Real-time continuous estimation of gas exchange by dual oximetry

Correlation and validity of imputed PaO2/FiO2 and SpO2/FiO2 in patients with invasive mechanical ventilation at 2600m above sea level

OBJECTIVE

To establish the correlation and validity between PaO2/FiO2 obtained on arterial gases versus noninvasive methods (linear, nonlinear, logarithmic imputation of PaO2/FiO2 and SpO2/FiO2) in patients under mechanical ventilation living at high altitude.

DESIGN

Ambispective descriptive multicenter cohort study.

SETTING

Two intensive care units (ICU) from Colombia at 2600m a.s.l.

PATIENTS OR PARTICIPANTS

Consecutive critically ill patients older than 18 years with at least 24h of mechanical ventilation were included from June 2016 to June 2019.

INTERVENTIONS

None.

VARIABLES

Variables analyzed were demographic, physiological messures, laboratory findings, oxygenation index and clinical condition. Nonlinear, linear and logarithmic imputation formulas were used to calculate PaO2 from SpO2, and at the same time the SpO2/FiO2 by severe hypoxemia diagnosis. The intraclass correlation coefficient, area under the ROC curve, sensitivity, specificity, positive predictive value, negative predictive value, positive and negative likelihood ratio were calculated.

RESULTS

The correlation between PaO2/FiO2 obtained from arterial gases, PaO2/FiO2 derived from one of the proposed methods (linear, non-linear, and logarithmic formula), and SpO2/FiO2 measured by the intraclass correlation coefficient was high (greater than 0.77, p<0.001). The different imputation methods and SpO2/FiO2 have a similar diagnostic performance in patients with severe hypoxemia (PaO2/FiO2 <150). PaO2/FiO2 linear imputation AUC ROC 0,84 (IC 0.81-0.87, p<0.001), PaO2/FiO2 logarithmic imputation AUC ROC 0.84 (IC 0.80-0.87, p<0.001), PaO2/FiO2 non-linear imputation AUC ROC 0.82 (IC 0.79-0.85, p<0.001), SpO2/FiO2 oximetry AUC ROC 0.84 (IC 0.81-0.87, p<0.001).

CONCLUSIONS

At high altitude, the SaO2/FiO2 ratio and the imputed PaO2/FiO2 ratio have similar diagnostic performance in patients with severe hypoxemia ventilated by various pathological conditions.

Correlation and validity of imputed PaO2/FiO2 and SpO2/FiO2 in patients with invasive mechanical ventilation at 2600m above sea level

OBJECTIVE

To establish the correlation and validity between PaO2/FiO2 obtained on arterial gases versus noninvasive methods (linear, nonlinear, logarithmic imputation of PaO2/FiO2 and SpO2/FiO2) in patients under mechanical ventilation living at high altitude.

DESIGN

Ambispective descriptive multicenter cohort study.

SETTING

Two intensive care units (ICU) from Colombia at 2600m a.s.l.

PATIENTS OR PARTICIPANTS

Consecutive critically ill patients older than 18 years with at least 24h of mechanical ventilation were included from June 2016 to June 2019.

INTERVENTIONS

None.

VARIABLES

Variables analyzed were demographic, physiological messures, laboratory findings, oxygenation index and clinical condition. Nonlinear, linear and logarithmic imputation formulas were used to calculate PaO2 from SpO2, and at the same time the SpO2/FiO2 by severe hypoxemia diagnosis. The intraclass correlation coefficient, area under the ROC curve, sensitivity, specificity, positive predictive value, negative predictive value, positive and negative likelihood ratio were calculated.

RESULTS

The correlation between PaO2/FiO2 obtained from arterial gases, PaO2/FiO2 derived from one of the proposed methods (linear, non-linear, and logarithmic formula), and SpO2/FiO2 measured by the intraclass correlation coefficient was high (greater than 0.77, p<0.001). The different imputation methods and SpO2/FiO2 have a similar diagnostic performance in patients with severe hypoxemia (PaO2/FiO2 <150). PaO2/FiO2 linear imputation AUC ROC 0,84 (IC 0.81-0.87, p<0.001), PaO2/FiO2 logarithmic imputation AUC ROC 0.84 (IC 0.80-0.87, p<0.001), PaO2/FiO2 non-linear imputation AUC ROC 0.82 (IC 0.79-0.85, p<0.001), SpO2/FiO2 oximetry AUC ROC 0.84 (IC 0.81-0.87, p<0.001).

CONCLUSIONS

At high altitude, the SaO2/FiO2 ratio and the imputed PaO2/FiO2 ratio have similar diagnostic performance in patients with severe hypoxemia ventilated by various pathological conditions.

Changes in breath sound power spectra during experimental oleic acid-induced lung injury in pigs

To evaluate the effect of acute lung injury on the frequency spectra of breath sounds, we made serial acoustic recordings from nondependent, midlung and dependent regions of both lungs in ten 35- to 45-kg anesthetized, intubated, and mechanically ventilated pigs during development of acute lung injury induced with intravenous oleic acid in prone or supine position. Oleic acid injections rapidly produced severe derangements in the gas exchange and mechanical properties of the lung, with an average increase in venous admixture from 16 ± 12 to 62 ± 16% (P < 0.01), and a reduction in dynamic respiratory system compliance from 25 ± 4 to 14 ± 4 ml/cmH2O (P < 0.01). A concomitant increase in sound power was seen in all lung regions (P < 0.05), predominantly in frequencies 150-800 Hz. The deterioration in gas exchange and lung mechanics correlated best with concurrent spectral changes in the nondependent lung regions. Acute lung injury increases the power of breath sounds likely secondary to redistribution of ventilation from collapsed to aerated parts of the lung and improved sound transmission in dependent, consolidated areas.

Diagnostic measures to evaluate oxygenation in critically ill adults: implications and limitations

Accurate assessment and treatment of disturbances in oxygenation are crucial to optimal outcomes in critically ill patients. Oxygenation is dependent upon adequate pulmonary gas exchange, oxygen delivery, and oxygen consumption. Each of these physiologic processes may vary independently in response to pathophysiologic conditions and therapeutic interventions. The author reviews diagnostic measures available to evaluate pulmonary gas exchange, oxygen delivery, and oxygen consumption in critically ill patients. Currently available tools and their potential value as well as key methodological limitations are addressed. Failure on behalf of clinicians to fully appreciate these limitations can lead to misdiagnoses and inappropriate treatment. The aim of this article is to help advanced practice nurses more fully understand the implications and limitations of these diagnostic measures to ensure accurate assessment and treatment of disturbances in oxygenation.

[Continuous monitoring of mixed venous blood oxygen saturation]

Mixed venous oxygen saturation (SvO2), measured on pulmonary artery blood, is a convenient indicator of matching between O2 transport (TaO2) and O2 body consumption (VO2). The measurement technique is based on the haemoglobin reflection spectrophotometry principle using two or three wave lengths. The Fick principle points out that SvO2 depends on five parameters: SvO2 = SaO2 - (VO2/CI x Hb x PO) where SaO2, CI and PO respectively represent arterial O2 saturation, cardiac index and O2 affinity. SvO2 does not always reflect tissue O2 tension: when considering a given value of SvO2, PvO2 will depend upon the position of the oxyhaemoglobin dissociation curve. It is impossible to establish in the absolute a "normal" value of SvO2. However, in most clinical circumstances, an SvO2 ranging from 60 to 80% attests that O2 tissue delivery is appropriate. Under certain conditions a continuous monitoring of SvO2 allows to assess another index such as ventilation-perfusion index or the O2 tissue extraction index. Usually SvO2 variations are more informative than the absolute SvO2 value. However, their interpretation should be cautious. First and foremost, the ability of each of the four main SvO2 determinants to influence the SvO2 is unequal as the numerical ranges of variation of these determinants are very different. Moreover, the attribution of a variation of SvO2 to one of its determinants implies that each of them is independent from the others, a feature which is very rarely seen in clinical practice. Finally as the mathematical relationship between SvO2 and its determinants is linear (SaO2 and VO2), or hyperbolic (CI and Hb), the weight of SaO2 or VO2 is independent of their absolute value, whereas CI or Hb weights will depend on their value. The limits of SvO2 monitoring are linked first to the occurrence of an anaerobic metabolism state when TaO2 becomes too low; SvO2 then just provides informations on the aerobic part of the metabolism. Moreover, SvO2 is just a global indicator for tissue O2 oxygenation status which does not give any indication about regional flow distribution. Therefore, SvO2 enables systemic imbalance supervision only. Finally, the existence of a right-to-left shunt will modify the SvO2 values through various mechanisms. However the SvO2 measured, in the pulmonary artery, remains reliable, whereas the presence of a left-to-right shunt will highly alter SvO2 basal value, only its time course remaining significant. SvO2 monitoring, element of diagnosis and monitoring, as well as a warning signal, has a priori specific indications poorly assessed, so far. (ABSTRACT TRUNCATED AT 400 WORDS)

Pulse oximetry

The pulse oximeter, a widely used noninvasive monitor of arterial oxygen saturation, has numerous applications in anesthesiology and critical care. Although pulse oximetry is considered sufficiently accurate for many clinical purposes, there are significant limitations on the accuracy and availability of pulse oximetry data. This article reviews both the clinical uses of the pulse oximeter and the limitations on its performance. The pulse oximeter is generally acknowledged to be one of the most important advances in the history of clinical monitoring.

Continuous monitoring of gas exchange and oxygen use with dual oximetry

The utility of integrated pulse and pulmonary artery oximetry, known as dual oximetry, was evaluated by monitoring 10 critically ill surgical patients for a total of 208 patient hours. The ventilation-perfusion index (VQI), an estimate of venous admixture, and the oxygen extraction index (O2EI), an estimate of tissue oxygen utilization coefficient, previously described, were calculated on-line from arterial and mixed venous oxyhemoglobin saturations using a computer. Effective monitoring was accomplished 85% of the total time. The dual oximetry device was nonfunctional owing to equipment failure only 15% of the time, even though no undue attention was given to instructing the staff on operation of the oximeters. Accuracy of VQI and O2EI was reconfirmed by this study. Drift in the saturations, VQI, and O2EI during the 6-h period between calibrations was negligible. The 95% range of random variability was +/- 2% for SaO2, +/- 3% for SvO2, +/- 5% for VQI, and +/- 0.04 for O2EI. Thirty-six episodes of arterial blood desaturation below 90% were detected by continuous oximetry. In contrast, 74 routine arterial blood samples revealed only four such episodes. Dual oximetry appears to be a technically reliable and accurate method of monitoring pulmonary gas exchange and tissue oxygen utilization. The equipment provided stable readings for at least six hours without recalibration. Random variability is sufficiently small to allow early detection of alterations in pulmonary and circulatory function without blood sampling.