Introductory Chapter: Arterial Blood Gases – Canary in the Mine?

1. Introduction

A woman in her sixties was brought to the ED with a history of heart failure for 8 years and diabetes for 26 years. She was found to have dyspnea, tachypnea, and tachycardia. Her complaints have worsened progressively over the past 3 to 4 days, especially when she lies down. She says that she had been fighting a cold 2 weeks ago, and a greenish sputum persisted thereafter. She appears anxious and agitated but denies chest pain, nausea, vomiting, diaphoresis, or fever. She is a smoker with 60 packs/years (40 years and 1.5 packs of cigarettes a day). She has been diagnosed with bronchitis several times and was hospitalized 3 years ago with pneumonia. She has gained around four kilograms over the last months. Her respiratory rate was 32 bpm, heart rate 116 bpm, SaO2 87% at 4 L/min O2 via nasal cannula on the ambulance. High-flow nasal cannula oxygen therapy increased SaO2 to 93% in the ED.

On the complete blood count, she has increased her leukocyte count 12.400 10⁹/L. Blood glucose is 270 mg/dL and serum creatinine is 1.6 mg/dL. Arterial blood gas values turns out to be pH 7.33, PaO2 88 mmHg and PaCO2 52 mmHg on the initial examination on room air.

Highlight: Almost everyday, emergency physicians encounter patients such as this lady, in whom many factors contribute to the cause of the ED visit and complicate the clinical picture. Routine investigations help enlighten the differential diagnosis, but oxygenation and ventilation can be obscured after these. Respiratory failure and clinical deterioration despite treatment, can be overlooked in the absence of arterial blood gases.

2. Role of arterial blood gases (ABG) in evaluation of critical patients

ABG analysis has an important role in the assessment of critical diseases and in determining the etiology and severity of many entities. ABG is useful in the management of various respiratory and metabolic disturbances.

Measurement of oxygen (PaO2) and carbon dioxide partial pressures (PaCO2), oxygen saturation (SaO2), pH and bicarbonate values ​​in arterial blood is performed by ABG analysis in the evaluation of acid-base and respiratory balances [1].

Two basic functions of respiration are examined in blood gas analysis: ventilation and oxygenation. Multiple pathological entities can be recognized in critically ill patients using analysis of the pH, PaO2, PaCO2, and SaO2, and comparing it to measured serum bicarbonate [2]. With the devices developed in recent years, glucose, electrolyte, kidney functions, bilirubin, and hemoglobin levels can be examined in the ABG analysis and information about the metabolic status can also be obtained. Table 1 gives a summary of ingredients of ABG analysis to consider in routine practice.

Indications of ABG analysis include conditions such as respiratory failure, ventilation-perfusion mismatch, metabolic acidosis or alkalosis, and acute respiratory diseases are typical indications for the use of ABG. Indications for ABG analysis can be summarized as follows:

• Diagnosis and follow-up of metabolic and respiratory acidosis and alkalosis

• Determining the type of respiratory failure

• Evaluation of the effectiveness of the given treatment

• Indication and follow-up of oxygen therapy

• Sudden-onset and unexplained dyspnea

• Cardiac arrest situations and the effect of procedures including CPR or post-ROSC states

Clinical estimates about oxygenation status is mostly erroneous and have poor prognostic power in the clinical setting. For example, one of the most important clinical signs, cyanosis is affected by the level of hemoglobin, skin color, perfusion and lighting status. In most circumstances, ABG and SpO2 offer an accurate measurement of oxygenation to guide diagnosis and management in the acute setting.

Likewise, SaO2 and SpO2 have their inherent deficiencies to demonstrate ventilatory function. An important criterion to oversee deterioration in the patient is to be informed on rising PaCO2 in respiratory failure or respiratory arrest. SaO2 and SpO2 heralds this kind of worsening status considerably lately in a sedated patient when consciousness is depressed or periarrest situations [3].

Sampling for ABG analysis triggers unwelcome experiences for patients. In addition, the procedure is accompanied by complications like arterial injury, thrombosis, air or clotted-blood embolism, arterial occlusion, hematoma, aneurysm formation, and reflex sympathetic dystrophy [4].

3. Interpretation of blood gases

Blood gas evaluation in clinical follow-up is a valuable tool that provides information on the severity of the disease. PaO2 represents oxygenation, and PaCO2 shows alveolar ventilation. The clinician must be aware of the normal values ​​in the examination of ABG, which has a pivotal place in the clinical approach. The patient’s clinical condition and other laboratory findings should also be taken into consideration in the clinical decision-making process in addition to ABG findings.

Blood gas measuring devices directly measure pH and PCO2 and calculate bicarbonate using the Henderson-Hasselbach equation.

PaO2 and PaCO2 give insight for gas exchange, while pH, PaCO2, and HCO3 are the parameters used to evaluate the acid-base status [1]. Of note is that the alveolar-arterial oxygen gradient is beneficial as a surrogate of pulmonary gas exchange, which can be abnormal in patients with a ventilation-perfusion mismatch [5].

In addition to evaluation of the patients’ ventilatory, respiratory, and acid-base status, analysis of ABG is useful for assessment of the response to modes of treatment and monitoring the degree and progression of cardiopulmonary disease processes [6, 7]. Of note, incompatible or discrepant values are a potential drawback of the analysis of ABG; therefore, clinicians should strive to eliminate potential sources of error [8].

Finally, it should be taken into account that ABG results may be inaccurate due to laboratory errors and errors during sample collection. Interpretation of the ABG findings allows evaluation of the severity of disturbances, whether the imbalances are acute or chronic, and whether the primary disorder is primarily metabolic or respiratory [9].

4. Summary

ABG evaluates the physiological functions of the respiratory system, acid-base balance, and oxygen-carrying capacity through a blood gas analysis taken from an arterial blood sample, in this way. It plays a pivotal role in the evaluation of patients with critical diseases. In most clinical scenarios, PaO2 is examined to assess oxygenation, while PaCO2 is examined to evaluate ventilation. P(A-a)O2 is calculated to evaluate gas exchange. Conditions such as respiratory failure, ventilation-perfusion mismatch, metabolic acidosis or alkalosis, and acute respiratory diseases are accepted indications for the use of ABG.

Acid-base balance gains priority in most critical patients to intervene before correcting secondary problems. ABG interpretations prevent unnecessary use of imaging modalities and medications. Therefore, the patient’s clinical condition and other laboratory findings should always be taken into consideration in the clinical decision-making process.

Conflict of interest

None.

Funding source

None.