Capnography: Part II
In part one of this 2 part course we discussed what capnography is, reviewed airway anatomy and physiology, and covered how the measurement of end tidal CO2 is done by the monitor. This class will examine the capnograph waveform and how to use this to assess the patient’s airway.
THE CAPNOGRAPH WAVEFORM
It has been said that using capnography without a waveform is like interpreting a cardiac rhythm without an ECG. Having just an end-tidal CO2 number does not tell the complete story; you need a waveform in order to get “the rest of the story”.
Before you can understand the abnormal waveform you must first be able to read a normal waveform.
The capnograph is a measurement of expired carbon dioxide over time. Horizontal axis is a measurement of time; the vertical axis is the amount of CO2. This means that you not only can tell the amount of CO2, but also the respiratory rate.
READING THE WAVEFORM
Each part of the waveform represents a different phase of the respiratory cycle.
- PHASE I: Respiratory Baseline, CO2 free dead space air, normally 0
- PHASE II: Expiratory Upstroke, rapid rise due to mixing of dead space air and alveolar air, should be steep
- PHASE III: Expiratory Plateau, exhalation of mostly alveolar air
- ☼: Peak Et CO2 Level , end of exhaled air, peak end tidal CO2 level, normally 35-45mmHg
- PHASE IV: Inspiratory Downstroke , inhalation of CO2 free gas, quickly returns to the baseline.
Changes in the capnograph or end tidal CO2 levels represent changes in metabolism, perfusion, the pt’s airway, or equipment
Interpreting the Capnograph
Just like when learning to interpret ECG’s, taking a systematic approach to interpreting the capnography waveform makes understanding the capnograph easier. The first question to ask is “Is there CO2 present?” This is seen by the presence of a wave form. If there is not defined waveform then there is an issue with the pt’s airway, if an advanced airway is place; the pt is not breathing, or there is an equipment malfunction. Next look at the respiratory base line, does the end tidal CO2 value return to zero during inhalation? If the waveform does not return to the baseline this is sign that the pt is rebreathing exhaled CO2. The next step is to look at the shape of the waveform. Does the waveform rise steeply, plateau, and then steeply return to the baseline? Sloping to the waveform, notching of the plateau, or a prolonged waveform are signs of possible abnormalities. Finally, read the end tidal CO2 level. Normal CO2 is 35-45 mmHg, above 45mmHg is hypercapnia and below 35mmHg is hypocapnia.
This is the normal capnograph. A wave form is present; the wave form begins at the base line, raises steeply, plateaus with a gradual upslope, and quickly returns to the baseline. Finally, the end tidal CO2 reading is within the normal range of 35-45 mmHg.
In hyperventilation, the capnograph starts normal but as the rate becomes faster the waveforms become closer together and the level of end tidal CO2 begins to decrease. The most common cause of hyperventilation is over zealous bagging of a patient with an advanced airway placed. When decreasing end tidal CO2 levels are noted, simply slow the rate in which the patient is being ventilated until end tidal CO2 levels return to normal. In the spontaneously breathing patient, increasing respiratory rate and decreasing end tidal CO2 levels can be a sign of hyperventilation syndrome or pulmonary embolism.
Hypoventilation is seen when there is increasing end tidal CO2 levels and the waveform still has a normal look. This is often seen in patients with decreased respiratory drive due to narcotic overdose, CNS depression, or heavy sedation. These patients are not breathing fast enough or deep enough to adequately remove CO2 from the lungs, resulting in increasing end tidal CO2 levels. Assisting ventilations with a BVM or administering a reversal agent, such as Narcan, will help improve ventilation and reduce CO2 levels.
Loss of Alveolar Plateau
The loss of a normal slightly upsloping alveolar plateau is an indication of an incomplete or obstructed exhalation. The waveform often will have a “shark fin” pattern. This waveform shows that exhalation is being slowed, often by bronchoconstriction. Common causes include asthma, COPD, or an airway obstruction. The use of bronchodilators or tracheal suctioning are often needed to correct this waveform.
Apnea/Loss of Waveform
The sudden loss of a waveform is indicates that no CO2 is present. In the spontaneously breathing patient this means that the patient is most likely gone into respiratory arrest. (If the patient is still breathing, check the equipment) If the patient has an advanced airway in place, this is an indicator that there is a problem with the airway. The airway may have become displaced or obstructed. Quickly reassess the airway and the patient with physical exam. If there is any doubt about the location or patency of the airway, remove it, ventilate the patient with a BVM and reestablish the airway if indicated.
A normal capnograph is the best available evidence that the ET tube is correctly positioned and that proper ventilation is occurring. When the ET tube is placed in the esophagus, either no CO2 is sensed or only small transient waveforms are present. Capnography is to be used every time that an advanced airway is place. This verification is now considered the “standard” for proper airway placement.
Elevation of the baseline indicates that there is incomplete inhalation and/or exhalation. The CO2 is not being completely washed out during inhalation. This is often seen with air trapping in patients with a history of asthma or COPD. Elevation of the base line can also occur when there is a malfunction in the exhalation valve of the BVM or ventilator. Increasing expiratory time will help remove excess CO2 in patients who are experiencing air trapping.
Clinical Applications of Capnography
Now that we have seen what the capnograph means, the big question is “How can I use this in the back of the ambulance?”
The first thing that comes to most people’s mind is the confirmation of advanced airway placement. The use of waveform capnography is now the standard for confirming airway placement after physical assessment. Not only does the use of continuous capnography allow for conformation, it also allows for ongoing monitoring of airway placement. A displaced airway can be instantaneously detected by the loss of the waveform on the capnograph. In the new 2010 American Heart Association’s guideline, the use of waveform capnography for airway confirmation is a Class I recommendation.
Waveform capnography can also be useful during cardiac arrest. The quality of CPR can be monitored using end tidal CO2 levels during compressions. Studies have shown that the greatest chance for the return of spontaneous circulation happens when CO2 levels are maintained above 20mmHg during cardiac arrest. CO2 levels below 20 mmHg may be an indicator of inadequate chest compression depth and rate, or that the person performing chest compressions has become fatigued and compressors need to be changed. During cardiac arrest our first priority is providing high quality CPR at all times. The 2010 AHA guidelines for CPR recommend that compressions for adult cardiac arrest should be at least 100 per minute at a depth of at least 2 inches. Interruptions in compressions should be limited to less than 10 seconds. Additionally, when end tidal CO2 levels are persistently low, less than 10mmHg despite high quality CPR, this can indicate that the return of circulation is unlikely. Inversely, if there is a sudden, sustained increase in end tidal CO2 levels, this can be an indication of a return of spontaneous circulation.
The use of waveform capnography is not limited to the patient who is in cardiac arrest or those who have an advanced airway in place. Using the Zoll CAPNO2 Mask with the E-series monitor we are able to monitor the end tidal CO2 and waveform of the spontaneously breathing patient. Being able to obtain waveform capnography in the spontaneously breathing patient can be very useful in the patient who is experiencing difficulty breathing. It is often difficult to hear breath sounds in these patients due to decreased tidal volume or ambient noise. Using waveform capnograph we can now see if there is obstruction in the airway. If the patient’s waveform has the “shark fin” pattern it is an indicator that there is an obstruction and the bronchodilators are indicated. After administering bronchodilators the alveolar plateau should begin to flatten back out, taking a more normal shape. If the waveform has a normal shape, the cause is likely not caused by bronchoconstriction and other causes of respiratory distress should be investigated.
Capnography can also be used to monitor a patient who is receiving medications that can affect respiratory drive, such as opiates or benzodiazepines. Because these medications can reduce respiratory effort, hypoventilation can result. If the patient is being monitored using capnography, a rise in end tidal CO2 levels can signal that the patient is not breathing deep enough or frequently enough to adequately remove carbon dioxide from the lungs. This is shown as hypoventilation on the waveform capnograph with rising end tidal CO2 levels. These patients will need to have their respirations supported or a reversal agent administered. Because the end tidal capnography was being monitored, the hypoventilation can be noted before the pulse oximetry begins to fall.
This two part article has shown what capnography is, reviewed respiratory anatomy/physiology, how the end tidal CO2 is measured, steps to reading the capnograph waveform, and finally it has shown some practical clinical applications where capnography can be useful. End tidal is quickly becoming an important measurement in the assessment and management of patients. Remember, the use of capnography is required by local medical control and state protocol every time that an advanced airway is placed, but your use of capnography does not have to be limited to these patients only. Using waveform capnography on patients receiving pain control medications or experiencing difficulty breathing can aid in their assessment and treatment.