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Assessing Ventilation and
Blood flow with Capnography


Tom Ahrens, PhD, RN, FAAN

Research Scientist 

Barnes Jewish Hospital

St. Louis, MO


Capnography has the unique ability to aid clinicians in assessing both ventilation and blood flow.  In this program, a review of the physiology that allows exhaled CO2 to monitor ventilation and perfusion is presented.  The emphasis in this program is using capnography to prevent over-sedation and monitoring of blood flow. Due to the ability to assess both ventilation and perfusion, capnography has been called the “15 second vital sign”.  In the midst of the pandemic, capnography has shown to detect severe respiratory failure (hypercarbia) and pulmonary emboli due to the clotting disorders in COVID. Actual clinical patient examples are used to illustrate the value of capnography in patient assessment.  The use of capnography is likely to continue to grow throughout the hospital, outpatient and pre-hospital settings.  


Learning Objectives:

After this activity, the participant will be able to:

  1. Recall the relationship between PaCO2 and PetCO2 levels

  2. Discuss how the reason why capnography is a rapid assessment of inadequate ventilation and perfusion

  3. Discuss how capnography can reflect a patient at risk for over-sedation

  4. Describe how to use capnography in monitoring blood flow

Faculty Bio

Tom Ahrens, PhD, RN, FAAN is a Research Scientist at Barnes Jewish Hospital in St. Louis, MO. Dr. Ahrens has more than 25 years of experience as a critical care nurse, and is the author of five books, more than 100 papers, and more than 40 scientific publications. His book Essentials of Oxygenation received the Book of the Year Award from the American Journal of Nursing. He is also a grant reviewer for the National Institutes of Health and formerly served on the board of directors for AACN. He is a recognized authority in sepsis and has given numerous lectures around the country on the subject. Dr. Ahrens is a Fellow of the AAN which named him one of its first Edge Runners in 2006; the award recognizes innovations resulting in better care for patients, families, and the community.

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