When a blood test reports “CO2,” it primarily refers to the level of bicarbonate (HCO3-) in the blood. Bicarbonate is a key electrolyte that plays a central role in the body’s acid-base balance, helping to maintain a stable pH level. A low CO2 reading indicates a lower than normal bicarbonate level, signifying an acidic state.
Understanding Blood CO2 Levels
Bicarbonate functions as an important buffer, neutralizing excess acids to keep the blood’s pH within a narrow, healthy range, typically between 7.35 and 7.45. This acid-base balance is important for bodily functions, as deviations can disrupt metabolic activities and cellular integrity. The body’s pH is constantly challenged by metabolic processes that produce acids.
The kidneys and lungs work together to regulate bicarbonate and maintain pH balance. Kidneys adjust pH by reabsorbing bicarbonate from urine into the bloodstream and excreting hydrogen ions. Lungs quickly influence blood pH by controlling exhaled carbon dioxide, as CO2 combines with water to form carbonic acid. A normal bicarbonate level for adults ranges from 22 to 29 milliequivalents per liter (mEq/L), though values can vary between laboratories.
Primary Causes of Low Blood CO2
Low bicarbonate, known as metabolic acidosis, means the body has too much acid or has lost too much base. One common cause is diabetic ketoacidosis (DKA), a complication of uncontrolled diabetes. Lacking sufficient insulin, the body breaks down fat for energy, producing acidic ketone bodies. These accumulated acids overwhelm the body’s buffering capacity, leading to decreased bicarbonate.
Kidney disease, including chronic kidney disease (CKD) and renal tubular acidosis, is a common cause of low bicarbonate. Healthy kidneys remove excess acid and conserve bicarbonate; however, impaired kidney function reduces their ability to excrete acids or reabsorb bicarbonate, leading to acid buildup. Severe diarrhea can also cause low bicarbonate due to significant loss of bicarbonate-rich fluids from the digestive tract.
Certain medications, such as carbonic anhydrase inhibitors (used for conditions like glaucoma), can lower bicarbonate levels. These drugs interfere with the kidney’s ability to reabsorb bicarbonate, promoting its excretion. Lactic acidosis, a buildup of lactic acid, decreases bicarbonate. This condition can arise from various factors, including poor tissue oxygenation (as seen in shock or sepsis), liver failure, or intense physical activity. Other less common causes include certain poisonings (e.g., methanol, ethylene glycol) and Addison’s disease.
Potential Implications and Next Steps
A low CO2 level signals metabolic acidosis, an imbalance where acids accumulate. Symptoms are typically not from the low bicarbonate itself, but from the underlying condition causing the acidic state. These symptoms can include rapid breathing, headache, confusion, fatigue, nausea, or vomiting.
If left unaddressed, chronic metabolic acidosis can lead to health concerns. It may contribute to increased bone loss, increasing fracture risk, and can cause muscle wasting. For individuals with kidney disease, metabolic acidosis can accelerate the progression of their condition.
When a blood test reveals low CO2, consult a healthcare professional. Further evaluations and tests will identify the root cause of the metabolic acidosis. Treatment focuses on addressing the underlying condition; for example, with kidney disease, oral bicarbonate supplementation might be recommended to restore balance.