Capnography: Part I
Over the last several years capnography has become a key measurement in the assessment of patients in the pre-hospital setting. Capnography is considered to be an important vital sign. This 2 part class with cover the basics of capnography and how capnography can be used to better assess your patient.
Part 1 will discuss what capnography is, review airway anatomy and physiology, and cover how the measurement of end tidal C02 is done by the monitor.
Part 2 will examine the Capnography wave form and how to assess the waveform to gain information in order to assess the intubated and non-intubated patient.
Capnography: It’s all about a Gas. [top]
Capnography is the measurement of exhaled carbon dioxide (C02). C02 is a product of cellular metabolism and is transported to the lungs to be removed. Measuring C02 levels provides a picture of the perfusion status of the patient, how well blood is being moved around the body, and a picture of the patient’s airway, how well air is moving in and out of the lungs. Capnography measures the partial pressure C02 being exhaled and provides a numeric value expressed in millimeters of Mercury (mmHg) and a graphical waveform of the exhaled C02.
Capnography vs. End Tidal C02 Detection [top]
Many people have asked why use Capnography when the use of end tidal C02 detectors can tell you the same thing. End tidal C02 detectors are a qualitative test for the presence of carbon dioxide. Detectors use special litmus paper, or specially treated capsules, that change color in the presence of C02. This does not provide an exact value of the amount of C02 being exhaled. In contrast, capnography is a quantitative measurement, giving both an exact numeric value and a graphical representation. Think of end tidal C02 detectors as feeling a patient’s radial pulse, you know the heart is beating but you cannot fully determine the cardiac rhythm. Capnography is like the ECG, telling you not only the heart rate, but also showing you the electrical activity of the heart.
Airway Anatomy and Physiology [top]
The airway is divided into 2 divisions, the upper airway and the lower airway.
The upper airway is made up of the nasopharnyx, oropharnyx, hard and soft palates, and the larynx, including the epiglottis and the vocal cords. The primary role of the upper airway is to warm, filter, and humidify air entering the lungs. The upper airway also provides Positive End Expiratory Pressure (PEEP), this help prevent alveolar collapse.
The lower airway consists or the trachea, bronchi, bronchioles, and alveoli. The airway from the nose to the bronchioles does not participate in the exchange of gases and is considered dead air space. All gas exchange takes place at the alveolar level.
Physiology Rule #1: Air must go in and out. [top]
Ventilation and Respiration
Ventilation is the mechanical movement of air in and out of the lungs. Oxygen is taken in and carbon dioxide is eliminated. Chemoreceptors in the medulla detect increased levels of C02 and trigger ventilation. This is known as the hypercarbic drive. The hypoxic drive (low 02 levels) is a secondary drive. When ventilation is triggered, the diaphragm contracts, moving downward, and the intercostal muscles spread the chest wall outward, increasing the volume inside the chest. This decreases the pressure inside of the chest and air enters the lungs. The amount of air entering the lungs, or tidal volume, is approx 500cc in adults. Of that volume, 350cc reaches the alveoli, the rest remaining inside the dead space.
Physiology Rule #2: Blood must go round and round. [top]
Not only is the exchange of gases needed, but those gases must be delivered to the tissue, and waste products returned to the lungs. This is achieved by perfusion, the movement of blood thru the body. In order to have adequate perfusion several things must happen.
- There must be adequate oxygen, see rule #1.
- There must be an adequate number of red blood cells. Hemoglobin in the RBC’s carry the 02 molecules.
- The RBC’s must be able to offload and take on 02.
- The blood pressure must be adequate to push the blood cells to the tissues
This is known as the Fick Principle.
The body likes to maintain a balance between Ventilation and Perfusion. When there is inadequate Ventilation and/or Perfusion a V/Q mismatch occurs. A V/Q mismatch occurs when there is ventilation of un-perfused lung area (P.E.), perfusion to unventilated lung (obstruction, overdose), or a combination of decreased perfusion and decreased ventilation (cardiac arrest).
Measuring End Tidal C02 [top]
Monitors use Infrared (IR) Spectroscopy to measure the amount of exhale carbon dioxide. As exhaled gases pass through the IR beam, C02 absorbs a specific wavelength of the IR beam. The beam is then analyzed by the monitor and end tidal C02 is measured by the amount of wavelength absorbed. Most devices use a broad spectrum IR beams, this can allow other gases such as oxygen or nitrogen to alter readings, if they are present in higher than normal concentrations. The sample can be measured several different ways: main-stream, side-stream, or micro-stream.
Main-stream sampling places the sensor directly in line with the airway. This produces almost instantaneous End Tidal C02 readings. Mainstream sensors tend to be bulk and can be difficult to use on the non-intubated patient. Sensors can also be blocked by secretions or moisture, causing erroneous readings.
Side-stream sampling is done by pulling a sample to the expired air through a sampling tube and into the monitor. The sample is then measured and an End Tidal C02 reading given. Side-stream sampling eliminates the need for a bulky sensor, making monitoring of the non-intubated patient easier. It is also affected less by secretions or moisture. Because the sample must be drawn into the monitor, there is a lag time in End Tidal C02 measurement, as much as 30 seconds in some models.
Micro-stream devices are the last advancement in side-stream sampling. Micro-streams draw a sample form the middle of exhaled air, at a faction of the volume of standard side-stream technology. Micro-stream technology also uses C02 specific IR detection, reducing the chance of altered readings.
Part I: Recap [top]
Part 1 has shown what capnography is, reviewed airway anatomy, and discussed different ways end tidal C02 is measured by the monitor. In part 2 we will look in depth at the capnography waveform and how to assess perfusion and airway status.