Understanding the Gap: The Difference Between Arterial and End Tidal CO2
Capnography monitoring, as a clinician tool to help enhance patient care, is used in multiple environments — from the emergency room (ER) to the ICU. To maximize the benefits of capnography, a solid knowledge of all aspects of capnography measurements is required including reasons behind a mismatch of a patient’s end-tidal carbon dioxide (etCO2) and arterial blood gas (ABG). Understanding this delta is imperative to help determine a patient’s overall status and how to best respond with intervention.
Breaking down the basics of gradient results within capnography
The gradient, is the difference between the arterial carbon dioxide partial pressure (PaCO2) and the etCO2 partial pressure is a result of the relationship between ventilation and perfusion or, rather, ventilation-perfusion matching (V/Q).
When calculating the gradient, the clinician is comparing the carbon dioxide (CO2) sampled from the ABG to the gas sample exhaled from the lungs and displayed on a capnograph or (etCO2). In normal, healthy lungs, the match of arterial carbon dioxide and exhaled CO2 is closely correlated.
With a normal match of alveolar ventilation and perfusion, this gradient is roughly 2 to 5 mmHg, where the arterial carbon dioxide is greater than the exhaled carbon dioxide.1-3 Clinicians may, however, observe a widened or increased gradient caused by physiologic dead space ventilation or low pulmonary circulation. These clinical changes result in a V/Q mismatch.1-3
Increased dead space ventilation occurs when areas of the lung are ventilated but not perfused. This is more common among patients with pulmonary embolism or decreased cardiac output and cardiac arrest.1,2
Another explanation for the widened gradient is low pulmonary circulation or shunted perfusion. This occurs when areas of the lung are perfused but not ventilated. This occurs among patients with bronchoconstriction, pulmonary edema, or atelectasis. A combination of dead space ventilation and shunted perfusion can occur simultaneously in many critical patients, which leads to an even wider gradient. 1-3
Related: Learn more about dead space, shunted, and normal match ventilation and perfusion.
The graphics below are examples of normal match of ventilation and perfusion, dead space ventilation, and shunted perfusion.
Recognizing the gradient benefit occurs once a baseline is determined. Using capnography trending data from a gradient baseline may help reduce the number of ABGs required. It may also support clinical decisions surrounding patient treatment options and the efficacy of the chosen course of treatment.
It’s also important to keep in mind that an increasing or widened gradient may indicate a worsening in the V/Q matching, whereas a decreasing or narrowing gradient may indicate V/Q matching improvement. Observing an increase or decrease in the gradient can also offer clinicians objective data to help determine if prescribed therapy is effective.
Impacts of waveform capnography, physiological changes, and clinical conditions on the gradient
The capnogram waveform, measured PaCO2 and etCO2, offers clinicians support in identifying patients showing signs of metabolic, respiratory, and cardiovascular changes.
Other important clinical conditions that may result in the etCO2 exceeding the PaCO2 include patient exercise and large tidal volumes. When this occurs, the alveoli come in contact with the venous side of circulation from the over expansion of the lung.
Read more on the below table showing various conditions and how the waveform and gradient compare.
1. Cheifetz IM, Myers TR. Respiratory therapies in the critical care setting. Should every mechanically ventilated patient be monitored with capnography from intubation to extubation? Respir Care. 2007;52(4):423-438.
2. Gabrielli A, Layon AJ. Ventilation/perfusion abnormalities and capnography. In: Gravenstein JS, Jaffe MB, Paulus DA, eds. Capnography. Clinical aspects. England. Cambridge University Press. 2004.
3. McSwain SD, Hamel DS, Smith PB, et al. End-tidal and arterial carbon dioxide measurements correlate across all levels of physiologic dead space. Respir Care. 2010;55(3):288-293.
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