notes by dr Claudio Italiano
The air is a mixture composed of nitrogen, an inert gas, 78%, oxygen for 20.95% and
carbon dioxide for 1%. The oxygen concentrations of the air we breathe, even if
so low, are sufficient to guarantee the oxygenation of the blood in healthy
people, in those that are in good health of the apparatuses:
a) Circulatory
b) Ematic
c) Respiratory
In fact, if the heart pump allows the implementation of a good blood circulation and the blood flows regularly in the vessels and capillaries of the tissues, if the hemoglobin rate is adequate and there are no conditions of anemia (see anemias), if, finally, the lung ventila and is perfused, then there are all the conditions for the body to use in the combustion of glucose, and therefore, for its metabolism, the right amount of oxygen. The physiological conditions, now described, vice versa, are modified when such diseases occur which cause an imbalance in the adequate oxygen supply to the tissues. Under normal conditions (subject that normally breathes in ambient air) FiO2 is 0.21 and the proportion of O2 present in the dissolved blood is about 2%, this value can rise to 8% with an inspiratory mixture with FiO2 equal to 1.00 . If a further oxygen supply is required, for example during CO poisoning (carbon monoxide), hyperbaric oxygen is used (see hyperbaric chamber ). The purpose of oxygen therapy, therefore, is to provide an adequate concentration of oxygen in the blood, with a consequent reduction of respiratory stress and stress of the myocardium, which, given its considerable demand for oxygen, could easily undergo hypoxia and therefore to ischemic facts (cf. ischemic heart disease).
In the acute patient, tissue hypoxia must be prevented, a condition that occurs, in fact, in the event of an imbalance between the transport of O2 and the consumption of O2. Oxygen therapy is, therefore, the administration of oxygen at higher concentrations than those present in the atmospheric environment in order to modify the arterial content of O2 (CaO2) by modifying the PaO2. The transport of oxygen to the tissues depends on factors such as cardiac output, the oxygen content at the arterial level, the concentration of hemoglobin and the metabolic demands, these factors are to be considered when administering oxygen therapy. The foundation on which O2 therapy is based is the increase in the pressure gradient of O2 across the alveolar-capillary membrane. For this reason, during O2 therapy, gaseous mixtures are administered with a variable O2 concentration (FiO2). Once the hemoglobin has been completely saturated, further increases in FiO2 increase the amount of O2 dissolved in the blood.
It is used in "hypoxemia", that is, when it is determined a partial decrease of
O2 in the arterial blood. This condition can manifest itself in acute, for
example, with changes in the state of vigilance of the CNS; Stroke stroke, with
sudden dyspnea with inadequate ventilation and / or perfusion, for example for
pulmonary embolism, with increased blood pressure or changes in heart rate,
heart failure, in chronic obstructive pulmonary disease, asthmatic disease,
neuromuscular disease
In particular:
- COPD
- Interstitiopatie
- Cystic fibrosis
- OSAS
- anemia
- carbon monoxide poisoning
- cyanide poisoning
Advantage of oxygen therapy.
- improvement of general clinical conditions
- reduction of dyspnea
- reduction of cardiac work
- reduction in time of polyglobulia
- reduction of pulmonary pressure
- increase in exercise tolerance
- improvement of sleep
- improvement of brain function
- reduction of days of hospitalization
- increase in survival
- improvement of the quality of life
The systems that allow oxygen therapy to the patient not intubated, ie to the
patient who has not been connected to a machine in the context of a
resuscitation division called "mechanical fan". Based on the influence of
ambient air in the system and the presence of inspiratory tanks, the various
systems are divided into two large groups:
1) low flow devices;
2) high flow devices;
The oxygen is led to the patient's bed by means of a union fitting and a device
called a flow meter which allows to measure the oxygen flow rate from 0 to 15
liters / min., Plus a glass through which oxygen bubbled and enriched with
moisture. Then follows the polyethylene connecting pipe and the final "cannula
or glasses" and "mask" system, with or without a reserve flask, in order to save
gas.
Nasal cannulas and simple masks are generally. The nasal cannulas have a maximum deliverable FiO2 capacity between 0.24 and 0.44 and the maximum flow is about 6 l / min. If flows of more than 4 l / min are used, the air must be humidified to prevent drying of the nasal mucosa. This method has the advantage of being well tolerated by the patient and allowing feeding. The simple masks give a maximum capacity of FiO2 that can be delivered is between 0.40 and 0.60, that is to say up to 3 times the air breathed in oxygen, when the flow of O2 must be between 5 and 8 l / min. The masks have side openings to prevent rebreathing and to ensure the influence of ambient air. With this method it is always necessary to humidify the inspired air, while the partial recirculation masks are equipped with a bag (reservoir) that must remain inflated in both the respiratory acts, higher oxygen concentrations can be administered, because , both the bag and the mask act as an oxygen reservoir. Among the high flow systems we mention the famous "Venturi mask". Venturi mask: it is the most used and reliable method to deliver precise concentrations of oxygen through a non-invasive method. The mask is constructed to allow a constant flow of ambient air to be mixed with a predefined flow of oxygen. It is used primarily in individuals with COPD, because it is able to deliver low levels of supplemental oxygen, thus avoiding to administer the hypoxic stimulus. The Venturi mask uses devices that change according to the concentration and liters of oxygen that must be administered, they are easily distinguishable through the colors:
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