In critically ill patients, CO 2 production may increase from such conditions as sepsis, fever, seizures, agitation, and carbohydrate overloading. Measured CO 2 also is influenced by CO 2 production. To visualize ventilation and perfusion, see Lung zones. This may result from such ventilatory problems as high mean airway pressure or inadequate exhalation time (resulting in overdistention), or from such circulatory problems as shock, massive fluid loss, or pump failure (resulting in hypotension). Dead-space ventilation results in ventilated alveoli with insufficient perfusion, which leads to low ETco 2. That’s why it’s important to know the patient’s arterial CO 2 baseline, track trends, and correlate ETco 2 values with ABG values in the event of sudden or dramatic deviations.Įnd-tidal clearance must be evaluated in the context of the patient’s perfusion status. (Not all alveoli participate in gas exchange at a given time, resulting in some shunting.) In acutely ill patients, especially those on mechanical ventilators, pronounced changes in ventilation and/or perfusion may occur, causing a much greater disparity between arterial and end-tidal values. Circulating blood CO 2 is slightly greater than exhaled CO 2 due to a ventilation-perfusion (.V/Q) mismatch. In patients with normal pulmonary function, CO 2 (normally 35 to 45 mm Hg) and ETco 2 should correlate closely, with a deviation of about 2 to 5 mm Hg. Ventilation/perfusion matchingĮTco 2 reflects metabolism, circulation, and ventilation. Because inspired air has negligible CO 2 amounts (less than 0.04%), exhaled CO 2 can be monitored and used as a correlate to assess CO 2 gas exchange and effective ventilation. As a result, oxygen from the alveolus (where Po 2 is about 100 mm Hg) diffuses into the blood, and CO 2 diffuses from the blood into the alveolus (where Pco 2 measures about 40 mm Hg). In the pulmonary artery, deoxygenated blood has a partial pressure of carbon dioxide (Pco 2) of approximately 46 mm Hg and a partial pressure of oxygen (Po 2) of roughly 40 mm Hg. Gases diffuse from areas of higher concentrations to areas of lower concentration. The lungs serve as a pump to promote the activity of ventilation. This process occurs in red blood cells, where HCO 3– is released back into the plasma ready to accept another hydrogen ion (H+), while CO 2 and H 2O are carried to the arterial-alveolar junction for release into the atmosphere. While ETco 2 monitoring doesn’t directly indicate acid-base balance, it can shed light on ventilation efficacy.ĬO 2 combines with water to create H 2CO 3, which can degrade to bicarbonate, water, and CO 2. Both pH and CO 2 are measured from arterial blood gas (ABG) samples. As HCO 3-, CO 2 influences blood pH direct CO 2 measurement indicates ventilatory effectiveness. Depending on blood pH at any given time, CO 2 converts either to carbonic acid (H 2CO 3, an acid) or to bicarbonate (HCO 3-, a base).ĬO 2 exists in three primary states in the blood: as HCO 3– (70%), bound to hemoglobin (20%), and dissolved in the plasma (10%). CO 2 plays an important role in acid–base buffering. Cells take in oxygen and glucose and release water, carbon dioxide, and energy. The science behind the techniqueĬO 2 is a byproduct of cellular metabolism. Keep in mind that ETco 2 findings are interpreted within the context of physical and other traditional assessment methods. Although ETco 2 is used in non-intubated patients, here we focus on its use in patients with endotracheal intubation. This article explains these concepts, discusses ETco 2 waveforms, and describes the assessment capabilities of this monitoring method. To fully optimize its use, clinicians must understand certain basic principles. Although the principles underlying ETco 2 monitoring have been known for many years, recent technological advances have made the technique feasible for routine clinical application. Wherever ETco 2 monitoring is used, it can enhance patient safety-as long as bedside clinicians understand the interplay among ventilation, perfusion, and CO 2 production. Author Guidelines and Manuscript Submission.
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