Just like in most fairy tales, some things are just too good to be true. In a 2012 study, Soleimanpour et al were able to show a strong correlation between EtCO2 and pCO2 in normal healthy adults, with only a small discrepancy of 2 to 5 mmHg less in EtCO2 as compared to pCO2 on ABG.⁸ In another study by Yosefy et al, EtCO2 successfully predicted pCO2 in emergency department patients in respiratory distress; however, these studies were conducted and proved a strong correlation between EtCO2and pCO2 in “normal, healthy” patients.⁹ In additional studies of capnography use in respiratory patients, researchers have found the correlation between EtCO2 and pCO2 is not nearly as strong when the patient has shunting or mismatch perfusion as a result of underlying pulmonary disease. It is theorized that the increased dead-space ventilation creates an increasing gradient among EtCO2 and pCO2, therefore changing the correlation between one another to a mere moderate level. In a 2011 study by Kartal et al, the agreement between EtCO2 and pCO2 in COPD patients was a discrepancy of 8.4 mmHg less in EtCO2 as compared to pCO2.¹⁰ A similar study in COPD patients in 2015 was conducted by Taghizadieh et al and showed an even greater discrepancy among EtCO2 and pCO2, as much as 23 mmHg less in EtCO2 compared to arterial pCO2.¹¹ Based on these small clinical trials looking specifically at COPD patients, the EtCO2 value is only minimally to moderately useful as a screening tool for hypercapneic respiratory failure. However, EtCO2 seems to remain consistently below pCO2 and may provide a starting point to monitor for improvement or worsening CO2 retention. Unlike asthmatics, the numerical end value of the EtCO2 capnogram can be used in your next COPD exacerbation patient to trend improvement or worsening respiratory failure, keeping in mind the correlation between EtCO2 and pCO2 is far from perfect in COPD patients.

In the next “Tricks of the Trade” column, we’ll review how to use end-tidal capnography to check orogastric/nasogastric tube placement and guide cardiopulmonary resuscitation.

Dr. D’Amore is an emergency medicine resident at St. Joseph’s Regional Medical Center in Paterson, New Jersey.

References

1. Godwin SA, Burton JH, Gerardo CJ, et al. Clinical policy: procedural sedation and analgesia in the emergency department. Ann Emerg Med. 2014;63(2):247-258.
2. Ma OJ, Rush MD, Godfrey MM, et al. Arterial blood gas results rarely influence emergency physician management of patients with suspected diabetic ketoacidosis. Acad Emerg Med. 2003;10(8):836-841.
3. Byrne AL, Bennett M, Chatterji R, et al. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014;19(2):168-175.
4. Bou Chebl R, Madden B, Belsky J, et al. Diagnostic value of end tidal capnography in patients with hyperglycemia in the emergency department. BMC Emerg Med. 2016;16:7.
5. Soleimanpour H, Taghizadieh A, Niafar M, et al. Predictive value of capnopgraphy for suspected diabetic ketoacidosis in the emergency department. West J Emerg Med. 2013;14(6):590-594.
6. 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):1373-1378.
7. Gilhotra Y, Porter P. Predicting diabetic ketoacidosis in children by measuring end-tidal CO2 via non-invasive nasal capnography. J Pediatr Child Health. 2007;43(10):677-680.
8. Soleimanpour H, Gholipouri C, Golzari SEJ, et al. Capnography in the emergency department. Emerg Med. 2012; 2(9):e123.
9. Yosefy C, Hay E, Nasri Y, et al. End tidal carbon dioxide as a predictor of the arterial PCO2 in the emergency department setting. Emerg Med J. 2004; 21(5):557-559.
10. Kartal M, Goksu E, Eray O, et al. The value of ETCO2 measurement for COPD patients in the emergency department. Eur J Emerg Med. 2011;18(1):9-12.
11. Taghizadieh A, Rahmani F, Soleimanpour H, et al. Comparison of end tidal carbon dioxide and arterial blood bicarbonate levels in patients with exacerbation chronic obstructive pulmonary disease. Thrita. 2015;4(2):e26169.