ABG analysis is a very important examination in acute and critical cases. In the past, PHS was used to collect ABG. However, its consistency with venous blood was often questioned by the clinicians [8]. Our study explored the accuracy and stability of indicators of venous blood and ABG collected by PHS and DABS in retrospective research and found that irrespective of the method used for collecting ABG blood sample, the consistency was high, especially ABG collected by DABS.
From the basic characteristics, there was almost no statistical difference in age, gender, type of admission, admission scores and venous blood results between PHS group and DABS group, suggesting that the population composition of the two groups was comparable though the collection duration was different. Because of the different ways of collecting arterial blood, the difference in arterial biochemical indicators between the two groups is what our study explored.
The qualitative analysis revealed that the correlation coefficient of the blood results between ABG and venous blood was markedly high, no matter which method, PHS or DABS, was used to collect ABG. It is worth mentioning that DABS had a significantly higher consistency in K+ and Na+, while PHS seemed to have a higher consistency in Cl-, Hb, and HCT. But after the sensitivity analysis, all five blood results had higher correlation coefficients in DABS after removing the extreme values.
The quantitative analysis, that is, from the difference of related indicators between venous blood and arterial blood, showed that all the results of arterial blood collected with DABS had less mean differences compared with the venous blood, especially in Hb, whose mean difference was too small to have a statistical difference, suggesting that DABS had a higher accuracy in collecting ABG. Regarding standard deviation, DABS had a smaller standard deviation in K+ and Na+. While PHS appeared to have a smaller standard deviation in Cl-, Hb, and HCT. But after the sensitivity analysis, all five blood results had a smaller standard deviation in DABS after removing the extreme values, suggesting that DABS had higher stability (Table 4).
The possible reasons for the difference in arteriovenous blood samples are as follows: Firstly, the use of heparin sodium saline dilutes the arterial blood samples in different degrees at each time of ABG collection. Therefore, the blood results of the arterial blood samples taken with PHS are generally lower than those of the venous blood samples. At the same time, the amount of heparin saline attached to the syringe tube wall is uncertain each time. This results in fluctuations with true value, which is consistent with the conclusion of this study. As a result, using DABS to collect ABG avoids such an error, so DABS is more accurate and stable. Secondly, there may be inherent differences in the biochemical indicators of arteriovenous blood [9], such as blood potassium. After capillary microcirculation, metabolites, such as lactic acid, entered the vein, making the PH of venous blood more acidic, affecting H+-K+ exchange, and resulting in higher potassium in venous blood than that in arterial blood. Further experiments are needed to analyze the arteriovenous blood on the same instrument at the same time. Thirdly, there could be accidental errors in the arteriovenous blood detection instrument, which might be the reason why our findings are some kind of different from other researches [2, 3] where sodium and potassium showed no significant difference between venous and arterial but Hb did. Fourthly, hemolysis may occur during blood collection [10]. Fifthly, potassium will release from platelets during coagulation [11]. Lastly, heparin sodium itself can bind positive ions. Heparin molecule contains sulfate groups and carboxyl groups, which is a strong acidic polyanion, and is capable of reacting with a cationic salt, including Ca2+, Na+, K+, etc. [12], thus reducing the corresponding cationic concentration.
It is strange that there are more extreme values in DABS group than in the PHS group. But it is not difficult to understand that the DABS group's data is more stable, so the quartile spacing is smaller, and the data falling three times out the quartile spacing naturally increases.
It is worth noting that when sensitivity analysis was conducted by removing the extreme values, Hb and HCT changed greatly, even reversing the previous conclusions. It attracted our attention on how extreme values affect Hb and HCT. Therefore, we further analyzed the extreme values of Hb and HCT. We found that the extreme values of Hb and HCT were highly consistent in both PHS and DABS groups (Figure 2a, 2b, 2d, 2e). For the extreme values, whether in PHS or DABS group, annihilation and gastrointestinal hemorrhage were the main common characteristics of these abnormal values. Chest pain, uremia, and others were also the common characteristics of the abnormal values (Figure 2c, 2f). Among them, the differences in Hb between venous blood and arterial in patients with gastrointestinal hemorrhage was obvious, which had never been reported before. We speculate that in the course of acute hemorrhage, the microcirculation of the body compensates for the replenishment of fluid in the blood vessels to dilute the venous blood, especially, the red blood cells could not pass through the blood vessel wall. As a result, Hb and HCT in the arteriovenous blood are quite different even though they are sampled at the same time. We call for a more in-depth study of the causes and mechanisms of extreme values in blood gas. It also suggests that when patients suffered from annihilation, gastrointestinal hemorrhage, chest pain, uremia, the indicators in ABG might have obviously deviated from the true value.
Limitation
In the study, values tested from venous blood were regarded as the golden standard, but there may be measurement errors in venous blood itself, which inevitably leads to bias. This is a real-world retrospective analysis, in which ABG and venous values are common emergency indicators, so it could not achieve absolute control like arterial-arterial comparison or venous-venous comparison. Some studies pointed out that venous blood gas could be used to replace ABG [13], but it could not judge the oxygen and carbon dioxide indicators. Although those studies have given the relevant formula [14, 15], the calculation is relatively complicated, and the results were not visualized. So, the accuracy and stability between ABG and venous blood indicators need to be further confirmed by prospective multi-center research.