Analysis of gases in inhaled and expired air

Capnometry (CO2 analysis) is the measurement of the concentration or partial pressure of carbon dioxide during breathing in the inhaled and exhaled gas mixture.

In most functional diagnostics rooms, only spirometry is performed from the study of external respiration today. Spirometry however reveals mainly obstructive disorders and then only when the obstruction affects the bronchi of medium and large caliber. The narrowing of small (less than 2 mm in diameter) airways has little effect on bronchial resistance and is practically not felt by the patient. Chronic obstructive pulmonary disease (COPD), as a rule, begins with the distal airways, proceeds for a long time without obvious clinical manifestations (cough, shortness of breath) and for a long time may not manifest obstructive disorders detected by spirometry. In this regard, methods that assess the state of the respiratory zone of the lungs, including the terminal sections of the airways and alveoli, are of undoubted interest.

The respiratory zone of the lungs ensures the exchange of gases during breathing, and its damage, depending on its severity, may be accompanied by severe respiratory failure, including with slightly reduced or even normal spirometric values. It is the defeat of the respiratory zone of the lungs that explains the severity of respiratory failure in diseases such as alveolitis, beryllium disease, some interstitial lung diseases, viral-associated pneumonia.

Capnometry (CO2 analysis) as a continuous measurement of the concentration (partial pressure) of carbon dioxide during calm breathing and when performing a series of breathing tests (holding the breath while inhaling, during deep exhalation, etc.) gas exchange in the lungs. Breathing is carried out in an open circuit with continuous sampling of part of the air to the gas analyzer, the principle of which is based on the control of the absorption of carbon dioxide in the infrared region.

CO2 analyzing, available as an option in the MAS2-C spirometer, allows evaluating:

– the concentration of carbon dioxide in the alveolar space (through the PETCO2 measurement), allowing to correlate the adequacy of alveolar ventilation to the metabolic rate with the possibility of detecting alveolar normo-, hypo- or hyperventilation;
– the slope of the alveolar plateau, thus assessing the uniformity of the distribution of ventilation and blood flow (ventilation-perfusion relations) in the lungs;
– the ratio of the value of the functional dead space (DS), i.e. the sums of the anatomical and alveolar MP and tidal volume (Vd/Vt);
– the patient’s breathing pattern, determining its frequency and depth, to reveal the state of hypercapnia, which is widely used in intensive care units and in anesthesia practice.

The most common cause of disturbance in the uniformity of distribution of ventilation and blood flow in the lungs is obstruction of small airways, expressed to varying degrees in different parts of the lung, violations of the elastic properties of the lungs, disturbances of microcirculation in the lungs, focal inflammatory and fibrotic processes in the lung tissue. When COPD, abnormalities on the capnogram can be detected at an early stage (pre-disease stage), when the spirometric parameters (FEV1, FEV1/FVC) are slightly reduced or are within normal limits.

Two approaches are used to calculate the capnogram: calculation from the curve of the dependence of the concentration (partial pressure) of CO2 on time and calculation from the curve of the dependence of the concentration (partial pressure) of CO2 on the volume of exhaled air (capnovolumetry). In the first case, the following indicators are assessed: РetСО2 – partial pressure of СО2 at the end of calm expiration, slope of the alveolar phase (ΔР/Δt), angle between the ascending part of the curve and the alveolar phase (angle α), the angle between the alveolar phase and the descending part of the curve (angle β), Tulou index – difference PetСО2 at the end of a deep and calm exhalation. The ratio of the volume of the dead space to the tidal volume (Vd/Vt) is also calculated.

With capnovolumetry (simultaneous recording of a spirogram and a capnogram is required), the slope of the alveolar phase during calm and deep exhalation, the magnitude of the alveolar and anatomical dead spaces, and a number of other indicators are calculated.