The Role of Capnography in Ventilation Monitoring

Through the expanded use of capnography, RCPs can rapidly, easily, and noninvasively detect changes in the status of patients with a variety of cardiopulmonary conditions.

The hypothetical patient is attempting to maximize the use of his accessory muscles and has a cyanotic tint to his lips and fingers. His rapid, wheezing respirations can be heard throughout the room. He is unable to speak beyond single words. The patient has severe chronic obstructive pulmonary disease (COPD) and has been intubated for respiratory distress in the past. Even with high-flow oxygen and nebulized medications, the patient’s oxygen saturation continues to decline. He is obviously tiring and the work of breathing is rapidly exceeding his ability to support his respiratory needs. Following a moderately difficult intubation, the RCP attempts to auscultate the lung fields, but is unable to hear any gas movement in the chest. The RCP quickly reaches for the secondary means of confirming intubation: a capnograph.

Capnography is the continuous, noninvasive measurement and graphical display of end-tidal carbon dioxide (ETCO₂). Capnography is an underutilized assessment tool in the management of respiratory patients. Managing the respiratory status of the intubated and ventilated patient may prove a challenge for the clinician.¹ Determining endotracheal tube placement has become a significant issue in many health care environments. The technology of capnography provides an assessment tool for ventilation management.

Capnography was used originally in mechanically ventilated patients to assess patient levels of carbon dioxide on a breath-by-breath basis, continuously and noninvasively. The capnography sample chamber or sensor, placed between the patient’s artificial airway and the ventilator, inspects the inhaled and exhaled gases for specific concentrations of carbon dioxide. The inhaled and exhaled concentrations of carbon dioxide are graphically displayed as a waveform on the monitor, with a corresponding numerical value. Today, capnography plays a key role in confirming intubation and verifying placement of an airway throughout intubation, ventilation assessment, and resuscitation. Hyperventilation often occurs preceding or following intubation. A real risk in continued hyperventilation is the associated cerebral vasoconstriction caused by the low carbon dioxide.

Various forms of capnography are available in health care facilities, enabling RCPs to identify problems with ventilation immediately. Capnography is also useful in monitoring nonintubated patients to assess ventilation and perfusion of the pulmonary vessels. The RCP can use capnography as a supplemental tool and an early warning system to identify trends in ventilation and perfusion.

The application of capnography is not yet universal in all health care facilities. This limitation reflects several factors, including cost, education,² and concerns over additional technology.

Carbon Dioxide
Carbon dioxide is the waste product of cellular metabolism. As cells consume oxygen, carbon dioxide is produced, transferred to the circulation, and delivered to the lungs via venous return.

Cellular production of carbon dioxide is a metabolic by-product of the oxidative breakdown of metabolic fuels. The higher the metabolic rate, the higher the carbon dioxide production rate. Carbon dioxide dissolves rapidly in the cells and easily diffuses out of the cells and into the venous blood. Carbon dioxide is carried by the poorly oxygenated venous blood through the right heart and into the pulmonary arteries to reach the capillaries surrounding each pulmonary alveolus.

As ambient air is drawn into the alveolus during inspiration, the carbon dioxide in the blood diffuses through the capillary and alveolar walls into the alveolar air sac. Under normal conditions, one pass of the blood through the alveolar capillary drives the alveolar Pco₂ nearly to match (usually within 5 mm Hg) the Paco₂. As expiration begins, the gas containing carbon dioxide is expelled from the alveoli to displace and mix with the air in the bronchial tree. As this mixture of gases reaches the upper airways and the capnography monitor, the measured Pco₂ rises sharply to a plateau and then slowly increases to a peak as the carbon dioxide level continues to increase. This peak Pco₂ at the end of expiration is known as the end-tidal carbon dioxide; in healthy individuals, it is generally within 5 mm Hg of the Paco₂. These differences can be affected by many patient factors, increasing, for example, in patients undergoing aggressive emergency procedures and in patients with significant cardiopulmonary disease.³

Once inspiration begins, the Pco₂ measured at the mouth or nose drops rapidly to almost zero. The rapid expiratory rise, slowly rising plateau, and drastic decrease at the beginning of inspiration constitute the characteristic waveform of the capnogram. The importance of the ETCO₂ waveform and numerical value resides in their ability to reflect the cardiopulmonary status of the patient. A capnograph (ETCO₂ monitor) provides this information to the prehospital clinician continuously

Excretion of carbon dioxide is the final common pathway of metabolism, and it provides a useful global indication of patient status. Ventilation must be adequate to carry oxygen into the lungs. Oxygen is transferred into the erythrocytes and transported to the cells at the tissue level. Transport is a function of the cardiovascular system. The process of aerobic metabolism consumes the oxygen and produces carbon dioxide. The carbon dioxide is transferred from the tissue into the red cells and is transported to the lungs for elimination. Hypoxemia, cerebral ischemia, and coronary ischemia are possible even in the presence of normal capnography waveforms and numerical values.

Capnogram Waveforms
The capnogram (capnograph waveform) displays concentration across time. The horizontal axis reflects time and the vertical axis displays the concentration of ETCO₂.

Capnographs manufactured today display the ETCO₂ as either a percentage (normal range: 5% to 6%) or as mm Hg (normal range: 35 to 45 mm Hg). Displaying the ETCO₂ value as a percent correlates with the measurement of oxygen delivery; displaying it in mm Hg correlates with the measurement of Paco₂.

Each waveform represents a single respiratory cycle, including both inspiration and expiration. The first segment of the waveform is the flat area near the beginning of exhalation; this is the dead space. The rapid rise in the waveform matches the mixture of air with gas from the alveoli. The waveform reaches the plateau as alveolar gas is exhaled. The ETCO₂ level is measured at the peak of this plateau. As the patient inspires, the waveform rapidly drops to the baseline. The baseline should remain at or near 0 mm Hg, as inspired gas should not contain carbon dioxide.

In addition to displaying a single breathing cycle, capnography can display a trend, compressing many breaths together so that changes over time can be seen easily.

Technology
The capnograph uses infrared light technology that incorporates a very sensitive emitter and a detector that identifies only the light-absorption signal of carbon dioxide. This specificity for carbon dioxide allows the use of capnography in the presence of other gases or aerosolized medications. The accuracy of ETCO₂ measurements can be confirmed using the waveform identifying the unique shape of the characteristic capnogram. Newer devices allow for accurate monitoring even with high respiration rates and respiratory low tidal volumes, as are often present in neonatal and pediatric patients.

Capnographs available for use today are small and lightweight, providing the RCP with a numerical display of ETCO₂ and respiration rate, a clear waveform, and audible and visible alarms. These capnographs are powered through internal batteries, 120-V AC power, or through the device (such as a cardiac monitor) in which they are mounted. Some devices transfer data to a printer, personal computer, or electronic patient information management system. Combined with the other monitored parameters, capnography provides an easy-to-use, cost-effective, and useful solution for patient monitoring. Capnography is valuable in the continuous monitoring of patients of all ages, from neonates through adults.

Recent technology changes have integrated ETCO₂ monitoring into cardiopulmonary monitors. This integration allows the RCP to initiate ETCO₂ monitoring simultaneously with electrocardiography, noninvasive and invasive blood pressure monitoring, and pulse oximetry.

Sidestream Analysis
Sidestream devices allow continuous, precise measurement of ETCO₂, inspired carbon dioxide, and respiratory rate in both intubated and nonintubated patients, correlating respiratory events with hypoxia or myocardial ischemia. Small samples (generally 30 to 200 mL/min) of the patient’s inhaled and exhaled gases are continuously aspirated into the monitor.⁴ Sidestream sampling systems use a pump to draw sample gas into the monitor for internal measurement. A physical limitation of sidestream monitoring is the time requirement for the gas sample to travel from the patient’s airway to the internal carbon dioxide analyzer. If the respiration rate is increased (as seen in infants, for example), inadequate time may exist for accurate sample measurement and display of the waveform. The monitor uses hydrophobic filters to separate the moisture content of the exhaled gases to prevent contamination of the analyzer.

Mainstream Analysis
Another method that uses infrared analysis is mainstream monitoring, which measures the concentration of carbon dioxide in a sample chamber on the airway, typically between the endotracheal tube and the ventilator circuit. Mainstream monitoring systems normally require patient intubation, as the sample analyzer is connected directly to the endotracheal tube. Some models allow for use in both sidestream and mainstream applications. Mainstream systems measure carbon dioxide at the patient’s airway, respond quickly to ETCO₂ changes, and may perform better at higher respiratory rates. Their primary limitation is the requirement of intubation.

Colorimetric Detectors
The chemical carbon dioxide detector is a single-patient-use indicator that can be used for several hours following intubation or initial use.⁵ It works by displaying the concentration of ETCO₂ according to a color scale: purple=<0.5%; tan=0.5 to 2%; and yellow=>2%.

Following intubation, the chemical detector is connected to the endotracheal tube or other airway and ventilation of the patient is initiated.⁶ If endotracheal-tube placement is correct, the small indicator area of the detector changes color from purple to yellow with each ventilatory exchange. A color change is a positive indication of correct tube placement or good ventilation combined with circulation.⁷ If the color does not change, and other assessment indicators are negative for proper endotracheal-tube placement, the clinician should immediately remove the tube and ventilate the patient. These devices do not produce the characteristic capnography waveform with diagnostic information, but are easy to use, relatively inexpensive, small, and lightweight. These devices are produced for adult and pediatric patients.⁸

Critical Differences
These technologies affect how capnography is used, the clinical decisions that can be based on the capnography values, and the cost of using capnography. Both sidestream and mainstream analysis methods sample inspired and expired gases as close as possible to the patient’s airway.

There are virtually no safety issues for the patient or the clinician related to the use of capnography. The sampling (aspiration) in sidestream analysis of the inspired and expired gas has no substantial clinical effect on patient ventilation.

Confirmation of Intubation
There is no such thing as minor airway management. The most common problems with airway management and ventilation can be detected using capnography. Capnography is a standard of care in many areas of health care where intubation and other forms of airway management are commonly performed.

The American Heart Association has identified capnography as a tool for secondary confirmation of intubation.⁹ Capnography provides several key advantages in patient assessment; it is easy to implement and use, noninvasive, easy to maintain and calibrate, and relatively inexpensive. In the past, capnography has been used primarily to confirm proper endotracheal intubation.¹⁰ Capnography also offers the opportunity to improve patient assessment in both intubated and nonintubated patients, to reduce costs through better clinical management of the patient, and to decrease potential legal liability.

Capnography has long played a role in confirming proper placement of the endotracheal tube.¹¹ In both elective and emergency intubations, ETCO₂ is a standard of care for confirming endotracheal intubation. Insertion of the endotracheal tube into the esophagus may lead to hypoventilation, hypoxemia, cerebral ischemia, and death. Standard methods of confirming endotracheal intubation include observation of chest excursion, auscultation, fogging of the endotracheal tube, and esophageal tube checking. Capnography confirms placement of the endotracheal tube by detecting the carbon dioxide in exhaled gases. A normal capnogram confirms proper placement of the endotracheal tube, with few exceptions. Intubation of the esophagus may provide a few normal-appearing waveforms, but the carbon dioxide introduced into the stomach during manual ventilation is rapidly depleted, resulting in a flat line on the display. The carbon dioxide measurement chamber or sample line should be attached prior to intubation. At the completion of intubation, the clinician can then initiate ventilation and confirm intubation using both primary and secondary methods.

Ventilation Efficacy
Analysis of Paco₂ is not available in the field, but it correlates well with ETCO₂ in most patients. Capnography functions as an excellent adjunct to other monitoring methods, including arterial blood gas (ABG) analysis and oximetry. While ETCO₂ levels in very ill patients should be interpreted with caution, trends in ETCO₂ correlate with changes in Paco₂ and can provide an early warning of metabolic or cardiorespiratory problems such as shunting, dead space, bronchoconstriction, or pulmonary embolism. Capnography allows for the trending of the ETCO₂ value and its subsequent comparison with ABG values.

Although capnographs can be used with pulse oximeters and are often combined with them in single units, these instruments present different views of oxygenation and ventilation. Pulse oximeters measure oxygen saturation of hemoglobin in arterial blood (Spo₂) at the sensor site and, thereby, provide additional information as to the adequacy of lung perfusion and oxygen delivery to tissues. Capnography continuously measures pulmonary ventilation and is able to detect small changes in cardiorespiratory function rapidly, before oximeter readings change.

The ETCO₂ is inversely proportional to alveolar ventilation. When ventilation decreases (when carbon dioxide production exceeds elimination through the lungs), the ETCO₂ increases. In this situation, the pH drops as the patient becomes acidic due to hypoventilation. As ventilation increases, the ETCO₂ decreases. In this state, Paco₂ falls, pH rises, and the patient becomes alkalotic due to hyperventilation.

In the presence of increased dead space (ventilation of portions of the lungs without perfusion and gas exchange), hyperventilation with increased tidal volumes is required to maintain adequate ventilation and sustain the ETCO₂ within normal limits.

Hypoxemia is a common cause of hyperventilation, and it may be seen in patients who have asthma, COPD, pneumonia, or traumatic injury. Anxiety or neurological disorders may also produce hyperventilation. Increased values (hypercapnia) are commonly found in advanced obstructive or restrictive diseases. Measurements in these patients are characterized by abnormal ventilation-perfusion patterns and the inability to maintain adequate alveolar ventilation. Whether carbon dioxide retention is the result of primary lung disease, trauma to the central nervous system, or neurological disease, pH is maintained at a level as close to normal as possible by an increase in the bicarbonate level in the patient’s blood. The renal system produces bicarbonate to buffer the increased acid load.

Hypoxemia is often present in patients who retain carbon dioxide (COPD patients). As alveolar carbon dioxide increases, alveolar oxygen decreases. If the cause of the hypercapnia is primary lung disease, either obstructive (COPD) or restrictive (asthma), the severity of the hypoxemia may increase due to ventilation abnormalities. These patients, in the home care environment, commonly use oxygen therapy. Changes in the ETCO₂ seen with supplemental oxygen require careful evaluation. Patients may have a decreased ventilatory response to carbon dioxide, and administering oxygen to these individuals may cause a reduction of the hypoxic stimulus to ventilation, resulting in a further increase in ETCO₂.

Trauma and Resuscitation
Respiratory assessment and monitoring of the trauma patient are of critical importance, as changes in vital signs and patient symptoms pose an increased risk to the patient’s stability. The level of assessment and treatment must be appropriate to the patient’s needs. Trauma-patient monitoring may result in significant and immediate changes, as obtaining other vital signs may be difficult in the immediate resuscitation period. In addition to conventional monitoring of heart rate, blood pressure, respiratory rate, body temperature, and arterial oxygen saturation, ETCO₂, as the sixth vital sign, should be monitored.

Prediction of survival after cardiac arrest (as measured by the return of spontaneous circulation) during cardiopulmonary resuscitation is difficult. Capnography has the potential to assist in making that prediction.¹² The potential for survival is dependent upon several factors, including the cause of cardiac arrest, the patient’s response to resuscitation efforts, the length of time elapsed from the onset of cardiac arrest, and the length of time since the initiation of resuscitation efforts. Objective methods for the assessment of return of spontaneous circulation (Doppler pulse detection and invasive blood-pressure measurement) are often limited to the ability to detect a pulse and circulation.

Capnography is an objective assessment tool for the prediction of survival following cardiac arrest.¹³ ETCO₂ levels correlate well with cardiac output, perfusion of peripheral tissues, and pulmonary circulation.¹⁴ As cardiac output increases, perfusion of peripheral tissue beds improves, sweeping carbon dioxide from the peripheral circulation and returning it to the pulmonary capillary membrane for exhalation. The concentration of carbon dioxide in the exhaled gases increases as cardiac output increases. In patients with return of spontaneous circulation, the level of ETCO₂ increases with cardiac output. Conversely, cardiac arrest can manifest itself as a sudden drop in ETCO₂ from normal levels to near zero.

The financial and emotional costs associated with futile resuscitation efforts can be high. While the numbers of patients undergoing prehospital resuscitation who survive to functional discharge is increasing, the overall survival rate is low.

Pulmonary Embolism and Asthma
Recent research has demonstrated a role for capnography in the assessment of asthma and pulmonary emboli. Asthma, a constrictive disease, limits the exhalation of carbon dioxide. Asthma may affect alveolar emptying and the release of carbon dioxide into the exhaled gas due to bronchiolar constriction. This constriction increases the slope of the plateau and rounds the peak of the capnogram. These distinctive features can be continuously monitored as an objective, quantitative measure of the response to bronchodilator therapy in children, as well as in adults.

All patients have natural dead space (areas of the airways and lungs that do not participate in ventilation). These areas do not come into contact with the blood at the pulmonary capillary membrane. Gases within the natural dead space do not exchange oxygen and carbon dioxide at the pulmonary capillary level. Some physical conditions increase the normal dead space to a level that may affect the ability of the patient to exchange gases. These conditions include pulmonary emboli. A pulmonary embolus may be caused by a variety of medical and traumatic conditions, such as surgery, changes in blood clotting, cigarette smoking, use of oral contraceptives, and prolonged periods of inactivity. With an increase in dead space, the reduction in pulmonary blood flow is reflected in the capnography waveform and numerical display. In exhalation in the presence of dead space, the concentration of exhaled carbon dioxide is decreased.

Procedural Sedation
Capnography is an ideal tool for monitoring the patient during sedation and/or pain management with the use of narcotics. Sedatives such as benzodiazepines have the potential to cause hypoventilation, apnea, and, as a result, hypoxemia. Monitoring of peripheral oxygen saturation provides information only on oxygenation, not ventilation. A decrease in oxygen saturation, as reflected in a drop in the Spo₂ displayed by the monitor, is a late indicator of hypoventilation and apnea. Patients have the ability to maintain an adequate Spo₂ level for prolonged periods in the presence of apnea, especially during the administration of supplemental oxygen.

The use of narcotics also has the potential to create respiratory depression. When used in conjunction with benzodiazepines, the medications have a synergistic effect in sedating the patient and producing respiratory depression.

To measure changes in ETCO₂ during sedation and pain management, capnography is invaluable. Continuous ETCO₂ waveforms and numeric values are displayed on the monitor, often in conjunction with Spo₂. If possible, clinicians should initiate capnographic monitoring prior to sedation and/or pain management to obtain baseline values. The primary change in patient status is commonly an increase in ETCO₂ levels. Transient Spo₂ desaturations may occur, but may be slow in circumstances involving apnea or hypoventilation. Significant increases in ETCO₂ may occur.

Advances in capnography have enabled clinicians to assess respiratory rate and adequacy of ventilation accurately in spontaneously breathing and sedated patients. The addition of ETCO₂ monitoring allows earlier identification of respiratory depression in this group of patients. Standard vital signs, oxygen saturation, and ETCO₂ are also monitored continuously. ETCO₂ monitoring provides an earlier indication of respiratory depression than pulse oximetry or respiratory rate alone.

Problem Solving
As with any single monitoring tool, clinicians must not rely solely on capnography for patient assessment. If the patient’s signs and symptoms fail to match the capnogram, they must intervene with the appropriate therapy to improve the patient’s condition. When used in combination with other monitoring tools, such as continuous patient assessment, spirometry, pulse oximetry, blood-pressure measurement, and electrocardiography, capnography can help ensure optimal oxygenation and ventilation and can allow the RCP to identify problems before the patient’s condition significantly deteriorates.

The shape of a single carbon dioxide–expiration curve provides significant help in troubleshooting because there is only one normal shape.

Capnography is an excellent tool for monitoring ventilation and respiratory efforts. The level of carbon dioxide in exhaled breath has, for many years, been appreciated as a valuable indicator of the clinical status of the metabolic, cardiovascular, and respiratory systems. Capnography is useful in confirming the success of intubation, tracking levels of respiratory effort, and monitoring the status of ventilated patients. Recent improvements in capnography technology have created lightweight, easy-to-use, and cost-effective instruments that significantly expand the clinical benefits of carbon dioxide monitoring.

As growing cost pressures and concerns about reimbursement continue to attract attention, health care facilities are reviewing quality-of-care issues. In part, these concerns have spurred a rise in public expectations for well-managed, integrated care using the best technologies. The value of capnography and other technologies that protect patients from potential problems increases. Capnography and pulse oximetry are standards of care within many areas of health care because proper use of these devices may prevent mishaps and untoward outcomes.

With recent advances in capnography, RCPs can meet often conflicting imperatives through its expanded use to detect changes in the status of patients with a variety of cardiopulmonary conditions rapidly, easily, and noninvasively. Capnography has the potential to improve patient care by reducing risks associated with patient management, by reducing costs associated with poor outcomes, and by improving the effectiveness of caregivers in enhancing patient outcomes.

EDITOR’S NOTE: The references for this article are posted with the online version of this story at www.respiratory-therapy.com.

Dan Hatlestad is a speaker, author, and trainer, Littleton, Colo.