Low CO2 Symptoms: Causes, Signs & What to Do
Low CO2 on a metabolic panel means metabolic acidosis -- a buildup of acid in the blood from causes like DKA, kidney disease, or diarrhea. This page covers the specific symptoms, likely causes, normal ranges, and when to act.
CO2 on a comprehensive metabolic panel (CMP) measures serum bicarbonate (HCO3-). A low CO2 result (below 22 mEq/L) indicates metabolic acidosis — an excess of acid in the bloodstream that has consumed the bicarbonate buffer. This is a significant finding that always warrants investigation into the specific cause, which can range from diabetic ketoacidosis to chronic kidney disease to simple diarrhea. See the Bicarbonate biomarker overview for how serum CO2 is measured.
What Low CO2 Means
Bicarbonate falls when the body is producing excess acid or losing bicarbonate directly. The body compensates by increasing the breathing rate to blow off CO2 gas and lower carbonic acid (this is Kussmaul breathing in severe cases). The cause of metabolic acidosis is identified by calculating the anion gap: high anion gap metabolic acidosis (from unmeasured anions like ketones, lactate, or toxins) versus normal anion gap metabolic acidosis (from direct bicarbonate loss or failure to excrete H+).
Symptoms of Low CO2
Mild metabolic acidosis (18-22 mEq/L):
- Often asymptomatic or minimal symptoms
- Mild fatigue and weakness
Moderate metabolic acidosis (10-18 mEq/L):
- Rapid or deep breathing (the body trying to blow off CO2 to compensate)
- Nausea, vomiting, and loss of appetite
- Fatigue and weakness
- Headache
- Confusion or difficulty concentrating
Severe metabolic acidosis (below 10 mEq/L):
- Kussmaul breathing — deep, labored, rapid breathing with a characteristic sighing quality
- Fruity breath odor (ketones, in DKA)
- Severe confusion and altered consciousness
- Cardiac arrhythmias
- Shock and cardiovascular collapse (in lactic acidosis)
What Causes Low CO2
High anion gap metabolic acidosis (acid production exceeds excretion):
- Diabetic ketoacidosis (DKA) — most common cause in type 1 and type 2 diabetes; ketones are unmeasured anions that widen the gap
- Lactic acidosis — from sepsis, shock, tissue hypoxia, metformin toxicity in kidney failure, or ischemia
- Starvation ketosis (milder than DKA, bicarb usually above 18 mEq/L)
- Salicylate toxicity (aspirin poisoning — complex mixed picture)
- Toxic alcohol ingestion (methanol or ethylene glycol)
- Uremia from kidney failure (impaired acid excretion and organic acid accumulation)
Normal anion gap metabolic acidosis (bicarbonate loss or failure to excrete H+):
- Diarrhea — direct loss of bicarbonate from the gut (intestinal secretions are alkaline)
- Renal tubular acidosis (RTA) — kidneys fail to excrete H+ or reabsorb bicarbonate
- Large volume saline infusion (dilutional acidosis)
- Adrenal insufficiency (low aldosterone reduces H+ excretion)
Normal CO2 (Bicarbonate) Levels
| Measure | Reference Range |
|---|---|
| Serum bicarbonate (CO2) | 22-29 mEq/L |
| Mild concern | 18-22 mEq/L |
| Moderate acidosis | 10-18 mEq/L |
| Severe acidosis | Below 10 mEq/L |
When to See Your Care Team
Book a 1:1 consultation with a licensed care team lead for CO2 below 22 mEq/L confirmed on repeat testing. CO2 below 18 mEq/L requires prompt evaluation — calculate the anion gap immediately. CO2 below 15 mEq/L with symptoms is a medical emergency. If the patient has diabetes, suspect DKA and check glucose and ketones urgently. Lactic acidosis in the context of sepsis or cardiovascular compromise is a critical emergency.
Frequently Asked Questions
What is the anion gap and why does it matter when CO2 is low?
The anion gap = sodium - (chloride + bicarbonate). Normal is 8-12 mEq/L. When extra acid (unmeasured anions like ketones or lactate) accumulates, the gap widens because these acids consume bicarbonate while the sodium stays the same. High anion gap acidosis = DKA, lactic acidosis, uremia, or toxins. Normal anion gap acidosis = diarrhea, renal tubular acidosis, or saline infusion. The distinction immediately narrows the cause.
What is Kussmaul breathing?
Kussmaul breathing is a deep, labored, and rapid breathing pattern the body uses to compensate for severe metabolic acidosis. By breathing faster and deeper, the body exhales more CO2 gas, which reduces carbonic acid in the blood and partially corrects the pH. It is a visible sign of severe metabolic acidosis and is classically associated with DKA.
Can diarrhea cause low CO2?
Yes. Unlike stomach acid (which is acidic), intestinal secretions are rich in bicarbonate. Severe or prolonged diarrhea directly removes bicarbonate from the body, producing normal anion gap metabolic acidosis. This is especially significant in children and people with cholera or secretory diarrhea, where bicarbonate losses can be massive.
Does metformin cause low CO2?
Not under normal conditions. However, metformin inhibits hepatic lactate clearance and, in patients with kidney failure (where metformin accumulates to toxic levels), it can cause severe lactic acidosis. This is the reason metformin is contraindicated in patients with eGFR below 30 mL/min/1.73m2 and should be used cautiously below 45 mL/min/1.73m2.