The Hayek Oscillator is a recent development in external high ,frequency centilution and is a useful mode of ventilatory support during anaesthesia, in chronic respiratory failure and weaning in intensi1.e care. The Hayek Oscillator is new and its application is growing, as are the number of patients who have benefited,from its use. Horcerer, there are no clear guidelines on how best to adjust the oscillator to achieve optimum ventilation. A simple method qf predicting changes in tidal volume and minute ventilation following adjustment wouldmake the oscillator more useful. W e present nomograms,fi)r tidal colume, minute oentilation andeffective alveolar ventilation when the three variables, oscillator,frequency , mean chamber pressure and peak-to-trough pressure span were adjusted. Thefrequency-tidal volume relationship was unaflected by a mean chamber pressure o f 0 ,-5,-10 crnH20, but altered with changes in peak-to-trough pressure span. W e have also determined the eflkct of' increasing negutiiie extrathoracic pressure on functional residual capacity. The relationship between tidal volume and ,frequency uas non-lineur and related to the peak-to-trough pressure span. Mean functional residual capacity signlficantlj, increased,from 2.25 1 (S E M 0.10) without the cuirass at rest to 2.61 1 (S E M 0.14) at-10 cmHzO dp < 0.05; n = 5) and 2.47 (S E M 0.12) at-20 c.mH20 of mean chamber pressure. Vital capacity was unchanged by increasing extrathoracic pressure as M'US total lung cupacity .

A previous volume history should be established prior to pressure- volume (P-V) curve measurement, however the effect of the volume history and the peak inspiratory pressure (PIP) during the P-V measurement has not been explored. Lung injury was created by lavage in nine sheep (25-35 kg). After stabilization, four P-V curves were sequentially obtained with PIP of 40, 50, 60, and 40 cm H2O. Prior to each P-V measurement the PIP delivered for 1 min was the same as during P-V measurement. We compared the lower inflection point (Pflex), upper inflection point (UIP), compliance below Pflex (Cstart), compliance between Pflex and UIP (Cinf), and compliance between UIP and peak pressure (Cend) for the inflation limb, and the point of maximum curvature on the deflation limb (PMC), compliance between peak pressure and PMC (Ctop), and maximum compliance (Cdef) for the deflation limb. In two sheep, Pflex at PIP 40 cm H2O could not be identified but appeared when PIP was raised. Pflex, Cstart, Cend, and Ctop were not affected by the PIP. However, UIP, PMC, Cinf, and Cdef increased as the PIP increased. Volume history and the PIP during P-V curve measurements affect both the inflation and deflation P-V curves.

These data indicate that HFO, ITPV, and PCV when applied with an open-lung protective ventilatory strategy results in the same gas exchange, lung mechanics, and hemodynamic response, but pilot data indicate that lung injury may be greater with PCV.

Peak volume history pressure must be considered when interpreting the inflation limb of the pressure-volume curve of the respiratory system beyond the inflection point. The peak pressure achieved during the pressure-volume curve is important during interpretation of deflation compliance and the point of maximum compliance change on the deflation limb.