Impact of mechanical power and positive end expiratory pressure on central vs. mixed oxygen and carbon dioxide related variables in a population of female piglets

INTRODUCTION

The use of the pulmonary artery catheter has decreased overtime; central venous blood gases are generally used in place of mixed venous samples. We want to evaluate the accuracy of oxygen and carbon dioxide related parameters from a central versus a mixed venous sample, and whether this difference is influenced by mechanical ventilation.

MATERIALS AND METHODS

We analyzed 78 healthy female piglets ventilated with different mechanical power.

RESULTS

There was a significant difference in oxygen-derived parameters between samples taken from the central venous and mixed venous blood (Sv ¯ $$ \overline{v} $$ O2 = 74.6%, ScvO2 = 83%, p < 0.0001). Conversely, CO2-related parameters were similar, with strong correlation. Ventilation with higher mechanical power and PEEP increased the difference between oxygen saturations, (Δ[ScvO2-Sv ¯ $$ \overline{v} $$ O2 ] = 7.22% vs. 10.0% respectively in the low and high MP groups, p = 0.020); carbon dioxide-related parameters remained unchanged (p = 0.344).

CONCLUSIONS

The venous oxygen saturation (central or mixed) may be influenced by the effects of mechanical ventilation. Therefore, central venous data should be interpreted with more caution when using higher mechanical power. On the contrary, carbon dioxide-derived parameters are more stable and similar between the two sampling sites, independently of mechanical power or positive end expiratory pressures.

Effects of red blood cell transfusion on global oxygenation in anemic critically ill patients

BACKGROUND

Consensus for transfusion in intensive care unit (ICU) patients recommends a restrictive strategy, based on a hemoglobin (Hb) concentration of 7 g/dL. Red blood cell (RBC) transfusion is used to prevent tissue hypoxia by improving oxygen transport (DO₂ ) and therefore oxygen consumption (VO₂ ). We studied the effects of RBC transfusion on systemic oxygenation parameters reflecting systemic oxygen extraction (EO₂ = DO₂ /VO₂ ): S(c)vO₂ , lactate level, venous-to-arterial carbon dioxide difference (Pv-aCO₂ ), and cardiac index/EO₂ (CI/EO₂ ) and evaluated their usefulness in guiding transfusion decisions in ICU patients.

STUDY DESIGN AND METHODS

Prospectively, all adult patients transfused were included except those with active bleeding or without a jugular or subclavian catheter. We measured O₂ parameters before and after transfusion. Patients were a priori grouped according to their initial S(c)vO₂ (< or ≥70%), treatment with vasopressors, cardiac function, and septic status.

RESULTS

A total of 62 patients received 105 RBC transfusions. For all, mean arterial pressure (77 [69-88] to 81 [73-91] mm Hg), Hb concentration (7.4 [7.0-7.8] to 8.4 [7.7-8.9] g/dL) and S(c)vO₂ (65% [59%-73%] to 69% [62%-75%]) increased after transfusion (all P < .001). S(c)vO₂ improved after transfusion only when initial S(c)vO₂ was less than 70% (62% [56%-65%] to 66% [61%-71%]; P < .001). In this group, Pv-aCO₂ , lactate concentrations, and CI/EO₂ did not change after transfusion. Cardiac function, sepsis, or vasopressor therapy did not affect these results.

CONCLUSIONS

Among systemic O₂ parameters, only a S(c)vO₂ < 70% in anemic ICU patients improves after transfusion. As S(c)vO₂ can reflect a DO₂ /VO₂ imbalance, it could be helpful when combined with the Hb concentration to decide whether to transfuse. However, the benefit on outcome should be further studied.

Measurement and correction of the bulk magnetic susceptibility effects of fat: application in venous oxygen saturation imaging

PURPOSE

To develop a correction method for the effects of the magnetic susceptibility of fat (χ_Fat ) on the calculation of venous oxygen saturation (SvO₂ ).

THEORY

The magnetic field shifts associated with the magnetic susceptibility of deoxyhemoglobin can be used to estimate SvO₂ , a measure of oxygen extraction and metabolism. However, the distinct magnetic susceptibility of fat surrounding targeted veins will give rise to magnetic field perturbations that will extend into the vein and surrounding tissues, potentially confounding the calculation of SvO₂ .

METHODS

Multi-echo modified Dixon fat-water separated imaging was used to quantify fat-water distributions around the superficial femoral vein (venous return from the lower leg). Fat fraction images were used to generate χ_Fat images, to calculate and remove the associated fat-susceptibility-induced magnetic field shifts before the estimation of SvO₂ . This approach was evaluated at rest and with plantar flexion exercise to evaluate calf muscle oxygen extraction in 10 healthy subjects.

RESULTS

The presence of fat around the vein resulted in complex magnetic field shifts and errors in estimated SvO₂ . Corrected resting SvO₂ values were significantly larger than those measured with conventional methods, at rest (72.6 ± 11.0% vs. 65.2 ± 12.2%, P < 0.05) and post-exercise (37.4 ± 12.3% vs. 31.7 ± 12.7%, P < 0.05), with larger errors in individuals and/or regions with increased fat volumes. Estimation and removal of the field-effects from χ_Fat enabled the use of fat tissues for the measurement and removal of the background magnetic field.

CONCLUSIONS

The magnetic susceptibility effects of fat can confound SvO₂ estimation, but the susceptibility field effects can estimated and removed with the use of modified Dixon fat-water separated imaging.