[show abstract][hide abstract] ABSTRACT: Overview of the mechanisms governing gas transport, mechanical factors influencing the transmission of pressure and flow to the lung, and the measurement of lung mechanics during high-frequency oscillatory ventilation (HFOV) in acute respiratory distress syndrome.
Studies indexed in PubMed illustrating key concepts relevant to the manuscript objectives. Pressure transmission during HFOV in the adult lung was simulated using a published theoretical model.
Gas transport during HFOV is complex and involves a range of different mechanisms, including bulk convection, turbulence, asymmetric velocity profiles, pendelluft, cardiogenic mixing, laminar flow with Taylor dispersion, collateral ventilation, and molecular diffusion. Except for molecular diffusion, each mechanism involves generation of convective fluid motion, and is influenced by the mechanical characteristics of the intubated respiratory system and the ventilatory settings. These factors have important consequences for the damping of the oscillatory pressure waveform and the drop in mean pressure from the airway opening to the lung. New techniques enabling partitioning of airway and tissue properties are being developed for measurement of lung mechanics during HFOV.
Awareness of the different mechanisms governing gas transport and the prevailing lung mechanics during HFOV represents essential background for the physician planning to use this mode of ventilation in the adult patient. Monitoring of lung volume, respiratory mechanics, and ventilation homogeneity may facilitate individual optimization of HFOV ventilatory settings in the future.
Critical Care Medicine 04/2005; 33(3 Suppl):S135-41. · 6.12 Impact Factor
[show abstract][hide abstract] ABSTRACT: Respiratory inductance plethysmography (RIP) and electrical impedance tomography (EIT) are two monitoring techniques that have been used to assess lung volume noninvasively.
RIP uses two elastic bands around the chest and abdomen to assess global changes in lung volume. In animal models, RIP has been shown to detect changes in lung mechanics during high-frequency oscillatory ventilation and has the potential to quantify lung volumes noninvasively. EIT measures regional impedance changes with 16 electrodes around the patient's chest, each of them injecting and receiving small currents. Impedance changes have been correlated with volume changes in animal models and in humans. In a recent animal model, EIT was shown to be capable of tracking lung volume changes during high-frequency oscillatory ventilation.
The promise of monitoring techniques such as RIP and EIT is that they will guide lung protective ventilation strategies and allow the clinician to optimize lung recruitment, maintain an open lung, and limit overdistension. EIT is the only bedside method that allows repeated, noninvasive measurements of regional lung volumes. In the future, it will be important to standardize the definitions of alveolar recruitment and ultimately demonstrate the superiority of EIT-guided ventilator management in providing lung protective ventilation.
Critical Care Medicine 04/2005; 33(3 Suppl):S163-9. · 6.12 Impact Factor
[show abstract][hide abstract] ABSTRACT: High frequency oscillatory ventilation (HFOV) has emerged over the past 20 years as a safe and effective means of mechanical ventilatory support in patients with acute respiratory failure. During HFOV, lung recruitment is maintained by application of a relatively high mean airway pressure with superimposed pressure oscillations at a frequency of 3 to 15Hz, creating adequate ventilation using tidal volumes less than or equal to the patient's dead space volume. The physiologic rationale for the application of HFOV in the clinical arena comes from its ability to preserve end-expiratory lung volume while avoiding parenchymal overdistension at end-inspiration and theoretically limiting the potential for ventilator-associated lung injury. Data in the neonatal population suggests significant benefits in pulmonary outcomes when HFOV is applied with a recruitment strategy in preterm infants with respiratory distress syndrome (RDS). Use of HFOV in the paediatric and adult populations has not as yet been associated with significant improvements in clinically important outcome measures.
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