This chapter describes oxygen cascade, and monitoring of gas exchange by pulse oximetry, capnography and blood gas analysis. Hypoventilation is a reduction in the volume of gas delivered to the alveoli causing hypoxia. Ventilation/perfusion mismatch is responsible for the hypoxaemia seen in pulmonary oedema, chronic obstructive pulmonary disease, pulmonary embolism and interstitial lung disease. Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) describe a spectrum of a critical illness syndrome involving severe inflammatory disease of the lung. Positive end expiratory pressure (PEEP) is used to improve oxygenation and prevent lung shear-stress injury associated with the opening and closing of collapsed alveoli. However, PEEP can also cause cardiovascular compromise and increased airway pressures and overdistension. The classification of pulmonary embolism by its clinical effects is: small pulmonary emboli, submassive pulmonary emboli, and massive pulmonary emboli. Surgical and catheter-based thrombectomy are alternative strategies in the management of massive pulmonary emboli.