الفهرس 
Anatomical PhysiologyOnly 14 pages are availabe for public view
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Abstract
Mechanical ventilation is one of the most challenging tasks facing physicians in the ICU. Weaning from mechanical ventilation is defined as the gradual process of transferring the respiratory work of breathing from the ventilator to the patient. It is easily obtained in about 70-80% of the patients. However 20–30% present with difficult weaning.
The respiratory system is divided into ventilatory organs (upper and lower air passages) and the alveoli. Regarding the internal structure of the lungs, respiratory bronchioles divide into terminal bronchioles, each leading into two alveolar ducts, which lead into alveolar sacs, which contain the alveoli, in which gas exchange takes place.
Breathing is regulated by the respiratory center in the medulla. Chemoreceptors sense the arterial PaO2, PaCO2, and pH. Aortic bodies and carotid body respond to reduction in arterial PaO2. Central chemoreceptors respond to raised carbon dioxide concentration and fall in blood pH. The driving forces for the exchange of gases between the alveoli and the ambient air are pressure differences.
The main objectives of mechanical ventilation are to reverse hypoxemia, to reverse acute respiratory acidosis so as to relieve life-threatening academia rather than to normalize PaCO2, and to relieve respiratory distress and elevated work of breathing.
Complications of mechanical ventilation include complications of intubation, ventilator-induced lung injury, ventilator-associated pneumonia, oxygen toxicity, and intrinsic PEEP.
The process of weaning begins with assessment of readiness for weaning, followed by a spontaneous breathing trial. For many patients, discontinuation of sedation is a critical step. The decision to remove the artificial airway in patients successfully passing an SBT requires further assessment of the patient’s ability to protect the airway.
Some of the objective parameters to predict weaning success include:
(a) PaO2/FiO2 ratio >150-200.
(b) Level of PEEP between 5-8 cm H2O.
(c) FiO2 level <50%.
(d) pH > 7.25.
(e) Ability to initiate spontaneous breaths.
Some of the subjective parameters include:
(a) Hemodynamic stability.
(b) Absence of active myocardial ischemia.
(c) Absence of clinically significant, vasopressor-requiring hypotension.
(d) Appropriate neurological examination.
(e) Improving or normal appearing chest radiogram.
(f) Adequate muscular strength allowing the capability to initiate/sustain the respiratory effort.
Recent guidelines for weaning include the following:
1) Patients should be categorized into three groups based on the difficulty and duration of the weaning process.
2) Weaning should be considered as early as possible.
3) A spontaneous breathing trial is the major diagnostic test to determine whether patients can be successfully extubated.
4) The initial trial should last 30 min and consists of either T-tube breathing or low levels of pressure support.
5) Pressure support or assist–control ventilation modes should be favored in patients failing initial trials.