Abstract:OBJECTIVES:Hemodynamic support is aimed at providing adequate O2 delivery to the tissues; most interventions target O2 delivery increase. Mixed venous O2 saturation is a frequently used parameter to evaluate the adequacy of O2 delivery.METHODS:We describe a mathematical model to compare the effects of increasing O2 delivery on venous oxygen saturation through increases in the inspired O2 fraction versus increases in cardiac output. The model was created based on the lungs, which were divided into shunted and n… Show more
“…It is possible that the magnitude of this phenomenon (here calculated as 4.9% points in SvO 2 per 100 mm Hg change in PaO 2 ) would be less prominent in the setting of diminished oxygen delivery (whether globally or regionally). As suggested previously by mathematical modeling, tissue hypoxia may cause excess dissolved oxygen to more rapidly diffuse to the mitochondrion, diminishing the magnitude of change in tPO 2 and therefore SvO 2 during hyperoxemia. Nonetheless, given these findings, SvO 2 cannot be viewed as a pure marker of the adequacy of DO 2 , but rather must be interpreted in the context of PaO 2 .…”
SvO acutely changes following changes in PaO even absent changes in measured DO . This may lead to errors in Fick estimates of CI. Further work is necessary to understand the impact of this phenomenon in disease states.
“…It is possible that the magnitude of this phenomenon (here calculated as 4.9% points in SvO 2 per 100 mm Hg change in PaO 2 ) would be less prominent in the setting of diminished oxygen delivery (whether globally or regionally). As suggested previously by mathematical modeling, tissue hypoxia may cause excess dissolved oxygen to more rapidly diffuse to the mitochondrion, diminishing the magnitude of change in tPO 2 and therefore SvO 2 during hyperoxemia. Nonetheless, given these findings, SvO 2 cannot be viewed as a pure marker of the adequacy of DO 2 , but rather must be interpreted in the context of PaO 2 .…”
SvO acutely changes following changes in PaO even absent changes in measured DO . This may lead to errors in Fick estimates of CI. Further work is necessary to understand the impact of this phenomenon in disease states.
Perspectives on invasive and noninvasive ventilatory support for critically ill
patients are evolving, as much evidence indicates that ventilation may have positive
effects on patient survival and the quality of the care provided in intensive care
units in Brazil. For those reasons, the Brazilian Association of Intensive Care
Medicine (Associação de Medicina Intensiva Brasileira - AMIB) and
the Brazilian Thoracic Society (Sociedade Brasileira de Pneumologia e
Tisiologia - SBPT), represented by the Mechanical Ventilation Committee
and the Commission of Intensive Therapy, respectively, decided to review the
literature and draft recommendations for mechanical ventilation with the goal of
creating a document for bedside guidance as to the best practices on mechanical
ventilation available to their members. The document was based on the available
evidence regarding 29 subtopics selected as the most relevant for the subject of
interest. The project was developed in several stages, during which the selected
topics were distributed among experts recommended by both societies with recent
publications on the subject of interest and/or significant teaching and research
activity in the field of mechanical ventilation in Brazil. The experts were divided
into pairs that were charged with performing a thorough review of the international
literature on each topic. All the experts met at the Forum on Mechanical Ventilation,
which was held at the headquarters of AMIB in São Paulo on August 3 and 4, 2013, to
collaboratively draft the final text corresponding to each sub-topic, which was
presented to, appraised, discussed and approved in a plenary session that included
all 58 participants and aimed to create the final document.
“…Indeed, our recent work on patients with circulatory failure requiring vasopressors showed that PaO 2 could increase ScvO 2 and mixed venous oxygen saturation in a very substantial fashion (up to 10% in some patients)-more than the effect associated with changes in cardiac output 31 . Mathematical modelling also supports the fact that ScvO 2 could be affected by PaO 2 in a substantial fashion 32 . This novel concept is important because any increases in ScvO 2 due to an increase in PaO 2 would confound its associations with the cardiac output status of patients, rendering a 'good' ScvO 2 uninterpretable as a marker of adequate cardiac output or oxygen delivery 33 .…”
Optimising perioperative haemodynamic status may reduce postoperative complications. In this prospective prevalence study, we investigated the associations between standard haemodynamic parameters and a low central venous oxygen saturation (ScvO 2 ) in patients after major surgery. A total of 201 patients requiring continuous arterial and central venous pressure monitoring after major surgery were recruited. Simultaneous arterial and central venous blood gases, haemodynamic and biochemical data and perfusion index were obtained from patients at a single time-point within 24 hours of surgery. A low ScvO 2 (<70%) was observed in 109 patients (54%). Use of mechanical ventilation, mean arterial pressure, central venous pressure, haemoglobin concentrations, arterial pH and lactate concentrations, arterial oxygen (PaO 2 ) and carbon dioxide tensions (PaCO 2 ) were all associated with a low ScvO 2 in the univariate analyses. In the multivariate analysis, only a higher perfusion index (odds ratio [OR] 0.87, 95% confidence interval [CI] 0.78 to 0.98), PaO 2 (OR 0.98 per mmHg increment, 95% CI 0.97 to 0.99) and PaCO 2 (OR 0.88 per mmHg increment, 95% CI 0.82 to 0.95) and a lower central venous pressure (OR 1.14 per mmHg increment, 95% CI 1.04 to 1.25) were significantly associated with a reduced risk of a low ScvO 2 , all in a linear fashion. In conclusion, PaO 2 , PaCO 2 , perfusion index and central venous pressure were significant predictors of a low ScvO 2 in patients after major surgery including cardiac surgery, suggesting that ScvO 2 should always be interpreted with the arterial blood gases and that liberal perioperative fluid therapy aiming at a high central venous pressure may be detrimental in optimising ScvO 2 .
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