End-Tidal CO2 Detectors 

End-tidal carbon dioxide (CO₂) monitoring (EtCO₂) was originally introduced into widespread clinical use as measured by mass spectroscopy. However, this technology was limited primarily to the operating room and became standard use within the field of anesthesia. With technological improvements, EtCO₂ can now be measured with infrared spectroscopy. This has allowed the measurement of EtCO₂ in a more portable fashion.

In infrared spectroscopy, beams are emitted from a light source into a sample. As the beam passes through the sample, CO₂ absorbs a specific wavelength of light (4.26 µm). This measurement is then used to calculate the amount of CO₂ in the sample.

This result can provide information on CO₂ production, pulmonary perfusion, alveolar ventilation, respiratory patterns, and elimination of CO₂. The use and applications of capnography have evolved beyond the operating room and into other areas such as the critical care setting, emergency department, and the prehospital setting.

Category

End-tidal CO₂ detectors

Device details

Various manufacturers produce capnography devices. Most newer patient monitors and portable monitor/defibrillators have the capability of measuring EtCO₂. Below is a partial list of manufacturers and devices that can measure EtCO₂.

Portable/bedside EtCO₂ monitors

BCI/Smith Medical

• Capnocheck PLUS capnograph
• Capnocheck SLEEP capnograph
• Capnocheck II Hand-Held capnograph

Nonin Medical

• RespSense capnograph
• LifeSense capnograph

Phasein

• EMMA capnometer

COSMO

• CO₂ SMO capnograph

DRE

• ECHO CO₂ capnograph

Patient monitors with EtCO₂ monitoring

Welch Allyn

BCI/Smith Medical

Critcare

Venni

Portable monitor/defibrillators with EtCO₂ monitoring

Zoll

Medtronic

Phillips

Currently, there are 2 basic types of CO₂ detectors: quantitative and qualitative.

Qualitative CO₂ detectors are colormetric detectors that contain material that reversibly reacts with CO₂. This reaction causes the color to change, most commonly, from purple to yellow. Quantitative CO₂ detectors give a measured value of EtCO₂. This numeric value is referred to as capnometry. Quantitative detectors can also be displayed as a waveform called a capnogram. This waveform of inspiratory/expiratory CO₂ can be displayed over time or volume and is referred to as a capnograph.

Qualitative capnography units can be broken down into mainstream and sidestream configurations. Mainstream units, or in-line units, are used for ventilated patients who are intubated endotracheally. The sensor is placed directly on an adapter attached to the endotracheal tube. From there, EtCO₂ can be directly measured. Sidestream units have a sensor that is located on the main unit itself. These systems aspirate the gas sample from the patient’s airway, which then measures the EtCO₂. In turn, sidestream units can be used in awake or intubated patients.^[1]

Indications for EtCO₂ measuring include the following:

• Endotracheal intubation
• Cardiac arrest
• Procedural sedation
• Asthma/chronic obstructive pulmonary disease (COPD)
• Metabolic acidosis
• Adequacy of mechanical ventilation
• Detection of spontaneous respiration
• Indirect estimate of cardiac output
• Malignant hyperthermia
• Adequacy of fresh gas flow
• Onset of neuromuscular blockade
• Shunts in cyanotic heart diseases
• Detecting circuit disconnection
• Waning of neuromuscular blockade
• Detection of pulmonary embolism
• Detection of breathing/sampling circuit leak
• Detection of blocked or kinked tracheal tube
• Adequate reversal of neuromuscular blockade
• Evaluation of dual lumen endotracheal tubes
• CO₂ absorption (laparoscopic)
• Functional analysis of rebreathing apparatus
• Gastric tube insertion
• Measurement of cardiac output
• Seizures
• Detection of dead space
• Nutritional support

While there are many ways to confirm proper endotracheal intubation, multiple studies show the reliability in using capnography,^{[2, 3, 4, 5]} including in the setting of cardiac arrest.^{[6, 7]} This has been emphasized by the American College of Emergency Physician (ACEP) clinical policy,^[8] as well as the American Heart Association (AHA) guidelines on the 2010 Advanced Cardiac Life Support.^[9]

Metabolic acidosis

Several studies have evaluated the use of EtCO₂ in predicting diabetic ketoacidosis.^{[10, 11]} In one study of pediatric patients presenting with gastroenteritis, EtCO₂ showed a strong correlation with serum HCO₃^-. An EtCO₂ level of 34 mm Hg or less yielded a 100% sensitivity for predicting an HCO₃^- level of 15 mmol/L or less, whereas an EtCO₂ level of 31 mm Hg or less yielded a 95% specificity for predicting metabolic acidosis.^[12]

A recent study showed that, in adult patients without COPD, an EtCO₂ level of 36 mm Hg or more had a sensitivity of 99% and a negative predictive value of 0.05 for metabolic acidosis, as defined by a HCO₃^- of 21 mmol/L or less.^[13]

Equipment

Necessary equipment includes the following:

• EtCO₂ monitoring unit
• In-line adapters for intubated patients (for sidestream or mainstream units)
• Nasal cannula unit that can sample exhaled air samples

Quantitative EtCO₂ monitoring units can be fixed or portable. Portable units can be a standalone handheld or can be bundled into another unit such as a defibrillator/portable unit.

Patient setup

Mechanically ventilated patients should be supine with the airway device in place. Nonintubated patients should be in a comfortable position (either lying down or sitting up).

Approach considerations

Approach 1 (qualitative C0₂)

Qualitative devices generally attach to the proximal portion of the endotracheal tube. The bag-valve-mask (BVM) is then connected to the device (see image below).

Patients should be ventilated with 6 breaths; adequate color change is then confirmed on the device. The color change is read on the device at end-expiration. The actual color change differs depending on the manufacturer. Most qualitative devices function for up to 2 hours after initial use. It is reasonable to detach the device from the endotracheal tube and BVM once endotracheal intubation is confirmed.

Approach 2 (quantitative capnography—intubated)

For ventilated patients, quantitative capnography devices connect in-line between the endotracheal tube and the ventilatory source (usually BVM or ventilator; see image below). The sampling port is then connected to the monitoring device, where the EtCO₂ level and waveform can be seen.

Approach 3 (quantitative capnography—nonintubated)

For nonventilated patients, quantitative capnography devices are integrated into specialized nasal cannulas that contain a CO₂ sampling port (see image below). The nasal cannula is placed in the nares and secured in the usual fashion around the ears. The sampling port is then connected to the monitoring device, where the EtCO₂ level and waveform can be ascertained.

Physiology

CO₂ is produced by tissues during aerobic metabolism. This byproduct passively diffuses out of cells and into the bloodstream, where it is bound primarily by HCO₃^-. However, some of the CO₂ is bound by hemoglobin and transported to the lungs. When hemoglobin is transported through the pulmonary circulation, the affinity for CO₂ decreases (the Haldane effect) as the partial pressure of oxygen (PaO₂) is increased. CO₂ then crosses the blood-gas barrier into the alveoli, where it is eliminated through respirations. The expired CO₂ can then be measured as EtCO₂.

In healthy individuals, the EtCO₂ is approximately 2-5 mm Hg less than the atrial partial pressure of CO₂ (PaCO₂). However, in certain pathologies, the EtCO₂ may not reflect the PaCO₂. In conditions with decreased perfusion, such as cardiac arrest, decreased blood flow to the lungs would not allow for proper gas exchange, thus lowering EtCO₂ levels. Likewise, interruptions in pulmonary blood flow such as during pulmonary embolism would cause a ventilation/perfusion mismatch. This, in turn, can cause an increase in physiological dead space and a larger gradient between EtCO₂ and arterial PaCO₂.

The normal capnogram pattern resembles a trapezoid and consists of 4 major phases in the respiratory cycle. In the first phase, exhalation has begun and the measured EtCO₂ is zero. In the second phase, the air from dead space is cleared and alveolar gases along with atmospheric gases are released, causing a sudden rise in EtCO₂. In the third phase, the EtCO₂ measured represents all alveolar gases and is the latter portion of exhalation. In the fourth phase, inspiration occurs and the CO₂ levels fall quickly to zero (see image below).^[14]

Interpretation

Confirmation and monitoring of endotracheal tube placement

While there are many ways to confirm proper endotracheal intubation, multiple studies show the reliability in using capnography,^{[2, 3, 4, 5]} including in the setting of cardiac arrest.^{[6, 7]} This has been emphasized by the American College of Emergency Physician (ACEP) clinical policy,^[8] as well as the American Heart Association (AHA) guidelines on the 2010 Advanced Cardiac Life Support.^[9]

Endotracheal tube placement can be confirmed either qualitatively, through color change, or quantitatively, through evaluation of waveform. Qualitative colorimetric EtCO₂ detector devices are frequently used to confirm placement of endotracheal tubes. These devices detect levels of CO₂ flowing through the endotracheal tube and give a yellow reading in the presence of high levels of CO₂ indicative of tracheal intubation. No color change or intermediate color change suggests esophageal intubation or poor pulmonary perfusion. An endotracheal tube that becomes dislodged during transport or a procedure results in a loss of colorimetric change.

During cardiac arrest, lower levels of CO₂ are delivered to the lungs, resulting in lower EtCO₂ readings. No color change on colorimetric end-tidal CO₂ detectors can mean either improper placement of the endotracheal tube or inadequate perfusion due to ineffective CPR. A partial or intermediate color change during a cardiac arrest can result from either retained CO₂ in the esophagus and improper endotracheal tube placement or poor tissue perfusion. If, after additional ventilation of 6-8 breaths, the color remains intermediate, the endotracheal tube is likely in proper position and the perfusion is inadequate.^[15]

False-positive results on colorimetric end-tidal CO₂ detector devices can occur in several situations. Preintubation bag mask ventilation can fill the stomach with air containing CO₂, leading to positive color change or intermediate color change in the presence of esophageal intubation. Over time, the concentration of CO₂ will decrease, however, so the change should be monitored over several breaths. Intubation of the hypopharynx can also result in a false-positive result if the CO₂ levels are sufficient. False-negative results can occur when the endotracheal tube is obstructed, despite its proper positioning.^[15]

Continuous capnography can also be used to detect the presence of a properly placed endotracheal tube. In a properly placed endotracheal tube, the waveform should appear to have the trapezoidal shape similar to that in the image below. During the exhalation phase, EtCO₂ levels rise and then plateau, while, during inhalation or ventilation, the tracing declines back to baseline. If the endotracheal tube is improperly placed, the waveform does not have the trapezoidal shape and may appear flat. When an endotracheal tube is displaced or a circuit disconnects, a loss of shape of the capnogram occurs.

The American Society of Anesthesiology (ASA) also has a policy on anesthesia monitoring, which includes the use of capnography for confirmation of endotracheal intubation, as well as during intubation for complications with ventilation while intubated.^[16]

Cardiac arrest

Continuous capnography waveforms during a cardiac arrest can provide information about the placement of the endotracheal tube, the effectiveness of CPR, and the return of spontaneous circulation (ROSC); it is recommended for use in the 2010 AHA ACLS guidelines.^[9] EtCO₂ levels of less than 10 mm Hg suggest improper endotracheal tube placement, provider fatigue, inadequate rate of compressions, or inadequate force of compressions. ROSC results in a sudden increase in EtCO₂ levels, and this technique is recommended as a way to detect ROSC without interrupting chest compressions (see image below).

In some instances, a pulse is present but not detectable owing to extreme hypotension. Example causes include pulseless electrical activity, tension pneumothorax, pericardial tamponade, massive pulmonary embolus, or hypovolemia. A capnography waveform with detectable but low end-tidal CO₂ levels in the absence of compressions should prompt the provider to consider those causes. However, when the EtCO₂ level remains below 10 mm Hg despite adequate chest compressions, survival is unlikely.^{[17, 18, 19]}

Procedural sedation

Continuous waveform capnography using sidestream methods via nasal-oral cannula that detects EtCO₂ is often used to monitor for decreased respiratory effort in patients undergoing procedural sedation.^[20] During hyperventilation, characterized by faster respiratory rates, the EtCO₂ level is low and the capnogram is narrow with low amplitudes (see image below).

Conversely, hypoventilation is demonstrated as a wide waveform with high amplitudes (see image below).

Shallow breathing, hypopnea, is characterized by lower levels of CO₂ concentration. In general, an EtCO₂ change of 10 mm Hg from baseline indicates respiratory depression.^{[21, 22]} Complete absence of a waveform indicates complete upper airway obstruction, laryngospasm, or apnea (see image below).

In the case of airway obstruction and laryngospasm, chest wall movement continues, but there are no breath sounds, and the capnogram disappears.^[14]

Capnography is recommended by the ASA for use during both moderate and deep sedation.^[16] ACEP clinical policy suggests that capnography can be considered for monitoring respiratory status in procedural sedation.^[23]

COPD, asthma, and respiratory distress

Capnography can be used to monitor respiratory status in various patients in the ED. In the setting of an upper airway obstruction or laryngospasm, chest wall movement may still occur, giving a measurement of a respiratory rate, although no effective ventilation is occurring. In this setting, capnography can signal ineffective ventilation as shown by a flattening of the capnogram.

The EtCO₂ level can be used as a surrogate marker for PaCO₂ in obstructive lung disease. In the presence of air trapping, as seen in obstructive air disease such as COPD or asthma, the capnogram has a more gradual upslope and a slight incline during plateau phase and will be wider, representing an extended expiratory phase (see image below).^[24] The EtCO₂ level can change depending on the ventilator status, with trends showing decreased EtCO₂ levels (improvement), stable EtCO₂ levels (no change), or increasing EtCO₂ levels (worsening).

Metabolic acidosis

EtCO₂ monitoring has been evaluated as a noninvasive method for predicting metabolic acidosis. This may help triage patients in determining those with potential severe acidosis without possible invasive testing, especially in children.

Several studies have evaluated the use of EtCO₂ in predicting diabetic ketoacidosis.^{[10, 11]} In one study of pediatric patients presenting with gastroenteritis, EtCO₂ showed a strong correlation with serum HCO₃^-. An EtCO₂ level of 34 mm Hg or less yielded a 100% sensitivity for predicting an HCO₃^- level of 15 mmol/L or less, whereas an EtCO₂ level of 31 mm Hg or less yielded a 95% specificity for predicting metabolic acidosis.^[12]

A recent study showed that, in adult patients without COPD, an EtCO₂ level of 36 mm Hg or more had a sensitivity of 99% and a negative predictive value of 0.05 for metabolic acidosis, as defined by a HCO₃^- of 21 mmol/L or less.^[13]