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.