Mechanical loads modulate tidal volume and lung washout during high-frequency percussive ventilation

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Abstract

High-frequency percussive ventilation (HFPV) has been proved useful in patients with acute respiratory distress syndrome. However, its physiological mechanisms are still poorly understood. The aim of this work is to evaluate the effects of mechanical loading on the tidal volume and lung washout during HFPV. For this purpose a single-compartment mechanical lung simulator, which allows the combination of three elastic and four resistive loads (E and R, respectively), underwent HFPV with constant ventilator settings. With increasing E and decreasing R the tidal volume/cumulative oscillated gas volume ratio fell, while the duration of end-inspiratory plateau/inspiratory time increased. Indeed, an inverse linear relationship was found between these two ratios. Peak and mean pressure in the model decreased linearly with increasing pulsatile volume, the latter to a lesser extent. In conclusion, elastic or resistive loading modulates the mechanical characteristics of the HFPV device but in such a way that washout volume and time allowed for diffusive ventilation vary agonistically.

Introduction

High-frequency ventilation (HFV) was introduced in clinical practice by Oberg and Sjöstrand (1969). High-frequency ventilation is based on two different concepts of gas transport: convective gas transport and nonconvective flow principles. Additionally, HFV is characterised by breathing frequencies higher than 1 Hz and tidal exchanges smaller than the combined set-up and anatomical dead spaces.

High-frequency percussive ventilation (HFPV) was introduced with the intent of overcoming the inconveniences of other variants of HFV (e.g., high-frequency oscillation, high-frequency jet ventilation) (Gallagher et al., 1989). HFPV associates the positive aspects of conventional mechanical ventilation (CMV) with those of HFV, and was initially used for the treatment of acute respiratory diseases caused by burns and smoke inhalation (Lentz and Peterson, 1996, Loring et al., 1993, Reper et al., 1998, Reper et al., 2002, Reper et al., 2003, Rodeberg et al., 1992, Rodeberg et al., 1994), and in the treatment of the newborn affected by hyaline membrane disease or infant respiratory distress syndrome (IRDS) (Baird et al., 1994, Campbell et al., 1991). Later on, it was employed in severe gas exchange impairment, where conventional mechanic ventilation (CMV) was useless (Paulsen et al., 2002). To date, the data reported in the literature have proved its effectiveness and safety in several types of disease, either affecting the respiratory system (e.g., ARDS, chest trauma) (Velmahos et al., 1999), or in head (Barrette et al., 1987) and multiple-trauma patients (Hurst et al., 1987), where the effects of conventional mechanical ventilation might compromise other organ functions (Ranieri et al., 1999, Ranieri et al., 2000).

Recently Lucangelo et al. (2004) reported the elastance and resistance dependence of flow, volume and pressure curves generated during HFPV. Nevertheless, considering that the HFPV device generates a tidal volume based on intermittent positive pressure ventilation associated with a superimposed continuous pulmonary washout of gas by means of high-frequency pulsatile mini-volumes, further analysis of the responses of these ventilation waveforms to imposed loads is still wanted. Therefore, this work aims to evaluate the effects of mechanical resistive and elastic loading on tidal volume and lung washout during HFPV.

Section snippets

Equipment

The same ventilator and mechanical model recently described were used (Lucangelo et al., 2004). High-frequency percussive ventilation (HFPV) was generated by a volumetric diffusive respirator (VDR-4®, Percussionaire Corporation, Sandpoint, Idaho, USA) that provides pulsatile flow, pulse frequency, pulse inspiratory and expiratory durations (i and e, respectively), and inspiratory and expiratory durations TI and TE, respectively and feeds an injector/exhalation valve. The Phasitron®, part of the

Results

Fig. 3 summarizes the effects of R and E on VT/Vtot, which increases with R but falls with E. Indeed, it can be seen in Fig. 3 and Table 1 that the smallest values of VT/Vtot (8.7%) were found when R = 0 and E = 100 cmH2O/l.

In Fig. 4A iso-elastance (E = 20 cmH2O/l) volume curves are displayed as a function of inspiratory time. From top to bottom R corresponded to 0, 5, 20, and 50 cmH2O/l/s. A tele-inspiratory plateau was reached in the first two conditions only, as shown in Table 1. One can also see in

Discussion

High-frequency percussive ventilation provides tidal volume as the difference between its pulsatile in- and out-volumes (Fig. 2). Clearly, some amount of gas is washed out of the lungs throughout inspiration. As previously discussed (Lucangelo et al., 2004), performance of the HFPV device vary according to the physiological/physical feedback of the model (or patient), but the interaction between loads and ventilator is poorly understood. Thus, the aim of this work is to evaluate the effects of

Acknowledgements

Dr. W.A. Zin is a member of the Millenium Institute: Global and Integrated Advancement of Mathematics in Brazil, Ministry of Science and Technology, a researcher of the Brazilian National Council for Scientific and Technological Deveolpment (CNPq/MCT), Brazil, and a Visiting Professor at the School of Critical Care, Facoltà di Medicina, Università degli Studi di Trieste, Italy. There are no conflicts of interest.

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