Blockade of pulmonary stretch receptors reinforces diaphragmatic activity during high-frequency oscillatory ventilation

During apneic periods elicited by high-frequency oscillatory ventilation (HFOV) a tonic diaphragmatic activity was observed, contrasting with the absence of diaphragmatic activity during apnea induced by lung inflation. To clarify the mechanism underlying the persistence of the diaphragmatic activity during HFOV-induced arrest of breathing the reflex responses to short periods of HFOV, and to periods of lung inflation with airway pressure (P _aw) equal to the meanP _aw and/or to maximalP _aw during HFOV were examined both before and after the blockade of slowly adapting stretch receptors (SR) by inhalation of sulphur dioxide (SO₂) in anaesthetized rabbits. In animals with intact SR, the HFOV-induced reflex apnea lasted longer than that induced by lung inflation, the associated diaphragmatic activity being in the most cases higher than the diaphragmatic activity during quiet expiration; inflation, however, completely inhibited diaphragmatic activity. After blockade of SR, spontaneous breathing continued during periods of lung inflation, i.e., the Hering-Breuer inflation reflex was abolished, whereas HFOV still led to a cessation of spontaneous breathing, the associated diaphragmatic activity even exceeding the level observed during quiet inspiration. From these results we conclude that only one part of the reflex response to HFOV is due to SR-stimulation and that in addition other vagal pulmonary receptors (irritant-and/or C-fibre-receptors) are involved. The stimulation of the latter counterbalances the concomitant stimulation of SR, giving rise to the tonic activity of the diaphragm.     

A system for integrated measurement of ventilator settings,lung volume change and blood gases during high-frequency oscillatory ventilation

To describe and validate a system for integrated measurement of ventilator settings and dependent physiological variables during high-frequency oscillatory ventilation (HFOV). A custom interface was built for data acquisition. Lung volume change was determined by respirator inductive plethysmography (RIP), modified to sampling rates of 140 Hz. Blood gas analysis was obtained using a continuous intra-arterial blood gas monitoring system. FIO2 was measured by means of an electrochemical sensor. Pressure at the airway opening and trachea (microtip transducer) were sampled. The data acquired were sent to a laptop computer for analysis, display and storage. The system was tested during a lung recruitment procedure in an animal model of respiratory distress. Linearity of the RIP was checked by gas volume injection using a supersyringe. The system operated successfully. Agreement between RIP-measured volume with injected volume was excellent; bias was 5 ml; limits of agreement were 1-9 ml. Graphs were obtained, showing the relationship between imposed mean airway pressure and lung volume change, and oxygenation. The integration of ventilator settings and dependent physiological variables may provide useful information for clinical, instructional and research application.     

Conventional gas ventilation, liquid-assisted high-frequency oscillatory ventilation, and tidal liquid ventilation in surfactant-treated preterm lambs

This study was designed to compare the efficacy and potential protective or injurious effects of tidal liquid ventilation (TLV), liquid-assisted high-frequency oscillatory ventilation (LA-HFOV), and high PEEP conventional mechanical ventilation (CMV) in neonatal respiratory distress syndrome. Preterm lambs (124-126 days gestation), prophylactically treated with natural surfactant, were allocated to one of the treatment modalities or to an untreated fetal control group (F), euthanised after tracheal ligation. LA-HFOV animals received an intratracheal loading dose of 5 mL x kg(-1) followed by a continuous intrapulmonary instillation of 12 mL x kg(-1);h(-1) FC-75 perfluorocarbon liquid. The ventilation strategies aimed at keeping clinically appropriate arterial blood gases for a study period of 5 hours. A histological lung injury score was calculated and semiquantitative morphometry was performed on lung tissue fixed by vascular perfusion. The alveolar-arterial pressure difference for O2 was significantly lower throughout the study in TLV compared to CMV lambs; at 1, 2, and 5 hours, oxygenation was better in TLV when compared to LA-HFOV. Total lung injury scores in TLV lambs were significantly lower than in either CMV or LA-HFOV animals, but higher when compared to F. CMV and LA-HFOV induced an excess of collapsed and overdistended alveoli, whereas in TLV alveolar expansion was normally distributed around predominantly normal alveoli. CMV and LA-HFOV, but not TLV, were associated with an excess of dilated airways. Thus, in the ovine neonatal RDS model, TLV compared favourably to either gas ventilation strategy by its more uniform ventilation, reduced lung injury, and improved gas exchange.     

Total and regional lung volume changes during high-frequency oscillatory ventilation (HFOV) of the normal lung

The effect of high-frequency oscillatory ventilation (HFOV) settings on the distribution of lung volume (V(L)) with changes in mean airway pressure (Paw), frequency (f(R)) and tidal volume (V(T)) remains controversial. We used computer tomographic (CT) imaging to quantify the distribution of V(L) during HFOV compared to static continuous positive airway pressure (CPAP). In anesthetized, supine canines, CT imaging of the entire lung was performed during CPAP and HFOV at Paw of 5, 12.5 and 20 cm H(2)O, f(R)=5, 10, 15 Hz. We found small, statistically significant decreases compared with CPAP in total and regional V(L) during HFOV that were greatest at lower f(R) and Paw. Apex and base sub-volumes underwent changes comparable to the lung overall. Increases in f(R) were accompanied by increases in Pa(O)(2). These finding provide additional insight into the impact of HFOV settings on the distribution of V(L) and suggest that there is low risk of occult regional over-distention during HFOV in normal lungs.     

The individuality of chest wall motion in tetraplegia

We have studied the motion of the chest wall in eight supine tetraplegic subjects using optical contour mapping. Measurements were obtained during quiet breathing, exaggerated breathing, during the course of a deep inspiration and during static inspiratory efforts. The pattern of motion showed a high degree of individual variability which did not appear to be related to the age of the patient, the duration of injury or the presence of intercostal muscle spasticity. By contrast, it did appear to be related to the presence of bony rib cage stiffness, and possibly the action of the neck accessory muscles. The pattern of rib cage motion in tetraplegia is more complex than conventionally thought.     

Humidification of base flow gas during adult high-frequency oscillatory ventilation: an experimental study using a lung model

In adult high-frequency oscillatory ventilation (HFOV) with an R100 artificial ventilator, exhaled gas from patient's lung may warm the temperature probe and thereby disturb the humidification of base flow (BF) gas. We measured the humidity of BF gas during HFOV with frequencies of 6, 8 and 10 Hz, maximum stroke volumes (SV) of 285, 205, and 160 ml at the respective frequencies, and, BFs of 20, 30, 40 l/min using an original lung model. The R100 device was equipped with a heated humidifier, Hummax Ⅱ, consisting of a porous hollow fiber in circuit. A 50-cm length of circuit was added between temperature probe (located at 50 cm proximal from Y-piece) and the hollow fiber. The lung model was made of a plastic container and a circuit equipped with another Hummax Ⅱ. The lung model temperature was controlled at 37℃. The Hummax Ⅱ of the R100 was inactivated in study-1 and was set at 35℃ or 37℃ in study-2. The humidity was measured at the distal end of the added circuit in study-1 and at the proximal end in study-2. In study-1, humidity was detected at 6 Hz (SV 285 ml) and BF 20 l/min, indicating the direct reach of the exhaled gas from the lung model to the temperature probe. In study-2 the absolute humidity of the BF gas decreased by increasing SV and by increasing BF and it was low with setting of 35℃. In this study setting, increasing the SV induced significant reduction of humidification of the BF gas during HFOV with R100.     

Gas transport in branched airways during high-frequency ventilation

A theoretical model of high-frequency ventilation (HFV) is presented based on the physical convective exchange process that occurs due to the irreversibility of gas velocity profiles in oscillatory flow through the bronchial airways. Mass transport during the convective exchange process can be characterized by a convective exchange length, , which depends only on the irreversibility of bronchial velocity profiles and can be measured by the experimental technique of photographic flow visualization in bronchial tree models. Using the exchange length and the molecular diffusivity, a simple model of overall bronchial mass transfer is developed. The model allows a prediction of the mean gas concentration profiles along the airways, the site of maximum mass transfer resistance, and overall flow rate of the gas of interest in or out of the lung as functions of the parameters of HFV. The results predicted by the model agree with the limited experimental data available for animals and humans. For normal unassisted ventilation, total bronchial cross-sectional area around the 15th Weibel bronchial generation is predicted to be the single most important parameter in controlling the total gas transport rate along the airways. For the breathing of room air, values of the respiratory quotient around 0.78 are predicted, which are insensitive to V_T and f. The model represents a fruitful combination of fluid mechanical theory and experiment with physiologic data to yield new and deeper insight into the operation of the human respiratory system during HFV and normal breathing.     

Intensification of gas exchange during high-frequency artificial ventilation

Institute of General Resuscitation, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR D. S. Sarkisov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 108, No. 8, pp. 142–144, August, 1989.     

Analysis of the gas exchange system dynamics during high-frequency ventilation

High-frequency ventilation (HFV) as a form of artificial respiration has attracted interest in recent years as a means of reducing the risk of barotrauma in clinical applications. This paper explores the high-grequency dynamics of the gas exchange system in order to obtain mathematical models that allow optimization studies aimed at answering the question: What is the optimum ventilatory waveform that secures a certain level of gas exchange while minimizing the resulting fluctuations in pleural or alveolar pressure? Two classes of input are considered: sinusoids and band-limited white noise. A model for the dynamic relation between tracheal flow and CO₂ tension is obtained from experimental data which, in combination with existing models relating tracheal flow to pleural or alveolar pressure, allows optimization of the input flow waveform for a given level of CO₂ elimination rate. The developed relation between CO₂ elimination rate and input was verified by experimentally measured arterial CO₂ tension.     

Flow transport and gas mixing during invasive high frequency oscillatory ventilation

A large Eddy simulation (LES) based computational fluid dynamics study was performed to investigate gas transport and mixing in patient specific human lung models during high frequency oscillatory ventilation. Different pressure-controlled waveforms (sinusoidal, exponential and square) and ventilator frequencies (15, 10 and 6 Hz) were used (tidal volume = 50 mL). The waveforms were created by solving the equation of motion subjected to constant lung wall compliance and flow resistance. Simulations were conducted with and without endotracheal tube to understand the effect of invasive management device. Variation of pressure-controlled waveform and frequency exhibits significant differences on counter flow pattern, which could lead to a significant impact on the gas mixing efficiency. Pendelluft-like flow was present for the sinusoidal waveform at all frequencies but occurred only at early inspiration for the square waveform at highest frequency. The square waveform was most efficient for gas mixing, resulting in the least wall shear stress on the lung epithelium layer thereby reducing the risk of barotrauma to both airways and the alveoli for patients undergoing therapy.     

Relative motion of lung and chest wall promotes uniform pleural space thickness

The pleural space is modeled in two dimensions as a thin layer of fluid separating a deformable membrane and a rigid surface containing a bump. We computed the steady-state membrane configuration and fluid pressure distribution during relative sliding of the two surfaces. For physiologically relevant values of membrane tension, shear flow-induced pressures near the bump and far-field pressure gradients are similar to those measured in vivo within the pleural space (e.g. Lai-Fook et al.) [J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 56 (1984) 1633-1639]. Deformation of the membrane over the bump suggests that the pressure field generated by the sliding motion promotes an even layer of fluid in the pleural space, preventing asperities from touching. Results also suggest a possible mechanism for pleural fluid redistribution during breathing, whereby irreversible fluid motion is associated with the deformability of the membrane.     

Gas transport during high-frequency ventilation: Theoretical model and experimental validation

We present a theoretical model of gas transport through the dead space during high-frequency ventilation (HFV) with volumes less than dead space volume. The analysis is based on the axial distribution of transit times of gas moving through the dead space. The model predicts that for tidal volumes (V) much less than dead space (V_d), gas exchange will be proportional to the product of frequency (f) and V². If gas transport is analyzed in terms of Fick's law, then the effective diffusion coefficient (D_eff) can be shown to be equal to fV² times a constant, whose value equals the square of the coefficient of dispersion of axial transit times through the dead space . Experimental results in straight tubes fit the predictions of this model quite well. A through the entire dead space of about 30% is more than sufficient to account for gas exchange during HFV in physical models or in intact animals. An axial dispersion of this magnitude can be measured directly from a typical Fowler dead space determination in healthy subjects.     

Compartmental chest wall volume changes during volitional normocapnic hyperpnoea

During increased ventilation, inspiratory rib cage muscles have been suggested to take over part of diaphragmatic work after the diaphragm fatigues. We investigated the extent to which this proposed change in muscle recruitment is associated with changes in the relative contribution of chest wall compartments to tidal volume (V(T)). Thirteen healthy subjects performed 1 h of fatiguing normocapnic hyperpnoea. Chest wall volumes were assessed by optoelectronic plethysmography. While breathing frequency increased (43±3 to 56±5 breaths min(-1), p=0.006) and V(T) decreased during normocapnic hyperpnoea (2.6±0.2 to 1.9±0.1l, p<0.001), the relative contribution of chest wall compartments to V(T) remained unchanged (pulmonary rib cage: 48±9 versus 51±14%; abdominal rib cage: 24±4 versus 23±9%; abdomen: 28±8 versus 26±9%; all p>0.05). In conclusion, fatiguing respiratory work is not associated with a change in compartmental contribution to V(T), even in the presence of a change in breathing pattern.     

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

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.     

Gas exchange during high-frequency ventilation in the pigeon (Columba livia)

We studied the effect of high-frequency ventilation (HFV) on the gas exchange of tracheotomized pigeons. The pigeons were artificially ventilated using a piston pump, which alternately connected the pigeons' airways to a constant-flow source. Two-minute periods of HFV were interposed between long periods of normal ventilation. The effect of HFV was assessed by the recorded changes in the PO2, PCO2 and pH of arterial blood and from the changes in the composition of the gas in the interclavicular air sacs. The results showed that HFV can augment gas exchange when the tidal volume (VT) is less than the volume of the anatomical dead space (VD). However, normal arterial gas composition can only be maintained if respiratory frequency is high (greater than 20 Hz). At the normal panting frequency of pigeons (7.8 Hz), gas exchange can thus only be maintained if tidal volume is approximately 125% of the dead space. When panting the VT must be greater than the VD. This finding agrees with the results of recent work showing flush-out- or compound-panting in birds: i.e. if, during panting, VT approaches close to the VD, intermittent interruptions, by taking deeper breaths in order to ensure a supply of fresh air to the lungs, are necessary.     

Dynamic dual-energy chest radiography: a potential tool for lung tissue motion monitoring and kinetic study

Dual-energy chest radiography has the potential to provide better diagnosis of lung disease by removing the bone signal from the image. Dynamic dual-energy radiography is now possible with the introduction of digital flat-panel detectors. The purpose of this study is to evaluate the feasibility of using dynamic dual-energy chest radiography for functional lung imaging and tumor motion assessment. The dual-energy system used in this study can acquire up to 15 frames of dual-energy images per second. A swine animal model was mechanically ventilated and imaged using the dual-energy system. Sequences of soft-tissue images were obtained using dual-energy subtraction. Time subtracted soft-tissue images were shown to be able to provide information on regional ventilation. Motion tracking of a lung anatomic feature (a branch of pulmonary artery) was performed based on an image cross-correlation algorithm. The tracking precision was found to be better than 1 mm. An adaptive correlation model was established between the above tracked motion and an external surrogate signal (temperature within the tracheal tube). This model is used to predict lung feature motion using the continuous surrogate signal and low frame rate dual-energy images (0.1-3.0 frames per second). The average RMS error of the prediction was (1.1 ± 0.3) mm. The dynamic dual energy was shown to be potentially useful for lung functional imaging such as regional ventilation and kinetic studies. It can also be used for lung tumor motion assessment and prediction during radiation therapy.