Venous Oxygen Saturation as a Physiologic Transfusion Trigger

Abstract

IntroductionVenous oxygen saturation is a clinical tool which integrates the whole body oxygen uptake-to-delivery (VO 2 -DO 2 ) relationship. In the clinical setting, in the absence of pulmonary artery catheter (PAC)-derived mixed venous oxygen saturation (SvO 2 ), the central venous oxygen saturation (ScvO 2 ) is increasingly being used as a reasonably accurate surrogate [1]. Central venous catheters (CVCs) are simpler to insert, and generally safer and cheaper than PACs. Th e CVC allows sampling of blood for measurement of ScvO 2 or even continuous monitoring if an oximetry catheter is being used. Th e normal range for SvO 2 is 68 to 77% and ScvO 2 is considered to be 5% above these values [2].A decrease in hemoglobin (Hb, g/dl) is likely to be associated with a decrease in DO 2 when cardiac output (CO) remains unchanged, since DO 2 = CO x CaO 2 , where CaO 2 is arterial oxygen content and is ≈ Hb × SaO 2 x 1.34 (where SaO 2 is the arterial oxygen saturation in%; and 1.34 is the oxygen-carrying capacity of Hb in mlO 2 /g Hb), when one ignores the negligible oxygen not bound to Hb [1]. A decrease in Hb is one of the four determinants responsible for a decrease in SvO 2 (or ScvO 2 ), alone or in combination with hypoxemia (decrease in SaO 2 ), an increase in VO 2 without a concomitant increase in DO 2 , or a fall in cardiac output.When Because it integrates Hb, cardiac output, VO 2 and SaO 2 , the venous oxygen saturation therefore helps to assess the VO 2 -DO 2 relationship and tolerance to anemia during blood loss. Venous oxygen saturation as a physiologic transfusion triggerWhen DO 2 decreases beyond a certain threshold, it induces a decrease in VO 2 . Th is point is known as the critical DO 2 (DO 2 crit), below which there is a state of VO 2 -DO 2 dependency also called tissue dysoxia. In humans, dysoxia is usually present when SvO 2 falls below a critical 40-50% (SvO 2 crit); this may, however, also occur at higher levels of SvO 2 when O 2 ER is impaired. Usually eff orts in correcting cardiac output (by fl uids or inotropes), and/or Hb and/or SaO 2 and/or VO 2 must target a return of SvO 2 (ScvO 2 ) from 50 to 65-70% [4]. In sedated critically ill patients in whom life support was discontinued, the DO 2 crit was found to be approximately 3.8 to 4.5 mlO 2 /kg/min for a VO 2 of about 2.4 mlO 2 /g/ min; O 2 ER reached an O 2 ERcrit of 60% [5] with SvO 2 crit being ≈ 40%.In a landmark study by Rivers et al. [6], patients admitted to an emergency department with severe sepsis and septic shock were randomized to standard therapy (aiming for a central venous pressure [CVP] of 8-12 mmHg, mean arterial pressure (MAP) ≥ 65 mmHg, and urine output ≥ 0.5 ml/kg/h) or to early goal-directed therapy where, in addition to the previous parameters, an ScvO 2 of at least 70% was targeted by optimizing fl uid administration, keeping hematocrit ≥ 30%, and/or giving dobutamine to a maximum of 20 μg/kg/min. Th e initial ScvO 2 in both groups was low (49 ± 12%), suggesting a hypodynamic condition before resuscitation was started.

Transfusión según cifras de hemoglobina o de acuerdo con objetivos terapéuticos

“…Dada la imposibilidad de establecer un trigger universal basado en cifras de Hb, la tendencia actual se decanta por los triggers transfusionales fisiológicos, bien basados en signos y síntomas (lactato, SvO 2 y ScvO 2 ), bien basados en la oxigenación tisular (segmento ST del ECG, Digit-Symbol Substitution Test y P300) 31,32 . Actualmente, la ScvO 2 parece ser uno de los triggers más prometedores.…”

“…En humanos, la disoxia o punto crítico de la SvO 2 se considera cuando se alcanza un valor de alrededor del 40-50%, aunque puede ocurrir en niveles más altos y el target de retorno debería estar alrededor del 65-70% en el caso de la ScvO 2 . Se acepta que si el rango de normalidad para la SvO 2 oscila entre el 68 y el 77%, el de la ScvO 2 se situará alrededor de un 5% por encima de estos valores 32 . La utilización del target transfusional de la ScvO 2 > 70% se ha asociado con un aumento de la transfusión una vez que los fluidos y los fármacos vasoactivos se han titulado correctamente, si bien también es cierto que la transfusión se ha relacionado con un aumento significativo de la ScvO 2 en los pacientes transfundidos con una ScvO 2 < 70% 33 .…”

Anemia and Transfusions in Surgical Patients: Current Concepts and Future Directions

“…Defined as the ratio of VO 2 [53]. In the intensive care unit setting, ScvO 2 measurements are frequently available and can be used to approximate SvO 2 .…”

Transfusion Triggers in Cardiac Surgery

“…Oksimetrik bir kateter kullanıldığında sürekli ölçüme izin verir. ScvO 2 'nin normal değerleri SvO 2 'nin % 5 üzerinde-dir (79)(80)(81) .…”