Constancy of physiological dead space during high-frequency ventilation

Gas Exchange Mechanism of High Frequency Ventilation: A Brief Narrative Review and Prospect

The high frequency ventilation (HFV) can well support the breathing of respiratory patient with 20%-40% of normal tidal volume. Now as a therapy of rescue ventilation when conversional ventilation failed, the HFV has been applied in the treatments of severe patients with acute respiratory failure (ARF), acute respiratory distress syndrome (ARDS), etc. However, the gas exchange mechanism (GEM) of HFV is still not fully understood by researchers. In this paper, the GEM of HFV is reviewed to track the studies in last decades and prospect for the next likely studies. And inspired by previous studies, the GEM of HFV is suggested to be continually developed with various hypotheses which will be testified in simulation, experiment and clinic trail. One of the significant measures is to study the GEM of HFV under the cross-disciplinary integration of medicine and engineering. Fully understanding the GEM can theoretically support and expand the applications of HFV, and is helpful in investigating the potential indications and contraindications of HFV.

High frequency oscillatory ventilation near resonant frequency of the respiratory system in rabbits with normal and surfactant depleted lungs

It has been suggested that high frequency oscillatory ventilation (HFV) might improve gas exchange and reduce the risk of pressure-related side-effects compared to conventional mechanical ventilation (CMV). Whereas most studies have used arbitrarily set frequencies for HFV, we evaluated the effects of HFV near resonant frequency (fr). Anaesthetised and tracheotomized adult rabbits (n = 10; 3.8-5.1 kg body weight) were ventilated by alternating periods of CMV and HFV near fr. Negative ventilator resistance was used for complete resistive unloading of the respiratory system before each HFV period. This enabled a continuous swinging at resonance thus allowing measurement of fr and selection of exactly that frequency for the HFV run. Intra-animal CMV-HFV comparisons (n = 4) were performed on each animal: with healthy lungs at a mean airway pressure (MAP) of 0.5 kPa and after saline lung lavage at MAPs of less than 1.5 kPa; 1.5-1.8 kPa; greater than 1.8 kPa. Surfactant removal caused total respiratory system compliance (Ctot) to decrease from 44 +/- 5 to 22 +/- 3 ml/kPa. Corresponding fr was 244 +/- 48 and 360 +/- 30 min-1, respectively. HFV produced effective pulmonary gas exchange but did not improve arterial oxygenation in comparison with CMV at matched MAPs both before and after surfactant depletion. Volume amplitudes of oscillation necessary to achieve normocapnia were slightly above the natural plus equipment (2 ml) dead space. Maximum intra-alveolar pressure (Pmax) was calculated for the HFV runs from MAP, Ctot, and the volume amplitude of oscillation. Pmax during CMV was nearly twice that during HFV at equivalent PaCO2 and equivalent MAPs throughout the experiments.

Frequency dependence of dead space during high-frequency ventilation in rhesus monkeys

A rotary valve ventilator, designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, rhesus monkey at ventilatory frequencies up to 20 Hz. A total of five studies were carried out on three animals. Determinations of the minute ventilation required to maintain a normal steady-state PCO2, together with the FECO2 and arterial PO2, PCO2, and pH, were made at a number of frequencies. The results so obtained were used to calculate the Bohr (physiological) dead space. Dead space remained approximately constant at close to the value determined during spontaneous ventilation for each individual animal in the range of ventilatory frequencies from below 1 to around 5 Hz, but decreased somewhat with increasing frequency above 5 Hz. The calculated physiological dead space at 15 Hz was about 70% of the value at normal respiratory frequencies. These findings in primates, obtained using a ventilator system which allows very accurate determinations of expired gas volume and content, when compared with those from our previous studies in rabbits and dogs, provide further evidence that the relationship between efficiency of gas exchange and ventilatory frequency during HFV is highly species-specific.

Effective dead space of differently shaped airways during high-frequency ventilation of a CO2-producing lung model

A simple lung model is described which simulates the mechanical properties and CO2 production of the lungs of a small animal, and which can be used with high-frequency ventilators. The model was ventilated by a rotary-valve ventilator and the system was used to determine the increment in 'physiological' (Bohr) dead space produced at various ventilatory frequencies between 0.5 and 15 Hz by the insertion of various additional volumes and configurations of 'anatomical' dead space in its 'upper airway'. It was found that the insertion of a straight tube with an internal diameter similar to that of the ventilator outlet tube produced an increment in 'physiological' dead space that remained commensurate with the volume of the added tube at all combinations of the tube volume and ventilatory frequency. The insertion of similar volumes of dead space in the form of tubes with smoothly expanded central portions produced increments in 'physiological' dead space which were commensurate with the volume of the added tube only at the lowest frequencies, and actually became negative at higher frequencies and volumes. These findings provide further evidence that the volume and configuration of the external portion of a high-frequency ventilator system may be of at least as much importance in determining its efficiency as is the lung structure of the animal or patient being ventilated.

Effect of resident gas density on CO2 elimination during high-frequency oscillation: a model study

In order to throw more light on the mechanisms governing the efficiency of intrapulmonary gas mixing during high-frequency oscillatory ventilation, an experimental, and theoretical, study was carried out on a model casting of the airways of a human lung that closely resembled the respiratory tract. The experiments were carried out under various conditions during high-frequency oscillation (HFO), by using alveolor resident gas mixtures of different densities. The efficiency of gas mixing was assessed by measuring the time constants of the CO2 alveolar washout which were compared to those obtained from simulations on a theoretical model based on a turbulent diffusional resistance concept. Our results showed that the decay in CO2 concentration was highly dependent on both frequency (f) and tidal volume (VT). Tidal volume was found to have a greater effect on efficiency of gas mixing than frequency. Moreover, even though there were statistically significant differences in the time courses of CO2 washout between N2 and He, N2 and SF6 or between He and SF6, this could not imply that gas mixing was limited by diffusion. Agreement between the experimental time constants of CO2 elimination during HFO and the predicted mixing time constants was satisfactory. It is concluded that turbulent augmented diffusion is the main factor responsible for effective gas transport during high-frequency oscillatory ventilation.

High frequency ventilation

High frequency ventilation (HFV) presents a new respiratory therapy modality that has taught us much about the theories of gas transport in the lung. Both experimental and clinical applications are summarized. Although the future clinical role of HFV remains uncertain, pediatric applications and investigation continue at the forefront of this new technology.

High-frequency ventilation

Over the last six years high-frequency ventilation has been extensively evaluated both in the clinical and laboratory settings. It is now no longer the great mystery it once was, and it is now no longer believed (as many had hoped), that it will solve all the problems associated with mechanical pulmonary ventilation. Although the technique is safe and appears to cause no harm even in the long term, it has not yet been shown to offer any major advantages over conventional mechanical ventilation.

Some criteria for laminar conditions during HFV

Assessment of the type of flow regime-laminar, transitional or turbulent-present in the central airways during high-frequency ventilation (HFV) can assist in identification of the predominant gas transport mechanisms for a particular species and set of HFV conditions. In this study, we use published empirical relationships, developed to identify the initial change from oscillating laminar flow to oscillating turbulent flow in tubes, to derive the limiting relationships between a dimensionless stroke volume and the Womersley number for maintenance of laminar conditions. When used with morphometric lung models for man and dog, these limiting relationships predicted maximum stroke volumes at experimental frequencies that either exceeded or agreed within 20% with the stroke volumes reported for steady-state ventilation of humans and dogs using HFV. Based on these limiting relationships, stroke volume-frequency combinations reported to demarcate the decline of PaO2 and alveolar ventilation were associated with nonlaminar conditions. It is expected that this approach may be useful in selecting the stroke volume-frequency pairs for HFV when a specific type of flow regime is desired as well as for analysis of HFV data.

Frequency dependence of dead space during high-frequency ventilation in dogs

A rotary valve ventilator, designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, mongrel dogs at ventilatory frequencies up to 20 Hz. A total of eleven studies were carried out on four animals. Determinations of the minute ventilation required to maintain a normal steady-state PCO2, together with the FECO2 and arterial PO2, PCO2, and pH, were made at a number of frequencies. The results so obtained were used to calculate the Bohr (physiological) dead space. Dead space remained approximately constant at close to the value measured during spontaneous ventilation for each individual dog in the range of ventilatory frequencies from below 1 to around 5 Hz, but decreased with increasing frequency above 5 Hz. The calculated physiological dead space at 15 Hz was about half of the value at normal respiratory frequencies. These findings in dogs, obtained using a ventilator system which allows very accurate determinations of expired gas volume and content, are consistent with published results for this species from other laboratories. They contrast with our earlier findings in rabbits that dead space remains constant with increasing frequency, suggesting that the effects of high-frequency ventilation on CO2 transport are species-dependent.

A new ventilator for physiologic studies during high-frequency ventilation

Research in the field of high frequency ventilation (HFV) has been hampered by the difficulty of collecting and analyzing expired gas, uncontaminated by gas of inspiratory composition, at ventilatory frequencies in the relevant range. A new mechanical ventilator is described, which has been designed to facilitate studies on the respiratory physiology of small animals during HFV. Ventilation is controlled by a single rotary valve, which alternately switches the airway to an approximately constant-flow gas source during inspiration, and to gas measurement and analysis apparatus during expiration, which takes place passively. Theoretical and experimental studies, using the techniques of mechanical and mathematical modelling, are used to measure the performance of the ventilator and to demonstrate that it allows the quantitative collection of expired gas during ventilation at rates up to at least 20 Hz. The advantages and limitations of this approach are discussed.