End-Tidal Capnography

End-tidal capnography refers to the graphical measurement of the partial pressure of carbon dioxide (in mm Hg) during expiration (ie, end-tidal carbon dioxide [EtCO2, PetCO2]). First established in the 1930s, clinical use of EtCO2 measurement became accessible in the 1950s with the production and distribution of capnograph monitors.[1, 2]

With continuous technologic advancements, EtCO2 monitoring has become a key component in the advancement of patient safety within anesthesiology, and the American Society of Anesthesiologists (ASA) has endorsed end-tidal capnography as a standard of care for general anesthesia and moderate or deep procedural sedation.[3, 4]  

Studies showed that during cardiac arrest, EtCO2 values of greater than 10-20 mm Hg are associated with return of spontaneous circulation (ROSC).[5, 6, 7, 8]  Accordingly, other specialties, including critical care and emergency medicine,[9] began to implement end-tidal capnography monitoring more frequently, though there remains room for greater uptake of this life-saving technology outside of the operating room.[10, 11, 12]

Continuous waveform capnography, combined with clinical assessment, is "the most reliable method of confirming and monitoring correct placement of an endotracheal tube," according to the American Heart Association (AHA).[13]  Capnography can also be used to ensure ventilation with supraglottic devices, as well as to confirm that a spontaneously ventilating patient is in fact breathing (eg, via face mask or nasal cannula sampling).

More generally, end-tidal capnography is used in the following settings:

• General anesthesia
• Procedural sedation, including sedation with monitored anesthesia care
• Analysis of ventilation (eg, in the intensive care unit [ICU])
• Cardiac arrest, to confirm tracheal intubation and adequacy of chest compressions ^{[6, 13]}

Cardiopulmonary resuscitation

The balance between production, delivery, and elimination of CO2 can be monitored by means of end-tidal capnography. In the event of cardiopulmonary arrest, cardiac output drops to zero, and thus, no transport of CO2 from the tissues to the lungs can occur. End-tidal capnometry of an artificially ventilated patient would, after several washout breaths, show a flat EtCO2 capnogram with EtCO2 equaling zero during the arrest. Once chest compressions are initiated, circulation of blood will again deliver CO2 to the lungs, and the EtCO2 capnogram will rise and fall with each breath as CO2 is ventilated.

EtCO2 levels of 20 mm Hg or greater indicate adequate chest compressions during cardiopulmonary resuscitation (CPR), and failure to achieve a level of at least 10 mm Hg after 20 minutes of CPR may help in making the decision to terminate resuscitative efforts.[6, 14, 13]

Complication prevention

Continuous EtCO2 monitoring can provide an early warning of impending hypoxemia. Several studies have demonstrated that respiratory depression is detected via end-tidal capnography 30-60 seconds before it is detected via oxygen saturation.[15, 16]

Best practices

In anesthesia and procedural sedation, end-tidal capnography has become the standard of care. Class IA evidence has established the indications for use, with several randomized trials demonstrating reductions in episodic hypoxia during procedural sedation.

Reading the capnogram

Cellular metabolism produces carbon dioxide, while the lungs work to eliminate it from the body. The balance between production and elimination can be followed in the rise and fall of EtCO2 as displayed by the capnogram. More specifically, EtCO2 waveforms provide clinicians with a tool for quick and reliable diagnoses of common pulmonary pathophysiology.

Generally, EtCO2 is displayed as a waveform with partial pressure of CO2 on the y-axis and time on the x-axis (see the images below).

The capnogram has four phases, as follows:

• Phase I represents the end of inspiration, and early expiration of gas from dead space (circuit tubing, anatomic)
• Phase II represents gas from late anatomic and alveolar dead space, upsloping as alveolar gas mixes in and raises the CO ₂
• Phase III (plateau) represents the partial pressure of carbon dioxide exchanged at the alveoli; the amplitude of the plateau is referred to as EtCO ₂; phase III normally has a very slight upslope due to physiologic V/Q mismatch
• Phase IV signifies the start of inspiration, and the capnogram reflects the transition at the sampler from alveolar air flow to fresh air flow, or “scrubbed” air, in a closed circuit ^[17]

Of note, the information captured by end-tidal capnography (partial pressure of CO2) is devoid of volumetric information. Therefore, capnography should be used with end-tidal volume measurements for a full assessment of ventilation parameters. (See the video below.)

Pathology and the capnogram

Causes of high EtCO2 include the following:

• Malignant hyperthermia
• Shivering
• Fever
• Sepsis
• Endocrine disease
• Hypoventilation (see the video below)

Causes of low EtCO2 include the following:

• Hypothermia
• Low  cardiac output
• Pulmonary embolism
• Hyperventilation (eg, with metabolic acidosis; see the video below)

Other waveform findings include the following:

• Curare clefts (see the image below) typically occur when a mechanically ventilated patient attempts to inspire (eg, when analgesia is inadequate); they may also represent surgeon manipulation of the chest or adbomen
• A long, steeply upsloping phase III "plateau" is indicative of obstructive airway disease (eg, chronic obstructive pulmonary disease [COPD] or bronchospasm)
• An elevated baseline during phases IV or I may indicate a failing CO ₂ absorber
• A less acute phase IV slope suggests a failing expiratory valve
• Cardiogenic oscillations are seen with prolonged expiratory time

Explain to patients that you are monitoring their breathing and that this should not have any effect on them. They may simply breathe normally.

Equipment employed in end-tidal capnography includes the following:

• End-tidal capnometer (fixed or portable)
• Mainstream or sidestream sampling (see  End-Tidal CO2 Detectors)
• Nasal cannula or mask adaptors (nonintubated patients)

The end-tidal capnometer must be prepared, calibrated, and tested prior to use. (See the image below).

The patient should be in a position that both facilitates airway patency and allows adequate exposure for the procedure being performed. Often, the anesthesia provider and the surgeon discuss which technique and position will minimize risks to the patient prior to the procedure. Ideally, if endotracheal intubation is indicated, the patient is placed in a supine position for induction of general anesthesia and airway management.

End-tidal carbon dioxide (EtCO2) can be monitored in several ways, each of which has its own advantages. Options to be considered include the following:

• Qualitative vs quantitative measurement
• Mainstream vs sidestream capnometry

Colorimetry

Carbon dioxide colorimetry uses acidic changes in expired air containing carbon dioxide to change colors. Often, these devices are used in the emergency department (ED) or by an emergency medical technician (EMT) to confirm endotracheal tube placement.

Mainstream capnometry

Mainstream capnometry refers to the use of a nondiverting sampling device. This form of capnography is newer to clinical use than sidestream capnography is, with technologic advances allowing the development of smaller and lighter sampling devices. The advantage of this approach is that a real-time display is available during exhalation because the device is part of the breathing circuit and samples at the distal end of the ventilatory circuit.[15]

Sidestream capnometry

Sidestream capnometry refers to the use of a diverting sampling device. This form of capnography has been in use for some time and probably is still the most commonly used approach. During exhalation, a small portion of air is diverted at a T-piece to an external EtCO2 analyzer.

Several disadvantages are noted, including the following[15] :

• Diversion of air to a distant external device creates extra dead space between the patient and the sampler, causing a delay in time between exhalation and readout on capnometry
• The air sampled is removed from the circuit and can require increased flow in closed-circuit ventilation
• The T-piece used at the point of sampling is rather small and can be occluded with patient secretions
• Because dead space is added in the circuit, some dead space air mixes with exhaled air; however, the significance is negligible

Portable capnometry

Portable capnometry[18, 19, 20] is newer than the methods previously mentioned and has not yet been as widely used, largely because of limitations imposed by cost and operative parameters.