Correction of Hypoventilation by High-Frequency Oscillation (HFO)

摘要

During the inhalation of a dense gas, pulmonary flow resistance increases and diffusivity decreases. Increased flow resistance at depth accounts for reductions in maximal minute ventilation and exercise ventilation. Low diffusivity hampers mixing of inspired gas with alveolar gas: the result is an apparent increase of dead space. The end result of dense gas breathing is often CO2 retention and hypercapnea. Severe bouts of dyspnea seemingly unrelated to chemical changes may result from respiratory muscle fatigue secondary to the increased work of breathing. Under such conditions some form of ventilatory assistance may be beneficial. A new method of ventilation, called high-frequency oscillation (HFO) (3-5), lacks the disadvantages of the older methods. High-frequency oscillation involves application of small pressure and flow oscillations to the trachea or chest wall at frequencies above 5 Hz and at stroke volumes far below anatomical dead space. High-frequency oscillation is distinguished from the older technology of high-frequency positive pressure ventilation (HFPPV) by its higher frequencies, smaller stroke volumes, and still somewhat mysterious mode of action. As used experimentally and clinically, HFO is administered to anesthetized, usually paralyzed and intubated subjects as a total mode of ventilation, coupled with a substantial flow of fresh gas (bias flow) to the upper airways. It is believed that the oscillations serve to enhance diffusion and extend the alveolar space toward the mouth.