Topic role of venous blood gases in patients of acute breathlessness presenting in emergency department

 

 

Abstract               In emergency departments, blood gas analysis is used for two main purposes: establishing acid-base state (mainly pH but also bicarbonate) and assessing respiratory function (mainly pCO2 but also pH and to a lesser extent pO2). The discomfort and possible risks for the patient during this procedure should not be overlooked. Research states the difference in pain experience between arterial and venous punctures and found arterial puncture to be significantly more painful than venous puncture. This test can result in patient discomfort as well as complications such as arterial injury, thrombosis or embolization, hematoma, aneurysm formation, and reflex sympathetic dystrophy Venous blood sampling is usually easier, less painful and more convenient. The aim is to determine the agreement for pH, pCO2, bicarbonate and base excess between arterial and venous blood samples and to identify unanswered questions in agreement between arterial and venous blood gas parameters. The main objective of this study is to examine the agreement between ABG and VBG samples for all commonly used parameters (pH, PCO2, and bicarbonate) in a acute breathless patients, specifically analyzing multiple paired arterial and venous samples from each patient.

Key Word: venous blood gases.

 

INTRODUCTION

Historically, values obtained via VBG have been criticized for a perceived lack of accuracy in all domains, however the fact is that increasing data shows that for most clinical indications, data from VBG correlates well, and are just as useful as that from ABG. While arterial punctures are not only painful but also cause thrombus, emboli, hematoma and predispose aneurysm formation, VBGs carry less risk of vascular injury, nerve damage, and cause much less pain to the patient along with lower risk for accidental needle-sticks as compared to ABGs.

 Several studies have looked at the correlation between values obtained with VBG and compared them to ABGs

• Zeserson et al conducted a prospective cohort study of 156 critically ill patients in the ED and ICU setting to evaluate the correlation between pH and pCO2 when derived from ABG vs VBG with added pulse oximetry for estimating PaO2 and concluded that arterial and venous pH and PCO2 had good correlation.

(Zeserson E, Goodgame B, Hess JD, Schultz K, Hoon C, Lamb K, Maheshwari V,Johnson S, Papas M, Reed J, Breyer M. Correlation of Venous Blood Gas and Pulse Oximetry With Arterial Blood Gas in the Undifferentiated Critically Ill Patient. J Intensive Care Med. 2016 Jun 9).

• Byrne et al conducted a meta – analysis of 1768 subjects from 18 individual studies and found that peripheral VBG correlates well with ABG with respect to pH but found an unacceptably wide 95% prediction interval when looking at the pCO2.

(Byrne, A. L., Bennett, M., Chatterji, R., Symons, R., Pace, N. L. and Thomas, P. S. (2014), Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology, 19: 168–175.

• A review article by Kelly AM summarized data comparing ABG and peripheral VBG variables in ED all-comers also concluded that venous pH had sufficient agreement however concluded with a word of caution: there is no data to support that this correlation is maintained in shock states.

(Kelly AM, McAlpine R, Kyle E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. Emerg Med J. 2001)

For most parameters, there is good correlation. However, there are a few important scenarios that may be exceptions. Not surprisingly, the major exception is PO2; venous PO2 readings do not correlate well with arterial PO2. A workaround to this limitation is to estimate arterial oxygenation using SpO2. The VBG analysis plus SpO2 provided accurate information on acid–base, ventilation, and oxygenation status for patients in undifferentiated patients ED and ICU

We decided to compare venous blood gas assessments for variables such as pH, pCO2, bicarbonate and base excess between arterial and venous blood samples.

 

MATERIAL AND METHODS

This Prospective study was conducted at Dr. D.Y.Patil Medical College, Hospital and Research Centre, Nerul, Navi Mumbai by Department OfEmergency Medicine

Study design: Cross sectional studies (with exclusion)

Study duration: 2 years

Eligibility Criteria:

Inclusion Criteria – Age above 12 years of either sex Male/Female.

Exclusion Criteria – Age below 12 years of either sex Male/Female.

Sample size: 50

OBSERVATIONS AND RESULTS

Table 1:

Its observed that the mean respiratory rate of the study conducted came out to be 31.1 breaths per minute.

Table 2:

The mean SpO2 of the patients observed to be 89.4 at room air.

 

Table 3: Comparison of pH

The mean pH in arterial blood came out to be 7.39. The mean pH of venous sample observed to be 7.33 The difference between the pH of both the samples is 0.06 with p value of .021.

 

Table 4: Comparison of PCo2

The mean pCO2 values of arterial blood observed to be 32.05 The mean pCO2 value of the venous blood samples was observed to be 39.52 The difference between the mean pCO2 values of both arterial and venous samples observed to be -7.47.

Table 5: Comparison of HCo3

The mean arterial HCO3 level in the sample came out to be 17.79 The mean venous HCO3 value in the sample came out to be 20.05 The difference between the meanof arterial and venous HCO3 level is -2.25

 

Table 6: Comparison of PO2

The mean PO2 level of arterial blood came out to be 91.74. The mean PO2 level of venous blood came out to be 64.45. The difference between the mean values of the PO2 level of both arterial and venous sample came out to be 26.2.

DISCUSSION

In evaluating the agreement between arterial and venous blood gas values in the emergency department we conducted the study which took place in an emergency department setting, and comparison between arterial and venous values were made if the treating physician deemed it necessary that an arterial blood sample was obtained.

PH: The study conducted at our emergency department has shown an agreement between the ABG and VBG pH values, the values has been grouped in the table below. The values has been reviewed and compared with the other studies conducted elsewhere. We listed the studies we reviewed in table below, representing the number of subjects each study researched, the blood gas values for which a comparison has been made and the clinical presentation of the researched patients.We grouped the findings for each included study in table below , which represents the values for mean arterial pH, mean venous pH , mean difference between arterial and venous pH and degree of correlation (r value).

Pooled data analysis (n=635) showed us a mean difference between venous and arterial pH of 0.043 pH units and in the three studies reporting 95% limits of agreement the ranges went from -0.11 to 0.13 pH unit

HCO3: The study conducted at our emergency department has shown an agreement between the ABG and VBG HCO3 values, the values has been grouped in the table below. The values has been reviewed and compared with the other studies conducted elsewhereIn reviewing the value of venous HCO3 in the emergency department setting we selected four studies which were compatible with our inclusion criteria and addressed our research question.

 

Pooled data analysis reveals a mean HCO3 difference of 1.51 mmol/l and three studies reporting agreement between arterial and venous pH report 95% limits of agreement ranging from -5.13 to 4.3 mmol/l hypercarbia

pCO2:The study conducted at our emergency department has shown an agreement between the ABG and VBGpCO2 values , the values has been grouped in the table below. The values has been reviewed and compared with the other studies conducted elsewhere in comparing venous to arterial pCO2. We selected 4 studies which applied to our research criteria. All the researched values will be expressed in mmHg unless stated otherwise.We summarized the reviewed results in table belowand calculated the pooled mean differences between arterial and venous pCO2

 

 

Pooled data analysis reveals a mean HCO3 difference of 1.51 mmol/l and three studies reporting agreement between arterial and venous pH report 95% limits of agreement ranging from -5.13 to 4.3 mmol/l hypercarbia

pCO2:The study conducted at our emergency department has shown an agreement between the ABG and VBGpCO2 values , the values has been grouped in the table below. The values has been reviewed and compared with the other studies conducted elsewhere in comparing venous to arterial pCO2. We selected 4 studies which applied to our research criteria. All the researched values will be expressed in mmHg unless stated otherwise.We summarized the reviewed results in table belowand calculated the pooled mean differences between arterial and venous pCO2

Pooled data analysis shows a pooled mean difference between venous and arterial pCO2 of 6.16 mmHg and three studies reported 95% limits of agreement ranging from -17.4 to 23.9 mmHg.

 

Arterial versus venous blood gas analysis: Arterial puncture is commonly used in the emergency department to evaluate the acid-base and respiratory status of the patient. The discomfort and possible risks for the patient during this procedure should not be overlooked. A systematic study performed on 271 patients by Giner et al. researched the difference in pain experience between arterial and venous punctures and found arterial puncture to be significantly more painful than venous puncture. In their research, venous blood gas was also retrieved more successful on first attempt. In another study on patient experience in the intensive care unit (ICU) arterial puncture was reported as the most frequent unpleasant experience. We found several studies reporting on the possible complications of arterial puncture. One case report mentioned the development of a large false aneurysm of the radial artery leading to compartment syndrome of the forearm due to repeated arterial punctures on the same site. In another report severe edema and ischemia occurred six hours after arterial puncture and was only corrected after surgical intervention. We wanted to find out the relative occurrence of these complications and found one study reporting on this topic. Mortensen et al. performed a study on 2360 patients of whom complete hospital records were available up to 24 hours after arterial puncture. They found a total complication rate of 13% of which 2.6% were major complications, which was defined as arterial puncture ending in long-term functional impairment, additional hospitalization or fatal complications. They stressed that most of the minor complications due to arterial puncture go unnoticed due to inadequate follow-up observations and therefore the incidence in clinical practice seems lower than it actually is. In their study they also researched predisposing factors in developing complications and found among other things that patients older than 65 years had an increased risk of complications. Other risk factors were hypertension, arteriosclerosis, the use of anticoagulants and aortic insufficiency. Some of these risk factors often occur with certain presentations in the emergency setting, and in our specific subset the link between COPD and high age resulting in an increased risk of complications from arterial puncture should be stressed. This is also the case for the combination of diabetes and arteriosclerosis. Arterial blood gas analysis is a well established an useful technique but should be used only when it is strictly necessary. Furthermore, venous blood gas analysis can, as we researched, often be a good alternative. Another advantage of venous blood gas samples is the ease and availability. Most patients in need of an arterial blood gas analysis also receive an intravenous line or additional venous blood tests which would enhance the ease of incorporating a venous blood gas sample to be taken during this procedure, relieving the burden on the patient as well as being more cost-effective.

CONCLUSION

The data we reviewed revealed us that venous blood gas analysis has the potential to be of great importance in the emergency department in general as well as for patients presenting with diabetic ketoacidosis or acute exacerbation of COPD. The blood gas values for pH and HCO3 showed excellent agreement and correlation and can be considered clinically interchangeable with arterial values. On venous pCO2 we found differing results and therefore suggest the possible implementation of arterialization of venous blood gas which will make all these values even more accurate and will allow the use of venous pCO2 in the clinical setting. The use of venous blood gas values has shown to be cost-effective and implementation of this procedure in the routine venous blood sampling will prove to be time and money saving, as it as well will reduce the risk factors for the patient and the health care worker. In presentations were pO2 values have to be determined precisely or in patients presenting with severe circulatory failure our data is not concise enough and therefore we still recommend arterial blood gas analysis in these situations.

 

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