5 things to know about capnography in cardiac arrest
Apply waveform capnography to make informed clinical decisions before, during, and after cardiac arrest patient care
By Bob Sullivan
Waveform capnography provides continuous feedback on CPR quality, confirmation of airway placement, ventilation, and a return or loss of circulation. Here are five things you should know about waveform capnography in cardiac arrest:
1. Loss of ETCO2 may be the first sign that CPR is needed
End-tidal CO2 (ETCO2) detection requires air movement in and out of the lungs (ventilation), CO2 production from cellular metabolism, and blood flow to the lungs to excrete the CO2. A change in any of those will be immediately detected with waveform capnography, which is valuable information while caring for critically ill and injured patients.
Besides constant palpation of a carotid pulse, waveform capnography is the most reliable prehospital monitoring device to detect the immediate loss of circulation. As soon as the heart stops, the capnography waveform will disappear and the ETCO2 reading will change to zero. This is even true for patients with good air movement while receiving positive pressure ventilation.
Waveform capnography is also useful for hypotensive patients with decreased mentation, where detection of a carotid pulse can be difficult. If a waveform is produced with exhalation, some circulation is present.
Other monitoring devices are less reliable for detecting cardiac arrest. Pulse-oximetry depends on circulation at the location where the probe is placed, which is inconsistent in patients in shock or with cold extremities.
The tracing on an ECG monitor may not even change after the patient loses pulses; the patient’s heart may still have electrical activity but no pumping action. This is especially true in bradycardic patients receiving transcutaneous pacing. If a capnography waveform is lost, check for a pulse immediately, and initiate CPR if one cannot be felt. If a pulse is felt, then check for breathing, because respiratory arrest will also cause a loss of ETCO2. If the patient has a pulse and is not breathing, initiate positive pressure ventilation. If the patient has a pulse and is breathing, look for a problem with the capnography circuit, such as secretions in the filter or kinks in the tubing.
2. Compression feedback devices measure CPR quality, ETCO2 measures how the body responds to it
Consistent, high quality chest compressions are essential for successful cardiac arrest resuscitation. The depth of each compression should be at least 2 inches, but not greater than 2.4 inches, the hands must come completely off the chest on the upstroke, compressions should be delivered at a rate of 100-120 time a minute, with pauses only for compressor changes.
Several devices are available that provide real-time feedback on compression quality. These prompt the compressor to adjust their technique, and detect the need to change compressors due to fatigue. ETCO2 provides feedback on how effective compressions are at perfusing vital organs. Both are important to guide treatment, and both should be applied as early as possible.
A higher ETCO2 reading during resuscitation correlates with improved cardiac output and patient outcomes. An ETCO2 reading above 15 mm HG indicates compressions are generating perfusion . The higher the ETCO2, the better the perfusion generated by CPR, and the better the chances of survival are. Compressions may also generate small capnography waveforms from passive air exchange between ventilations, which also provides feedback on compression rate.
Low ETCO2 (below 10 mm HG) may be caused by either poor compression technique, or from low perfusion and metabolism after a long downtime or shock despite good compressions. Compression feedback helps determine whether compression adjustments can make perfusion better, and ETCO2 shows when good compressions do not generate effective perfusion.
3. Waveform capnography guides ventilation rates and confirms airway placement
Excessive positive pressure ventilation frequently occurs during resuscitation and is extremely harmful . Delivering ventilation too frequently or too forcefully increases intrathoracic pressure, which decreases the amount of blood circulated with compressions. Hyperventilation also causes vasoconstriction and decreased perfusion to the brain.
The AHA recommends that ventilations should be delivered every 6 seconds (approximately 10 per minute), that the BVM be squeezed slowly, over one second, and only enough tidal volume (about 600 mL) should be delivered to make the chest rise . Timing ventilation can be difficult under stress during resuscitation, and when applied, waveform capnography has been shown to reduce the incidence of patient hyperventilation .
Through a circuit connected to a BVM or advanced airway device, each ventilation will produce a waveform and the respiratory rate will be displayed with the ETCO2 reading. When an advanced airway is placed, waveform capnography is the most reliable method to confirm placement, and documented proof that the airway remained in place throughout the course of care.
4. Capnography helps determine when to terminate resuscitation — and when to continue efforts
Determining when to stop resuscitation efforts is one of the most difficult decisions EMS providers face. ETCO2 can help differentiate when efforts should continue and when they are futile. In patients receiving high-quality chest compressions, who have an advanced airway placed, a persistent ETCO2 reading below 10 mm HG after 20 minutes of resuscitation is an indication to terminate efforts . However, a high ETCO2 shows that CPR is providing effective circulation to vital organs — essentially doing the heart’s job while defibrillation and medications are administered to get it beating on its own.
Data is also emerging that shows an ETCO2 reading above 15 mm HG, or one that increases from baseline, is an indication to continue resuscitation efforts . There are several cases of patients who survived neurologically intact after an hour or more of resuscitation on scene with a high ETCO2 reading . The use of capnography to guide the length of resuscitation efforts is an exciting area for future research.
5. A spike in ETCO2 reading is the first sign of ROSC, and a drop in ETCO2 is the first sign it is lost
A sudden increase in ETCO2 is often the first sign of return of spontaneous circulation (ROSC), even before a carotid pulse can be detected. Because impaired circulation during arrest causes CO2 to build up in the bloodstream, the initial ETCO2 reading may initially be higher than the normal 35-45 mm Hg range as it gets washed out of the lungs. Remember that a waveform with ventilation and no compressions means that there must be circulation taking place, so pause compressions and check a pulse if a spike in ETCO2. A few minutes after the initial increase, use ETCO2 to titrate the ventilation rate and tidal volume to maintain an ETCO2 reading between 35 and 40 mm HG .
Patients with ROSC are at high risk for rearrest, especially in the first 10 minutes. With ventilation at the appropriate rate, a steady decline in ETCO2 indicates that CPR may soon be needed, or that a vasopressor is needed to support blood pressure. If the ETCO2 is abruptly lost, it may be caused by rearrest or displacement of the advanced airway. First check a pulse, and then confirm airway placement.
Waveform capnography provides information about both circulation and ventilation before, during, and after cardiac arrest. Apply the device early in resuscitation, and apply the information it provides to clinical decisions.
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