Capnometry is the measurement of carbon dioxide in expired gas. Capnometers measure carbon dioxide using one of two techniques, each with its own advantages and disadvan- tages. The more common form of capnometry in intubated patients is referred to as main- stream. The mainstream capnometer is placed in-line with the endotracheal tube circuit. It utilizes a light-emitting detector that is positioned on either side of an airway adaptor attached to the top of the endotracheal tube. It uses infrared light absorbance to detect carbon dioxide.
Because of its in-line positioning, it allows for rapid breath-to-breath analysis of carbon dioxide. Although it does not depend on the aspiration of gas, it is susceptible to interference by secretions or humidity. Moreover, because of the need of an added adaptor, it may add to the dead space ventilation. This is usually not problematic except in the smallest of infants.
Finally, the sensor used by most mainstream capnometers is large and heavy relative to the endotracheal tube, and therefore, may place undue tension on the tube.
Sidestream sampling is the other form of capnometry. It is less commonly used in intu- bated patients, but is increasingly being utilized in non-intubated circumstances. The side- stream technique continuously aspirates a small amount of gas as the patient ventilates either spontaneously or through a mechanical ventilator. The advantage of this method is that the apparatus adds no additional dead space or weight to an endotracheal tube. The disadvan- tage, particularly in smaller patients, is that it may decrease minute ventilation due to the aspiration of gas. Also, because of the method of sampling, mucous and water may be inad- vertently aspirated into the monitoring device obstructing optimal gas fl ow. Finally, because the gas has to be pulled out of the endotracheal tube/ventilator circuitry, there is a delay in the response time to changes in carbon dioxide. It should be noted, however, that some gas aspirating systems utilize an adapter positioned between the ventilator circuit and the endo- tracheal tube, adding to system dead space similar to mainstream capnometers.
Capnometry has become an important component of pediatric critical care monitoring.
First, and perhaps foremost, it has become a standard of care to confi rm correct placement of an endotracheal tube after intubation. This may be accomplished in one of two ways. The fi rst, and perhaps the simplest, involves attaching the endotracheal tube to a colorimetric capnom- eter that will change colors when exposed to carbon dioxide usually from purple to yellow.
The colorimetric capnometers contain a disc that when exposed to carbon dioxide produces hydrogen ions. The increase in hydrogen ions, and the resultant change in pH, results in the color change of the disc. If no carbon dioxide is detected, the colorimetric capnometer will remain purple. If carbon dioxide is detected, the disc will change color from purple to yellow.
This method may only be used for short term confi rmation of exhaled carbon dioxide. The second method involves graphically displaying the detected level of carbon dioxide. The sec- ond method, capnography, may be used to quantify the amount of carbon dioxide detected and may refl ect the level of carbon dioxide at any given point in the respiratory cycle.
There are situations in which capnometry/capnography may provide misleading informa- tion regarding the appropriate positioning of an endotracheal tube. For example, the ingestion of carbonated beverages prior to intubation may result in the detection of carbon dioxide with esophageal placement of the tube. In addition, vigorous bag-valve mask ventilation prior to intubation may result in an air-fi lled stomach allowing for the detection of carbon dioxide
Capnometry has become a standard of care to confi rm appropriate endotracheal intubation.
Capnography may be used to estimate the percentage of dead space ventilation.
with an esophageal intubation. Furthermore, tube placement above the vocal cords in the hypopharynx may allow for suffi cient ventilation such that carbon dioxide may be detected despite the tube not being positioned in the trachea. In contrast, in the setting of cardiac arrest or extreme hypoperfusion, carbon dioxide may not be delivered to the lungs, and thus, there is little to no carbon dioxide in the exhaled breaths. Consequently, the capnometer/capno- graph will not detect carbon dioxide although the endotracheal tube is properly positioned in the trachea. Large air leaks around the endotracheal tube or obstructed tubes may also result in diminished amounts of carbon dioxide being detected despite appropriate positioning of the endotracheal tube. It is recommended that capnometry/capnography be assessed over at least the fi rst six breaths of ventilation to minimize the risk of misinterpretation.
In addition to confi rming endotracheal intubtion, capnometry may be used to non-inva- sively monitor arterial carbon dioxide content. Physiologically, carbon dioxide readily dif- fuses across the alveolar capillary membrane such that the concentrations of arterial and alveolar carbon dioxide quickly equilibrate. Consequently, the partial pressure of carbon dioxide in the alveolus closely approximates the partial pressure of carbon dioxide in the arterial blood. Once in the alveolus, the gas moves into a terminal bronchiole, a subsegmen- tal bronchus, a main bronchus, the trachea, the endotracheal tube, and out of the body. During that entire transit, very little additional gas exchange occurs. Consequently, under ideal cir- cumstances, by measuring the peak concentration of carbon dioxide (end tidal) as it exits the endotracheal tube or nasopharynx, it is possible to estimate the concentration of carbon dioxide in the alveolus, and therefore, the partial pressure of carbon dioxide in the arterial blood. This is the foundation upon which the development of capnometry was developed. In the patient without cardiopulmonary disease, the system works well and exhaled end tidal carbon dioxide approximates PaCO 2 . In fact, the end tidal carbon dioxide is usually 2–5 mm Hg lower than the PaCO 2 because of anatomic dead space ventilation and the expected, mild ventilation perfusion mismatch in the upper lung fi elds (West Zone I). In those upper lung fi elds, ventilation is slightly greater than perfusion because of the gravitational forces favor- ing blood fl ow to the lower, more dependent lung fi elds.
However, as might be anticipated, there are many clinical situations common to the pedi- atric intensive care unit where the premise of balanced ventilation and perfusion is invalid, and thus, capnometry provides erroneous estimates of arterial carbon dioxide. As the end tidal carbon dioxide (EtCO 2 ) represents the average partial pressure of ventilated alveoli and the PaCO 2 represents the same for perfused alveoli, any alteration in ventilation perfusion matching will result in an inaccurate EtCO 2 estimate of the PaCO 2 . For example, in any set- ting of an increased ventilation to perfusion ratio (e.g. increased dead space secondary to decreased cardiac output, pulmonary embolus, etc.), the EtCO 2 will underestimate the PaCO 2 (Fig. 1-6 ). For example, if the PaCO 2 is 40 mm Hg, and only half of the alveoli are being effectively perfused, the carbon dioxide coming out of the perfused alveoli will be 40 mm Hg. In contrast, if the other 50% of alveoli are not being perfused at all, the carbon dioxide coming out of these alveoli would be zero. When the gas from the two sets of alveoli meet and mix in the trachea, the resulting concentration of carbon dioxide detected at the capnom- eter would be 20 mm Hg (as opposed to the true arterial value of 40 mm Hg). In light of this, end tidal carbon dioxide monitoring is being utilized as a method to help assess adequacy of pulmonary blood during cardiopulmonary resuscitation. Similarly, in the setting of a decreased ventilation perfusion ratio, where alveoli are being perfused, but not ventilated, the carbon dioxide in these non-ventilated alveoli will never be detected by the capnometer.
Therefore, the EtCO 2 detected by capnometry will refl ect only those alveoli that are actively participating in ventilation.
In addition to the absolute numbers provided by the capnogram, the waveform may also be used to detect problems within the cardiopulmonary system. A normal capnogram consists of four stages (Fig. 1-7 ). First, there is an inspiratory baseline (I) where atmospheric air at the sensor has little to no carbon dioxide thereby providing a baseline value of zero. Once exhala- tion begins, and the air from the anatomic dead space is cleared (no or minimal carbon diox- ide present), the second stage is characterized by a rapid rise (steep) in the measured carbon dioxide as alveolar air rich with carbon dioxide rushes past the sensor (II). During exhalation, the concentration of carbon dioxide quickly stabilizes and the level of carbon dioxide roughly fl attens. The highest recorded value of carbon dioxide at the end of exhalation is recorded as
Normal
Deadspace Shunt
Arterial blood V/Q = 1
V/Q = 0 V/Q = ∝
Mixed venous blood
PETCO2 = 40
PETCO2 = 21 PinCO2 = 0 PETCO2 = 36 PinCO2 = 0 PinCO2 = 0
Alveolus 1 PACO2 = 40
PACO2 = 0 PACO2 = 42
PaCO2 = 42
Alveolus 2 PACO2 = 40
PACO2 = 36 PACO2 = 44
PaCO2 = 40
PaCO2 = 40
PvCO2 = 44 PvCO2 = 44
PvCO2 = 44
FIGURE 1-6
The relationship between end tidal carbon dioxide and arterial carbon dioxide at different ratios of ventilation and perfusion (Cordova and Marchetti ( 2002 ) )
the end tidal carbon dioxide. During the fi nal stage of the respiratory cycle, inspiration occurs.
With the fresh rush of carbon dioxide free air across the sensor, the carbon dioxide level quickly plummets to zero (IV). The capnogram waveform may be used to detect conditions associated with increased airway resistance. For example, waveforms associated with a wider angle between the upslope and the plateau stages of exhalation suggest slower carbon dioxide removal and increased airway resistance. The same is true for an uprising stage III plateau.
Capnography is also being used for the monitoring of the non-intubated patient particu- larly in the setting of procedural sedation. Because the medications required for such seda- tion may be associated with respiratory compromise, close monitoring of the respiratory system is of paramount importance. Traditionally, oxygenation has been monitored with pulse oximetry and ventilation has been assessed with clinical observation alone. Sidestream capnography, by means of a nasal oral cannula which simultaneously monitors exhaled car- bon dioxide and delivers low fl ow oxygen, allows for a more precise and detailed assess- ment. Monitoring of the capnogram allows for the continuous monitoring of airway obstruction, apnea, and hypercarbia (Fig. 1-8 ). It also allows for a more exact measurement of the respiratory rate than traditional thoracic impedance devices. Capnography has also been used in non-intubated patients to monitor the respiratory status in the setting of sei- zures, altered mental status, and overdoses.
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III
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I CO2 Partial Pressure (mm Hg)
FIGURE 1-7
A normal capnogram consists of four stages. First, there is an inspiratory baseline ( I ) where atmospheric air at the sensor has little to no carbon dioxide thereby providing a baseline value of zero. Once exhalation begins, and the air from the anatomic dead space is cleared (no or minimal carbon dioxide present), the second stage is character- ized by a rapid rise (steep) in the measured carbon dioxide as alveolar air rich with carbon dioxide rushes past the sensor ( II ). During exhalation, the concentration of carbon dioxide quickly stabilizes and the level of carbon dioxide roughly fl attens. The highest recorded value of carbon dioxide at the end of exhalation is recorded as the end tidal carbon dioxide. During the fi nal stage of the respiratory cycle, inspiration occurs and with the fresh rush of carbon dioxide free air across the sensor, the carbon dioxide level quickly plummets to zero ( IV ) (Cordova and Marchetti ( 2002 ) . Original reference Airway Management. Philadelphia: Lippincott-Raven, 1996)
a 40
0
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Time Time Time
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40 [CO2][CO2][CO2][CO2]
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FIGURE 1-8
Capnograms during procedural sedation in non-intubated patients. ( a ) Normal waveform.
( b ) Patient with bradypneic hypoventilation, with normal tidal volume but slowed respira- tory rate. ( c ) Hypopneic hypoven- tilation with decreased tidal volume resulting in increased dead space ventilation. ( d ) Loss of a waveform consistent with either complete laryngospasm or apnea (From Krauss and Hess ( 2007 ) )
Finally, capnography is also being recommended in the setting of pediatric cardiopulmo- nary arrest to assess the adequacy of perfusion to the lungs. Although a specifi c value has not been uniformly defi ned, providing cardiopulmonary resuscitation to maintain the end tidal carbon dioxide level above a specifi ed value for each patient will help assure adequacy of pulmonary blood fl ow with compressions and minimize the chance of potentially deleterious hyperventilation.