What are the causes of low bicarbonate levels in a Venous Blood Gas (VBG) analysis?

From the Guidelines

Low bicarbonate in a venous blood gas (VBG) is primarily caused by metabolic acidosis, which can result from various conditions, including diabetic ketoacidosis, lactic acidosis, renal tubular acidosis, and severe diarrhea, as well as certain medications or toxins. When evaluating low bicarbonate, it's essential to assess the anion gap to differentiate between causes with normal anion gap (hyperchloremic) and elevated anion gap acidosis 1.

Common Causes of Low Bicarbonate

• Diabetic ketoacidosis, where insulin deficiency leads to ketone production, as seen in patients with plasma glucose 250 mg/dl (13.9 mmol/l) and serum bicarbonate level 15 mEq/l 1
• Lactic acidosis from tissue hypoxia in conditions like sepsis or cardiac arrest
• Renal tubular acidosis due to kidney dysfunction, which can be managed by maintaining serum bicarbonate at or above 22 mmol/L, a goal that can be achieved through higher dialysate and/or oral supplementation with bicarbonate salts 1
• Severe diarrhea causing bicarbonate loss through the GI tract
• Certain medications or toxins like salicylates, methanol, or ethylene glycol

Treatment Approach

Treatment should target the underlying cause while monitoring electrolytes and acid-base status.

• In severe cases with pH below 7.1-7.2, sodium bicarbonate administration may be considered, though this remains controversial and should be approached cautiously as it can cause fluid overload, hypokalemia, and paradoxical CNS acidosis.
• Normalization of the predialysis or stabilized serum bicarbonate concentration can be achieved by higher dialysate and/or by oral supplementation with bicarbonate salts, with an oral dose of sodium bicarbonate usually about 2 to 4 g/d or 25 to 50 mEq/d 1

From the FDA Drug Label

CLINICAL PHARMACOLOGY Intravenous sodium bicarbonate therapy increases plasma bicarbonate, buffers excess hydrogen ion concentration, raises blood pH and reverses the clinical manifestations of acidosis. Plasma concentration is regulated by the kidney through acidification of the urine when there is a deficit or by alkalinization of the urine when there is an excess.

The causes of low bicarbonate in VBG (Venous Blood Gas) are not directly stated in the provided drug label. However, it can be inferred that kidney dysfunction or excess hydrogen ion concentration may lead to low bicarbonate levels.

• Kidney dysfunction: The kidney plays a crucial role in regulating plasma bicarbonate concentration.
• Excess hydrogen ion concentration: This can lead to acidosis, which may cause low bicarbonate levels. 2

From the Research

Causes of Low Bicarbonate in VBG

• Low bicarbonate levels in venous blood gas (VBG) analysis can be caused by various factors, including metabolic acidosis 3
• Metabolic acidosis can occur due to the accumulation of endogenous acids that consume bicarbonate, such as lactic acidosis, ketoacidosis, and renal failure 3
• High anion gap metabolic acidosis, which includes lactic acidosis, ketoacidosis, and renal failure, can lead to low bicarbonate levels 3
• Hyperchloremic metabolic acidosis, caused by gastrointestinal bicarbonate loss, renal tubular acidosis, and drug-induced hyperkalemia, can also result in low bicarbonate levels 3
• Lactic acidosis, which is common in diabetic ketoacidosis (DKA), can contribute to low bicarbonate levels 4, 5, 6
• Renal failure, which can lead to the accumulation of acids and the loss of bicarbonate, can also cause low bicarbonate levels in VBG analysis 7, 3

Relationship Between Arterial and Venous Bicarbonate Levels

• Studies have shown a strong correlation between arterial and venous bicarbonate levels, suggesting that VBG analysis can be a reliable alternative to arterial blood gas (ABG) analysis in certain situations 7
• The Pearson correlation coefficient between arterial and venous bicarbonate was found to be 0.842 (p<0.001) in a study of patients with renal failure 7
• This correlation suggests that low bicarbonate levels in VBG analysis can be a reliable indicator of metabolic acidosis and other conditions that affect bicarbonate levels 7