Fig. 28.1
ATP output from a mole of glucose is different in the presence or absence of oxygen at the cellular level
So, energy (ATP) requirements, and therefore oxygen requirements, increase as activity levels increase. A measure of the metabolic cost of activities can be described in terms of the metabolic equivalent or the Metabolic Equivalent of Task (MET). One MET is defined as the expenditure of 1 kcal/kg/h, which corresponds to oxygen requirement of approximately 3.5 ml/kg/min. One MET is the level of metabolism at rest or when sitting quietly – which is not the same as the basal metabolic rate. At 2 MET, twice the amount of energy is expended and oxygen requirement increases to 7 ml/kg/min. Light activities score <3 MET, moderate activities like brisk walking and bicycling MET 3–6 and vigorous activities (jogging) MET >6. Used clinically, the MET values are only estimates of level of activity, which also vary between individuals according to physiological factors such as age, weight and level of fitness.
Pharmacokinetics
Oxygen moves into the body by bulk flow, as part of the air we inhale enters the alveoli. Whilst breathing air, the oxygen tension in the alveoli is about 100 mmHg. This then diffuses across from the alveoli to the blood in the pulmonary capillaries. The pulmonary artery which brings in mixed venous blood (the collective venous return from all the organs in the body to the heart) to the alveoli, has an oxygen tension of only 40 mmHg. This allows a huge gradient for diffusion of oxygen across the alveolar-capillary membrane. The amount of oxygen transferred across from the lungs to the blood is dependent not only on the diffusion gradient but also on the match between the alveolar ventilation and the pulmonary blood flow. Oxygen is then moved by bulk flow in the blood to the tissues by the pumping action of the heart (called the cardiac output), mainly in combination with hemoglobin in red blood cells. Under normal atmospheric pressure the dissolved oxygen constitutes only 0.003 ml/mmHg PO2 i.e. only about 0.3 ml of oxygen per 100 ml of blood, whereas the amount carried by the haemoglobin in a patient with a haemoglobin of 15 g% is about 19.5 ml per 100 ml of blood.
As oxygen delivery is mainly reliant on the level of hemoglobin, increasing this with the use of erythropoietin is a strategy to improve oxygen delivery in those with anemia. In a hyperbaric environment (usually 3 atmospheres), the amount of dissolved oxygen in the blood can be increased to 6 ml of oxygen per 100 ml of blood and this may be a method to increase tissue oxygen delivery in very unusual circumstances. At the tissue capillaries, oxygen diffuses from the capillary bed to the cells and the mitochondria. The oxygen tension in the mitochondria can be as low as 2–3 mmHg and yet support aerobic metabolism.
In those with severe life threatening respiratory dysfunction where the lungs can no longer sustain oxygen transfer, oxygen can be administered directly into the blood through the use of an Extracorporeal Membrane Oxygenation (ECMO) circuit or a cardiac bypass machine. This may also be used in patients with impossible to manage complete airway obstruction as a way of temporarily securing the delivery of oxygen to the tissues.
Sources of Oxygen
Oxygen can be obtained from the environment around us, from cylinders of compressed oxygen or from pipeline supplies drawn from liquid oxygen tanks, usually found in hospitals where usage is high.
Oxygen from the Environment
Oxygen from the air around us can be used to ventilate and oxygenate a patient with the use of self inflating ventilation devices like the Ambu Bag. In desperate situations when patients are unable to breathe adequately, these self inflating ventilation devices can be utilised to deliver air (21 % oxygen) to the lungs and can be life saving. Oxygen from the environment can be enriched from 21 % to nearly 100 % with the use of a device called an oxygen concentrator. Oxygen concentrators essentially pass air through zeolite which absorbs nitrogen, and concentrates the oxygen to a higher percentage. Oxygen concentrators are used for domiciliary purposes for patients with chronic conditions requiring long term oxygen therapy who can cope with care in the home environment.
Oxygen from Cylinders
Oxygen is stored in cylinders under pressure. The pressure of a full cylinder of oxygen is 134 atmospheres. Oxygen cylinders come in various sizes and smaller ones are used for transportation of patients requiring oxygen therapy. A regulator brings the pressure down to 4 atmospheres, after which the oxygen is passed through a flow meter or other equipment.
Liquid Oxygen
Liquid oxygen is stored in huge insulated cylinders at temperatures below zero degrees centigrade. Oxygen vaporises from the liquid state in these special cylinders and flows in the hospital pipelines to oxygen wall outlets. At these points, flow meters can be attached to enable delivery of oxygen directly to patients, or to equipment such as mechanical ventilators or anesthetic machines. These liquid oxygen tanks are refilled from cylinders sent by trucks from the manufacturers.
Dosaging
Oxygen is administered as a percentage of oxygen in air or other carrier gas. It is described either as a percentage or fraction of inspired oxygen (FiO2). So, the inspired oxygen concentration while breathing air is 21 % or FiO2 is 0.21. If pure oxygen is administered, then the concentration is 100 % or FiO2 is 1. Oxygen analysers are used to measure the concentration of oxygen delivered from mechanical ventilators and anaesthetic machines.
Methods of Administration
Oxygen can be administered via nasal prongs, oxygen masks, breathing systems, resuscitation devices and ventilators.
The flow meters allow a maximum oxygen delivery of 15 L/min. The rate of airflow into the upper respiratory tract during inspiration is around 30 L/min. Thus, administering 15 L/min of oxygen with an ordinary oxygen mask will provide an inspired concentration of around 50–60 % only, because the rest of the air inspired is entrained from the atmosphere, diluting the administered oxygen. When using nasal prongs, we normally administer oxygen at a rate of 3–5 L/min, which supplements inspired oxygen concentration to about 25–30 % depending on the state of opening of the mouth (breathing with open mouth allows more atmospheric air the be entrained into the airway, diluting the administered oxygen more).
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