What end-tidal CO2 (etco2) level should you expect in an opioid overdose?

Expected End-Tidal CO2 Levels in Opioid Overdose

In opioid overdose, you should expect elevated end-tidal CO2 (ETCO2) levels above 50 mmHg, which indicates significant respiratory depression and hypoventilation. 1

Pathophysiology of Opioid-Induced Ventilatory Impairment

Opioid overdose causes ventilatory impairment through three primary mechanisms:

1. Depression of the respiratory center in the brainstem, leading to reduced alveolar ventilation (decreased respiratory rate and/or tidal volume)
2. Reduced oropharyngeal muscle tone, resulting in upper airway obstruction
3. Depression of the hypothalamus, causing increased arousal thresholds and sedation 1

These mechanisms lead to type-2 respiratory failure with elevated arterial partial pressures of carbon dioxide (hypercapnia), which is reflected in increased ETCO2 levels.

ETCO2 Values in Opioid Overdose

• Normal ETCO2: 35-45 mmHg
• Opioid overdose: ETCO2 > 50 mmHg 1
• Severe respiratory depression: ETCO2 > 50 mmHg, absent waveform, or absolute change from baseline > 10 mmHg 1

In a study of patients receiving procedural sedation with opioids, all patients with respiratory depression demonstrated an ETCO2 greater than 50 mmHg, an absent waveform, or an absolute change from baseline in ETCO2 greater than 10 mmHg 1.

Monitoring Considerations

ETCO2 vs. Other Monitoring Methods

• ETCO2 monitoring is superior to pulse oximetry alone for detecting opioid-induced respiratory depression. In one study, pulse oximetry would have identified only 11 of 33 patients meeting criteria for respiratory depression 1.

• Respiratory rate is an unreliable indicator of opioid-induced ventilatory impairment. Small case series have demonstrated that neither respiratory rate nor oxygen saturation correlated with arterial partial pressures of carbon dioxide, while sedation levels did 1.

• Sedation level is a better clinical indicator of opioid-induced ventilatory impairment than respiratory rate 1.

Research Evidence on ETCO2 in Opioid Use

A study on prescription opioid overdose showed that:

• Fentanyl had the highest risk (83.3%) of causing severe respiratory depression
• Codeine had the lowest risk (3.6%) 2

Another study examining heroin users found that:

• Significant respiratory depression (indicated by ETCO2 increases) was observed in 8/10 patients 30 minutes after receiving their prescribed injectable opioid
• SpO2 (oxygen saturation) showed significant depression in only 4/10 patients
• ETCO2 was more reliable than SpO2 for detecting respiratory depression 3

Clinical Application

When managing a patient with suspected opioid overdose:

1. Use capnography (ETCO2 monitoring) as the primary method to detect hypoventilation
2. Look for ETCO2 > 50 mmHg as an indicator of significant respiratory depression
3. Monitor for changes from baseline > 10 mmHg in ETCO2, which also indicates respiratory depression
4. Assess sedation level as it correlates better with ventilatory impairment than respiratory rate
5. Do not rely solely on pulse oximetry or respiratory rate as they may not detect significant hypoventilation, especially if supplemental oxygen is being administered

Pitfalls and Caveats

• Hypoxemia may be a very late sign of hypoventilation, especially if the patient is receiving supplemental oxygen 1
• ETCO2 values obtained through capnography may not always correlate perfectly with arterial partial pressures of carbon dioxide 1
• Patients with underlying lung disease (particularly chronic airflow obstruction) may be at higher risk for acute opioid-induced respiratory depression 3
• The specific opioid involved significantly affects the risk of respiratory depression, with synthetic opioids like fentanyl carrying the highest risk 2

Remember that opioid-induced ventilatory impairment is potentially fatal if unrecognized and untreated, but harm is preventable in most cases if detected and managed at an early stage 1.