PT - JOURNAL ARTICLE AU - Johan Petersson AU - Robb W. Glenny TI - Gas exchange and ventilation–perfusion relationships in the lung AID - 10.1183/09031936.00037014 DP - 2014 Oct 01 TA - European Respiratory Journal PG - 1023--1041 VI - 44 IP - 4 4099 - http://erj.ersjournals.com/content/44/4/1023.short 4100 - http://erj.ersjournals.com/content/44/4/1023.full SO - Eur Respir J2014 Oct 01; 44 AB - This review provides an overview of the relationship between ventilation/perfusion ratios and gas exchange in the lung, emphasising basic concepts and relating them to clinical scenarios. For each gas exchanging unit, the alveolar and effluent blood partial pressures of oxygen and carbon dioxide (PO2 and PCO2) are determined by the ratio of alveolar ventilation to blood flow (V′A/Q′) for each unit. Shunt and low V′A/Q′ regions are two examples of V′A/Q′ mismatch and are the most frequent causes of hypoxaemia. Diffusion limitation, hypoventilation and low inspired PO2 cause hypoxaemia, even in the absence of V′A/Q′ mismatch. In contrast to other causes, hypoxaemia due to shunt responds poorly to supplemental oxygen. Gas exchanging units with little or no blood flow (high V′A/Q′ regions) result in alveolar dead space and increased wasted ventilation, i.e. less efficient carbon dioxide removal. Because of the respiratory drive to maintain a normal arterial PCO2, the most frequent result of wasted ventilation is increased minute ventilation and work of breathing, not hypercapnia. Calculations of alveolar–arterial oxygen tension difference, venous admixture and wasted ventilation provide quantitative estimates of the effect of V′A/Q′ mismatch on gas exchange. The types of V′A/Q′ mismatch causing impaired gas exchange vary characteristically with different lung diseases. A review of ventilation–perfusion relationships and gas exchange, basic concepts and their relation to clinical cases http://ow.ly/wMUwq