Venous blood gases: is it useful in COPD?

Introduction

Arterial blood gas evaluation is considered to be very important in treating patients seeking emergency service for acute exacerbation of chronic obstructive pulmonary disease (AECOPD). Procedural operations and these preliminary results will affect the treatment procedures. The values obtained from arterial blood gas analysis have significant parameters that shape common treatment procedures and end emergency service process. However, the procedural applications conducted to obtain arterial blood gas have complication risks such as local hematoma, infection, aneurysm, dissection, thrombosis in distal arterials or embolic ischemia. While this process is difficult to conduct, it also may require repeated operations. Furthermore, when the blood is obtained from radial artery in the wrist, it is a painful procedure for conscious patients who receive no sedoanalgesia implementation. Obtaining the sample from peripheral surface veins is known to be an alternative application since it is much easier and has fewer risks compared to arterial blood sampling. Venous blood sampling is also less painful for patients. In addition, acquiring venous blood will reduce some complication risks such as thrombosis, emboly or hematoma. While some studies conclude that there is a considerable correlation between arterial blood gas values and venous blood gas values of patients with AECOPD, other studies claim that the correlation between arterial blood gas values and venous blood gas values is not significant [1], [2], [3], [4].

The aim of the study is to find out whether values of venous blood gas sampling can be used instead of arterial blood gas values in the evaluation of blood gas for the emergency service patients with AECOPD.

Materials and methods

Ethics approval for the study was attained from Cukurova University Medicine Faculty Ethics Committee. The study had a prospective design. The patients consulting the emergency service between 01.01.2015 and 30.05.2015 with AECOPD diagnosis were included in the study. Individual approvals from the patients were also obtained. AECOPD, refers to the case when patients have increasing coughing, wheezing, purulent phlegm with increased volume, breathing difficulties and need antibiotics or corticosteroid for treatment [5]. The patients who did not approve participation; those who were transferred from other institutions; and the ones with other lung pathology diagnoses that cause breathing complications such as hemothorax, pneumothorax, or pulmonary embolism and cardiac disease such as acute myocardial infarction, acute heart failure were not included in the study.

The emergency service accepts 350,000 patients annually and has 900 inpatient bed availability. Adana city center has 35 m altitude above sea level.

Ninety patients diagnosed with COPD and identified to have AECOPD based on AECOPD criteria participated in the study. Following the acceptance of the patients to the emergency service and upon receiving their history and completing their physical examination, arterial blood gas sampling from radial artery and comorbid venous blood gas sampling from the established vascular access without tourniquet application were obtained. The samples were taken with heparinized injectors under anaerobic conditions. The analysis was conducted ABL 800 FLEX gas analyzer (radiometer, made in Denmark) made device in 2 min. The results for pH, partial oxygen pressure (pO₂), partial carbon dioxide pressure (pCO₂) and bicarbonate (HCO₃) were recorded for the study. Also, the patients’ age, gender, vital values, oxygen saturation, and their respiratory functions at the emergency service were recorded.

Findings

The study included 90 patients with 93.3% being males (n=84). The average age for the sample was 63.52±11.08 years (min=36, max=89). Patients’ vital signs and arterial and venous blood gas average values are given in Tables 1 and 2.

Eighty five point six percent of the participants (n=77) consulted the emergency service for difficulties in breathing while 14.4% complained for coughing (n=13).

The results suggest that there were significant correlations between venous blood gas pH, pO₂, pCO₂ and HCO₃ values and arterial pH, pO₂, pCO₂ and HCO₃ values of the patients. Especially, the correlation results for pH, pCO₂ and HCO₃ was observed to be high (p<0.0001). Bland-Altman analysis was performed to determine whether there is difference between methods or not (Table 3) (Figures 1–8).

Figure 1:

The correlation between arterial and venous pH values.

Figure 2:

Bland Altman plots of arterial and venous pH (average vs. difference).

Figure 3:

The correlation between arterial and venous HCO₃ values.

Figure 4:

Bland Altman plots of arterial HCO₃ (AHCO₃) and venous HCO₃ (VHCO₃) values (average vs. difference).

Figure 5:

The correlation between arterial and venous pO₂ values.

Figure 6:

Bland Altman plots of arterial and venous pO₂ (average vs. difference).

Figure 7:

The correlation between arterial and venous pCO₂ values.

Figure 8:

Bland Altman plots of Arterial pCO₂ (ApCO₂) and venous pCO₂ (VpCO₂) (average vs. difference).

In order to be able to obtain arterial blood gas values via venous blood gas values, logistic regression analysis was conducted (Table 4).

Discussion

Arterial blood gas analysis is a standard, routine clinical implementation for the patients suffering from chronic obstructive pulmonary disease (COPD) held in emergency services. However, arterial blood gas analysis is known to have both some technical difficulties and complication risks. Owing to those technical difficulties and complications, alternative methods are required to lessen the effects significantly.

The foremost aim in dealing with the patients suffering from AECOPD in emergency services is the recognition of hypoxia and hypercapnia. While pulse oximetry oxygen saturation non-invasively measured as transcutaneous can give clinicians very close information to the actual value, an invasive operation is definitely required to recognize hypercapnia [6]. In the literature, many studies conducted have signified that there is a positive correlation between arterial and venous blood gas values. Despite the studies mentioned, there exist some studies about using venous blood gas instead of arterial blood gas, which have put forward different results. For example, Gennis et al. state that they could not come to a clear decision about evaluating only venous blood gas since there is a great difference range between arterial and venous blood gas values although it shows a high correlation [7]. However, Gennis et al. suggested using venous blood gas although they obtained similar findings in another study which examined the patients suffering from diabetic ketoacidosis [8]. In another study conducted by Kelly et al. HCO₃ in arterial and venous blood gases of respiratory and metabolic diseases in emergency services has been examined and as a result it has been claimed that the venous HCO₃ value can be a suitable parameter to predict the arterial values [9]. Similar to the findings of Kelly et al’s study, a significant correlation between the venous and arterial HCO₃ values has also been detected in our study (r=0.8229% 95 CI 0.7423–0.8800, p<0.0001). In addition, it has been found out and mathematically formulated that the arterial HCO₃ value can be calculated by using the venous HCO₃ value depending on logistic regression analysis (Arterial HCO₃=8.5264+0.6647×venous HCO₃). Nevertheless, none of the aforementioned studies has been conducted on the patients, suffering from AECOPD that really need blood gas analysis. Our study has been carried on the patients, who need immediate blood gas analysis and therefore our participants are the patients who may encounter possible further complications.

In some studies about AECOPD patients, there has been some disputes on pH, HCO₃ and pCO₂. It has been found out that while there is a strong correlation between pH and HCO₃ values, a poor correlation has been detected between venous pCO₂ values and arterial values. For this reason, the clinical usage of venous pCO₂ has not been suggested [10], [11], [12]. Besides those studies, some research also exists, which has put forward a strong correlation between venous pCO₂ and arterial pCO_2.However, those studies do not present an analysis regarding correlation degree [10], [13], [14], [15]. Kelly et al’s study on 201 acute respiratory patients examining venous pCO₂ and pH hypercarbia monitorization has concluded that there is a strong correlation between arterial and venous pCO_2.Kelly et al. have also found out that the sensitivity becomes 100% when the cut-off value is determined as 45 mm-Hg for pCO₂ in venous blood gas. The results of the study have shown that the need to obtain arterial blood gas sample in their clinics has regressed to 41 % in the patients concerned [16]. However, no mathematical formulation has been put forward despite ROC analysis held in the study. In our study, a strong correlation has been found between arterial and venous pCO₂ by using Bland Altman Bias Plots method (r=0.8229% 95 CI 0.7423–0.8800, p<0.0001). By carrying out logistic regression analysis and using venous pCO₂, a mathematical formulation has been obtained to calculate pCO₂ value (Arterial pCO₂=21.9590+0.9208×venous pCO₂). Yet, a strong correlation has been obtained in the analysis regarding pH (r=0.7951% 95 CI 0.7039–0.8605, p<0.0001). In addition, a mathematical formulation has been reached to estimate arterial pH by using venous pH value depending on the analysis of logistic regression (Arterial pH=1.0312+0.8645×venous pH).

In the evaluation of respiratory functions, pCO₂, pH and other indicators of oxygenation are the basic measurements to determine the severity of the disease and the accuracy of the suitable treatment as well. Meta-analysis studies regarding pulse oxymeter measurements put forward that there is a strong correlation between arterial blood gas measurements and pulse oxymeter. Those studies point out that pulse oxymeter perform accurate predictions ranging between 2% (± a standard deviation) or 5% (±2 standard deviations) in 70–100% oxygen saturation range [17]. Ak et al. conducted a study focusing on 132 patients and although the researchers have determined formulations to calculate arterial blood gas values depending on venous blood gas values in AECOPD, they do not suggest estimating arterial pO₂ and oxygen saturation depending on venous pO₂ values in clinic contexts [3]. In our study, a strong correlation between arterial and venous pO₂ could not be obtained although a statistically significant relation exists between their values (r=0.2693% 95 CI 0.06586–0.4512, p=0.0103).

Limitations of the study

The scope of the study is limited to the number of the participants which may hinder to obtain reliable values. Besides, the state of participant patients’ comorbid diseases (diabetes mellitus, hypertension, coronary artery disease, cardiac dysfunction) can be thought to affect the blood gas parameters during the acute exacerbation period due to various biochemical and metabolic issues.

Conclusions

The results of the study suggest that some formulations can be used to estimate pH and pCO₂ values for the evaluation of emergency service seeking patients’ pulmonary with AECOPD. By the use of those formulations, arterial puncture treatments can be minimized in emergency services. However, the formulation developed to estimate pO₂ values in this study is not considered suitable to put into clinical use since there is a poor correlation between artery and vein values. Combining pulse oxymeter and venous pO₂ values can be beneficial in clinical use, however further research can contribute and put forward the benefits of this combination values.

Further studies having multi-center should be arranged and especially studies involving patients AECOPD, who do not suffer from other metabolic diseases, should be conducted. Thus, arterial interventions will reduce significantly and venous sampling will be utilized as an alternative method.