David Hudon, BS, NRP
Test your knowledge on hyperventilation, hypoventilation and reactive airway disease capnography waveforms
End-tidal carbon dioxide (CO2) detection is an essential tool for EMS providers. CO2 is the byproduct of cellular respiration, the process of breaking down glucose and oxygen into energy. CO2 is removed from the body through the lungs and respiratory system. Capnography is the practice of using a capnographer to measure the amount of CO2 being removed from the body. The capnographer can produce a waveform representation of the exhaled CO2 and a numerical value measured in millimeters of mercury (mmHg). A normal CO2 range is between 35 and 45 mmHg.
Like an electrocardiograph (ECG), the waveform produced by the capnographer can assist EMS providers in selecting the best treatment for the patient. Capnography can also show the effectiveness or ineffectiveness of the chosen treatment
A normal capnography waveform is comprised of four different phases. Phase 1 is the respiratory baseline and represents a patient's inhalation. The air we breathe contains about 21% oxygen and 78% nitrogen, meaning that there is virtually no CO2 for the capnographer to pick up. In phase 1, the waveform appears as a flat line at or only slightly above the 0 mmHg measurement.
Phase 2 represents the first portion of the exhalation process. During this phase, the waveform rises sharply up from 0 as CO2 travels up from the alveoli and mixes with gas in the anatomical dead space. Anatomical dead space represents gas in the pulmonary system that did not make it to the alveoli or bronchioles for gas exchange.
During phase 3, concentrated CO2 from the alveoli begins being measured. The phase 3 waveform is much more shallow than phase 2 but still rises slightly to an apex. It is at this apex that the CO2 is measured numerically.
Lastly, phase 0 is a sharp drop back to the respiratory baseline at exhalation is complete, and a new breathe begins.
Figure: Normal capnography waveform
Providers may see several different capnography waveforms while treating patients. Some of the most common waveforms seen are hyperventilation, hypoventilation, and reactive airway disease.
When an adult patient is hyperventilating, they are breathing at a rate above 20 breaths per minute. Some patients will even be breathing twice, or even three times the standard rate. When breathing this fast, patients are exhaling far more than they are inhaling, which leads to a drop in CO2 picked up by the capnography. The waveform seen on the capnography will be smaller and more frequent than those with a standard respiratory rate. The CO2 will also register at less than 35 mmHg.
Figure: Hyperventalation capnography waveform
Patients that are not breathing fast enough or less than 12 breaths per minute will show the opposite waveform than those mentioned above. The lack of sufficient exhalation means that CO2 builds up in the body. The capnographer picks up on the high concentration of CO2 and registers values greater than 45 mmHg.
Figure: Hypoventilation capnography waveform
Reactive airway disease is used to describes conditions such as COPD, emphysema, and asthma. In these diseases, the lower airways constriction causes the patient's expiratory phase to be longer than average. The alveoli also empty inconsistently, depending on the swelling. These factors create a slurred phase 2 and no discernable phase 3 plateau. This waveform's appearance looks similar to a shark's dorsal fin and is often referred to as a “Shark fin waveform.”
Figure: Airway disease (shark fin) capnography waveform
Capnography is one of the best resources that EMS providers have in their toolbox that can assist them with monitoring a patient's respiratory effort and narrow in on a differential diagnosis. Recognizing waveforms is key to providing proper and timely treatments to patients with respiratory disease and complaints.
Test your knowledge. Take the test below to see how your capnography knowledge measures up.