"Closing volume" during high-frequency ventilation in anesthetized dogs

Airway closure, mean airway pressure, gas exchange and different modes of artificial ventilation were investigated in anesthetized and paralyzed dogs with clinically healthy lungs. The animals were ventilated with either intermittent positive pressure ventilation (IPPV), continuous positive pressure ventilation (GPPV, positive end-expiratory pressure (PEEP) = 0.49 kPa) or high-frequency jet ventilation (HFJV, open system) of 2 and 30 Hz with an inspiratory to expiratory (I/E) - ratio of 30/70 and 60/40. Closing volume (CV) was determined by a modified technique, submitting the lung to constant subatmospheric pressure after an inspiratory vital capacity of oxygen. Two different tests for CV were used: the foreign gas bolus (FGB) with helium as nonresident gas and the single breath nitrogen dilution technique (SBO2). During conventional mechanical ventilation, CV decreased significantly (P less than 0.05) after establishing a PEEP of 0.49 kPa. During HFJV, CV increased significantly (P less than 0.01). This effect was predominantly dependent on I/E duration time ratio and to a lesser extent on ventilatory frequency. There were significant differences between CV obtained by the FGB-method (CV(helium] and CV derived from the SBO2-test (CV(SBO2], although both tests revealed the same proportional changes of CV during the different modes of ventilation. The elevated CV was associated with a decreasing Pao2 and increasing Aa-Do2 and Paco2, indicating substantial hypoventilation and mismatching of ventilation and perfusion. Mean airway pressure increased with both CPPV and HFJV, revealing a dissociation between airway pressure and regional FRC distribution during HFJV. It is concluded that certain modes of high-frequency ventilation lead to impaired distribution of inspired gas to dependent lung regions.(ABSTRACT TRUNCATED AT 250 WORDS)

[ABS 800: expanding the possibilities of respiratory therapy]

A new module of the Dräger modular device system provides a means for demand CPAP (to increase FRC) and assisted spontaneous breathing. Both features together may largely increase the potency of ventilatory therapy for spontaneously breathing patients. The machine may reduce the need for full ventilatory support in acute respiratory failure, but also provides a valid and rational tool for respiratory therapy.

Viscoelasticity of tracheobronchial secretions in high-frequency ventilation

The effects of high-frequency ventilation (HFV) on the rheological properties of tracheobronchial secretions were investigated in vitro. The mucus was obtained by suction from 16 patients after major surgery of the larynx or hypopharynx. After short centrifugation, the mucus was treated with high-frequency vibrations of 3 or 30 Hz for 10 min in a chamber saturated with water vapor. The mucus was separated by a nonpermeable membrane from the ventilator's gas stream. Maximal, minimal, and mean viscosity were determined by a modified capillary viscosimeter. Under high-frequency vibrations, maximal, minimal, and mean viscosity slightly increased (p less than or equal to .001). Conversely, the mean viscosity of control samples did not change significantly. These findings indicate that high-frequency vibrations do not improve mucous rheology. Observed beneficial effects of HFV on mucociliary clearance may be caused by changes in the mucous subphase during HFV.

[Flow resistance in pediatric oro- and nasotracheal tubes]

Flow resistances of different endotracheal tubes used in paediatric anaesthesia were determined with varying gas flows of 3 to 10 lit. per minute. We found no significant differences between preformed orotracheal and nasotracheal tubes and ordinary endotracheal tubes consisting of PVC plastic and spiral tubes made of silicone material, respectively. Woodbridge tubes showed significantly higher resistance to all gas flows used. Because of their less favourable material and flow resistance properties, Woodbridge tubes cannot be recommended for paediatric anaesthesia.