A noninvasive, continuous measurement of exhaled CO2 and may be used to identify changes in a patient’s hemodynamic or ventilatory status, capnography is becoming an essential tool for monitoring patients in the emergency department.
The Joint Commission strongly recommends its use in procedural sedation. And studies have demonstrated the significant impact of capnography in the care of intubated patients, patients in cardiac arrest, and as an adjunctive therapy in the management of mechanically-ventilated patients.
Among many other professional organizations that support the use of capnography include: the American Association for Respiratory Care Practice Guidelines recommends capnography be used for all patients on mechanical ventilation; the Emergency Nursing literature supports capnography use during procedural sedation; and in the 2005 Guidelines for CPR and Emergency Cardiac Care, the American Heart Association recommends monitoring exhaled CO2 for endotracheal intubation.1
Prehospital research reports the use of capnography as a valid early indicator of return of spontaneous circulation (ROSC) 2,3
Important Contribution
The complex process of tissue perfusion results in many end products, one of which is the production of carbon dioxide (CO2). This cellular metabolic byproduct diffuses into the blood and is eliminated by the lungs with exhalation.
Perhaps the most important contribution of capnography is the ability to trend CO2. Alterations in the level of CO2 in the blood, or partial pressure of arterial CO2 (PaCO2), would also be reflected in alterations in the level of CO2 measure at end-exhalation, EtCO2. Capnography allows for a continuous, noninvasive and immediate monitoring of the patient’s response to treatment or changes in status.
Products that provide measurements of capnnography include nasal cannula and endotracheal devices. These devices use the absorption of infrared light to calculate the CO2 concentration. Endotracheal devices and many cannula devices allow for the use of supplemental oxygen. Specific nasal cannula and masks are also available to allow the monitoring of CO2 while the patient is receiving continuous positive airway pressure (CPAP). A colorimetric CO2 detector uses the acidic nature of CO2 and pH-sensitive paper to identify the presence of CO2 to confirm proper endotracheal tube placement in a newly intubated patient.
Physiology
Partial pressure of carbon dioxide (PCO2) is an index of the effectiveness of alveolar ventilation. An arterial sample directly reflects the exchange of inhaled air with blood in the lungs whereas capnography indirectly measures this exchange.
CO2, once it has traveled through the blood into the pulmonary capillaries, readily diffuses into the alveoli. During normal ventilation-perfusion matching in the lungs, the blood supply to the alveoli is unimpeded and the alveoli are unrestricted allowing CO2 to easily mix with the gases within the lungs. Therefore, the amount of CO2 in the pulmonary capillaries would be nearly equal to the amount of CO2 in the alveoli.
The comparison of the PaCO2 obtained from an arterial blood gas specimen to the partial pressure of exhaled CO2, more frequently called EtCO2 and obtained from an exhaled gas specimen, would be nearly identical.
Difference Between PaCO2 & EtCO2
The difference between PaCO2 and EtCO2 measurements occurs because, even with normal respirations during a ventilation-perfusion steady-state, not every alveoli participate in gas exchange. The capillary may carry CO2 near the alveoli which may not be in a state to allow the gas to cross the alveolocapillary membrane. Consequently, the concentration of CO2 in the total number of alveoli ready for exhalation would be less than the concentration of arterial carbon dioxide. The normal difference between PaCO2 and PetCO2 is 2-5 mm Hg (PaCO2: 35-45 mm Hg, PetCO2: 37-50 mm Hg).
Waveform
The waveform, or capnogram, is important not only as a diagnostic tool but also to establish the accuracy of the reported capnography number, just as the pulse oximetry waveform demonstrates SpO2.
The capnogram reflects the course of CO2 elimination during exhalation. A normal capnogram consists of several segments that reflect the various stages of exhalation and inhalation. EtCO2 represents the highest concentration of CO2 reached at the end of exhalation and is assumed to represent the CO2 concentration of alveolar gas (and subsequently the CO2 concentration of capillary blood).
Normally, gas eliminated from the upper airways is first to leave the lungs. During early exhalation this dead-space gas has not undergone exchange at the alveolocapilary membrane and therefore contains no CO2. The PCO2 at the onset of exhalation is therefore negligible.
As exhalation continues, gas from the alveoli begins to contribute to the exhaled gas. Carbon dioxide concentration rises sharply and rapidly. The sensor now detects gas that has undergone exchange, producing measurable quantities of CO2.
The final stages of alveolar emptying occur during late exhalation. During the alveolar plateau phase, CO2 concentration rises more gradually because alveolar emptying is more constant.
The point at which the EtCO2 value is determined is the end of exhalation, when CO2 concentration peaks. An alveolar plateau must be present to accurately estimate alveolar CO2.
During inhalation, the CO2 concentration declines sharply to zero.
Assess the capnogram for height, frequency, rhythm, baseline, and shape to assist in the evaluation of gas exchange, such as CO2 concentration, breath by breath respiratory rate, regularity of respirations, retained CO2, and obstruction respectively.
By the Numbers
The amount of CO2 in the exhaled gases is determined by alveolar ventilation, perfusion status and metabolic rate.
Increased EtCO2
A reduction in alveolar ventilation increases EtCO2. Classic hypoventilation, most commonly seen after sedation administration, is demonstrated by longer and higher waves. The gas exchange is normal but due to the slower respiratory cycle (longer wave), more CO2 is allowed to diffuse in to the alveoli before exhalation resulting in a greater concentration (higher wave) to be measured.
Rebreathing, such as poor head/neck alignment, draping near the airway, inadequate oxygen flow with a mask or insufficient expiratory time in the ventilated patient, increases EtCO2. This is easily diagnosed by a rising baseline. As the gas inhaled has a higher concentration than atmospheric, the downstroke of the wave does not return to zero.
Changes in perfusion may increase the transport of CO2 to the lungs. Capnography can be used to monitor changes in cardiac output during volume resuscitation for hypovolemic shock and during cardiopulmonary resuscitation as an initial indication of ROSC.
Increased metabolism may cause increased CO2 production, as in fever, pain and hypermetabolic states such as trauma, seizures or shivering. Absorption of CO2 from exogenous sources, like administration of sodium bicarbonate may cause a transient elevation.
Decreased EtCO2
Any condition that limits the flow of CO2-rich gas from the alveoli to the external sensor will result in lower numbers, such as airway obstruction and apnea. Increasing the amount of exhaled gas that has not interfaced with blood (physiologic dead space) and allowing CO2 to diffuse, will decrease the measured EtCO2, as seen with pneumonia and atelectasis.
This external sensor explains why both hypo- and hyperventilation may be the cause of a decreased EtCO2 level. Hypoventilation associated with shallow breathing does not allow all alveoli to empty their CO2 content, whereas hyperventilation (“blowing off” CO2) exhales the CO2 so quickly very little may accumulate in the alveoli.
Any condition that impairs pulmonary blood flow, such as pulmonary embolus and decreased cardiac output, will decrease EtCO2. A decrease in CO2 production will also decrease EtCO2, as in hypothermia and decreased muscle activity caused by heavy sedation or paralytic agents.
Valuable Tool
Capnography is an easy and reliable tool used alone, with procedural sedation, or in conjunction with serial arterial blood gases in the complex critical patient. The continuous monitoring feature allows early recognition of a change in patient status with minimal blood sampling.
References for this article can be accessed here.
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Denise Thomas is clinical nurse specialist in emergency, trauma and critical care services at Santa Rosa Memorial Hospital and Petaluma Valley Hospital, Santa Rosa, CA.
