Capnography - The "Other" Vital Sign (Part-1)

Capnography refers to the amount and concentration of carbon dioxide (CO2) measured in exhaled gas. But to understand capnography you must first understand that oxygenation and ventilation are two entirely different parameters. While it’s true that they are both related to the respiratory system the two parameters are not related, nor is one affected by the other because they are referring to two very different mechanisms. Oxygenation refers to the amount of oxygen present in arterial blood, and it relies on the inspiratory phase of respiration. Ventilation refers to the amount of carbon dioxide present in arterial blood, and it relies on the expiratory phase of respiration.

Oxygenation can be measured invasively by obtaining a sample of arterial blood (ABG) and measuring the partial pressure of oxygen (PaO2), which is the concentration of oxygen dissolved in the blood plasma. Oxygenation can be measured non-invasively by obtaining an oxygen saturation level with a pulse-oxymeter, which is how well the hemoglobin is saturated with oxygen.

Ventilation can be measured invasively by obtaining an arterial blood sample (ABG) and measuring the partial pressure of carbon dioxide, which is the concentration of carbon dioxide dissolved in the blood plasma. Ventilation is measured non-invasively by measuring the amount or concentration of carbon dioxide in exhaled air. That practice is known as capnography.

Taking a patients’ vital signs has been a standard practice in all areas of healthcare, from pre-hospital to extended care, for many years. These “vital” signs, which can be an indicator a patients’ cardiac and respiratory stability, are the measurement of some very basic parameters that include heart rate, respiratory rate, blood pressure, and oxygen saturation. These basic parameters give a good overall picture of the cardio-respiratory, and oxygenation status of your patient but it says nothing about your patients’ ventilation status. Ventilation status is at least equally important information and it is easily obtained by use of capnography.

For many years it was believed by many health care workers that capnography was intended to estimate your patients PaCO2. That misconception has earned capnography a bad rap in many areas of allied health care. My intention with this article is to give you a more complete understanding of what capnography really is, what kind of information we can learn from it, what to do with that information, and how to use it to your patients advantage.

As we know, carbon dioxide is a by-product of cell metabolism. In very simple terms, our body’s burn sugar, oxygen, and other nutrients to produce energy in the form of ATP (adenosine triphosphate). The main waste products are water, and carbon dioxide. The water that is not used by the body is routed through the kidneys, and eventually excreted from the body by urination. Carbon dioxide is carried by the red blood cells to the lungs, crosses the alveolar-capillary membrane, and is exhaled from the body. The CO2 measured in arterial blood is important to know. Equally important is the CO2 measured in exhaled gas AND the difference between the two. (Pa-ETCO2).




Definitions

• Capnography – A graphical display of CO2 concentration over time or expired volume.
• Capnogram – CO2 waveform either plotted against a volume (CO2 expirogram) or against time (time capnogram).
• PACO2 – Partial pressure of carbon dioxide in the alveoli.
• PaCO2 – Partial pressure of carbon dioxide in arterial blood.
• PETCO2 – Partial pressure of carbon dioxide at the end of expiration.
• (a-ET)PCO2 – The difference between PaCO2 and PETCO2. Also shown as P(a-ET)CO2. Sometimes called the “CO2-diff”, or “CO2 gradient”.
• PVCO2 – Partial pressure of carbon dioxide in mixed venous blood.


Mainstream: The infrared sensor is in the direct path of the gas source, and connected to the monitor by an electrical wire.

Sidestream:The sample of gas is aspirated into the monitor via a lightweight airway adapter and a 6ft length of tubing. The actual sensor is inside the monitor.

Clinical Uses of Capnography:

Over the year’s capnography has grown into a complicated, and very effective, tool in many areas of health care, including pre-hospital, emergency medicine, critical care medicine, pediatrics, pulmonary and respiratory medicine, anesthesia, and many more.

• Pre-Hospital: Colorimetric, or carbon dioxide detecting devices have become increasingly popular to help ensure proper placement of an endotracheal tube during intubation. This has proven to be a very helpful tool in the pre-hospital setting where it is often difficult to auscultate lung sounds. They’re fast and easy to use, they’re small and lightweight, and they’re inexpensive. The Drawbacks: 1- they are short loved. They only last about 5 or 10 minutes and have to be thrown away. 2- they cannot accurately measure an amount or concentration or carbon dioxide in exhaled gas. They merely detect the presence of carbon dioxide.
• Emergency Medicine: In addition to ensuring proper placement of an endotracheal tube during intubation, capnography can be used to ensure that cardiac output is still adequate. In the emergency department, the critically ill patient is unstable until proven otherwise. Even with cardiac monitors it can be very difficult to ‘catch’ cardiac arrest when it actually happens. A patient in PEA (pulse-less electrical activity) can have a very good heart rhythm on the monitor for quite some time. Capnography is the best non-invasive tool to use to ensure that cardiac output is adequate. And during cardiac arrest you can use capnography to determine the adequacy of cardiac output during chest compressions, and as a predictor of outcome. (See attached article). Capnography can also be used to help diagnose several conditions and disease processes that cause a change in the ventilation/perfusion ratio. A good example of this would be an APE (Acute Pulmonary Embolus), which will increase dead-space ventilation by decreasing pulmonary perfusion around the effected part of the lung. This causes an increase A-aDCO2 (The difference between ETCO2 and PaCO2, which is normally 3 – 5 mmHg). Another condition that would cause an acute increase in A-aDCO2 is an acute episode of CHF. A trending increase in A-aDCO2 can often indicate a progression into ARDS.
• Critical Care Medicine: Capnography can be effectively used for all of the above, plus be used as a comparison and trending tool to help us manage and trend the ventilatory status of patients suffering from one of MANY medical conditions, and disease states.
• Anesthesia: This is where it all started back in the 1970’s. It has been known for many years now that capnography is the most effective non-invasive way to measure the adequacy of ventilation, and cardiac output during anesthesia. It is now standard practice to use capnography during all cases where anesthesia is used, all across the country.
• Procedural Sedation: Capnography is the fastest, and most accurate indicator of hyperventilation, hypoventilation, and apnea. This is very helpful when a non-intubated patient is being sedated during a medical procedure. In such cases it is often difficult to ‘see’ your patient breath, and oxygen saturation is usually very slow to decrease in such cases and therefore would not be a good indicator of ventilation.