Background

The use of intraoperative pulse oximetry (SpO2) enhances hypoxia detection and is associated with fewer perioperative hypoxic events. However, SpO2 may be reported as 98% when arterial partial pressure of oxygen (PaO2) is as low as 70 mm Hg. Therefore, SpO2 may not provide advance warning of falling arterial oxygenation until PaO2 approaches this level. Multiwave pulse co-oximetry can provide a calculated oxygen reserve index (ORI) that may add to information from pulse oximetry when SpO2 is >98%. This study evaluates the ORI to PaO2 relationship during surgery.

Methods

We studied patients undergoing scheduled surgery in which arterial catheterization and intraoperative arterial blood gas analysis were planned. Data from multiple pulse co-oximetry sensors on each patient were continuously collected and stored on a research computer. Regression analysis was used to compare ORI with PaO2 obtained from each arterial blood gas measurement and changes in ORI with changes in PaO2 from sequential measurements. Linear mixed-effects regression models for repeated measures were then used to account for within-subject correlation across the repeatedly measured PaO2 and ORI and for the unequal time intervals of PaO2 determination over elapsed surgical time. Regression plots were inspected for ORI values corresponding to PaO2 of 100 and 150 mm Hg. ORI and PaO2 were compared using mixed-effects models with a subject-specific random intercept.

Results

ORI values and PaO2 measurements were obtained from intraoperative data collected from 106 patients. Regression analysis showed that the ORI to PaO2 relationship was stronger for PaO2 to 240 mm Hg (r = 0.536) than for PaO2 over 240 mm Hg (r = 0.0016). Measured PaO2 was ≥100 mm Hg for all ORI over 0.24. Measured PaO2 was ≥150 mm Hg in 96.6% of samples when ORI was over 0.55. A random intercept variance component linear mixed-effects model for repeated measures indicated that PaO2 was significantly related to ORI (β[95% confidence interval] = 0.002 [0.0019-0.0022]; P < 0.0001). A similar analysis indicated a significant relationship between change in PaO2 and change in ORI (β [95% confidence interval] = 0.0044 [0.0040-0.0048]; P < 0.0001).

Conclusions

These findings suggest that ORI >0.24 can distinguish PaO2 ≥100 mm Hg when SpO2 is over 98%. Similarly, ORI > 0.55 appears to be a threshold to distinguish PaO2 ≥150 mm Hg. The usefulness of these values should be evaluated prospectively. Decreases in ORI to near 0.24 may provide advance indication of falling PaO2 approaching 100 mm Hg when SpO2 is >98%. The clinical utility of interventions based on continuous ORI monitoring should be studied prospectively.

Background

Red blood cell (RBC) transfusion is common during infant cardiac surgery. A previous report of pediatric heart transplant recipients showed that increased RBC transfusion volume was independently associated with increased length of intensive care unit stay. It is unclear whether transfusion to infants as a subgroup carries similar risks. This study investigated relationships between intraoperative RBC transfusion during heart transplantation and postoperative length of stay (LOS), morbidity, and mortality in infants.

Methods

Retrospective analysis of medical records from infants <1 year old undergoing primary heart transplantation at Loma Linda University Medical Center from 1985 to 2012 was conducted. Exclusion criteria included preoperative exchange transfusion or extracorporeal membrane oxygenation. Data sought included patient characteristics; intraoperative RBC transfusion volume and cardiopulmonary bypass details; and postoperative vasoactive support, ventilator support, morbidity, LOS, and 30-day mortality. The relationship of RBC transfusion volume (mL/kg) to these postoperative variables was assessed by univariate analysis. Multiple regression analysis of postoperative LOS included variables that were independent predictors of LOS or associated with ≥10% change in the β-estimate for RBC effect.

Results

Data from 307 infants showed that most (66.8%) had single-ventricle physiology. Median age at transplant was 50 days, weight 3.95 kg, and intraoperative transfusion volume 109 mL/kg. Transfusion volume was inversely related to age and weight. Median postoperative LOS was 18.2 days. Univariate linear regression analysis of transfused volume showed no relationship to log-transformed postoperative LOS (F(1,305) = 0.00; P = 0.960; R = 0.000; β-coefficient = 0.004; 95% confidence interval = -0.1542 to 0.1623). Transfused volume was not related to 30-day mortality (difference -0.162; -0.048 to 0.371 mL/kg; P = 0.112) or to postoperative ventilator support (R = 0.047), but was greater in patients who required reoperation (difference -0.246; -0.494 to -0.025; P = 0.004). Multiple regression analysis for all patients revealed age, preoperative ventilator support, prolonged postoperative ventilatory or vasoactive support, transplant year, and 30-day mortality, but not major adverse events, to be significant confounding variables. Adjusting for these variables, transfused volume was not associated with prolonged postoperative LOS.

Conclusions

In contrast to a prior report, we found no correlation between intraoperative RBC transfusion and postoperative LOS when studying only infants. Infants have maturing organ systems, less physiologic reserve, and increased surgical blood loss (evaluated as mL/kg) during cardiac surgery than their larger, older counterparts, distinguishing them from the general pediatric population. These differences require additional studies to determine the outcome impact of transfusion strategies in the infant subgroup.