation:231213141516 Cbo2=13.4×Hb×So2/100+0.00326×Po2where Hb is hemoglobin concentration in grams per deciliter,So2 is O2 saturation in percent, and Po2 is O2 partial pressure inmillimeters of mercury.

Plasma CO2 contents(Cplco2) were calculated from thestandard formula derived from the Henderson–Hasselbachequation17181920: Cplco2=2.226×Hb×s×Pco2×(1+10pH-pK)where 2.226 is the conversion factor

Exercise Studies and Blood Values

The subject’s physical characteristics and the work rate at ATand maximal exercise of test A and test B are shown in Table 1. There was no significant difference between the two tests. V˙o2 increased at a rate of10.30 ± 0.70 m, L/min/W during exercise.

Because the duration of exercise varied slightly between subjects, blood values were grouped according to exercise intensity (Table 2). Both arterial and venous Hb significantly increased from restingvalues during exercise from AT to maximal

Comparison of CO[o2] WithCO[co2]

Confirming the Fick principle,1 the ratio of the meanvalues of CO[co2] was similar tothat of CO[o2], both at rest(0.99 ± 0.04) and during exercise (1.00 ± 0.13), when accuratemeasures and correct formulas and calculations were used (Fig 3 and Table 3). The overall difference of these two methods for COcalculation was negligible. Inasmuch as, V˙co2 is a simpler measurementthan V˙o2, it would be temptingto suggest that CO2 might be the more optimaltest gas for CO measurements.

References (49)

  • CK Mahutte et al.

    Effect of measurement errors on cardiac output calculated with and modified Fick methods

    J Clin Monit

    (1995)
  • DY Sue et al.

    Measurement and analysis of gas exchange during exercise using a programmable calculator

    J Appl Physiol

    (1980)
  • WL Beaver et al.

    A new method for detecting the anaerobic threshold by gas exchange

    J Appl Physiol

    (1986)
  • G Hu¨fner

    Neue versuche zur bestimung der saueratoffkapazita¨t des blutfarbstoffes

    Arch Anat Physiol

    (1894)
  • GG Davies et al.

    Continuous Fick cardiac output compared to thermodilution cardiac output

    Crit Care Med

    (1986)
  • ML Foss et al.
  • NC Gonzalez et al.

    Increasing maximal heart rate increases maximal O2uptake in rats acclimatized to simulated altitude

    J Appl Physiol

    (1998)
  • JL Teboul et al.

    Value of the venous-arterial Pco2gradient to reflect the oxygen supply to demand in humans: effects of dobutamine

    Crit Care Med

    (1998)
  • AR Douglas et al.

    Calculation of whole blood CO2content

    J Appl Physiol

    (1988)
  • R Hachamovitch et al.

    Respiratory and circulatory analysis of CO2output during exercise in chronic heart failure

    Circulation

    (1991)
  • GJR McHardy

    The relationship between the differences in pressure and content of carbon dioxide in arterial and venous blood

    Clin Sci

    (1967)
  • JF Nunn

    Nunn's applied respiratory physiology

    (1997)
  • WH Austin et al.

    Solubility of CO2in serum from 15 to 38°C

    J Appl Physiol

    (1963)
  • RG Kelman

    Digital computer procedure for the conversion of Pco2into blood content

    Respir Physiol

    (1967)
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    Supported in part by the Milly Liang Liu, MD, and Steve C. K. Liu, MD,Research Fund.

    {altfoot}*From the Division of Respiratory and Critical Care Physiology and Medicine, Department of Medicine, Harbor-UCLAMedical Center, St. John’s Cardiovascular Research Center, Torrance,CA.

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