A low level of carbon dioxide (Total CO2) reported on a blood test primarily reflects a low concentration of bicarbonate (HCO3-). Bicarbonate is an electrolyte essential for maintaining the body’s internal stability. A reading below 23 milliequivalents per liter (mEq/L) suggests an imbalance in the acid-base system. This result signals that the body may be struggling to regulate its acidity, requiring further medical investigation.
The Role of Bicarbonate in Acid-Base Balance
Bicarbonate functions as the body’s primary chemical buffer, neutralizing acids and bases to keep the blood’s pH within the healthy range of 7.35 to 7.45. This buffering is achieved when bicarbonate binds to excess hydrogen ions (acid) to form carbonic acid, which then breaks down into water and carbon dioxide for the lungs to exhale. The kidneys mainly regulate bicarbonate concentration by reabsorbing or excreting it to adjust blood alkalinity.
If a significant acid load enters the bloodstream, the bicarbonate buffer system is consumed. A low bicarbonate level indicates the body has used up this buffering capacity, leading to metabolic acidosis. This state causes the blood’s pH to fall, creating an acidic environment that affects physiological processes.
The body attempts to compensate through hyperventilation, increasing the breathing rate to expel more carbon dioxide and slightly raise the blood pH. This compensatory mechanism helps mitigate the effects of the acidic environment.
Medical Conditions That Cause Low Carbon Dioxide
A low bicarbonate level signals metabolic acidosis, stemming from three main issues: increased acid production, bicarbonate loss, or impaired acid excretion.
Increased Acid Production
Excessive acid creation occurs in conditions like diabetic ketoacidosis (DKA). Lack of insulin causes the body to break down fat, producing acidic ketone bodies that overwhelm the buffer. Lactic acidosis arises when cells lack sufficient oxygen and rely on anaerobic metabolism, building up lactic acid during severe shock, sepsis, or intense exercise. Certain intoxications, including poisoning from methanol, ethylene glycol, or high doses of aspirin (salicylates), also introduce strong acids.
Bicarbonate Loss
The second major cause is the direct loss of bicarbonate, often through the gastrointestinal tract. Severe or prolonged diarrhea excretes significant amounts of bicarbonate, shifting the blood toward acidity. Specific kidney disorders, known as Renal Tubular Acidosis (RTA), cause the kidneys to improperly reabsorb bicarbonate or fail to excrete acid into the urine.
Impaired Acid Excretion
The third mechanism involves the kidneys’ inability to effectively remove the normal acid load produced daily by metabolism. This frequently complicates chronic kidney disease (CKD), where declining nephron function impairs hydrogen ion excretion. The excess acid must then be buffered by the body’s bicarbonate, causing a chronic decline in the measured Total CO2 level.
Diagnosis and Treatment Approaches
When a low Total CO2 result is found, healthcare providers perform further tests to pinpoint the exact cause of metabolic acidosis. The Anion Gap is calculated immediately to determine if the acidosis is due to an excess of unmeasured acids (like ketones or lactate) or a loss of bicarbonate. An Arterial Blood Gas (ABG) test confirms the diagnosis by precisely measuring the blood’s pH and the partial pressure of carbon dioxide (PaCO2).
Treatment focuses entirely on resolving the underlying condition that created the acid-base imbalance. For instance, DKA requires insulin and fluid rehydration to stop ketone production. Lactic acidosis necessitates improving oxygen delivery to tissues, such as treating sepsis or improving circulation in shock.
For bicarbonate loss, like chronic diarrhea, rehydration and treating the gastrointestinal issue are the primary focus. Severe acidosis may require intravenous sodium bicarbonate to temporarily restore a safer blood pH. Chronic kidney disease treatment often includes dietary adjustments and oral alkali therapy to maintain stable bicarbonate levels.