![]() It is not exhaustive, undoubtedly, some techniques have been missed and others incompletely described. The goal of this article is to provide a physiologic understanding of the basis of techniques that can be used to measure V ˙ A / Q ˙ matching. Even in the presence of diffusion equilibrium, when ventilation and perfusion are not perfectly matched the net result is a difference between alveolar and arterial partial pressure of oxygen. Similarly, the alveolar O 2 and CO 2 contents are the ventilation-weighted sum of the individual gas concentrations leaving the lung. The concentration of oxygen and carbon dioxide in end-capillary blood leaving the lung is the blood flow weighted sum of the individual oxygen and carbon dioxide contents from each individual lung unit. This necessitates almost equal volumes of air and blood reaching the gas exchange portions of the lung ( 252). In the healthy lung, the overall V ˙ A / Q ˙ ratio is close to 1 and the oxygen content of room air (20.9 mL/100 mL air) is similar to that of the capacity of the blood to carry oxygen (~20.6 mL/100 mL blood). In a real lung, there are many units, each with a different V ˙ A / Q ˙ ratio-including the extremes-units of dead space that are ventilated but not perfused (including anatomical dead space from the conducting airways) and in some individuals, units with shunt that are perfused but not ventilated. ![]() Gas exchange in a single lung unit is determined by V ˙ A / Q ˙ ratio of the unit (“compartment”) and the absolute amounts of associated ventilation and blood to that unit ( 269). Impaired V ˙ A / Q ˙ matching is a hallmark of many lung diseases including chronic obstructive pulmonary disease (COPD) ( 321), pulmonary hypertension ( 214), asthma ( 322), pulmonary edema ( 245), pulmonary fibrosis ( 4), and acute respiratory distress syndrome ( 259). Shunt can occur as a result of intracardiac or intrapulmonary shunts, but significant shunt is much less common than V ˙ A / Q ˙ mismatch (see Ref. In healthy subjects, diffusion limitation is typically only observed during high-intensity exercise at sea level in some highly trained athletes or in normal subjects during heavy exercise at high altitude or in hypoxia ( 176, 267, 309, 325). This is because diffusion limitation is rarely observed in resting individuals with lung disease, the exception being in interstitial lung disease ( 170). Although the AaDO 2 can be increased by diffusion limitation of oxygen transport and shunt, ventilation-perfusion ( V ˙ A / Q ˙ ) matching, such that regions of the lung that receive fresh gas also receive deoxygenated capillary blood, is the most important mechanism affecting gas exchange ( 334). Many lung diseases are characterized by reduced gas exchange efficiency, manifest as an increase in the alveolar-arterial difference for oxygen ( AaDO 2). The physiological considerations for each of the techniques along with advantages and disadvantages are briefly discussed. The fundamental equations of pulmonary gas exchange are first reviewed to lay the foundation for the gas exchange techniques and some of the imaging applications. The focus is on the physiological basis of these techniques that provide quantitative information for research purposes rather than qualitative measurements that are used clinically. ![]() This article discusses the measurement of V ˙ A / Q ˙ matching with three broad classes of techniques: (i) those based in gas exchange, such as the multiple inert gas elimination technique (MIGET) (ii) those derived from imaging techniques such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT), and electrical impedance tomography (EIT) and (iii) fluorescent and radiolabeled microspheres. Ventilation-perfusion ( V ˙ A / Q ˙ ) matching, the regional matching of the flow of fresh gas to flow of deoxygenated capillary blood, is the most important mechanism affecting the efficiency of pulmonary gas exchange.
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