Primary assessment of patients admitted to the emergency ward using arterial blood gas

Sanjay Mulay; Rajesh Ursekar; Jerry Mathew; Krista Pinto

doi:10.4103/mgmj.mgmj_222_22

ORIGINAL ARTICLE

Year : 2022 | Volume : 9 | Issue : 4 | Page : 548-552

Primary assessment of patients admitted to the emergency ward using arterial blood gas

Sanjay Mulay, Rajesh Ursekar, Jerry Mathew, Krista Pinto
Department of Emergency Medicine, Bharati Vidyapeeth (Deemed to be University), Medical College, Pune, Maharashtra, India

Date of Submission	11-Nov-2022
Date of Acceptance	21-Nov-2022
Date of Web Publication	29-Dec-2022

Correspondence Address:
Dr. Rajesh Ursekar
Department of Emergency Medicine, Bharati Vidyapeeth (Deemed to be University), Medical College, Pune 411043, Maharashtra
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mgmj.mgmj_222_22

Abstract

Background: Arterial blood gas study (ABG) is a point-of-care testing (POCT) diagnostic tool that can furnish metabolic and respiratory aberrations. This study was conducted systematically, to assess the metabolic and respiratory aberrations quickly and the scope for corrective treatment so that metabolic and respiratory abnormalities get corrected. Materials and Methods: A prospective cross-sectional study was done among 150 cases admitted to the Department of Emergency Medicine during a three-month period where the study on ABG was done. Data was collected in the prescribed format and a stepwise interpretation of the ABG was done. The four primary disorders taken into consideration are metabolic acidosis, metabolic alkalosis, respiratory alkalosis, and respiratory acidosis Results: Out of 150 cases 82 had respiratory alkalosis, 51 had metabolic acidosis, ten had respiratory acidosis, and seven had metabolic alkalosis as a primary disorder. Conclusion: ABG analysis is a POCT diagnostic tool for analyzing various metabolic and respiratory aberrations and can also guide us in the scope for correction of the disorder.

Keywords: Arterial blood gas, emergency medicine, point of care testing

How to cite this article:
Mulay S, Ursekar R, Mathew J, Pinto K. Primary assessment of patients admitted to the emergency ward using arterial blood gas. MGM J Med Sci 2022;9:548-52

How to cite this URL:
Mulay S, Ursekar R, Mathew J, Pinto K. Primary assessment of patients admitted to the emergency ward using arterial blood gas. MGM J Med Sci [serial online] 2022 [cited 2023 Feb 7];9:548-52. Available from: http://www.mgmjms.com/text.asp?2022/9/4/548/365994

Introduction

Arterial blood gas study (ABG) is a point of care testing (POCT), a diagnostic tool used quite frequently in the Emergency Medicine Department (EMD). The information one can gather regarding the patient’s condition is quite extensive and quick. With a little drop of blood, it can furnish the metabolic and respiratory aberrations under various parameters mentioned in the ABG chart.

Apart from the history and clinical examination, the primary assessment of the patient in EMD is done by

Recording the vital parameters

Oxygen saturation (Saturation of Peripheral Oxygen)

Random blood Sugar

Blood investigations (Serum Creatinine)

Radiological investigations – X-rays, Computed T/Magnetic Resonance Imaging, Ultrasonography

The duration of stay in the Emergency Department is, on average around 2–3 hours within which one needs to identify

Hypoxemic status of the patient

Academia / Alkalemia.

Electrolyte imbalance.

A primary disorder causing metabolic aberrations and necessary correction.

Effectivity of intervention undertaken by the Emergency Medicine Physician

The ABG studies can help

In restoring Acid-base balance

In establishing the clinical diagnosis

Plan the treatment protocol and modify it as per the need of the patient

In ventilator management

The results of ABG studies need to be integrated with clinical data so that proper treatment can be devised. The ABG studies alone can do little. The accurate results of ABG analysis can be achieved by the proper collection of blood samples, handling of the same, and proper interpretation and analysis of ABG. Since this is a POCT, one should not be concerned about the storage and transport of the specimen. The risk of coagulation can be avoided by the use of proper anticoagulant and proper mixing of the anticoagulant.^[1]

There is quite a lot of controversy about the role of ABG or Venous Blood Gas. The respiratory physicians object to Venous Blood Gas analysis on the pretext that the values of the blood gasses get altered in Venous Blood Gas. However, most emergency physicians do not find any objection.^[2] Since this study is about ABG analysis, the debate between ABG versus Venous Blood Gas seems to be beyond the scope of this study.

Therefore, we conducted this study to interpret the ABG results systematically, to assess the metabolic and respiratory aberrations quickly, and to the scope for corrective treatment so that metabolic and respiratory abnormalities get corrected. However, the definitive management is started only after consultation with the concerned Specialist / Nephrologist as the case may be.

Materials and methods

Study period: August 2020 to October 2020

This is a prospective cross-sectional study of 150 cases, admitted to the EMD from August 2020 to October 2020, where the ABG study was done. The data was collected in the prescribed format and the interpretation was done under the following headings mentioned in the datasheet.

Acidemia /Alkalemia by assessment of pH

Assessment of Primary and Secondary disorder by identifying the rate of change in HCO₃^-and PaCO₂

Assessment of compensation.^[1]

Stepwise approach for Interpretation of ABG:

Step 1: Assess the oxygenation status of the patient by measuring PaO₂.

If PaO₂ is less than 60 mm Hg, treat hypoxia. Once PaO₂ reaches 100 mm of Hg, then reduce FiO₂.

Step 2: Identify the Primary Problem.

This is done by primarily calculating pH, PaCO₂, PaO₂, and HCO₃^-. [Table 1]

Table 1: Acid-base changes

Click here to view

Step 3: Assess the adequacy of the Compensatory response.

Metabolic disorders

The expected value of PaCO₂ is calculated by two digits after the decimal point of pH. For example, if the pH is 7.23, the expected value of PaCO₂ is 23 ± 5. i.e., 18–23.

Respiratory disorders

In respiratory acidosis, for every 10 mmHg rise of PaCO₂, there has to be a rise of 1 mEq/L of HCO₃^[3] If the actual value is within this expected range, then the disorder is labeled as a single disorder if it is not then it is labeled as a mixed disorder. Thus, in normal ABG -

pH and PaCO₂ move in the opposite direction

HCO₃^-and PaCO₂ move in the same direction

When pH and PaCO₂ move in the same direction (which normally should not), then the primary problem is metabolic. If they move in the opposite direction, but PaCO₂ is normal, the primary problem is respiratory.

If HCO₃^-and PaCO₂ move in opposite directions, then it is a mixed disorder, irrespective of pH, PaCO_2, and HCO₃^- values.

If the trend of change in PaCO₂ and HCO₃^- is the same, then calculate the percentage of the change. Greater the percentage, dominant the disorder.^[1]

The criteria for hemodynamic stability considered were as under:

Pulse rate: 70–100/min

Respiratory rate: 12–16/min

Blood Pressure: Systolic BP > 100mmHg

No artificial life supports

SPO2 > 94%

The four primary disorders taken into consideration are metabolic acidosis, metabolic alkalosis, respiratory alkalosis, and respiratory acidosis.

Inclusion criteria

Medical emergencies in adults.

Polytrauma

Exclusion criteria

Malignancy

Pregnancy

Ophthalmic emergencies

Results

This is the study of 150 patients who had presented to the emergency medicine department. It had 101 males and 49 females between the age group 18–95 years. 114 patients were hemodynamically stable whereas the remaining 36 patients required corrective measures to attain hemodynamic stability. Considering the clinical manifestations with major presentation taken into account, 38 had gastrointestinal, 18 had cardiovascular, 50 had a central nervous system, and 44 had pulmonary involvement.

In the analysis done, it was found that

51 had metabolic acidosis (18 compensated, 33 uncompensated)

7 had metabolic alkalosis (5 compensated, 2 uncompensated)

82 had respiratory alkalosis (47 compensated, 35 uncompensated)

10 had respiratory acidosis (4 compensated, 6 uncompensated)

Overall, the maximum number of patients on analysis had respiratory alkalosis as the primary disorder.

Among the study group after proper clinical assessment along with other diagnostic methods 53 patients requiring critical care admission, 64 were admitted and followed up inwards after admission, 33 were managed on an outpatient basis and 1case from critical care died.

Discussion

ABG is an integral part of the assessment of the metabolic and respiratory status of a patient in the emergency department.^[4],[5] A prospective observational study was done among 150 patients who presented to the emergency department and were deemed to require an ABG analysis as per the emergency physician’s opinion.^[6] ABG results were clinically interpreted to assess the primary disorder. The primary disorder was interpreted as compensated if pH was between 7.35 and 7.45.^[4]

The acid-base imbalances may be compensated or uncompensated and the compensation is mainly regulated by the renal and respiratory systems. Blood pH levels are regulated by the respiratory system by regulating the rate of respiration. The renal system regulates pH, through the excretion of hydrogen and bicarbonate ions.^[4] The primary disorders assessed were metabolic acidosis, metabolic alkalosis, respiratory alkalosis, and respiratory acidosis.

Metabolic acidosis

Metabolic acidosis is defined as HCO₃^- less than 22 mEq/L with a pH less than 7.35.^[5] Metabolic acidosis is characterized by an increase in hydrogen ions in the body with or without a decrease in the level of HCO₃^-. This occurs due mainly due to 3 major mechanisms:

Increased acid formation

Decreased acid secretion

Loss of bicarbonate^[6]

Metabolic acidosis is mainly seen in patients with renal failure, diabetic ketoacidosis, starvation ketoacidosis, and severe diarrhea.^[5],[7],[8] In isolated metabolic acidosis the extent of acute respiratory compensation can be determined by Winters formula.

PaCO2 = (1.5 X HCO₃^- + 8) +/- 2^[6]

The treatment of metabolic acidosis should be primarily targeted in the correction of the underlying disorder except in very few cases. Although bicarbonate administration is the most common method used for correction apart from treating the primary disorder, it has been found that the administration of bicarbonate has limited value and can be harmful.^[7] As per the Emergency Department protocol, we have done bicarbonate correction only when the pH is <7.1. Administration of THAM or delivering a base during dialysis might improve the acidosis and would also not cause adverse effects of sodium bicarbonate.^[9]

Metabolic alkalosis

Metabolic alkalosis is defined as an elevation of HCO₃^- levels >26meq/L along with a pH>7.45.^[4],[5]The main causes of metabolic alkalosis are:

Gain of HCO₃^-

Loss of non-volatile acid from extracellular fluid^[10]

Metabolic alkalosis is mainly seen in patients with gastric losses (vomiting, mechanical drainage), use of loop diuretics like chlorothiazide, cystic fibrosis, primary aldosteronism, severe hypertension, and in patients post dialysis.^[5],[10]

The correction of metabolic alkalosis should be primarily targeted at correcting the basic underlying pathophysiology causing the acid-base imbalance. In severe cases, reversal of alkalosis by administration of carbonic anhydrase inhibitors, acid infusion, and low bicarbonate dialysis is recommended.^[5],[11]

Respiratory alkalosis

Respiratory alkalosis is defined as a pH greater than 7.45 along with a PaCO2 level less than 35 mm Hg.^[4],[5] The main cause of respiratory alkalosis in a patient in the emergency department is hyperventilation. Along with hyperventilation they normally present with tachycardia or nonischemic chest pain.^[12]

Respiratory alkalosis is mainly seen in patients presenting with anxiety, pain, sepsis, pregnancy, and some central nervous system lesions. These patients should be closely watched out for respiratory muscle fatigue.^[5] Mild respiratory alkalosis is usually indicative of an underlying disease.

Treatment of respiratory alkalosis is mainly targeted at the correction of the underlying disorder.^[12]

Respiratory acidosis

Respiratory acidosis is defined as a pH less than 7.35 with a PaCO2 greater than 45 mm Hg.^[4],[5] The main cause of respiratory acidosis is an increase in PaCO₂. The major causes of respiratory acidosis include

Increased carbon dioxide production

Alveolar hypoventilation

Abnormal respiratory drive

Respiratory acidosis is mainly seen in patients with COPD, pulmonary edema, spinal cord injury, drug overdose, a massive pulmonary embolism, and bronchial obstruction.^[13] The patients normally present with respiratory distress, headache, and tachycardia and if CO₂ levels are extremely high, the patient may be coming to the emergency department in a drowsy or unresponsive state.^[5]

The primary modality of treatment may include invasive or non-invasive ventilatory support and other measures to correct the underlying pathophysiology may also be required.^[13]

Limitations of ABG studies:

ABG analysis studies can have the following limitations:

ABG sampling is an invasive procedure and may result in complications like arterial injury or thrombosis.^[14]

Findings could be different on another blood gas analyzer

It is a single institutional study.^[15]

The sample size is too small. It should be a multi-centric study where the results of different centers can be compared.

Conclusion

It can be concluded that ABG analysis is a POCT diagnostic tool that can guide us in analyzing various metabolic and respiratory aberrations of the patient in a very short period. It can also guide us in the scope for the correction of metabolic and respiratory aberrations. ABG analysis has to be correlated with clinical findings and other supportive investigations to arrive at a proper clinical diagnosis.

Ethical consideration

The Institutional Ethic Committee of Bharti Vidyapeeth (Deemed to be University) Medical College, Pune, Maharashtra, India had reviewed the research project and approved the undertaking of the study protocol vide their letter no. BVDUMC/IEC/8 dated 05 October 2020.

Financial support and sponsorship

Nil

Conflicts of interest

There are no conflicts of interest.

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