Systematic instrumental errors between oxygen saturation analysers in fetal blood during deep hypoxemia

Basing intubation of acutely hypoxemic patients on physiologic principles

The decision to intubate a patient with acute hypoxemic respiratory failure who is not in apparent respiratory distress is one of the most difficult clinical decisions faced by intensivists. A conservative approach exposes patients to the dangers of hypoxemia, while a liberal approach exposes them to the dangers of inserting an endotracheal tube and invasive mechanical ventilation. To assist intensivists in this decision, investigators have used various thresholds of peripheral or arterial oxygen saturation, partial pressure of oxygen, partial pressure of oxygen-to-fraction of inspired oxygen ratio, and arterial oxygen content. In this review we will discuss how each of these oxygenation indices provides inaccurate information about the volume of oxygen transported in the arterial blood (convective oxygen delivery) or the pressure gradient driving oxygen from the capillaries to the cells (diffusive oxygen delivery). The decision to intubate hypoxemic patients is further complicated by our nescience of the critical point below which global and cerebral oxygen supply become delivery-dependent in the individual patient. Accordingly, intubation requires a nuanced understanding of oxygenation indexes. In this review, we will also discuss our approach to intubation based on clinical observations and physiologic principles. Specifically, we consider intubation when hypoxemic patients, who are neither in apparent respiratory distress nor in shock, become cognitively impaired suggesting emergent cerebral hypoxia. When deciding to intubate, we also consider additional factors including estimates of cardiac function, peripheral perfusion, arterial oxygen content and its determinants. It is not possible, however, to pick an oxygenation breakpoint below which the benefits of mechanical ventilation decidedly outweigh its hazards. It is futile to imagine that decision making about instituting mechanical ventilation in an individual patient can be condensed into an algorithm with absolute numbers at each nodal point. In sum, an algorithm cannot replace the presence of a physician well skilled in the art of clinical evaluation who has a deep understanding of pathophysiologic principles.

Validation of an alternative technique for RQ estimation in anesthetized pigs

BACKGROUND

Respiratory quotient (RQ) is an important variable when assessing metabolic status in intensive care patients. However, analysis of RQ requires cumbersome technical equipment. The aim of the current study was to examine a simplified blood gas-based method of RQ assessment, using Douglas bag measurement of RQ (Douglas-RQ) as reference in a laboratory porcine model under metabolic steady state. In addition, we aimed at establishing reference values for RQ in the same population, thereby generating data to facilitate further research.

METHODS

RQ was measured in 11 mechanically ventilated pigs under metabolic steady state using Douglas-RQ and CO-oximetry blood gas analysis of pulmonary artery and systemic carbon dioxide and oxygen content. The CO-oximetry data were used to calculate RQ (blood gas RQ). Paired recordings with both methods were made once in the morning and once in the afternoon and values obtained were analyzed for potential significant differences.

RESULTS

The average Douglas-RQ, for all data points over the whole day, was 0.97 (95%CI 0.95-0.99). The corresponding blood gas RQ was 0.95 (95%CI 0.87-1.02). There was no statistically significant difference in RQ values obtained using Douglas-RQ or blood gas RQ for all data over the whole day (P = 0.43). Bias was - 0.02 (95% limits of agreement ± 0.3). Douglas-RQ decreased during the day 1.00 (95%CI 0.97-1.03) vs 0.95 (95%CI 0.92-0.98) P < 0.001, whereas the decrease was not significant for blood gas RQ 1.02 (95%CI 0.89-1.16 vs 0.87 (0.80-0.94) P = 0.11.

CONCLUSION

RQ values obtained with blood gas analysis did not differ statistically, compared to gold standard Douglas bag RQ measurement, showing low bias but relatively large limits of agreement, when analyzed for the whole day. This indicates that a simplified blood gas-based method for RQ estimations may be used as an alternative to gold standard expired gas analysis on a group level, even if individual values may differ. In addition, RQ estimated with Douglas bag analysis of exhaled air, was 0.97 in anesthetized non-fasted pigs and decreased during prolonged anesthesia.

Non-invasive capnodynamic mixed venous oxygen saturation during major changes in oxygen delivery

Mixed venous oxygen saturation (SvO₂) is an important variable in anesthesia and intensive care but currently requires pulmonary artery catheterization. Recently, non-invasive determination of SvO₂ (Capno-SvO₂) using capnodynamics has shown good agreement against CO-oximetry in an animal model of modest hemodynamic changes. The purpose of the current study was to validate Capno-SvO₂ against CO-oximetry during major alterations in oxygen delivery. Furthermore, evaluating fiberoptic SvO₂ for its response to the same challenges. Eleven mechanically ventilated pigs were exposed to oxygen delivery changes: increased inhaled oxygen concentration, hemorrhage, crystalloid and blood transfusion, preload reduction and dobutamine infusion. Capno-SvO₂ and fiberoptic SvO₂ recordings were made in parallel with CO-oximetry. Respiratory quotient, needed for capnodynamic SvO₂, was measured by analysis of mixed expired gases. Agreement of absolute values between CO-oximetry and Capno-SvO₂ and fiberoptic SvO₂ respectively, was assessed using Bland–Altman plots. Ability of Capno- SvO₂ and fiberoptic SvO₂ to detect change compared to CO-oximetry was assessed using concordance analysis. The interventions caused significant hemodynamic variations. Bias between Capno-SvO₂ and CO-oximetry was + 3% points (95% limits of agreements – 7 to + 13). Bias between fiberoptic SvO₂ and CO-oximetry was + 1% point, (95% limits of agreements − 7 to + 9). Concordance rate for Capno-SvO₂ and fiberoptic SvO₂ vs. CO-oximetry was 98% and 93%, respectively. Capno-SvO₂ generates absolute values close to CO-oximetry. The performance of Capno-SvO₂ vs. CO-oximetry was comparable to the performance of fiberoptic SvO₂ vs. CO-oximetry. Capno-SvO₂ appears to be a promising tool for non-invasive SvO₂ monitoring.

Low pO2 Contributes to Potential Error in Oxygen Saturation Calculations Using a Point-of-Care Assay

OBJECTIVES

The present study addressed the accuracy of calculated oxygen saturation (sO2) using point-of-care (POC) testing compared with measured values on a blood gas analyzer.

METHODS

In total, 3,323 sO2 values were measured in 1,180 patients using a CO-oximeter (ABL 800 Flex; Radiometer, Copenhagen, Denmark). Measured parameters were then used to calculate an expected sO2 for the POC method (Abbott i-STAT; Abbott POC, Princeton, NJ). Cases in which calculated sO2 differed from measured sO2 by 10% or more were analyzed.

RESULTS

Of the 3,323 comparisons performed, 260 (8%) showed discrepancies (± ≥10%) between measured and calculated sO2 values. Ninety-four of discrepant measurements (245 of 260) occurred when pO2 was less than 50 mm Hg. pH and bicarbonate distributions shifted to lower values in discrepant vs nondiscrepant cases.

CONCLUSIONS

Our results suggest that the likelihood of discrepant sO2 is 27% among patients with pO2 less than 50 mm Hg. Direct measurement of sO2 by CO-oximetry is strongly suggested in this clinical scenario.

Operational evaluation of pulse oximetry in NICU patients with arterial access

OBJECTIVE

To investigate pulse oximetry in neonates who require arterial access as represented by the clinical data recorded to manage their care.

STUDY DESIGN

Analysis of simultaneous SpO(2) and SaO(2) from: 7-year historical NICU data (N=31905); 4-month prospective NICU data (N=566); verification data using two hemoximeters (N=52); and NICU data from two collaborating centers (N=95 and 168). The bias function (SpO(2)-SaO(2)) was regressed against the measured "gold" standard, SaO(2).

RESULTS

A significant negative correlation was found for each of the data sets between the bias function and SaO(2). This bias was similar for devices from several manufacturers (Datex-Ohmeda, Masimo, Nellcor, and Spacelabs). Maximum operational performance occurred with peaks between 92 and 97% SaO(2), but declined markedly above and below this narrow range. In all, 71 to 95% of patients exhibited data with significant bias(.)

CONCLUSION

These operational data suggest that with the methodology and devices currently in use, SpO(2) values in most all neonates who require arterial lines inaccurately correlate with measured arterial saturation.

Accuracy of fetal pulse oximetry

BACKGROUND

The goal of this investigation was to evaluate the agreement of fetal pulse oximetry to saturation readings from hemoximetry at low oxygen saturation.

METHODS

Fetal oxygen saturation measurements obtained by pulse oximetry were compared with those obtained by hemoximetry in fetal scalp blood samplings. The prospective observational trial included fetuses with non-reassuring fetal heart rate tracings suggestive of hypoxia and requiring fetal scalp blood samplings. Arterial oxygen saturation was determined by a blinded pulse oximeter (N400, FS14; Nellcor Puritan Bennett, Pleasanton, CA, USA) and continuously stored on a notebook computer. Saturation from fetal scalp blood samples was measured by hemoximetry (Bayer Diagnostics 865; ABL 625, Radiometer). Data analysis focussed on the absolute and relative difference between hemoximetry and pulse oximetry of fetuses, showing the most distinct difference in neonatal outcome. Normal outcome was defined as spontaneous delivery and umbilical artery pH >or= 7.20 + Apgar 5 >or= 7 (n = 42). In contrast, a group of neonates with combined respiratory and metabolic acidemia at birth was defined by pH <or= 7.16 and an additional base excess <or= -9.4 mmol/l in the umbilical artery (n = 18).

RESULTS

The correlation coefficient between hemoximetry and pulse oximetry measurements was r = 0.72; p = 0.002 in the acidemic group. The median of absolute differences in saturation was + 5.2% (95%CI 2.5-10.3) saturation. The absolute differences ranged from -21% to + 36% saturation. The median of relative differences amounted to 23% (95%CI 15.1-55.0). The relative differences ranged from -30% to + 217%. Observing the saturation distribution in the two groups (hemoximetry and pulse oximetry, respectively) presented a median hemoximetry pulse oximetry of 38% (42%) in the normal and 26% (39%) in the acidemic group. In this small group of cases, the correlation coefficient between pH and saturation from pulse oximetry in fetal scalp blood from the samplings was r = 0.19.

CONCLUSION

Fetal pulse oximetry tends to overestimate arterial oxygen saturation compared with hemoximeter values.