An Overview of Capnography: A Ventilatory Monitor for Sedation in Dentistry

Monitoring the dental patient’s breathing is an essential element of procedural sedation. Using parameters such as oxygenation, ventilation, circulation, temperature, and response to stimulation is integral to patient monitoring.¹ Guidelines are often implemented to improve patient safety, reduce risks, and standardize practice.² In July 1, 2011, the American Society of Anesthesiologists (ASA) endorsed the use of end-tidal carbon dioxide (ETCO₂) monitoring by capnography for moderate sedation, deep sedation, and general anesthesia. The American Association of Oral and Maxillofacial Surgeons, in its 2012 practice guidelines, recommended capnography for use with deep sedation and general anesthesia in outpatient practices.³ The American Dental Association has recently proposed requiring capnography as a monitor for assessing the patient’s ventilatory status for moderate sedation in the Guidelines for the Use of Sedation and General Anesthesia by Dentists.  

The goal of this article is to present the clinician with an overview of capnography, which is a ventilatory monitor that measures the carbon dioxide at the end of exhaled breath, or ETCO₂. Capnography is both a quantitative and qualitative indicator of the adequacy of ventilation. Several other techniques can be used as a supplement to capnography; the advantages and disadvantages of each technique will be reviewed. The clinical considerations regarding the technique and use of capnography will be discussed.

Levels of Sedation

Although sedation and anesthesia exist on a spectrum, the Joint Commission on Accreditation of Healthcare Organizations’ Comprehensive Accreditation Manual for Ambulatory Care provides a definition for four levels of sedation and anesthesia: minimal sedation (anxiolysis), moderate sedation (formerly known as conscious sedation), deep sedation, and general anesthesia.⁴ As illustrated in the Table, as one progresses from minimal to deeper levels of sedation, central nervous system (CNS) and cardiovascular depression becomes pronounced, and the ability to maintain a patent airway becomes more impaired.

These levels of sedation can be achieved through using any medication by any route of administration. Almost all sedative medications that are administered for dental sedation are capable of inducing respiratory depression.^2,5-8 The medication, dose, patient’s level of anxiety, and presence of any preexisting conditions (such as sleep apnea) that render the patient more susceptible to respiratory depressants are all contributing factors to a patient’s unique response to the medication.⁹ For example, the intended level of sedation may have been minimal or moderate sedation. However, the patient may lose the ability to maintain a patent airway and may no longer respond to purposeful command and light tactile stimulation but is responsive only to painful stimulation. This patient would be deeply sedated.

Rationale for Monitoring Ventilation

Monitoring provides several benefits. Poor trends in vital signs can be detected before they evolve into more serious consequences and medical emergencies. Monitoring also enables sedation providers with the ability evaluate the efficacy of their interventions to reverse such trends. Monitoring also allows for the recognition of acute medical emergencies. Trained and attentive dentists and their team members are essential to the successful use of any monitoring device.¹⁰

The implementation of monitoring techniques such as capnography may improve the ability to rapidly detect and correct any ventilatory disturbances before the onset of life-threatening sequelae, such as hypoxic brain injury or cardiac arrest.⁵ This is especially important because the most common reason for the transfer of a sedated dental patient to a hospital’s emergency department is respiratory distress.^6,9 Most adverse outcomes may have been prevented with “timely monitoring” and “effective response.”^2,5,6 Prior to the ASA’s inclusion of capnography as a standard monitor, one study concluded that monitoring with capnography was 17.6 times more likely to detect respiratory depression than compared with standard monitoring, using methods described below.²

Monitors of Ventilation

Clinical Observation

Several qualitative monitoring techniques may still be used as an adjunct to capnography. Clinical observation of ventilation in a spontaneously breathing sedated patient can be performed by observing for chest rise, noting the fogging of a dental mirror, or feeling the exchange of warm air from the mouth and nose on exhalation.⁶ Verbal communication with the patient is another form of clinical observation. The ability to phonate also indicates that ventilation is occurring. Continued ventilation facilitates the perfusion of the CNS with oxygenated blood, meaning the CNS is intact and the patient is capable of performing the intricate functions necessary to understand and respond to verbal command. It also indicates that the patient is mild to moderately sedated.¹⁰ Some aspects of observation may be deceiving: the patient may still attempt to breathe against an obstructed airway and it may be difficult to visualize the chest when draped for a dental procedure.^2,10 Clinical observation is not precise; in one study, competent observers consistently recognized hypoxemia at an oxygen saturation of 70% or less.⁸

Precordial Stethoscope

Although the use of a precordial stethoscope is less sensitive to the detection of hypoventilation than capnography, using both techniques concomitantly can provide advantages.⁶ The monitor is placed on the suprasternal notch or jugular notch and the sounds of airflow during breathing are auscultated. Qualitative differences in sound quality may indicate whether there is a partial airway obstruction, complete airway obstruction, or a foreign body (such as secretions) in the airway. Cardiac rate and rhythm can also be assessed with a precordial stethoscope.¹⁰ The amplification of verbal communication and ambient noises in a dental operatory may be loud in the earpiece.^5,10

Pulse Oximetry

Pulse oximetry is not a true monitor of ventilation; rather it is an indirect method of monitoring ventilation, which has several major limitations. One can reason that if ventilation (the exchange of gases) is compromised, then the oxygen levels in the blood will fall. Pulse oximetry is used to monitor the level of oxyhemoglobin saturation in the blood (SpO₂). Hypoventilation will result in an increase in alveolar carbon dioxide (PACO₂), which will then lead to a decrease in alveolar oxygen (PAO₂). Desaturation of oxyhemoglobin will be detected as the arterial oxygen tension (PAO₂) drops below 100 mmHg. Administering supplemental oxygen, however, will help maintain oxygen saturation despite hypercapnea secondary to hypoventilation.² This can lead to the inherently risky practice of withholding supplemental oxygen to be able to detect desaturations. Doing so is not advised as a patient who has hypoxemia and respiratory depression may have even more difficulty ventilating and receiving needed oxygen.^2,6,11 There is a lag between the onset of respiratory depression, which can place the patient at risk for severe desaturation and hypoxemia.^9,11

Capnography

Capnography is a noninvasive monitor used for ventilation. It measures the end-tidal carbon dioxide both with a numerical value expressed in millimeters per mercury and with a qualitative waveform. Unlike pulse oximetry, capnography offers immediate detection of respiratory depression and may allow early identification of an airway issue (such as obstruction, a foreign body, or drug-induced respiratory depression) and intervention with airway support before serious hypoxemia occurs.^2,5,9,11

ETCO₂ is the amount of carbon dioxide exhaled as measured at the end of an exhalation. Capnography machines work by analyzing the exhaled gases using infrared light, which is passed through the analyzer. Carbon dioxide will absorb specific wavelengths of infrared light and the remainder of the light will reach the sensor in the analyzer. The device then calculates the amount of exhaled carbon dioxide.⁸ The range for the normal value is 35 mmHg to 40 mmHg. The arterial carbon dioxide tension (PACO₂) is usually 2 mmHg to 5 mmHg higher than ETCO₂ levels due to normal dead-space ventilation. The physiologic dead space is the part of the airway that participates in ventilation but not perfusion (and, therefore, no gas exchange can occur).¹⁰

The shape of the capnography waveform offers insights as to the differential diagnosis of impaired ventilation (Figure 1). The portion labeled “1-2” is the first part of the capnographic waveform. It is a flat line for which no carbon dioxide is exhaled because this is exhaled gas comes from the patient’s dead space. In the segment labeled “2-3,” the waveform then slopes upward as increasing amounts of alveolar gases containing carbon dioxide are exhaled. This is the expiratory upslope. A slow rate of rise in the expiratory upstroke can be attributed to airway obstruction. This may be due to an acute bronchospasm or chronic obstructive pulmonary disease. In segment “3-4,” a plateau is reached as pure alveolar gases are exhaled. At the end of the plateau, at point “4,” or at the end of a tidal volume, ETCO₂ levels are measured. The plateau should be nearly flat or be slightly upsloping. Following the plateau and the end of an exhalation, the waveform returns to point “1” (nearly zero carbon dioxide is measured) down an inhalation and the next waveform begins.^8,12 

Changes in ETCO₂ levels can reflect either metabolic changes in the production of carbon dioxide or changes in the elimination of carbon dioxide, such as with ventilation and/or perfusion. A differential diagnosis for decreased ETCO₂ levels includes declines in carbon dioxide production (including hypothermia and low metabolic rate). ETCO₂ levels can also be lowered if the patient is hyperventilating and has already exhaled and “breathed off” much of the carbon dioxide. Another reason for decreased ETCO₂ levels includes hypoperfusion of the alveoli, which is dead space in the ventilation in the absence of perfusion or diminished cardiac output. An embolism can also be responsible for poor perfusion, making ETCO₂ levels drop as well. Another reason for a loss of waveform and drop in ETCO₂ levels is the disconnection or total obstruction of the sample line, mouth breathing if sampled at the nasal cannula, apnea, or complete airway obstruction.

Increases in ETCO₂ levels can be assessed along a similar differential diagnosis. ETCO₂ levels may be increased due to increased production; a hypermetabolic state, excessive muscular activity, malignant hyperthermia, and sepsis are all potential causes. ETCO₂ levels can also be elevated due to hypoventilation such that when the obstruction is relieved, the exhaled breath will contain higher amounts of carbon dioxide. Rebreathing of exhaled gases can also contribute to elevated ETCO₂ levels.^7,12

Although a thorough discussion of advanced cardiac life support (ACLS) is beyond the scope of this article, dental professionals providing sedation should have current ACLS certification. In cardiac arrest, high-quality chest compressions are delivered and the ETCO₂ levels should be approximately 10 mmHg to 20 mmHg (in intubated patients). When resuscitative efforts have resulted in the return of spontaneous circulation, ETCO₂ levels will increase quickly as the circulation is bringing back deoxygenated blood containing carbon dioxide to the lungs for ventilation.^11,12

Capnography may be implemented in the dental setting using several techniques. Sampling lines are available both separate from the nasal cannula and integrated in the nasal cannula. The separate sampling line can be placed at the nare or in front of the mouth to detect mouth breathing, as well. A sample line can also be placed under the nasal hood. In an open system, such as with nasal cannula or nasal hood, the entire exhaled breath may not be captured. Although quantitative ETCO₂ values may be lowered due to dilution with high flow rates of supplemental oxygen, supplemental oxygen will not cause gross deviations in capnography. Flow rates of 2 L/min in patients who are moderately sedated will not interfere with capnography and will provide supplemental oxygen in a comfortable and non-irritating manner.⁷ Similarly, the scavenging system in a nitrous oxide-oxygen unit may cause potentially minor deviations in the measured quantitative ETCO₂ values.^5,6,11 Overall, capnography, when sampled with a nasal cannula, is highly sensitive in detecting changes in ventilation.⁶

Conclusion

The implementation of technological advances in dentistry, such as in imaging, diagnostics, instrumentation, and materials sciences, is far from new. Innovations in monitoring of ventilation with capnography have a role alongside established monitoring techniques. It is arguable that ventilation is the most important parameter for the monitoring of healthy patients, especially in children, for whom neurologic and cardiovascular sequelae are often late indicators of and attributable to respiratory distress. Although a modest initial cost in implementing capnography may be necessary, the benefits of improved monitoring, early intervention, and correction of hypoventilation may outweigh perceived disadvantages.

ABOUT THE AUTHOR

Dr. Saraghi is a dentist-anesthesiologist in the greater New York City Area.

DISCLOSURE

Dr. Saraghi has not received any honoraria for writing this article.

References

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12. Barash PG, Cullen BF, Stoelting RK, et al. Clinical Anesthesia. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 2009.