ELSEVIER *Assistant Professor, Department of Anesthe- siology tAssociate Professor, Departments of Anes- thesiology and Physiology IAssistant Professor, Department of Surgery §Research Engineer, Department of Anes thesiology Address reprint requests to Dr. Barnas at the Anesthesiology Research Laboratories, Uni- versity of Maryland School of Medicine, Room 534, MSTF Building, 10 South Pine Street, Baltimore, MD 21201. Received for publication May 12, 1995; re- vised manuscript accepted for publication July 31, 1995. Supported by the National Heart, Lung and Blood Institute Grant, HL-44128. Effects of Trendelenburg and Reverse Trendelenburg Postures on Lung and Chest Wall Mechanics Brenda G. Fahy, MD, * George M. Barnas, PhD,t Sheryl E. Nagle, MD,* John L. Flowers, MD,x Mary J. Njoku, MD,* Manish Agarwal, BS§ Departments of Anesthesiology, Physiology, and Surgery, University of Maryland Hospital, Baltimore, Maryland Study Objective: To test whether the Trendelenburg (“head-down’3 or reverse Trendelen- burg (“head-up ‘3 postures change lung and chest wall mechanical properties in a clinical condition. Design: Unblinded study, each patient se-rving as own control. Setting: University of Maryland at Baltimore Hospital, Baltimore, Maryland. Patients: 15 patients scheduled for laparoscopic surgery. Interventions: Patients were anesthetized and paralyzed, tracheally intubated and me- chanically ventilated at 10 to 30 per minute and at a tidal volume of 250 to 800 ml. Measurements were made bejore surgery in supine, head-up (10 degrees from horizontal) and head-down (15 degrees from horizontal} postures. Measurements and Main Results: Airway flow and airway and esophageal pressures were measured. From these measurements, discrete Fourier transformation was used to calculate elastances and resistances of the total respiratory system, lungs, and chest wall. Total respiratoq elastance and resistance increased in the head-down posture compared with supine due to increases in lung elastance and resistance (p c 0.05); but chest wall elastance and resistance did not change (p > 0.05). Lung elastance also exhibited a negative dependence on tidal volume while head-down that was not observed in the supine posture. The change in lung elastance compared with supine was positively cowelated to body mass index (weight/heigh?) and negatively cowelated to tidal volume. Lung and chest wall elastance and resistance were not affected 4 shiftingfiom supine to head-up (p > 0.05). Conclusions: The Trendelenburgposture increases the mechanical impedance of the lung to inflation, probably due to decreases in lung volume. This effect may become clinically relevant in patients predisposed with lung disease and in obese patients. Keywords: Compliance; elastance; lung, mechanics; position; resistance Introduction The mechanical impedances to inflation (i.e., elastance and resistance) of the lungs and chest wall determine the potential adverse effects of inflation pres- sure. High lung impedance will increase the possibility that inflation pressure will cause lung injury because of increased alveolar pressure,’ while high chest wall impedance will increase intrathoracic pressure and decrease cardiac output. Journal of Clinical Anesthesia 8:236-244, 1996 0 1996 by Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010 0952.8180/96/$15.00 PI1 SO952-8180(96)00017-7