Capnography

Chapter 207 Capnography



Bruno H. Pypendop, Dr.Med.Vet., Dr.Vet.Sci., DACVA



KEY POINTS



• Capnography allows the continuous, noninvasive assessment of partial pressure of arterial carbon dioxide.

• The normal gradient between end-tidal and arterial partial pressure of carbon dioxide (PCO2) is less than 5 mmHg in small animal species.

• An increased end-tidal-to-arterial PCO2 gradient indicates increased alveolar dead-space ventilation.

• Careful examination of a capnogram allows the detection of various abnormalities related to the equipment or the patient.

• Although capnography can be used in awake, nonintubated patients, it is more accurate in anesthetized and intubated patients.


INTRODUCTION


Capnometry is defined as the measurement and display of carbon dioxide concentration in the respiratory gases on a monitor. Maximum inspired and expired carbon dioxide concentrations during a respiratory cycle are displayed. Capnography is defined as a graphic display or recording of carbon dioxide concentration versus time or expired volume during a respiratory cycle (carbon dioxide waveform or capnogram). Time capnograms are most common and will be the only ones discussed here; volume capnograms can be used to measure the dead-space volume. A capnograph thus provides more information than does a capnometer; the interpretation of waveforms gives indications on the status of the patient and, in some cases, of the equipment.


Capnography can be used to confirm endotracheal tube placement, to assess ventilation and carbon dioxide elimination in a noninvasive manner, and (in combination with blood gas analysis) to estimate alveolar dead-space ventilation. Capnography has also been used to monitor the efficacy of cardiopulmonary resuscitation.



NONDIVERTING AND DIVERTING MONITORS


Two types of capnographs are available: nondiverting (mainstream) and diverting (sidestream).1 As its name indicates, a nondiverting capnograph measures carbon dioxide concentration directly in the breathing system, whereas a diverting device samples gas from the breathing system and measures the carbon dioxide concentration in that gas in the main unit. In the nondiverting monitors, the patient’s respiratory gas passes through a chamber with two windows. The sensor (light source and detector) fits over that chamber. The sensor also contains a heater to prevent water condensation on the windows. Advantages of mainstream devices include fast response time, no requirement for scavenging gas, ease of calibration (with a sealed chamber containing gas of known carbon dioxide concentration), and use of few disposable items. Disadvantages include the necessity to place the sensor near the patient (usually at the endotracheal tube connection), increase in apparatus dead space, potential for leaks, disconnection, or obstructions, potential for the sensor to become dislodged from the chamber, exposure of the sensor to damage, and measurement of carbon dioxide only.


In diverting monitors, the sensor is located in the main unit, remote from the breathing system. A pump samples respiratory gas at a constant flow, via sampling tubing. Advantages include minimal added dead space, lightweight patient interface, potential for the measurement of multiple gases, and possibility of use in places where the monitor needs to be remote from the patient. Disadvantages include potential for sampling problems, delayed response (especially with long sampling tubing), removal of gas at a given rate from the circuit, necessity to scavenge gas, potential for change in gas composition (depending on technology used), need for calibration gas, and potential for gas mixing in the sampling tubing (especially if the tubing is long).



TECHNOLOGY


Various techniques can be used to measure the concentration of carbon dioxide in the expired gas. These include infrared absorption, mass spectrometry, and Raman scattering.1


Infrared absorption is the most widely used technique. This technique is based on the concept that gases that have two or more dissimilar atoms in the molecule have unique and specific absorption spectra of infrared light. Infrared absorption can therefore be used to measure not only carbon dioxide but also nitrous oxide and the halogenated anesthetics.


Infrared monitors have a short warm-up time, and a quick response time, allowing them to measure inspired and expired concentrations. There is, however, some overlap between the absorption of carbon dioxide and nitrous oxide, and older devices need to be manually compensated when nitrous oxide is used. Water vapor must be removed from the expired gas (e.g., using Nafion tubing, water traps) because it absorbs infrared light at many wavelengths. Infrared absorption is the only technique available in nondiverting (mainstream) devices. It is also available in many diverting units.


Mass spectrometry is not used commonly to measure carbon dioxide concentration in respiratory gases. The mass spectrometer spreads gases and vapors of different molecular weights into a spectrum, according to their mass-to-charge ratios. By designing the device properly, it is then possible to direct these different gases and vapors toward targets that can count the number of molecules. The mass spectrometer can be used to measure not only carbon dioxide, but also oxygen, nitrogen, nitrous oxide, and the volatile anesthetic agents. Depending on the type of mass spectrometer, measurement of a new agent may require hardware or software adaptation. Mass spectrometry can be used to measure gas concentration from one or several locations (up to 31). The device measures gases as concentrations (contrary to infrared analyzers and Raman spectrometers, which measure partial pressures), and therefore assumes that the sum of the gases it measures equals 100%. This may result in errors if a gas that is not measured is present in significant concentrations.


The basis of Raman spectrometry is that when laser light interacts with a gas molecule that has interatomic bonds, some of its energy is converted into vibrational and rotational modes, and a fraction of that energy is reemitted at various wavelengths characteristic of the molecule. Venkata Raman won the Nobel Prize in physics in 1930 for the discovery of this phenomenon. Raman scattering is used in one clinical monitor: the Ra scal II. This device measures oxygen, nitrogen, carbon dioxide, nitrous oxide, and up to five anesthetic agents. It has a fast response time and a very fast startup time. It requires little maintenance and is very accurate.

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

Stay updated, free articles. Join our Telegram channel
Tags:
Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Capnography

Full access? Get Clinical Tree
Get Clinical Tree app for offline access