From the Guidelines
For an elderly patient with a CO2 level of 27 mmol/L, which is within the normal range, and an anion gap of 1, no immediate intervention is necessary, but the low anion gap warrants further investigation. The normal range for CO2 (bicarbonate) is typically 22-29 mmol/L, and the normal anion gap range is 8-12 mEq/L, though some labs consider values as low as 3-11 mEq/L normal. According to the American Thoracic Society clinical practice guideline 1, a serum bicarbonate level of 27 mmol/L is within the range where the diagnosis of obesity hypoventilation syndrome (OHS) is unlikely if the pretest probability is low. However, the low anion gap of 1 is unusual and warrants further investigation. Potential causes include laboratory error (most common), hypoalbuminemia, multiple myeloma, bromide or lithium intoxication, or severe hypernatremia.
Recommended Next Steps
- Repeating the electrolyte panel to confirm these values is essential.
- If confirmed, additional testing should include:
- Serum protein electrophoresis
- Albumin levels
- A complete medication review
- Close monitoring of the patient's clinical status and further diagnostic workup are appropriate next steps, considering the patient's age and potential for underlying metabolic disturbances.
- The BTS guideline for oxygen use in adults in healthcare and emergency settings 1 provides valuable insights into managing patients with potential respiratory issues but does not directly apply to the management of electrolyte imbalances or anion gap abnormalities.
- The guideline for ventilatory management of acute hypercapnic respiratory failure in adults 1 also does not directly address the issue at hand but emphasizes the importance of careful oxygen therapy in patients at risk of hypercapnic respiratory failure.
While these values don't require emergency treatment, they suggest an underlying metabolic disturbance that should be evaluated. The elderly are particularly susceptible to electrolyte abnormalities due to age-related changes in renal function, medication effects, and comorbidities.
From the Research
Acid-Base Disorders
- Acid-base disorders are common in the intensive care unit and can be divided into four major categories: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis 2.
- Metabolic acidosis is subdivided into anion gap and non-gap acidosis, with anion gap acidosis being caused by the accumulation of organic anions from sepsis, diabetes, alcohol use, and numerous drugs and toxins.
Respiratory Alkalosis
- Respiratory alkalosis occurs when alveolar ventilation exceeds that required to eliminate the carbon dioxide produced by tissues, resulting in concurrent decreases in Paco2, increases in pH, and compensatory decreases in blood HCO3- levels 3.
- Respiratory alkalosis can be acute or chronic, with metabolic compensation initially consisting of cellular uptake of HCO3- and buffering by intracellular phosphates and proteins.
- Chronic respiratory alkalosis results in longer-lasting decreases in renal reabsorption of HCO3-, with the arterial pH approaching near-normal values 3.
CO2 Levels and Age
- There is no direct information provided about the relationship between CO2 levels and age, particularly in elderly patients.
- However, it is mentioned that respiratory alkalosis is the most frequent acid-base disturbance encountered in clinical practice, particularly in critically ill patients, and that the degree of hypocapnia directly correlates with adverse outcomes 4.
Treatment of Respiratory Alkalosis
- Correction of a respiratory alkalosis is best performed by correcting the underlying etiology 5.
- Mild respiratory alkalosis often serves as a marker of an underlying disease and may not require therapeutic intervention, while severe respiratory alkalosis should be approached with a sense of urgency and be aggressively corrected 4.