End Tidal CO2 Not All Its Cracked Up To Be The Limitations of PETCO2 In Sedation Analgesia

End Tidal CO2 Not All Its Cracked Up To Be The Limitations of PETCO2 In Sedation Analgesia

Tidal Volume Noninvasive monitoring of ventilation and exhaled carbon dioxide of a patient End Tidal CO2 Produces A number A waveform (capnogram) Two basic types Mainstream the sensor is directly placed on the patient s air passage and the sensor directly performs CO2 2 readings Sidestream the air exchange is taking place via a circuit placed in the patient s air passage and the sensor is reading CO2 values from a sampling port 2 connected to this circuit Rate Accuracy similar

End Tidal CO2 Questions Changes in respiratory rate and tidal volume will change the shape (slope, period and amplitude) of the capnogram and the number (PETCO2) How sensitive is this technology for changes in respiratory status in procedural sedation and analgesia? How accurately does the PETCO2 represent the arterial pco2 during procedural sedation and analgesia? What is the correlation between clinically significant respiratory changes and the capnogram / PETCO2 in procedural sedation and analgesia? Is end tidal CO2 monitoring predictive of respiratory events during procedural sedation and analgesia How does end tidal capnography compare to pulse ox in procedural sedation and analgesia

Problems with ETCO2 and Procedural Sedation 1. The PETCO2 and arterial pco2 do not correlate 2. The PETCO2 may increase or decrease with hypoventilation 3. Capnogram morphology change may be too small to detect significant changes in respiratory status 4. Capnogram morphology may not reflect patient respiratory status

1. PETCO2 and Arterial pco2 Do Not Correlate PETCO2 tends to underestimate the true pco2 Wright et al: Ann Emerg Med 1992 Plewa et al: Acad Emerg Med 1995 Corbo et al : Ann Emerg Med 2005 Delerme et al: Am J Emerg Med 2010 Cinar et al : Am J Emerg Med 2012 Some studies showed good correlation None were perfect All that showed good correlation were with conscious patients Best correlation was with forced expiration and mouthpiece Impossible with sedation analgesia Rule 1. The PETCO2 number is not the pco2

1. PETCO2 and Arterial pco2 Do Not Correlate Why? Air leak Its all about dead space About 150ml + the circuit Constant As tidal volume goes down dead space remains the same thus increasing the dead space fraction The sensor is distant Dead Space Fraction Dead Space Volume Tidal Volume Tidal volume of 500 (350) vs Tidal volume 200 (<50)

2. The PETCO2 May Increase or Decrease with Hypoventilation During procedural sedation analgesia hypoventilation may present in deferent ways Decreased PETCO2 (most common) Increased PETCO2 No change in PETCO2 Burton et al : Acad Emerg Med 2006 Krauss et al : Ann Emerg Med 2007 Deitch et al : Ann Emerg Med 2010 Sivilotti et al : Can J Emerg Med. 2010 Rule 2. The change in PETCO2 number can not be used to determine hypoventilation in procedural sedation

2. The PETCO2 May Increase or Decrease with Hypoventilation Why? Dead space Bradypnea vs hypopnea Dead Space Alveolar Ventilation Dead Space Alveolar Ventilation Respiratory Rate Normal Tidal Volume Appropriate CO2 Washout PETCO2 Normal Respiratory Rate Tidal Volume CO2 Washout PETCO2

Burton et al : Acad Emerg Med 2006

3. Capnogram morphology change may be too small to detect significant changes in respiratory status Data show that a 10% change in ETCO2 is near 100% sensitive for detecting hypoxic event but only 64% specific Deitch et al : Ann Emerg Med 2010 Data also show that physicians have difficulty detecting larger changes in capnogram (30%) Sivilotti et al : Can J Emerg Med. 2010 Rule 3. The capnogram alone can not be used to predict hypoxia in procedural sedation

3. Capnogram morphology change may be too small to detect significant changes in respiratory status Why?

4. Capnogram Morphology May Not Reflect Patient Respiratory Status The capnogram may be flat or have low amplitude with a normal respiratory rate and tidal volume Why? In procedural sedation analgesia a nasal piece is used Some, little or no exhaled air may reach the sensor Rule 4. The ultimate assessment in respiratory status is multifactorial

How Does ETCO2 Compare With Pulse Patient on oxygen Oximetry in Sedation Analgesia ETCO2 can detect respiratory depression before SpO2 Deitch et al : Ann Emerg Med 2010 Patient on room air The pulse oximeter can detect respiratory depression before ETCO2 Sivilotti et al : Can J Emerg Med. 2010

How Does ETCO2 Compare With Pulse Oximetry in Sedation Analgesia Reproduced from Sivilotti et al : Can J Emerg Med. 2010

Summary PETCO2 and arterial pco2 are not equal The number is generally not helpful Extreme numbers may have meaning - > 80 or < 20 The change PETCO2 number can not be used to determine hypoventilation in procedural sedation In hypoventilation the number may go up or down The capnogram alone can not be used to predict hypoxia in procedural sedation Subtle changes may indicate significant changes in respiratory status Total patient / capnogram mismatch is possible

Take Home Capnography has significant limitations Use it only in conjunction with other monitoring modalities like pulse ox Consider sedation on room air rather than oxygen

Questions?