3 things to know about capnography and advanced airways
Understand how using end-tidal carbon dioxide to confirm placement of advanced airways and guide patient care
Placement of an advanced airway device in critically ill and injured patients is a high-risk procedure, and permanent disability or death is likely if the airway is misplaced and not recognized. Here are three things you need to know about capnography and advanced airways.
1. Waveform capnography provides DNA-level evidence of airway placement
Waveform capnography is the most reliable tool to confirm placement of an advanced airway, both for endotracheal (ET) tubes and supraglottic-airway devices. When a device is placed correctly, a waveform and end-tidal CO2 (ETCO2) reading appear within seconds after the first ventilation is delivered to the patient. Capnography provides objective, breath-to-breath feedback on ventilation through the duration of patient contact, and changes immediately if the airway is displaced or a problem with ventilation develops.
Several studies have found unacceptably high rates of misplaced ET tubes by paramedics when capnography was not used, even when other confirmation techniques indicated that it was. One study showed that "equal breath sounds" documented in 95 percent of patients with unrecognized misplaced ET tubes, and "visualized cords" documented in 52 percent . Another study found unrecognized misplaced ET tubes in 23.8 percent of patients when capnography was not used, and none in intubated patients where it was .
The esophogeal detection device has been shown to be unreliable for airway confirmation in obese patients and patients who aspirated before attempted intubation. Colormetric CO2 devices are less sensitive than waveform capnography, and may take several breaths before a color change registers . All of these tools rely on paramedic self-reporting and leave no record of validation like capnography does.
Pulse-oximetry is another tool that should be used when an advanced airway is placed, but it also has limitations for confirming airway placement. In patients who were preoxygenated before an airway was placed, it may take several minutes for their pulse-oximetry to change if the airway is misplaced. Pulse-oximetry readings also depend on circulation to the area where the probe is attached, which makes it less reliable in shock and cardiac arrest than capnography.
Waveform capnography is equally important to assess the placement of supraglottic airway devices, such as the King airway, laryngeal mask airway, air-Q, i-Gel, and Combitube. These can be placed blindly and are increasingly popular at the BLS level. While they have a high rate of successful placement, capnography ensures that they are inserted at the correct depth and direct air into the trachea. Capnography is especially important for the Combitube to prove that the patient is ventilated through the correct port.
The American Heart Association assigned its strongest recommendation class to the using capnography, along with a clinical assessment, to confirm ET tube placement. The AHA guidelines state that a waveform should be observed immediately after placement, after patient movement to the ambulance, and after patient transfer to the ED bed .
2. Apply capnography before advanced airway placement, and use other confirmation techniques as an adjunct
As reliable as capnography is for confirming airway placement, it does not replace a physical exam. There are a few circumstances when no ETCO2 will be detected with a correctly placed airway. These can be mitigated by applying capnography early and performing a physical exam at key points in airway management.
Inadequate chest compression during cardiac arrest care is one cause of ETCO2 failing to register with a correctly placed airway. ETCO2 depends on air movement in and out of the lungs, as well as circulation of CO2 to the lungs. Compressions that are not fast or deep enough, not done for a period while the airway is placed, or when down time has been so long that even good compressions do not generate CO2 production, will not produce a waveform from a correctly placed device.
Monitoring capnography and listening to breath sounds in the initial stages of resuscitation provides a baseline to compare readings after placement of an advanced airway. Once the advanced airway is placed, listen with a stethoscope over the patient’s epigastrium and look for a waveform on the monitor when the first ventilation is delivered. There should be no epigastric sounds, a waveform should appear, and the ETCO2 level should be the same or higher than before if the airway is placed correctly. A loss of ETCO2 after the first ventilation indicates that the airway is likely misplaced and should be removed immediately.
If an ET tube is placed too deep in the trachea, it will enter the right mainstem bronchus and only ventilate the right lung. While not as devastating as an esophageal intubation, it is still important to correct. An ETCO2 reading and waveform would be generated, but it may be lower than before intubation was attempted. If a waveform appears after the first ventilation, move the stethoscope to the left side of the chest and listen to the second ventilation. A waveform with breath sounds on the left side ensures that the tube is in the trachea and both lungs are being ventilated.
3. Use capnography to guide care after the airway is placed
In addition to confirming airway placement, capnography should be used to guide ventilation rate and tidal volume. ETCO2 will decrease as ventilation increases and more CO2 is blown off, and rise with slower ventilation as less CO2 is eliminated.
Patients are often incorrectly and mistakenly hyperventilated after intubation, which impedes circulation from CPR in cardiac arrest  and decreases cerebral perfusion in head injured patients , both of which are associated with poor outcomes. The AHA states that real-time feedback of ventilation during resuscitation may improve guideline adherence , and one study on intubated head-injured patients showed a lower incidence of hyperventilation and better outcomes when capnography was used . Use feedback from capnography to ventilate cardiac arrest patients at 8-10 breaths per minute , and to follow local protocols for ventilation rate and ETCO2 targets in other patients.
Capnogrpahy can also be used to monitor shock states after intubation. In cardiac arrest, ETCO2 can be used to monitor the quality of chest compressions (the higher the better), and a spike in ETCO2 is also the first sign of return of circulation. In patients with a pulse, progressive decline in ETCO2 with a consistent ventilation rate is a sign of worsening shock, or that a post-arrest patient is in danger of losing pulses again.
If ETCO2 is suddenly lost in an intubated patient, first check a pulse, as loss of circulation will cause an immediate loss of exhaled CO2. Next apply the DOPE mnemonic to troubleshoot airway problems — dislodgement, obstruction, pneumothorax, and equipment failure. Reassess breath sounds to check for airway dislodgement and pneumothorax, and use a suction catheter to assess for an obstruction in the tube. Next check for equipment failure if a mechanical ventilator is being used.
Once clinical explanations for a loss of ETCO2 have been ruled out, look for problems with the capnography circuit. Ensure that the circuit is clear of secretions and replace the circuit if needed. Also ensure that the capnography tubing is not pinched closed, and that it is attached to the monitor correctly. It is important to document the reason for any interruptions in ETCO2 when an advanced airway is in place, and the actions taken to correct it.
Waveform capnography is the standard of care for confirming advanced airway placement. ETCO2 level and ventilation rate can be continuously monitored through all phases of patient contact, and trends can be archived for documentation and quality assurance afterward. Failure to use capnography with an advanced airway puts patients at an unacceptable risk of harm, and EMS providers and services at risk of litigation.
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