A 55-year-old woman with a history of Crohn disease is admitted to the hospital due to perforated appendicitis. The patient quickly develops respiratory difficulty, and acute respiratory distress syndrome is diagnosed. She is intubated and mechanically ventilated with positive pressure ventilation. Despite the use of high positive end-expiratory pressure, the patient's condition continues to deteriorate. She is intermittently placed in the prone position while mechanically ventilated. Which of the following is most likely to occur due to this position change?

Question

A 55-year-old woman with a history of Crohn disease is admitted to the hospital due to perforated appendicitis.  The patient quickly develops respiratory difficulty, and acute respiratory distress syndrome is diagnosed.  She is intubated and mechanically ventilated with positive pressure ventilation.  Despite the use of high positive end-expiratory pressure, the patient's condition continues to deteriorate.  She is intermittently placed in the prone position while mechanically ventilated.  Which of the following is most likely to occur due to this position change?

Quizplus · Accepted Answer

11ec160b_3b3b_f934_9249_61114582433e_MD0001_00 Prone positioning (lying face down) is sometimes used to improve arterial oxygenation in patients with severe acute respiratory distress syndrome. It likely does so by reducing atelectasis of posterior alveoli to reduce intrapulmonary shunting and improve ventilation-perfusion matching throughout the lungs. Because the heart occupies anterior space in the chest cavity, the majority of lung mass is located posteriorly. When a patient is in the supine position (lying face up), the weight of the heart and anterior lung segments compresses the posterior lung segments and limits ventilation of posterior alveoli. In addition, abdominal organ and tissue mass weighs considerably on the posterior portion of the diaphragm, displacing it cranially and further contributing to compression of the posterior basal lung segments. The anterior alveoli, subjected to minimal compression and ample ventilation in the supine position, are often hyperinflated, increasing the risk of barotrauma in those regions. Prone positioning causes the heart and anterior lung mass to rest on the anterior chest wall, alleviating compression of the posterior lung segments. In addition, the freely hanging abdomen removes pressure on the diaphragm, allowing the diaphragm to move caudally and further open up collapsed posterior basal alveoli. With a higher percentage of total alveoli opened, ventilation is more evenly distributed, improving arterial oxygenation and reducing alveolar hyperinflation and hyperdistension (which may decrease the risk of barotrauma) (Choice A). (Choices B and D) Prone positioning may increase venous return (ie, right ventricular preload) and, consequently, cardiac output by positioning the heart slightly below the large reservoir of venous blood in the splanchnic circulation (ie, additional stores of venous blood are mobilized to the right atrium). (Choice C) By reducing posterior lung compression and decreasing atelectasis, prone positioning increases functional residual capacity (ie, the volume of air in the lungs at the end of tidal expiration). Educational objective: Prone positioning may be used to improve arterial oxygenation in patients with severe acute respiratory distress syndrome. The improvement likely results from reduced compression of the posterior lung segments, where the majority of alveoli are located. This leads to more evenly distributed ventilation throughout the lungs with reduced intrapulmonary shunting and improved ventilation-perfusion matching. __________ References: Treatment of ARDS with prone positioning. (http://www.ncbi.nlm.nih.gov/pubmed/27400909)

A 55-Year-Old Woman with a History of Crohn Disease Is