Capnography: A vital sign for the 21st century

For the past 100 years, vital signs have remained the same. We still record blood pressure, heart rate, respiratory rate, and temperature for every patient. But these signs reveal limited information about the patient’s physiology.

Take respiratory rate. A patient can have a normal rate but inadequate gas exchange. And a patient can be tachypneic or bradypneic and have either effective or ineffective gas exchange. Measurements of alveolar ventilation would tell us much more than respiratory rate does.
Blood pressure has similar limitations. A patient with a normal blood pressure can have inadequate tissue perfusion. And a patient with a low blood pressure can have adequate perfusion.

Today, newer technology allows us to be much more precise about a patient’s condition. With capnography, we can quickly and easily evaluate both pulmonary and cardiovascular status.

What is capnography?
Capnography is the measurement of exhaled carbon dioxide (CO2). A capnogram is the visual display of the exhaled CO2. It measures the CO2 level during four phases of breathing. At the end of phase 4, the alveolar emptying phase, the capnogram measures the end tidal CO2, also called PetCO2 for pressure of CO2 at end tidal or end expiration. The PetCO2 level is displayed on monitors as a numerical value.

The importance of PetCO2
PetCO2 is one of the most valuable aspects of the capnogram. With normal ventilation and perfusion in the lungs, the PetCO2 level reflects arterial CO2 (PaCO2) levels. The PetCO2 measurement is slightly below (1 to 5 mm Hg) the PaCO2. Why the difference? Because exhaled air is mixed with dead space air, which has a lower CO2 level.

As long as ventilation and perfusion are normal, the PetCO2 level doesn’t change. If ventilation to the alveoli decreases, as in respiratory depression or respiratory failure, the PaCO2 and PetCO2 levels will rise. The greater the increases in these levels, the worse the alveolar ventilation.

The danger level for these elevations isn’t known, but clinicians often use an increase of 10 mm Hg over baseline or an absolute level of 50 mm Hg or higher. Such increases may indicate respiratory acidosis, which causes a drop in pH and a disruption of cellular ion control mechanisms.

When breathing is normal, a sudden decrease in the PetCO2 level indicates a loss of perfusion from a sudden decrease in blood flow to the lungs. Conditions such as pulmonary embolism and a loss of cardiac output from bleeding or sudden heart failure are the most likely causes of a drop in PetCO2.

PetCO2 looks promising as a vital sign because it rapidly reflects changes in ventilation and perfusion. A change in PetCO2 will occur before a change in any other sign, particularly clinical signs, such as respiratory rate and heart rate.

Using capnography
You can use capnography on patients who are intubated or on nasal cannula. The two technologies for measuring exhaled CO2 are:

• chemical analysis (colorimetric technique)
• gas analysis, such as infrared spectro­-photometry.

The gas analysis technique has the advantage of being able to show the waveform and display the numeric value. For gas analysis, one of two methods can be used. With the sidestream method, a small amount of CO2 is drawn into a measurement device. With the mainstream method, CO2 is measured at the sampling site.

You can take measurements on any patient simply by placing the CO2 sampling device where the patient exhales—the nose or the endotracheal tube. Most capnography devices are lightweight and portable.

Capnography has many applications. (See The many uses of capnography.) The two that can be used for most patient assessments—the evaluations of ventilation and blood flow—are very simple. All you have to do is observe the PetCO2 for a sudden change.

Evaluating ventilation
The loss of a capnogram waveform and a drop of the PetCO2 level to zero suggest the patient isn’t breathing. If the patient is on a ventilator, these signs suggest a disconnection from the ventilator or extubation. Whether the patient is intubated or not, these signs indicate a medical emergency. The patient needs an immediate assessment.

Another reason PetCO2 is such a valuable measure of ventilation, even better than pulse oximetry, is this: changes in oxygen concentration (FIO2) will increase pulse oximeter (SpO2) readings and therefore will make the SpO2 insensitive to a reduction in ventilation. Since PetCO2 values are not affected by changes in FIO2, reduced ventilation will still be accurately measured by PetCO2 levels.

Keep in mind that the PetCO2 won’t accurately predict the PaCO2. (Remember the 1 to 5 mm Hg difference.) But that’s not important when using PetCO2 as a vital sign. What is important is viewing PetCO2 as a trend indicator of PaCO2. If the PetCO2 is increasing, you can safely assume that PaCO2 is increasing, too, though the PaCO2 may be increasing faster than the PetCO2.

Evaluating blood flow
If the PetCO2 suddenly decreases without a change in minute ventilation or breathing, evaluate cardiac output. Decreased PetCO2 usually results from decreased cardiac output. If the capnogram goes flat, the patient may have developed respiratory and cardiac arrest. The decrease in the PetCO2 reflects the reduced lung perfusion relative to ventilation, which allows more inspired air to be exhaled without having CO2 added.Reviewing case examples
To better understand capnometry analysis in assessing ventilation and perfusion disturbances, review the 3 case examples below. At the end of each scenario, you’ll be asked to determine what is happening. At the end of each case example, you’ll find the correct answer.

Case 1
A 56-year-old man is admitted to the outpatient procedure area for a follow-up colonoscopy. Three years ago, he had a colonoscopy during which a precancerous polyp was removed. The patient required higher-than-normal doses of a sedative and had a prolonged recovery. For the current procedure, the physician elects to use propofol (Diprivan) instead of midazolam (Versed) because of its quicker elimination and shorter recovery time. Because propofol can suppress respirations, the physician decides to monitor the patient using capnography with a nasal cannula. Twenty minutes into the procedure, you note that the PetCO2 has risen from the baseline of 37 mm Hg to 48 mm Hg.

What would you do?
Answer: Notify the physician of the increase in PetCO2. The physician should then decrease the propofol dosage until the PetCO2 returns to less than 47 mm Hg, which is 10 mm Hg higher than the patient’s baseline level.

Case 2

A 76-year-old woman is being weaned from mechanical ventilation. She has a mainstream PetCO2 analyzer in her ventilator circuit. After 15 minutes of weaning, her PetCO2 rises from a baseline of 35 mm Hg to 51 mm Hg.

What would you do?
Answer: The patient isn’t tolerating the weaning attempt because the elevated PetCO2 indicates that she’s not ventilating properly. The patient should be placed back on the ventilator, and her ability to breathe on her own should be reassessed later.

Case 3
A 29-year-old man is admitted to the emergency department after a motor vehicle collision. He has severe pain in his pelvic and left femur areas. There’s no obvious external bleeding; his breath sounds are clear; and his heart sounds are clear, not muffled.
He’s alert and oriented but says he really hurts and is very tired. He is placed on O2 at 2 L/minute. Sidestream capnography reveals a PetCO2 of 18 mm Hg.

What action would you take?
Answer: One of the most dangerous causes of a low PetCO2 is decreased cardiac output. To rule out a low PaCO2 as the cause of the patient’s low PetCO2, obtain arterial blood gas results. A noninvasive cardiac output measurement may also be obtained.

Advantages over traditional signs
With capnography, you can accurately assess ventilation and perfusion within seconds. Unlike traditional vital signs, capnography yields clear, specific information regarding the patient’s condition and quickly alerts you to life-threatening disturbances.

Plus, it’s portable, so it can be used anywhere in the hospital, and it’s easy to use: An increase in PetCO2 may signal inadequate ventilation, and a decrease in PetCO2 signals decreased perfusion. With all these advantages, capnography should be one of the new vital signs of the 21st century.

Selected references
Ahrens T, Sona C. Capnography application in acute and critical care. AACN Clinical Issues. 2003;14:123-132.

Burton JH, Harrah JD, Germann CA, Dillon DC. Does end-tidal carbon dioxide monitoring detect respiratory events prior to current sedation monitoring practices? Acad Emerg Med. 2006;13(5):500-504.

Krauss B, Heightman AJ. 15-second triage tool. The use of capnography for the rapid assessment & triage of critically injured patients & victims of chemical terrorism. JEMS. 2006;31(6):60-62, 64-66, 68.

Lightdale JR, Goldmann DA, Feldman HA, Newburg AR, DiNardo JA, Fox VL. Microstream capnography improves patient monitoring during moderate sedation: a randomized, controlled trial. Pediatrics. 2006;117(6):e1170-e1178.

Zwerneman K. End-tidal carbon dioxide monitoring: a VITAL sign worth watching. Crit Care Nurs Clin North Am. 2006;18(2):217-25, xi.

For a complete list of selected references, please see March 2007 references.

Tom Ahrens, DNS, RN, FAAN, is a Research Scientist at Barnes-Jewish Hospital in St. Louis, Missouri.

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