الفهرس 
Anatomical Considerations of LungsOnly 14 pages are availabe for public view
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Abstract
Intraoperative atelectasis is defined as pulmonary collapse that occurs after anesthetic induction being characterized, clinically, by a reduction in lung elastic recoil and compromised arterial oxygenation. It occurs in 85%–90% of healthy adults in the dependent parts of lungs within minutes after the induction of general anesthesia. It is seen both during spontaneous breathing and after muscle paralysis and irrespective of whether intravenous or inhalational anaesthetics are used.
Three sets of mechanisms have been proposed that may cause or contribute to the development of atelectasis, including compression of lung tissue as a result of loss of respiratory muscle tone leading to cephalad shift of the diaphragm permitting the intra-abdominal pressure to be transmitted to the thoracic cavity and increase of intrapleural pressure with subsequent compression of basal lung tissue. The second mechanism is absorption of alveolar gas which occurs by two ways; the first way is complete occlusion of the small airways and the second way, in the absence of airway occlusion, is when ventilation – perfusion ratio is low. The third mechanism is impairment in the function of pulmonary surfactant. The incidence of atelectasis is more in morbidly obese patients and when the fraction of oxygen used in the inspired gas is high.
Development of atelectasis is associated with the development of several pathophysiologic effects, including impairment of oxygenation with varying degrees of perioperative hypoxemia as a result of shunt of blood flow through non-ventilated lung tissue, decreased compliance as a result of decreased functional residual capacity, increased pulmonary vascular resistance, and development or potentiation of lung injury. Atelectasis may also increase the incidence of postoperative pneumonia.
While computed tomography is emerging as the preferred method for imaging the lung and detecting atelectasis, ultrasonography has been suggested as a useful aid to the rapid diagnosis of atelectasis and the use of a specific ultrasonographic detection of the cardiac impulse, termed the lung pulse, has been reported to be a highly sensitive early indicator of the presence of atelectasis. Contrast-enhanced sonography is a recent method of ultrasonography that allows visualization of lung atelectasis and may characterize pulmonary and bronchial arterial vascularity. Magnetic resonance imaging is an alternative to the computed tomography scanner where it combines the high image resolution and three-dimensionality of the latter but does not subject patients to radiation.
A vital capacity or recruitment maneuvers, defined as lung inflation to an airway pressure of 40 cm H2o maintained for 8-15 seconds, can effectively expand collapsed lung tissue and improve intraoperative gas exchange. An alternative to this is the cycling maneuver which consists of a step-by-step increase in inspiratory pressure and positive end expiratory pressure every 2-3 respiratory cycles, maintaining a constant differential pressure (inspiratory pressure - positive end expiratory pressure) of 20-25cm H2o until a peak inspiratory pressure of 40 and a positive end expiratory pressure of 20cm H2o is reached. This is maintained for about one minute, followed by a reduction, which is also progressive, of the pressures until finally the optimal positive end expiratory pressure is attained. This successfully can resolve atelectasis completely and restore the normal aeration of the lungs during and after surgical operations. This effect can be prolonged by the use of lower oxygen concentrations during and after the maneuver or when it is followed by positive end-expiratory pressure.
Also the use of continuous positive airway pressure at about 5 - 6 cmH2O can re-establish functional residual capacity and reduce atelectasis and the incidence of hypoxemia by increasing intrathoracic pressure and reducing respiratory effort.
Techniques or devices that either encourage or force patients to inspire deeply are of most clinical importance and aim to produce a large and sustained increase in transpulmonary pressure distending the lung and re-expanding collapsed lung units. Also, postoperative analgesia particularly, regional analgesic techniques, can significantly alter postoperative respiratory muscle function and improve diaphragm activity and respiratory mechanics after thoracic and upper abdominal surgery.
Although the use of low oxygen concentrations may decrease the incidence of development of atelectasis during general anesthesia, it is not recommended because it increases the risk of hypoxemia particularly during induction, extubation and the early postoperative period.
Another method for prevention of atelectasis during general anesthesia is the use of Bi-level Positive Airway Pressure Ventilation which consists of inspiratory and expiratory positive airway pressure. The inspiratory positive airway pressure allows recruitment of zones of alveolar collapse and results in a more homogeneous distribution of ventilation. Furthermore, the inspiratory positive airway pressure minimizes the work of breathing.
The association of low tidal volume plus moderate or high positive end-expiratory pressure seems more favourable in terms of preventing collapse than a high volume associated with a low positive end-expiratory pressure.
Finally, atelectasis that develops during general anesthesia may lead to perioperative pulmonary complications. Prevention of atelectasis formation is therefore an important goal.