emDOCs Podcast – Episode 84: Capnography

Background:

• 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

Interpretation

• 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 ↓

Common uses: 

• 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

Other indications:

EtCO2 should only be used as an adjunct at the bedside in these conditions. 

• Trauma
  • 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
• DKA
  • ↓ EtCO₂ levels = ↑likelihood of diagnosis.   
    • • EtCO₂ > 35 = 98% sens for ruling out DKA
      • EtCO₂ < 24 = 90% sens/spec for diagnosing DKA
• Sepsis
  • EtCO₂ < 31 associated with high mortality
• PE
  • Increased dead space = ↓ EtCO₂  
  • EtCO₂ > 32–36 + low probability Wells score – may rule out PE
  • Another tool  with pre-test probability and D-dimer
  • More studies needed
• 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
• Seizure
  • Useful for monitoring of respiratory status

Limitations:

• 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.

Pearls:

• 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.

Further Reading:

1. http://www.emdocs.net/capnography-useful-ed-part/
2. http://www.emdocs.net/capnography-useful-ed-part-ii/

References: 

1. Long B, Koyfman A, Vivirito MA. Capnography in the Emergency Department: A Review of Uses, Waveforms, and Limitations. J Emerg Med. 2017;53(6):829-842. PMID: 28993038
2. Manifold CA, Davids N, Villers LC, Wampler DA. Capnography for the nonintubated patient in the emergency setting. J Emerg Med. 2013;45(4):626-632. PMID: 23871325
3. Nassar BS, Schmidt GA. Capnography During Critical Illness. Chest. 2016;149(2):576-585. PMID: 26447854
4. Godwin SA, Burton JH, Gerardo CJ, et al. Correction: Correction to ‘Clinical Policy: Procedural Sedation and Analgesia in the Emergency Department’ [Annals of Emergency Medicine 63 (2014) 247-258.e18]. Ann Emerg Med. 2017;70(5):758. PMID: 28395927
5. Panchal AR, Berg KM, Hirsch KG, et al. 2019 American Heart Association Focused Update on Advanced Cardiovascular Life Support: Use of Advanced Airways, Vasopressors, and Extracorporeal Cardiopulmonary Resuscitation During Cardiac Arrest: An Update to the American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2019;140(24):e881-e894. PMID: 31722552
6. Soleimanpour H, Taghizadieh A, Niafar M, Rahmani F, Golzari SE, Esfanjani RM. Predictive value of capnography for suspected diabetic ketoacidosis in the emergency department. West J Emerg Med. 2013;14(6):590-594.  PMID: 24381677
7. Bou Chebl R, Madden B, Belsky J, Harmouche E, Yessayan L. Diagnostic value of end tidal capnography in patients with hyperglycemia in the emergency department. BMC Emerg Med. 2016;16:7. Published 2016 Jan 29. PMID: 26821648
8. Hemnes AR, Newman AL, Rosenbaum B, et al. Bedside end-tidal CO2 tension as a screening tool to exclude pulmonary embolism. Eur Respir J. 2010;35(4):735-741. PMID: 19717480
9. Riaz I, Jacob B. Pulmonary embolism in Bradford, UK: role of end-tidal CO2 as a screening tool. Clin Med (Lond). 2014;14(2):128-133. PMID: 24715122