The word “capnography” is derived from a Latin twist based on an original Greek concept. The Greeks first wrote about combustion centers throughout the body that they said released a byproduct called “Kapnos,” or in Latin, “Capnos”; both are words for “smoke.” What the Greeks called “combustion,” we now call metabolism and what they called “smoke,” today is termed carbon dioxide. Capnography is a device that is placed at the lips to detect the presence of carbon dioxide in exhaled air, called “End-Tidal Carbon Dioxide” or ETCO2 for short. Older, qualitative “colorimetric” capnography devices simply changed color when sensing the presence of carbon dioxide. Today’s EMS monitors use quantitative technology that combines a numerical readout (capnometer) with a waveform to measure and display the pressure of carbon dioxide in exhaled air. ETCO2 is a pressure measurement in mmHg and not a percentage or “count” like parts per million.
Capnography is able to sample exhaled air through two different technologies: sidestream and mainstream. Sidestream capnography samples and analyzes the patient’s breath “off to the side,” away from the patient’s direct path of exhalation; the breath is literally pulled into the monitor for analyzing. Mainstream capnography places the CO2 analyzer within the airway circuit, a system configured only for intubated patients. Sidestream is now the mainstay technology for intubated and non-intubated applications and has replaced most mainstream devices throughout all areas of medicine. It wasn’t until sidestream capnography was developed that we could use a cannula for patient assessment.
There are two types of waveform capnography: time and volumetric. Both time and volumetric capnography display ETCO2 pressure in millimeters of mercury (mmHg) on the vertical axis. However, in time capnography, CO2 pressure is measured over milliseconds of time on the horizontal axis and in volumetric capnography CO2 pressure is measured over milliliters of tidal volume on the horizontal axis. Time capnography is used in the prehospital setting as volumetric capnography requires equipment that is too cumbersome for EMS. However, volumetric capnography can be useful to pulmonologists to gain insight into certain disease processes.
Even though we measure ETCO2 pressure at the lips, we are measuring a byproduct that goes way deeper than that. Carbon dioxide literally travels from the cells, where it is made, to the lips where it is measured. Any changes along the way can change the measurement at the lips; more than “breathe fast goes down and breathe slow goes up.” To understand the multiple uses for capnography, you must fully understand the carbon dioxide transport chain. CO2 is a metabolic byproduct created at the cellular level that diffuses into the blood stream (perfusion), is delivered to the lungs by the circulatory system (cardiac output), crosses the alveolar membrane (respiration) and is finally exhaled out to the sensor on the lips (ventilation). Understanding the CO2 transport chain allows the provider to use capnography to assess more than just respiratory patients with ventilation issues. ETCO2 is also an established tool for assessing a patient’s perfusion status in shock, as well as metabolic status in diabetic patients.
CO2 is produced after bicarbonate buffers the acidic byproducts of metabolism, assuming the appropriate concentration of strong ions and enzymes are present to catalyze the reaction. The diffusion rate of CO2 is 20 times faster than that of oxygen due to its higher solubility. This is an important concept to grasp when discussing capnography changes in the presence of pulmonary edema. This high solubility allows CO2 to diffuse at the same rate through fluid as it does through air. Below is an oversimplified equation illustrating CO2 production (may the chemistry gods forgive me). The equation is reversible and can move left to right (increasing pH) or right to left (decreasing pH) in order to maintain a homeostatic acid/base balance.
Acid (H+) + Bicarbonate (HCO3–) <=> CO2 + H2O
For the sake of conceptual learning and at the expense of chemistry, think of CO2 as an acid in gas form. The body is constantly eliminating acid by converting it to CO2, which allows it to be exhaled. During hypoventilation CO2 builds up in the blood, decreasing pH (respiratory acidosis). During hyperventilation the rate of CO2 elimination is too high, increasing pH (respiratory alkalosis). Furthermore, if hypoventilation or hyperventilation persists long enough and/or a metabolic issue ensues that overwhelms the respiratory system, the renal system will have to manipulate bicarbonate levels in order to maintain a homeostatic pH (metabolic acidosis/alkalosis). While only blood gas analysis can quantitatively determine if a patient is in respiratory or metabolic acidosis/alkalosis, capnography can give insight into these conditions.
ETCO2 and waveform capnography assessment have become a standardized tool in EMS with multiple uses from airway and ventilatory assessment to shock and metabolic insights. Hopefully, this article has primed the mind for more detailed discussions in the future on the various uses in different patient subsets.