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Mechanical ventilation has been associated with pulmonary edema in the clinical setting, but the pathophysiological mechanisms of this process have not been clearly defined. Experimental studies have shown that high transpulmonary pressures resulting from ventilation may damage the capillary walls, thereby leading to edema. Knowledge of the stress distribution within the alveolar septa would be an important step in understanding this phenomenon. A newly developed saline-filled alveolar sac model was utilized for analysis of septal stresses in young and aging healthy lungs, in order to examine their vulnerability to pulmonary edema during ventilation. Significant stress concentrations were shown to develop near highly curved regions (small local radii of less than 4 mum in a lung inflated to 80% could be as high as 25 times that of average septal stresses. The combination of elevated stress sites that are formed in the stiffer parenchyma of the aging lung, together with the cyclic loading of ventilation, may explain the gaps and breaks previously observed in pulmonary edema.

Sound signals of respiratory airflow represent summations of acoustic waves of various frequencies, which basically depend on the characteristics of the flow and on those of the surrounding tissue. This study was designed to examine the capability of time-frequency distribution (TFD) of respiratory signals in order to differentiate between unobstructed and obstructed upper airways. In order to investigate the TFD characteristics of defined upper airway geometry we conducted a controlled basic study in a laboratory system with an in vitro isolated airway model, which was either unobstructed or had concentric obstructions of various degrees at different locations along the tube. Pressure fluctuations were acquired with a microphone proximal to the airway opening. A short-term Fourier transform was used to study the TFDs of these signals. The results of the in vitro study showed that the energy of the higher frequencies increased for relatively small incremental changes in: i) reduction of the lumen cross-section, ii) decrease of distance from measurement site to obstruction, and iii) increase of breathing effort. Further development of this method may lead to noninvasive clinical techniques for early diagnosis of upper airway obstructions.