Here's a new series on this site: our senior residents have now begun their journey into FOAMed and are posting quality material. Here's one from Dr Pek Jen Heng of Singhealth Residency (EM). End-tidal CO2 (EtCO2) is the maximum CO2 concentration at the end of each tidal breath. The relationship of CO2 concentration to time is represented by the capnogram.
Capnography provides information about:
Ventilation (elimination of CO2 by the pulmonary system)
Perfusion (transport of CO2 through the vascular system)
Metabolism (production of CO2 by cellular metabolism)
I) Endotracheal tube (ETT) placement
The presence of a waveform indicates the ETT is through the vocal cords. A normal waveform can also occur when the tube has been placed in the right mainstem bronchus. A waveform resembling tracheal placement may be present for a few breaths with an ETT placed in the hypopharynx just above the vocal cords, but these waveforms are likely to become erratic due to movement of the ETT. A flatline waveform generally indicates esophageal placement but can occur in prolonged cardiac arrest, tracheal placement with inadequate pulmonary blood flow, or ETT obstruction.
In addition to verification and monitoring of ETT placement, capnography provides similar information about the placement of alternate advanced airway devices such as laryngeal mask airway.
II) Effectiveness of CPR and prognosis
During cardiac arrest, EtCO2 reflects pulmonary blood flow and can be used as a gauge on the effectiveness of cardiac compressions. An effective CPR leads to a higher cardiac output, causing rise in EtCO2 will rise due to increase in perfusion. This correlates with increased ROSC and survival1.
EtCO2 levels of ≤10 mmHg measured 20 minutes after the initiation of advanced cardiac life support accurately predicted death in adult patients with cardiac arrest2.
III) Return of spontaneous circulation
EtCO2 is the earliest indicator of the return of spontaneous circulation3. When the heart restarts, the dramatic increase in cardiac output, and resulting increase in perfusion, leads to a rapid increase in EtCO2. Capnography eliminates the need to interrupt chest compressions to for pulse check.
IV) Cause of cardiac arrest
EtCO2 may also be helpful in determining the etiology of cardiac arrest. Patients with asphyxia (initial rhythm of asystole or pulseless electrical activity associated with conditions such as airway foreign body, aspiration) had higher EtCO2 levels compared with patients in the ventricular tachycardia/fibrillation group (associated with acute myocardial infarction)4.
V) Head injury and suspected increased intracranial pressure (ICP)
High CO2 levels (≥50 mmHg) result in cerebral vasodilation and raised ICP, while low CO2 levels (≤30 mmHg) result in cerebral vasoconstriction and is associated with worse neurologic outcome in severely brain-injured patients. Ventilation rates should be set to achieve eucapnia5.
VI) Determining prognosis in trauma
EtCO2 is a marker of pathophysiological states encountered in trauma since it reflects cardiac output. EtCO2 values obtained 20 minutes after intubation distinguished the great majority of survivors from nonsurvivors who required prehospital intubation. Median EtCO2 was higher among survivors6.
SPONTANEOUSLY BREATHING PATIENTS
I) Procedural sedation
Capnography can rapidly detect the adverse airway and respiratory events during procedural sedation including: apnea, upper airway obstruction, laryngospasm, bronchospasm, and respiratory depression.
Respiratory depression is detected by EtCO2 well before pulse oximetry detects falling oxyhemoglobin saturation, especially in patients receiving supplemental oxygen.
II) Obtunded or unconscious patients
These patients may have impaired ventilatory function as a result of conditions ranging from drug overdose to postictal state. Capnography can differentiate between patients with effective ventilation and those with ineffective ventilation.
In particular, EtCO2 provides a reliable assessment of a patient’s ventilatory status in actively seizing and post-ictal patients, and is more sensitive than pulse oximetry in predicting a trend toward respiratory failure.
III) Acute respiratory distress
Capnography provides dynamic monitoring of ventilatory status in patients with acute respiratory distress. Respiratory rate can be measured directly from the cannula, providing a more reliable reading than impedance respiratory monitoring. EtCO2 trends will provide sufficient information to determine whether the patient’s ventilation is worsening despite treatment (increasing EtCO2), stabilizing (stable EtCO2), or improving (decreasing EtCO2).
IV) Metabolic acidosis
There is a linear correlation between serum bicarbonate (HCO3) and EtCO2 in pediatric patients with diabetic ketoacidosis8 and gastroenteritis9. As the patient becomes acidotic, HCO3 decreases, which results in a compensatory respiratory alkalosis and a decrease in EtCO2. Capnography can be used as an indicator of metabolic acidosis in these patients.
In addition, EtCO2 can be used to distinguish diabetics in ketoacidosis (metabolic acidosis, compensatory tachypnea, and low EtCO2) from those who are not (nonacidotic, normal respiratory rate, and normal EtCO2).
V) Providing prognostic indicators in patients with sepsis or septic shock
There is an inverse relationship between EtCO2 and lactate levels in sepsis, severe sepsis, and septic shock. EtCO2 performs similarly to lactate as a predictor for mortality in patients with suspected sepsis10.
EtCO2 is safe, noninvasive, inexpensive, and rapidly performed at the bedside. It is an essential tool for evaluating patients in the emergency setting. Consider using it in your next patient for assessment of ventilation, perfusion and metabolism.
Sheak KR, Wiebe DJ, Leary M et al. Quantitative relationship between end-tidal carbon dioxide and CPR quality during both in-hospital and out-of-hospital cardiac arrest. Resuscitation. 2015 Apr;89:149-54.
Levine RL, Wayne MA, Miller. End-tidal carbon dioxide and outcome of out-of-hospital cardiac arrest. Engl J Med. 1997;337(5):301.
Garnett AR, Ornato JP, Gonzalez ER et al. End-tidal carbon dioxide monitoring during cardiopulmonary resuscitation. JAMA. 1987;257(4):512.
Grmec S, Lah K, Tusek-Bunc K. Difference in end-tidal CO2 between asphyxia cardiac arrest and ventricular fibrillation/pulseless ventricular tachycardia cardiac arrest in the prehospital setting. Crit Care. 2003;7(6):R139.
Brain Trauma Foundation. Guidelines for the management of severe traumatic brain injury. https://www.braintrauma.org/uploads/06/06/Guidelines_Management_2007w_bookmarks_2.pdf
Deakin CD, Sado DM, Coats TJ et al. Prehospital end-tidal carbon dioxide concentration and outcome in major trauma. J Trauma. 2004;57(1):65.
Deitch K, Miner J, Chudnofsky CR et al. Does end tidal CO2 monitoring during emergency department procedural sedation and analgesia with propofol decrease the incidence of hypoxic events? A randomized, controlled trial. Ann Emerg Med. 2010;55(3):258.
Fearon DM, Steele DW. End-tidal carbon dioxide predicts the presence and severity of acidosis in children with diabetes. Acad Emerg Med. 2002;9(12):1373.
Nagler J, Wright RO, Krauss B. End-tidal carbon dioxide as a measure of acidosis among children with gastroenteritis. Pediatrics. 2006;118(1):260.
Hunter CL, Silvestri S, Dean M et al. End-tidal carbon dioxide is associated with mortality and lactate in patients with suspected sepsis. Am J Emerg Med. 2013 Jan;31(1):64-71.