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Mixed venous O2 saturation: Measured by co-oximetry versus calculated from P\(\bar v\) o 2

Abstract

Objective. The objectives of our study were (1) to compare mixed venous saturations calculated by a blood gas machine with those measured directly by a co-oximeter; and (2) to compare the sensitivities and specificities of\(\dot V\)o2s derived from these values.Methods. Charts were retrospectively reviewed of all MICU patients [n=16] between December 1, 1991 and January 31, 1992, who required pulmonary artery catheters for their usual care and who had hemoglobin saturations of mixed venous blood concurrently measured by both a co-oximeter (Co-Ox Model 482, Instrumentation Lab, Lexington, MA) and a blood gas analyzer (Nova Biomedical StatLab5, Waltham, MA) which uses a variant of the Severinghaus equation to calculate S\(\bar v\)O2 from P\(\bar v\)O2). Data used at the time of each S\(\bar v\)O2 measurement to calculate oxygen consumption (\(\dot V\)O2) further was collected.Results. Available for analysis were 118 mixed venous blood samples. Although the S\(\bar v\)O2 values had a correlation coefficient of 0.807 (95% confidence interval [CI] 0.736 to 0.861, Fisher's z-transform), when\(\dot V\)O2s were calculated, the blood gas analyzer calculated saturations had a sensitivity of only 58.3% and a specificity of 89%, when compared with those calculated using the saturations measured by the co-oximeter. Attempts to mathematically improve upon the Severinghaus equation and upon an additional four regression equations used by other blood gas analyzers resulted in universally worse sensitivity.Conclusion. If S\(\bar v\)O2s calculated by a blood gas machine—rather than those co-oximetrically measured—are used to calculate\(\bar v\)O2s, 42% of patients with low O2s will be misclassified as normal and 11% of normals will be misclassified as low. This total error appears to be the result of measurement error by the Po2 electrode of the blood gas analyzer and shifts of the oxyhemoglobin dissociation curve, which are not accounted for in the equation that is used to calculate saturation from measuredPO2. We were not able to improve mathematically the sensitivity of any of the available regression equations used by blood gas analyzers to calculate S\(\bar v\)O2 from P\(\bar v\)O2. Therefore, it remains necessary to use co-oximetrically measured saturations when calculating\(\dot V\)O2.

Abstrakt

Ziel. Die Ziel unserer Untersuchung waren (1) der Vergleich der Berechnungen der gemischtvenösenO2-Sättigung aus Blutgasanalysen mit den Meßwerten aus einem CO-Oximeter und (2) der Vergleich der Sensitivität und Specifität der aus diesen Mssungen abgeleiteten Werte fürVO2.Methoden. Die Behandlungsdaten von 16 zwischen dem 1.12.91 und 31.1.92 auf einer allgemeinen Intensivstation behandelten Patienten wurden retrospektiv untersucht. Einschlußkriterien waren (1) durchgehende pulmonal-arterielle Katheterisierung und (2) gleichzeitig Messung der gemischtvenösen Sauerstoffsättigung durch ein CO-Oximeter (Co-Ox Model 482, Instrumentation Lab) und einen Blutgasanalysator (Nova Biomedical StatLab 5). Der BG-Analysator nutzt eine Variante der Severinghaus-Gleichung zur Berechnung es S\(\bar V\)O2 aus dem P\(\bar V\)O2. Die bei jeder S\(\bar V\)O2 Messung erhobenen Zustandsdaten zur Berechnung desO2-Verbrauches wurden aufgezeichnet.Ergebnisse. Es standen 118 gemischtvenöse Blutproben zur Verfügung. Obwohl die S\(\bar V\)O2 Werte beider Methoden eine Korrelation von 0.807 aufwiesen (95% Konfidenzintervall bei 0.736–0.861, z-Transformation nach Fisher), hatten bei derO2-Berechnung die aus de Blutgasanalyse berechneten Sättigungswerte nur eine Sensitivität von 58.3% und eine Spezifitä von 89% im Vergleich zu den Berechnungen aus den Messungen des CO-Oximeters. Versuche, die Ergebnisse durch Anpassung der Severinghaus-Gleichung unddurch vier zusätzliche Regressionsgleichungen zu verbessern, führten jeweils zu einer Verschlechterung der Sensitivität.Schlußfolgerung. Werden die von einem Blutgasanalysator ermittelten S\(\bar V\)O2-Werte anstelle direkter CO-oximetrisch gewonnener Daten zur Berechnung des\(\dot V\)O2 herangezogen, dann werden 42% der Patienten mit niedrigerO2-Sättigung als normal, 11% der Normalwerte alszu niedrig klassifiziert. Dieser Fehler scheint einerseits auf den Meßfehler der\(\overline P\)O2-Elektrode des Blutgasanalysators, andererseits auf cinc in der Berechnung unberücksichtigte Verschiebung der Sauerstoffbindungskurve des Hämoglobins zurückzuführen zu sein. Wir konnten bei keiner der von Blutgasanalysatoren benutzten Regressionsgleichungen die Sensitivität mathematisch verbessern. Deshalb bleibt es unseres Erachtens notwendig, die CO-oximetrisch gemessenen Sättigungswerte fr die\(\dot V\)O2 Berechnung zu nutzen.

Resumen

Objetivo. Los objectivos de nuestro estudio fueron (1) comparar las saturaciones venosas mezcladas calculadas por una màquina analizadora de gases en sangre con las saturaciones medidas directamente usando un co-oximetro; y (2) comparar la sensibilidad y especificidad de\(\dot V\)O2s derivados a partir de dichos valores.Mètodos. Se revisaron retrospectivamente las fichas de todos los pacientes de la Unidad de Cuidados Intensivos Mèdicos [n=19] que requirieron catèter de arteria pulmonar como parte de su manejo habitual, entre el 1 de diciembre de 1991 y el 31 de enero de 1992, y en quienes se midiò saturaciòn de hemoglobina en sangre venosa mezclada usando al mismo tiempo un co-oximetro (Co-Ox Model 482, Instrumentation Lab) y un analizador de gases en sangre (Nova Biomedical StatLab5; este equipo usa una variante de la ecuaciòn de Severinghaus para calcular S\(\bar v\)O2 a partir de la P\(\bar v\)O2). Además, se registró información usada para calcular consumo de oxigeno en el momento de mediciòn de S\(\bar v\)O2.Resultados. 118 muestras de sangre venosa mezclada estuvieron disponibles para el anàlisis. Aunque los valores de S\(\bar v\)O2 presentaban un coeficiente de correlaciòn de 0.807 (intervalo de confianza de 95% 0.736 a 0.861, transformaciòn z de Fisher), cuando los O2s fueron calculados, el analizador de gases en sangre calculò saturaciones con sensibilidad de solamente 58.3% y especificidad de 89%, al ser comparadas con las calculadas a partir de las mediciones del co-oximetro. Intentos de mejorar matemàticamente la ecuaciòn de Severinghaus y cuatro ecuaciones de regresiòn adicionales usadas por otros analizadores de gases en sangre resultaron siempre en peor sensibilidad.Conclusiòn. Si se usa las S\(\bar v\)O2s calculadas por una màquina de gases en sangre, en lugar de las medidas en el co- oximetro, para calcular\(\bar v\)O2s, 42% de los pacientes con O2s bajos serian erròneamente clasificados como normales y 11% de los normales serian erròneamente clasificados como bajos. Este error total parece resultar de errores de mediciòn lel electrodo dePO2 del analizador de gases en sangre, y de desplazamientos de la curva de disociaciòn de la oxihemoglobina que no son consideradas en la ecuaciòn usada para calcular saturaciòn a partir de laPO2 medida. No fuimos capaces de mejorar matemàticamente la sensibilidad de ninguna de las ecuaciones de regresiòn disponibles usadas por analizadores de gases en sangre para calcular S\(\bar v\)O2 a partir de la P\(\bar v\)O2. En consecuencia, sigue siendo necesario utilizar el co-oximetro para medir saturaciones al calcular\(\dot V\)O2.

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References

  1. Gutierrez G, Bismar H. Oxygen transport and utilization. In: Dantzker DR, ed. Cardiopulmonary critical care. Philadelphia: WB Saunders, 1991:199–229

    Google Scholar 

  2. Nelson LD. Mixed venous oximetry. In: Snyder JV, Pinsky MR, eds. Oxygen transport in the critically ill. Chicago: Year Book Medical Publishers, 1987:235–248

    Google Scholar 

  3. Kandel G, Aberman A. Mixed venous oxygen saturation. Its role in the assessment of the critically ill patient. Arch Intern Med 1983;143:1400–1402

    PubMed  Article  CAS  Google Scholar 

  4. Tuchschmidt J, Fried J, Astiz M, Rackow E. Elevation of cardiac output and oxygen delivery improves outcome in septic shock. Chest 1992;102:216–220

    PubMed  Article  CAS  Google Scholar 

  5. Shoemaker WC, Appel PL, Kram HB. Role of oxygen debt in the development of organ failure sepsis, and death in high risk surgical patients. Chest 1992;102:208–215

    PubMed  Article  CAS  Google Scholar 

  6. Tentative standard for definitions of quantities and conventions related to blood pH and gas analysis. In: National committee for clinical laboratory standards. 1982;2(10):342–344

  7. Breuer HWM, Groeben H, Breuer J, Worth H. Oxygen saturation calculation procedures: A critical analysis of six equations for the determination of oxygen saturation. Intensive Care Med 1989;15:385–389

    PubMed  Article  CAS  Google Scholar 

  8. Severinghaus JW. Blood gas calculator. J Appl Physiology 1966;21:1108–1116

    CAS  Google Scholar 

  9. Severinghaus JW. Simple, accurate equations for human blood O2 dissociation computations. J Appl Physiol 1979;46:599–602

    PubMed  CAS  Google Scholar 

  10. Kamal GD, Starr JM, Symreng T. Continuous in vivo pulmonary venous admixture from fiberoptically measured hemoglobin saturations. Crit Care Med 1990;18:1419–1422

    PubMed  CAS  Article  Google Scholar 

  11. Heiselman D, Jones J, Cannon L. Continuous monitoring of mixed venous oxygen saturation in septic shock. J Clin Monit 1986;2:237–245

    PubMed  Article  CAS  Google Scholar 

  12. Bland JM, Altman DG. Statistical methods for assessing the agreement between the two methods of clinical measurement. Lancet 1986;1(8476):307–310

    PubMed  CAS  Google Scholar 

  13. Schechter CB, Adler RS. Bayesian analysis of diastolic blood pressure measurement. Med Decis Making 1988;8:182–190

    PubMed  Article  CAS  Google Scholar 

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This study was presented in abstract form at the American College of Chest Physicians 58th Annual Scientific Assembly, Chicago, IL, October 28, 1992.

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Nierman, D.M., Schechter, C.B. Mixed venous O2 saturation: Measured by co-oximetry versus calculated from P\(\bar v\) o 2 . J Clin Monitor Comput 10, 39–44 (1994). https://doi.org/10.1007/BF01651465

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Key words

  • Measurement techniques
  • oximetry
  • Oxygen consumption
  • Equipment
  • catheters
  • pulmonary
  • arterial
  • Intensive care