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Introduction: The most important function of the respiratory system is the exchange of oxygen and carbon dioxide - moving oxygen in and moving carbon dioxide out. The Oxygen Cascade is a useful way to look at many aspects of Respiratory Physiology. It describes the steps involved in moving oxygen from the atmosphere all the way to our cells, where oxygen is utilised. During this talk, I will focus on these steps and relate them to how we anaesthetise and monitor patients.
Atmosphere to Alveoli: The partial pressure of oxygen (pO2) in dry atmospheric air at sea level is 160 mmHg (21%). Higher concentrations of oxygen are usually administered during general anaesthesia, and are measured using a fuel cell or paramagnetic analyser.
During normal inspiration, air is drawn into the lungs by increasing the intrathoracic volume and making the intrathoracic (and therefore alveolar) pressure subatmospheric. The situation is completely different during anaesthetic or ICU ventilation where positive pressure ventilation...