SummaryThe 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 .
Key wordsEquipment; Hayek Oscillator, nomograms.With the introduction of the Hayek OscillatorTM (Flexco Medical Instruments AG, Zurich, Switzerland) ( Fig. 1) external high frequency oscillation (EHFO) is a practical option in ventilatory support. It has been used to improve gas exchange in adults with acute respiratory failure on the intensive therapy unit [l, 21, has been shown to increase pulmonary blood flow in children following cardiac surgery [3] and has even been used to ventilate anaesthetised and paralysed patients undergoing microlaryngeal surgery, as tracheal intubation was not necessary [4, 51.The Hayek Oscillator can operate around a negative pressure baseline. Previous attempts at EHFO with compression alone have resulted in a gradual reduction in functional residual capacity [6] and lung collapse [7]; they relied upon the elastic recoil of the chest wall to restore normal lung volumes. By operating around a negative baseline there is more control over end expiratory lung volume and the reduction in functional residual capacity can be minimised.The mean pressure within the cuirass is termed mean chamber pressure. A positive end expiratory pressure of + 5 cmHIO will compress the chest and a negative inspiratory pressure of -5 cmHzO will expand it, resulting in an overall mean chamber pressure of OcmH20. The pressure difference between end expiratory (+ 5 cmH,O) and end inspiratory chamber pressure ( -5 cmHzO) is termed, the span. Changes in these two variables together with variations in frequency have been shown to alter minute and tidal volume in an animal model [8]. However, the effect of these changes has not been characterised in humans. We report the frequencyltidal volume and frequency/minute
