Capnography
Capnography is a medical technique employed to measure the concentration of carbon dioxide (CO2) in a patient's exhaled air, providing crucial insights into their respiratory function. By utilizing a device known as a capnograph, healthcare professionals, including anesthesiologists and emergency responders, can assess whether a patient is receiving adequate oxygen. This tool is especially valuable in situations where patients are unable to communicate their symptoms, as it captures real-time data on lung performance and overall respiratory health.
The process involves directing exhaled air through a beam of light, which measures how much CO2 is present based on the light absorption. Normal CO2 levels in a healthy individual range from 35 to 45 mmHg, with lower readings potentially indicating oxygen deprivation. Capnography has a range of applications, from monitoring patients during surgery to verifying the correct placement of breathing tubes, particularly in emergency medical scenarios. Its growing prevalence in ambulances and paramedic settings has significantly improved emergency care, enabling responders to make informed decisions during critical moments, such as in cases of cardiac arrest. Overall, capnography serves as an essential tool in modern medicine, enhancing patient assessment and treatment outcomes.
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Capnography
Capnography is a medical procedure that uses a device called a capnograph to measure the amount of carbon dioxide, also known as CO2, in the bloodstream. This helps medical professionals including physicians, anesthesiologists, and first responders to determine if a person is getting enough oxygen. It can provide information on how well a person's lungs are functioning and other important details about a patient's respiratory system that can guide a person's treatment. Capnography is a useful diagnostic tool because it can help medical professionals gather information even if the person's respiratory condition makes it difficult or impossible for the patient to talk.
Background
The average human takes twenty thousand breaths a day. Each inhale brings in fresh oxygen, a colorless, odorless gas that is essential to life. The lungs transfer the oxygen to the blood in the circulatory system, where the heart pumps it throughout the body. The body uses the oxygen as part of the process of metabolism, which converts nutrients into energy in all living beings.
One of the waste products of the metabolism process is carbon dioxide, another colorless, odorless gas. Carbon dioxide is expelled from the lungs with each breath out, or exhale. The process is similar to that in a car, where gasoline is burned, or combusted, in the engine to produce energy to move the car and a waste product, exhaust, is expelled from the car through the tail pipe.
Car mechanics can test a car's exhaust emissions to determine how well the engine is functioning by measuring what is in the exhaust. In a similar way, medical professionals can measure the amount of carbon dioxide that remains in a person's blood to determine how well the body is handling the conversion of oxygen to energy to carbon dioxide.
To do this, physicians and other medical professionals use a tool called a capnograph. It is named after the Greek word capnos, which means "smoke." The Greeks realized that the air that was breathed in and the gas that was breathed back out were different. They theorized that, much like the process of combustion produces a smoky waste product, the gas that living beings breathe out is also a byproduct of a process that happens inside the body.
On June 11, 1754, a Scottish chemist named Joseph Black noticed that if he heated calcium carbonate, a gas was released that was denser than air and could not support combustion. He called his discovery fixed air. Researchers would later determine that he had discovered carbon dioxide.
It was not until the 1850s that anyone was able to determine a way to measure how much carbon dioxide was present. This was accomplished by John Tyndall, an Irish scientist who found a way to quantify carbon dioxide by measuring how well it absorbs infrared radiation. The technique was in use in some clinical settings in the 1930s. However, it would take decades before a device would be developed in the 1950s that was small and convenient enough to use routinely in medical settings.
Overview
The technique developed by Tyndall uses a beam of light directed through the gas to begin measuring carbon dioxide. The beam is aimed at a sensor on the other side of the gas. The sensor measures how much of the light passes through the gas.
Gases such as oxygen, nitrogen, and carbon dioxide are quantified by a measure known as partial pressure. This is the percentage of the gas that is present as measured in millimeters of mercury, or mmHg. For example, if a sealed glass container has a mix of gas that is 80 percent oxygen and 20 percent nitrogen, the partial pressure would be roughly 600 millimeters of mercury for the oxygen and 150 millimeters of mercury for the nitrogen. The normal partial pressure of carbon dioxide in a healthy person who is calmly seated would be between 35 and 45 mmHg. A reading below 30 mmHg could be a sign that a patient is not getting enough oxygen.
To use capnography, exhaled air is captured and directed into the capnograph, where it moves between the light and sensor. The carbon dioxide in the exhaled air absorbs the light; the more carbon dioxide that is present, the less light gets through to the sensor. The reading is provided as a number in mmHg and as a series of waves that lets the technician see the pattern of the carbon dioxide levels over a series of breaths.
Capnography has multiple uses. Anesthesiologists use it to make sure a patient is getting enough oxygen during surgery. It can be used in emergency rooms to help determine if a patient's respiratory system is functioning properly; it can be used in the same way in non-emergency settings as well, especially with patients with conditions such as asthma or other breathing disorders. It can be particularly helpful when the patient is unconscious or otherwise unable to speak. Capnography can also be important in determining if endotracheal tubes, or breathing tubes, have been properly inserted.
Initially, capnography was used only in clinical medical settings, such as hospitals. In contemporary times, capnographs have become standard equipment in many ambulances and paramedic units. In these settings, they can help determine if breathing tubes are inserted correctly. Emergency responders have used them while administering cardiopulmonary resuscitation (CPR) to determine if the procedure is allowing the patient to get enough oxygen. If the brain is continuing to get oxygen, the emergency responders know that CPR is worthwhile and can continue the lifesaving treatment until the patient can reach more advanced medical treatment, even if the patient has remained unresponsive. This has saved lives in cases when CPR might have otherwise been discontinued.
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