Spot Check Analysis of Gas Exchange: Invasive versus Noninvasive Methods

Abstract

Background: Correct measurement of PO₂ and PCO₂ is essential to establish appropriate therapy such as long-term oxygen therapy (LTOT) in patients suffering from respiratory failure. Objectives: We aimed to compare common invasive and noninvasive methods for assessing blood gas components for spot check analysis. Methods: Arterial (P_aO₂, P_aCO₂) and capillary blood gas (P_CBGO₂, P_CBGCO₂) measurements were taken consecutively in a randomized order and were compared with noninvasive measurements obtained from the transcutaneous monitoring of PO₂ and PCO₂ (P_tcO₂, P_tcCO₂, sensor-temperature 44°C). Capillary samples were taken from both arterialized earlobes, where samples of right earlobes were defined as a reference value. Pain assessment of all measurements was evaluated by each subject using the 100-mm visual analogue scale. Results: 83 patients and 17 healthy subjects were included. The mean difference between P_aO₂ and P_tcO₂ was 11.9 ± 15.0 mm Hg, with lower limits of agreement (LLA) of -17.4 mm Hg (95% confidence interval (CI) -22.5 to -12.3 mm Hg), and upper limits of agreement (ULA) of 41.1 mm Hg (95% CI 36.0-46.2 mm Hg). The comparison of P_aO₂ with P_CBGO₂ showed a mean difference of 5.6 ± 7.2 mm Hg (LLA -11.0; ULA 19.6 mm Hg). The mean difference between P_aCO₂ and P_tcCO₂ was 1.1 ± 4.9 mm Hg (LLA -8.6; ULA 10.8 mm Hg) and that between P_aCO₂ and P_CBGCO₂ was 0.7 ± 2.0 mm Hg (LLA -3.3; ULA 4.8 mm Hg). The analysis of capillary blood gases (36.2 ± 22.3 mm) was rated as more painful than the analysis of arterial blood gases (26.1 ± 20.6 mm), while transcutaneous measurement was rated as the least painful method (1.9 ± 7.4 mm; all p < 0.0001). Conclusions: The comparison of different methods for blood gas measurements showed substantial differences between capillary and arterial PO₂ and between transcutaneous and arterial PO₂. Therefore, arterial PO₂ analysis is the essential method evaluating indication for LTOT. Nevertheless, comparative analysis further indicated capillary PCO₂ as an adequate surrogate for arterial PCO₂.

Capillary PO2 does not adequately reflect arterial PO2 in hypoxemic COPD patients

“…1,2,[5][6][7] Different trials have evaluated the agreement between ABG and CBG in terms of PO 2 measurements, with conflicting results; whereas one trial reported close agreement between ABG-versus CBG-derived values over a wide range of PO 2 values, 7 other trials showed wide limits of agreement (LOA). 8,9 A 2007 meta-analysis comparing ABGs to CBGs emphasized that capillary PO 2 (P c O 2 ) (earlobe sampling) may be an appropriate replacement for arterial PO 2 (P a O 2 ); 2 however, the authors also pointed out that CBG might not be appropriate if precision is required, based on the fact that the residual standard error (SE) in the regression equation was 6 mmHg. 2 This is of great importance, since, for example, long-term oxygen therapy (LTOT) is based on absolute values of PO 2 .…”

Efficacy and Safety of Using High-Flow Nasal Oxygenation in Patients Undergoing Rapid Sequence Intubation

“…Although transcutaneous oxygen (PtO 2 ) was monitored in our study (14), SpO 2 assessment has been found to be at least more favourable than PtO 2 as a non-invasive method to analyse blood oxygenation (14). Additionally, we can say that SpO 2 is more important than PaO 2 regarding the effect on oxygen delivery to tissues (14). Mir et al (12) did not report any systematic SpO 2 value during the procedure.…”

“…Measurements of pre-oxygenation have focused on EtO 2 of >90% because this is the most readily measurable, whereas a more accurate measure of the efficacy of pre-oxygenation is the arterial oxygen partial pressure. Although transcutaneous oxygen (PtO 2 ) was monitored in our study (14), SpO 2 assessment has been found to be at least more favourable than PtO 2 as a non-invasive method to analyse blood oxygenation (14). Additionally, we can say that SpO 2 is more important than PaO 2 regarding the effect on oxygen delivery to tissues (14).…”

“…Mir et al (12) did not report any systematic SpO 2 value during the procedure. We speculate that SpO 2 assessment shows benefits compared with the analysis of multiple arterial sampling (14), which would not be realistic in the context of RSI. (2) The study was not randomised as opposed to facemask oxygenation performed by Mir et al (12).…”

“…Heated humidification also facilitates secretion clearance, decreases bronchospasm and maintains mucosal integrity (8). Although HFNO has been used in several clinical fields, only few studies (12)(13)(14) have used HFNO as transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) at 70 L min −1 to prolong the apnoea time in anaesthetised and paralysed patients. Therefore, we designed an efficacy and safety study on the use of HFNO during RSI by monitoring the oxygenation status and incidence of unwanted events.…”