Oxygenation & Ventilation Are Not the Same Thing

I recently observed a paramedic who made one of the most difficult prehospital clinical decisions I have seen during my 16 years of involvement in prehospital care. We were notified that a motor vehicle crash patient was inbound by helicopter, Category A (the highest in Maryland). Details about the crash were scant, and while the patient was swiftly rolled into our trauma resuscitation unit, I nearly stepped on a slippery, blood-laden tubular object that had fallen off the backboard—the endotracheal tube.

The flight paramedic proceeded to tell me that the patient had been intubated in the field and although there had been an appropriate colorimetric change on the CO2 detection device, lung sounds were difficult to appreciate en route and capnographic waveforms were absent. Yet, the patient’s pulse oximeter continued to read 99% the entire time. Nevertheless, the paramedic pulled the tube shortly before arrival, and proceeded to mask ventilate the patient with an oral airway. One might ask, “What on Earth was this paramedic thinking?”

As it turned out, the paramedic made a difficult but supremely commendable and 100% appropriate decision to extubate the patient. The medic later admitted that he struggled with the decision to remove what was thought to be a “perfectly good endotracheal tube.” But in the end, he knew the difference between ventilation and oxygenation, and based on his assessment, he knew that the former was not being accomplished, and that failure of the later would quickly ensue. These two separate—although highly related—processes are often confused and frequently misunderstood, and comprehending the difference is critical in the prehospital arena. Advanced technologies have an important role in monitoring ventilation and oxygenation, and an understanding of the limitations of these devices is a prerequisite for effective use.

Ventilation vs. Oxygenation
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 by 2 to 5 mmHg.(2) Use of capnography is not limited to intubated patients; nasal cannulas and face masks can be modified to detect EtCO2.

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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 it is expired or if the tube is clogged with secretions. Causes of increased or decreased EtCO2 are listed in Table 1.(2) 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) For a more in-depth discussion of the physics and use of capnography in the prehospital setting, visit www.capnography.com.

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) An easy way to remember the correlation between SpO2 and approximate partial pressure of oxygen in arterial blood (PaO2) is presented in Table 2.

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)

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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
1. Neumar RW, Otto CW, Link MS, et al. Part 8: Adult advanced cardiovascular life support, 2010 American Heart Association Guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2010;122[Suppl 3]:S729–S767.
2. Galvagno SM, Kodali BS. Critical monitoring issues outside the operating room. Anesthesiology Clin. 2009;27(1):141–156.
3. Lewis LM, Stothert J, Standeven J, et al. Correlation of end-tidal carbon dioxide to cerebral perfusion during CPR. Ann Emerg Med. 1992;21(9):1131–1134.
4. Callaham M, Barton C. Prediction of outcome of CPR from end-tidal carbon dioxide concentration. Crit Care Med. 1990;18(4):358–362.
5. Sanders AB, Atlas M, Wy GA, et al. Expired PCO2 as an index of coronary perfusion pressure. Am J Emerg Med. 1985;3(2):147–149.
6. Galvagno SM. Emergency Pathophysiology. Jackson, Wyo.: Teton NewMedia, 2004.
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, Møller AM. Pulse oximetry for perioperative monitoring. Cochrane Database Syst Rev. 2003;3: CD002013.
11. Tinker J, Dull D, Caplan R, et al. Role of monitoring devices in prevention of anesthetic mishaps: a closed claims analysis. Anesthesiology. 1989;71(4): 541–546.