الفهرس 
ANATOMY AND PHYSIOLOGY OF THE RESPIRATORY SYSTEMOnly 14 pages are availabe for public view
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Abstract
M
echanical ventilation refers to any method of breathing in which a mechanical apparatus is used to maintain adequate, but not necessarily normal, gas exchange. It is indicated when the patient’s spontaneous ventilation is not adequate to sustain life or when it is necessary to take control of the patient’s ventilation to prevent impending collapse of other organ functions.
Modes of MV are described by the relationships between the various types of breaths and by the variables that can occur during the inspiratory phase of ventilation. Each mode of ventilation is distinguished by how it initiates a breath (trigger), how it sustains a breath (limit), and how it terminates a breath (cycle). The question of which mode is the so called right mode of ventilation for respiratory failure of a particular cause has no simple answer because there are many therapeutic options. There are two basic modes of ventilation: ventilation limited to the delivery of a specified volume (volume controlled) and ventilation limited by a pressure target (pressure controlled).
Although mechanical ventilation could be life saving it is also associated with numerous complications. The incidence of some complications increases with duration of mechanical ventilation. This together with the observation that complications increase length of stay, increase mortality, and increase costs provides a compelling rationale for efforts to reduce the duration of mechanical ventilation. Weaning from mechanical ventilation can be defined as the process of abruptly or gradually withdrawing ventilator support. This is usually simple in patients with known good pulmonary functions prior to an acute, reversible event. It is not simple in patients recovering from a prolonged ventilatory or respiratory illness that required mechanical ventilation or when a short acute ventilatory illness is superimposed on a chronic condition that may compromise respiratory reserve.
Determining when a patient is ready to wean continues to be a concern of clinicians and investigators. Premature attempts of weaning and extubation can be deleterious to patient physiologic and psychological well-being. Weaning indices are objective criteria that are used to predict the readiness of patients to maintain spontaneous ventilation. Despite the popularity of many of these predictors, little evidence indicates that they are in fact predictive. In general, they are good negative predictors but poor positive predictors.
There are four conventional modes of discontinuing patients from mechanical ventilation: (a) trials of spontaneous breathing (SB) with or without the addition of CPAP, (b) SIMV, (c) PS, and (d) noninvasive positive-pressure ventilation. Although most physicians generally use one of first three modes alone, some have used them in combination. With recent advances in technology, new features on ventilators have been also developed including (a) Automatic tube compensation (ATC), (b) Proportional-assist ventilation (PAV), and (c) Servo-controlled ventilation.
The percentage of patients who required weaning decreased from 80 to 10% when physician judgment was replaced by protocol management. Protocol-directed daily screening of respiratory function and trials of SBT decrease the time required for extubation, the incidence of self-extubation, the incidence of tracheostomy and ICU costs, and results in no increase or even a decrease in the incidence of reintubation.
A significant number of patients will experience prolonged weaning failure despite multiple weaning attempts following SBTs. Assuming that reversible factors have been optimized (e.g. cardiac, metabolic, etc.), the eventual clinical outcome of patients with prolonged weaning failure will depend on the long-term trajectory of the underlying disease. A significant number of patients with prolonged weaning failure remain ventilator dependent, requiring long-term ventilatory support, which may now be provided as NIV in the home setting.