Ventilation and oxygenation are separate physiological processes. Ventilation is the act or process of inhaling and exhaling. To evaluate the adequacy of ventilation, a provider must exercise eternal vigilance. Chest rise, compliance (as assessed by the feel of the bag-valve mask), and respiratory rate are qualitative clinical signs that should be used to evaluate the adequacy of ventilation. Capnography, long the standard of care in the operating room and intensive care unit, can also be used to assess ventilation. Also, continuous quantitative waveform capnography has become the standard of care for monitoring endotracheal tube placement.(1) Capnography can be used to assess end-tidal carbon dioxide (EtCO2) concentration or tension. Normal values of EtCO2 are 35—37 mmHg, and in normal lungs, the EtCO2 approximates the arterial CO2 concentration in the blood with a value that is usually lower 2—5 mmHg.(2) Use of capnography is not limited to intubated patients; nasal cannulas and face masks can be modified to detect EtCO2.
EtCO2 can be measured by colorimetry and capnography. Colorimetric devices provide continuous, semi-quantitative EtCO2 monitoring. A typical device has the following three color ranges:
- Purple–EtCO2 is less than 0.5%
- Tan–EtCO2 is 0.5—2%
- Yellow–EtCO2 is greater than 2%
Normal EtCO2 is greater than 4%; hence, the device should turn yellow when the endotracheal tube is inserted in patients with intact circulation.(2) False positives may occur when the device is contaminated with acidic substances, such as gastric acid, lidocaine or epinephrine. The device will not provide an accurate reading if it is expired or if the tube is clogged with secretions. One of the most common causes of increased EtCO2 is hypoventilation, since CO2 cannot be removed from the body when air exchange is impaired.
Capnography provides both a waveform and digital reading (mmHg of CO2 in exhaled gas). Capnography is no longer merely a standard for the operating rooms; it is a standard for ensuring ventilation after intubation anywhere, and it is now a fundamental objective means for assessing the adequacy of CPR.(1) For example, if the EtCO2 is less than 10 mmHg, the American Heart Association recommends optimizing chest compressions to improve the quality of CPR.(1,3—4) Capnography has prognostic value for trauma and cardiac arrest patients, and it correlates well with such other physiologic parameters as coronary perfusion pressure and cardiac output.(5)
Oxygenation refers to the process of adding oxygen to the body system. There is no way to reliably measure arterial oxygenation via clinical signs alone. Cyanosis, pallor and other physical findings are not reliable. The pulse oximeter, which relies on a spectral analysis of oxygenated and reduced hemoglobin as governed by the Beer-Lambert law, represents the principle means of assuring adequate oxygenation in a patient.(2) Saturation of peripheral oxygen (SpO2) levels measured with a pulse oximeter correlate highly with arterial oxygenation concentrations.(6)
Despite years of use in a wide variety of settings, even experienced physicians and nurses have significant knowledge deficits regarding the limitations and interpretation of pulse oximetry.(7—9) Pulse oximetry has several limitations. Hypoxia follows hypoventilation, and it may take 30 seconds or more for the pulse oximeter to reflect conditions of life-threatening hypoxia. Relying on the pulse oximeter alone can decrease the margin of safety because corrective actions taken after the pulse oximeter falls may be too late. Hypovolemia, vasoconstriction, peripheral vascular disease or nail polish may cause false readings. It should be noted that pulse oximetry, while a significant technological advance over the past 20 years, has not been reliably shown in all studies to improve outcomes.(10) However, in studies based on closed claims data (i.e., lawsuits), the use of pulse oximetry, at least in the operating room, has been suggested to reduce the serious mishap rate by at least 35%.(11)
Conclusion
Ideally, when monitoring ventilation and oxygenation in the prehospital environment, capnography should be combined with pulse oximetry. With capnography, providers are able detect respiratory insufficiency early and are able to institute early interventions, thereby preventing arterial oxygen desaturation. However, as with any monitoring technology, the best “monitor” is the provider. Pulse oximeters and capnometers do not treat patients. Integrating the information from your monitors and clinical assessment to make sound clinical decisions is the key to successful airway management. As evidenced by the astute assessment and action of a paramedic, knowing the difference between ventilation and oxygenation is a critical concept that must be understood.
References
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4. Callaham M, Barton C. Prediction of outcome of CPR from end-tidal carbon dioxide concentration. Crit Care Med. 1990;18(4):358—362.
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7. Sinex JE. Pulse oximetry: Principles and limitations. Am J Emerg Med. 1999;17(1):59—67.
8. Elliot M, Tate R, Page K. Do clinicians know how to use pulse oximetry? A literature review and clinical implications. Aust Crit Care. 2006;19(4):139—44.
9. Stoneham M, Saville G, Wilson I. Knowledge about pulse oximetry among medical and nursing staff. Lancet. 1994;344(8933):1339—1342.
10. Pedersen T, Dyrlund Pedersen B, Mà¸ller AM. Pulse oximetry for perioperative monitoring. Cochrane Database Syst Rev. 2003;3: CD002013
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