The arterial blood gas (ABG) is one of the most powerful and frequently used tests in critical care and in the operating room. An ABG may be ordered to obtain information about the patient’s acid/base status, arterial carbon dioxide tensions (PaCO2) and arterial oxygen (PaO2) tensions. Frequently, other information such as the calculated sodium bicarbonate, base deficit, hemoglobin, basic metabolic profile, dyshemoglobins (methemoglobin and carboxyhemoglobin), and lactic acid levels may also be measured in conjunction with traditional ABG values. However, this chapter will focus only on the information obtained from a traditional ABG (pH, PaCO2, PaO2).
The clinician may choose to obtain an ABG in a variety of clinical scenarios. In the intensive care unit, the ABG can diagnose a variety of metabolic acid/base disorders, perturbations of ventilation and hypoxemia. After a therapy has been initiated, a repeat ABG can determine the efficacy of the intervention (e.g., when mechanical ventilation is initiated for respiratory failure.) The ABG can also suggest the degree of degree of respiratory and renal compensation for a given acid/base disorder. In the operating room, ABGs are particularly helpful when acid/base status may change dynamically due to the procedure being performed, such as during operations requiring cardiopulmonary or veno-veno bypass, one-lung ventilation, transplant surgery, certain urologic procedures, and trauma. ABGs are also helpful when tight control of the partial pressure of CO2 is important for improved patient outcomes, such as in neurosurgical cases where carbon dioxide’s effect on intracranial pressure (ICP) can be critical.
The normal range for pH is 7.35–7.45. Lower values indicate an acidosis. Higher values signify an alkalosis. The next step in pH interpretation is to determine whether the acidosis or alkalosis is metabolic or respiratory in origin.
Although the cause of a metabolic acidosis cannot be determined solely by the ABG, a metabolic acidosis can be identified with a pH value of less than 7.35 with a PaCO2 below 40 mmHg. It can also be characterized as a decrease in the strong ion difference. A metabolic acidosis signifies an overproduction, ingestion or inadequate excretion of hydrogen (H+) ions in a variety of forms. If the cause is an increase in anions or nonvolatile acids not usually present in the blood, the acidosis is termed an “anion gap” acidosis. The anion gap is the difference between primary measured cations (sodium [Na+] and potassium [K+]) and the primary measured anions (chloride [Cl–] and bicarbonate [HCO3–]) in serum. A normal anion gap is less than 11 mEq/L. The normal gap does not reflect a permanent imbalance between cations and anions, but rather acknowledges the contribution of albumin as a significant negative change contribution to electrical neutrality.
Causes of an anion gap metabolic acidosis: