emDOCs Podcast – Episode 84: Capnography
- Aug 22nd, 2023
- Brit Long
Today on the emDOCs cast with Brit Long, MD (@long_brit), we discuss capnography.
Episode 84: Capnography
- Capnography is the noninvasive measurement of carbon dioxide partial pressure.
- Displayed as colorimetric/qualitative, quantitative and waveform
- Normal range for EtCO2 35-45 mm Hg
- Waveform has several different phases:
- Phase 0 – inspiration – clearing of CO2
- Phase I – expiration – anatomical dead space, should not contains CO2
- Phase II – expiration – rapid rise in CO2 concentration, as breath reaches upper airway from the alveoli
- Phase III – expiration – CO2 concentration reaches uniform levels in the airway, height and slope of the line offer important information
- Three aspects of capnography:
- EtCO2 maximum number/plateau
- Shape of the capnogram
- Difference/gradient between EtCO2 and arterial CO2 pressure
- Capnography reading affected by:
- CO2 production & transport, ventilation, and vent-perf ratio changes
- Vent settings/malfunctions, tubing obstruction/disconnection/leaks & monitor malfunction
- ETCO2 correlates with blood pressure, lactate & base excess in the critically ill.
- Mortality rate ↑ as EtCO2 levels ↓
- Confirmation of ETT placement
- Sensitivity ~ 100% for placement in trachea
- More reliable than fogging of ETT, chest wall movement, breath sounds
- Monitoring ETT during transport – quickly detect displacement
- Cardiac arrest
- EtCO2 assesses chest compression efficacy
- The level of EtCO2 = cardiopulmonary blood flow
- ACLS guidelines: quantitative waveform to monitor compressions & ROSC
- Level >20 mmHg → higher chance of ROSC
- Levels <10 mmHg → lower chance of survival
- Gradual↓ in EtCO2 = compressions not optimal or other complications (hemorrhage, pneumothorax, tamponade, myocardial infarction)
- Sudden ↑ ≥10 mm Hg is specific for ROSC
- Does not determine cause of arrest
- PQRST Mnemonic for capnography in cardiac arrest
- Position of tube – more reliable than other options
- Quality of compressions – valuable feedback in real time
- ROSC – sudden ↑ in value suggests ROSC
- Strategy for further treatment – trends + other diagnostics help decide on diagnosis & treatment (vasopressors, fluids, thrombolytics – trying to optimize perfusion)
- Termination of resuscitation – <10 mmHg associated with death
- Procedural sedation
- Assess ventilation at the bedside
- Detects hypoventilation faster than pulse oximetry
- Respiratory depression manifests as↑EtCO2
- ACEP recommends capnography as adjunct to pulse oximetry
EtCO2 should only be used as an adjunct at the bedside in these conditions.
- Associated with severity of injury and need for blood transfusion
- ↓ EtCO2 = ↑ likelihood of needing OR
- EtCO2 < 25 mm Hg =↓ cardiac output, ↓ BP & ↑ mortality
- Fluid responsiveness
- 2 mmHg ↑ EtCO2 with straight leg raise = patient will be fluid responsive
- EtCO2 < 31 associated with high mortality
- Obstructive airway disease
- Bronchospasm – steep phase III (shark fin capnogram)
- Early on ↓ EtCO2 – breathing off extra CO2
- Later ↑EtCO2 – poor ventilation, severe exacerbation
- Monitor treatment – shark fin disappears with improvement of bronchospasm from nebulizer/steroids
- Useful for monitoring of respiratory status
- Works well for a specific underlying issue – mixed pathophysiology makes interpretation challenging.
- In cardiac arrest, EtCO2 is dependent on compressions AND the underlying cause.
- Other organ system involvement & hypotension/low perfusion affect EtCO2.
- False positive CO2 detection can occur in esophageal intubations if the patient ingested carbonated beverages.
- Acidic solution exposure, such as stomach content fluid or vinegar, can result in qualitative color change.
- Capnography is an overall assessment of the path of CO2 through the entire body.
- Capnography picks up hypoventilation faster than pulse oximetry.
- ETCO2 correlates with blood pressure, lactate, and base excess in critically ill patients.
- Capnography has several novel uses in other disease states commonly seen in the ED such as trauma, DKA, sepsis and obstructive lung disease.
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