When a standard blood test, such as a Basic Metabolic Panel, reports a low level of “Carbon Dioxide” (\(\text{CO}_2\)), the result does not refer to the carbon dioxide gas expelled by the lungs. This measurement is instead an estimate of the total \(\text{CO}_2\) content in the blood, which is overwhelmingly composed of the bicarbonate ion (\(\text{HCO}_3^-\)). Bicarbonate is an electrolyte and a negatively charged ion that plays a fundamental part in maintaining the body’s internal balance. A low total \(\text{CO}_2\) value is therefore interpreted as a low bicarbonate level, signaling a potential disruption in the body’s acid-base status. This provides an important initial clue about how well the body is managing the acidic byproducts of its metabolic processes.
The Role of Bicarbonate in the Body
The bicarbonate ion is the primary component of the bicarbonate buffer system, the body’s defense against changes in acidity. Metabolic activities constantly produce acids, and this buffer system neutralizes them to keep blood \(\text{pH}\) within a narrow range of approximately 7.35 to 7.45. Bicarbonate works by absorbing excess hydrogen ions (\(\text{H}^+\)), preventing the blood from becoming too acidic, which could impair the function of various enzymes and proteins.
The body manages bicarbonate levels using a coordinated effort between the lungs and the kidneys. The lungs convert bicarbonate back to carbon dioxide and exhale it, providing a rapid mechanism for acid regulation. The kidneys are responsible for long-term control, either reabsorbing bicarbonate back into the bloodstream or excreting excess acid. If the body gains too much acid or loses too much base, the bicarbonate buffer stores are used up, leading to a drop in the measured total \(\text{CO}_2\) level.
What Low Carbon Dioxide Levels Indicate
A low total \(\text{CO}_2\) or bicarbonate level points toward a condition called metabolic acidosis. This is the physiological state where the body has either produced or accumulated too much acid, or lost too much of the alkaline base, overwhelming the buffering capacity. The normal range for total \(\text{CO}_2\) in adults is between 22 and 29 milliequivalents per liter (\(\text{mEq/L}\)), though laboratory ranges can vary slightly. A result below \(22\ \text{mEq/L}\) suggests that the body’s alkali reserve is depleted.
Metabolic acidosis means that the \(\text{pH}\) of the body fluids is trending toward the acidic side. The body attempts to compensate for this imbalance by increasing the rate and depth of breathing to expel more carbon dioxide, reducing the acid load. This rapid, deep breathing pattern, known as Kussmaul respirations in severe cases, is the lungs’ attempt to restore the acid-base balance. A low bicarbonate level indicates that the body is working harder to manage an underlying acid load, and a level below \(12\ \text{mEq/L}\) may indicate severe metabolic acidosis that demands urgent attention.
Primary Reasons for Low Bicarbonate
Low bicarbonate levels can result from two primary mechanisms: a significant gain of acid in the body or a direct loss of bicarbonate.
Acid Gain
One of the most common causes of acid gain is diabetic ketoacidosis (DKA), which occurs when uncontrolled diabetes produces acidic ketone bodies. Lactic acidosis is another cause, where excess lactic acid builds up due to insufficient oxygen supply to tissues. This is often seen in conditions like severe infection, shock, or cardiac failure. Certain toxic ingestions, such as methanol or ethylene glycol, also generate highly acidic metabolites that consume bicarbonate stores.
Bicarbonate Loss
Bicarbonate loss occurs primarily through the gastrointestinal tract or the kidneys. Severe, prolonged diarrhea is a frequent cause, as the lower intestinal tract naturally secretes bicarbonate, leading to base depletion. Kidney problems can also cause low bicarbonate if the kidneys fail to excrete the normal daily acid load or waste bicarbonate into the urine. This is relevant in chronic kidney disease, where the declining ability to remove acid relies on bicarbonate stores for buffering. Specific kidney disorders, like renal tubular acidosis, involve a failure to properly reabsorb bicarbonate or excrete acid, resulting in persistent base loss.
When to Seek Medical Attention
A low bicarbonate result on a routine blood test warrants follow-up with a healthcare provider, but the urgency depends on how low the value is and whether symptoms are present. For mild decreases, such as values between 19 and \(21\ \text{mEq/L}\), the first step is often to investigate the cause through clinical history and monitoring. The doctor may use other electrolyte levels from the blood panel to calculate the Anion Gap, which helps determine whether the low bicarbonate is due to an added acid or a loss of base, narrowing down the potential causes of metabolic acidosis.
If the low bicarbonate level is severe (below \(18\ \text{mEq/L}\)) or accompanied by concerning symptoms, immediate medical attention is necessary. Warning signs associated with severe acidosis include confusion, extreme fatigue, or a noticeable change in breathing, such as rapid, deep, labored breaths. These symptoms indicate that the body’s compensatory mechanisms are struggling to maintain a safe \(\text{pH}\) level. Seeking prompt care allows for the underlying cause to be identified quickly and for appropriate measures to be taken to restore the acid-base balance.