Guide — Emergency Respiratory Care
Carbon Monoxide Poisoning & Smoke Inhalation
The poisoning a pulse oximeter cannot see — how carbon monoxide binds hemoglobin and left-shifts the curve, why SpO₂ reads falsely normal, the role of co-oximetry and 100% oxygen, and the airway threat of smoke inhalation and inhalation injury. When the monitor looks fine and the patient does not, this is the picture to recognize.
9 min read · Emergency Respiratory Care
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Carbon monoxide (CO) is a leading cause of poisoning death. It binds hemoglobin with roughly 200–250 times the affinity of oxygen, forming carboxyhemoglobin (COHb), which both reduces oxygen-carrying capacity and left-shifts the oxyhemoglobin dissociation curve — impairing oxygen release at the tissues. Smoke inhalation adds thermal and chemical airway injury and the possibility of cyanide toxicity. Real emergencies follow current ACLS and burn protocols and physician orders; this guide builds the recognition pattern behind them.
Key Concepts
- CO pathophysiology. CO forms COHb, which means fewer hemoglobin binding sites for oxygen and a leftward shift of the dissociation curve, producing tissue hypoxia despite a normal PaO₂.
- Pulse oximetry is falsely normal. A standard two-wavelength oximeter cannot distinguish COHb from oxyhemoglobin, so SpO₂ reads high while the patient is hypoxic. Diagnosis requires CO-oximetry, which measures COHb directly.
- Know the levels. Normal COHb is under about 3% in nonsmokers and up to about 10% in smokers. Symptoms progress roughly from headache and nausea (mild), to confusion and dyspnea (moderate), to syncope, seizure, and coma (severe).
- The ABG misleads too.The PaO₂ is often normal because dissolved oxygen is unaffected — another reason the gas misleads. A calculated SaO₂ also misses it; you need a measured COHb (co-oximetry).
- Smoke inhalation is layered. Upper-airway thermal injury (stridor and edema — secure the airway early before it closes), lower-airway chemical injury, and possible cyanide poisoning from burning synthetics, which causes a lactic acidosis.
| COHb | Group | Findings |
|---|---|---|
| Under ~3% | Normal (nonsmoker) | Baseline; no symptoms attributable to CO |
| Up to ~10% | Normal (smoker) | Chronic low-grade elevation from tobacco smoke |
| Mild | Symptomatic | Headache and nausea |
| Moderate | Symptomatic | Confusion and dyspnea |
| Severe | Symptomatic | Syncope, seizure, and coma |
Assessment & Findings
- History points the way.A fire, faulty heater, enclosed space, or multiple victims, with headache, nausea, or confusion. The classic “cherry-red” skin is a late and unreliable sign.
- Trust the discrepancy, not the SpO₂. A falsely normal SpO₂ with clinical hypoxia should raise suspicion for CO — confirm with co-oximetry (a COHb level).
- Airway-burn red flags. Facial burns, singed nasal hairs, carbonaceous sputum, hoarseness, and stridor — intubate early.
RT Priorities & Interventions
- 100% oxygen immediately. Give 100% oxygen by non-rebreather (or through an endotracheal tube) — it dramatically shortens the CO half-life, from about 4–5 hours on room air to roughly 60–90 minutes on 100% oxygen, and to about 20–30 minutes with hyperbaric oxygen.
- Consider hyperbaric oxygen for severe poisoning. Loss of consciousness, neurologic signs, COHb over 25%, cardiac involvement, or pregnancy with elevated COHb.
- Secure the airway early in inhalation injury. Intubate before edema closes the airway; anticipate bronchospasm and the need to clear secretions and casts (bronchoscopy, suctioning, bronchodilators, humidification).
- Think cyanide in an enclosed-space fire. Consider cyanide toxicity (treated with hydroxocobalamin) when a persistent lactic acidosis follows an enclosed-space fire.
| Therapy | CO Half-Life |
|---|---|
| Room air | ~4-5 hours |
| 100% oxygen (non-rebreather / ETT) | ~60-90 minutes |
| Hyperbaric oxygen | ~20-30 minutes |
Common Pitfalls
- Trusting the pulse oximeter in suspected CO poisoning — it reads falsely normal; use co-oximetry.
- Waiting for airway edema to declare itself in a burn patient — intubate early while you still can.
- Taking reassurance from a normal calculated SaO₂ on the ABG, which is derived from PaO₂ and misses COHb.
Board Exam Pearls
- CO binds hemoglobin about 200–250 times more tightly than oxygen, forms COHb, and left-shifts the curve.
- SpO₂ is falsely normal — diagnose with CO-oximetry.
- Treatment is 100% oxygen (which shortens the CO half-life); hyperbaric oxygen for severe cases.
- Secure the airway early in inhalation injury (progressive edema).
- Suspect cyanide (enclosed-space fire plus lactic acidosis) and treat with hydroxocobalamin.
FAQ
Why is SpO₂ normal in carbon monoxide poisoning?
A standard pulse oximeter uses two wavelengths and cannot tell carboxyhemoglobin from oxyhemoglobin, so it counts CO-bound hemoglobin as “saturated” and reads falsely high. The patient is hypoxic; diagnosis needs co-oximetry, which measures carboxyhemoglobin directly.
How does oxygen treat CO poisoning?
100% oxygen displaces CO from hemoglobin and shortens its half-life from about 4-5 hours on room air to roughly 60-90 minutes, and to about 20-30 minutes with hyperbaric oxygen. Start a non-rebreather (or intubate) immediately on suspicion.
When should I worry about the airway in a burn patient?
Facial burns, singed nasal hairs, carbonaceous sputum, hoarseness, or stridor signal inhalation injury with progressive airway edema. Secure the airway early - once swelling advances, intubation can become impossible.
Does a normal PaO₂ rule out CO poisoning?
No. CO does not change the dissolved oxygen, so the PaO₂ stays normal and a calculated SaO₂ misses the problem. Only a measured carboxyhemoglobin level (co-oximetry) reveals it.
Put it to work
The gas plus co-oximetry tell the real story when the pulse oximeter lies. Practice the interpretation.
Open the ABG Interpreter →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Medical gas therapy, carbon monoxide, and hyperbaric oxygen chapters.
- Rose JJ, Wang L, Xu Q, et al. Carbon Monoxide Poisoning: Pathogenesis, Management, and Future Directions of Therapy. Am J Respir Crit Care Med. 2017;195(5):596-606.