Factors influencing mechanical performance of neonatal high-frequency ventilators

Factors influencing the mechanical performance of neonatal high-frequency ventilators of diverse design were assessed under controlled conditions. Each of eight ventilators was coupled to in vitro models of the neonatal respiratory system simulating disease of varying severity. The principal performance characteristics examined were frequency dependence and load dependence of tidal volume delivered, peak inspiratory flow rate, and waveforms of pressure at either end of the endotracheal tube. Despite wide diversity of ventilator designs, including jets, flow interrupters, and oscillators, common features emerged. In almost all devices tidal volume increased with endotracheal tube size, was invariant with respiratory system compliance, and decreased with frequency of oscillation. Peak inspiratory flow rates for a given tidal volume and frequency were smallest in the group of oscillators compared with jets and flow interrupters. Proximal pressure was a poor indicator of distal pressure. These findings suggest that delivered tidal volume may be sensitive to endotracheal tube size and airway patency but relatively insensitive to changes in lung tissue or chest wall mechanical properties. In these regards high-frequency ventilation differs from pressure-limited conventional mechanical ventilation. Comparison of data obtained at different clinical centers using high-frequency ventilators of varying design may be possible by taking these factors into account.     

Oxygen cost of breathing during fatiguing inspiratory resistive loads

When a subject breathes against an inspiratory resistance, the inspiratory pressure, the inspiratory flow, and the lung volume at which the breathing task takes place all interact to determine the length of time the task can be sustained (Tlim). We hypothesized that the mechanism actually limiting tasks in which these parameters were varied involved the rate of energy utilization by the inspiratory muscles. To test this hypothesis, we studied four experienced normal subjects during fatiguing breathing tasks performed over a range of pressures and flows and at two different lung volumes. We assessed energy utilization by measuring the increment in the rate of whole body O2 consumption due to the breathing task (VO2 resp). Power and mean esophageal pressure correlated with Tlim but depended also on lung volume and inspiratory flow rate. In contrast, VO2 resp closely correlated with Tlim, and this relationship was not systematically altered by inspiratory flow or lung volume. The shape of the VO2 resp vs. Tlim curve was approximately hyperbolic, with high rates of VO2 resp associated with short endurance times and lower rates of VO2 resp approaching an asymptotic value at high Tlim. These findings are consistent with a mechanism whereby a critical rate of energy utilization determines the endurance of the inspiratory pump, and that rate varies with pressure, flow, and lung volume.     

Pulmonary mechanics during rapid mechanical ventilation in rabbits with saline-lavaged lungs

Infants with respiratory failure are frequently mechanically ventilated at rates exceeding 60 breaths/min. We analyzed the effect of ventilatory rates of 30, 60, and 90 breaths/min (inspiratory times of 0.6, 0.3, and 0.2 s, respectively) on the pressure-flow relationships of the lungs of anesthetized paralyzed rabbits after saline lavage. Tidal volume and functional residual capacity were maintained constant. We computed effective inspiratory and expiratory resistance and compliance of the lungs by dividing changes in transpulmonary pressure into resistive and elastic components with a multiple linear regression. We found that mean pulmonary resistance was lower at higher ventilatory rates, while pulmonary compliance was independent of ventilatory rate. The transpulmonary pressure developed by the ventilator during inspiration approximated a linear ramp. Gas flow became constant and the pressure-volume relationship linear during the last portion of inspiration. Even at a ventilatory rate of 90 breaths/min, 28-56% of the tidal volume was delivered with a constant inspiratory flow. Our findings are consistent with the model of Bates et al. (J. Appl. Physiol. 58: 1840-1848, 1985), wherein the distribution of gas flow within the lungs depends predominantly on resistive factors while inspiratory flow is increasing, and on elastic factors while inspiratory flow is constant. This dynamic behavior of the surfactant-depleted lungs suggests that, even with very short inspiratory times, distribution of gas flow within the lungs is in large part determined by elastic factors. Unless the inspiratory time is further shortened, gas flow may be directed to areas of increased resistance, resulting in hyperinflation and barotrauma.     

Increased lung volume limits endurance of inspiratory muscles

We examined the influence of lung volume on the ability of normal subjects to sustain breathing against inspiratory resistive loading. Four normal subjects breathed on a closed circuit in which inspiration was loaded by a flow resistor. Subjects were assigned a series of breathing tasks over a range of pressures and flows. In each task there was a specified resistor and also targets for either mean esophageal or airway opening pressure, respiratory frequency, and duty cycle. Endurance was assessed as the length of time to failure of the assigned task. The prime experimental variable was lung volume, which was increased by approximately 1 liter during some tasks; 8 cmH2O continuous positive airway pressure was applied to increase lung volume without increasing elastic load. As previously shown (McCool et al.J. Appl. Physiol. 60: 299-303, 1986), for tasks that could be sustained for the same time, there was an inverse linear relationship of mean esophageal pressure with inspiratory flow rate. This trade-off of pressure and flow was apparent both with and without the increase of lung volume. Comparable tasks, however, could not be sustained as long at the higher lung volumes. This effect of volume on endurance was greater for tasks characterized by high inspiratory pressures and low flow rates than for tasks that could be sustained for the same time but that had lower inspiratory pressures and higher flow rates. This is probably due to the effects of shortening of the sarcomere on fatiguability. Increased lung volume, per se, may contribute to respiratory failure because of increased inspiratory muscle fatiguability by mechanisms independent of elastic load.     

Determinants of skin contact pressure formation during non-invasive ventilation

There is no published data about mask features that impact skin contact pressure during mask ventilation.To investigate the physical factors of skin contact pressure formation.We measured masks with original and reduced air cushion size and recorded contact pressure. We determined cushion contact and mask areas by planimetric measurements.Contact pressures necessary to prevent air leakage during inspiration exceed inspiratory pressure by 1.01±0.41 hPa independent of cushion size.Contact area, ventilator pressure and mask area during inspiration and expiration impact contact pressure. Mask contact pressures are higher during expiration. The contact pressure increases with increase in inspiratory pressures independent of the ventilator cycle. During expiration, the contact pressure will increase in proportion to the expiratory pressure reduction of the ventilator. The mask with reduced air cushion size developed higher contact pressures.Contact pressure can be reduced by selecting masks with a small mask area in combination with a large mask cushion.     

Flow and volume dependence of pulmonary mechanics in anesthetized cats

The effects of inspiratory flow rate and inflation volume on pulmonary mechanics were investigated in six anesthetized-paralyzed cats ventilated by constant-flow inflation. Pulmonary mechanics were assessed using the technique of rapid airway occlusion during constant-flow inflation which allows measurement of the intrinsic pulmonary resistance (RLmin) and of the overall "pulmonary flow resistance" (RLmax), which includes the additional pulmonary pressure losses due to time constant inequalities within the lung and/or stress adaptation. We observed that, at fixed inflation volume, 1) RLmin fitted Rohrer's equation, 2) RLmax was higher at low than intermediate flows, and 3) RLmax-RLmin decreased progressively with increasing flow. At fixed flow, RLmax increased, whereas RLmin decreased with increasing volume. We conclude that during eupneic breathing in cats, the pulmonary flow resistance as conventionally measured includes a significant component reflecting stress adaptation.     

Assessment the protection performance of different level personal respiratory protection masks against viral aerosol

New viral disease such as SARS and H1N1 highlighted the vulnerability of healthcare workers to aerosol-transmitted viral infections. This paper was to assess the protection performance of different level personal respiratory protection equipments against viral aerosol. Surgical masks, N95 masks and N99 masks were purchased from the market. The masks were sealed onto the manikin in the aerosol testing chamber. Viral aerosol was generated and then sampled simultaneously before and after the tested mask using biosamplers. This allows a percentage efficiency value to be calculated against test phage SM702 aerosols which surrogates of viral pathogens aerosol. At the same time, the masks face fit factor was determined by TSI8020. The viral aerosol particles aerodynamic diameter was 0.744 μm, and GSD was 1.29. The protection performance of the material of all the tested masks against viral aerosol was all >95 %. All the five surgical masks face fit factor were <8. F model N95 mask and H model N99 mask face fit factor were all >160. G model N95 mask face fit factor was 8.2. The protection performances of N95 or N99 masks were many times higher than surgical mask when considering the face fit factor. Surgical masks cannot offer sufficient protection against the inhalation of viral aerosol because they cannot provide a close face seal.     

Respiration during exercise in conscious laryngectomized humans

We compared respiratory patterning at rest and during steady cycle exercise at work rates of 30, 60, and 90 W in 7 male chronically laryngectomized subjects and 13 normal controls. Breathing was measured with a pneumotachograph and end-tidal PCO2 by mass spectrometer. Inspired air was humidified and enriched to 35% O2. Peak flow, volume, and times for the inspiratory and expiratory half cycles, time for expiratory flow, minute ventilation, and mean inspiratory flow were computer averaged over at least 40 breaths at rest and during the last 2 min of 5-min periods at each work rate. During the transition from rest to exercise and with increasing work rate in both groups, there was an increase in respiratory rate and depth with selective and progressive shortening of expiratory time; these responses were not significantly different between the two groups, but there was a suggestion that respiratory "drive" as quantitated by mean inspiratory flow may limit in the laryngectomized subjects at high work rates. Time for expiratory flow increased on transition from rest to exercise and then decreased in both groups as the work rate increased; it was shorter in the laryngectomy than control group at all levels. In the laryngectomized subjects there was significantly more breath-by-breath scatter in some variables at rest, but there was no difference during exercise. It is concluded that chronic removal of the larynx and upper airways in mildly hyperoxic conscious humans has only subtle and, therefore, functionally insignificant effects on breathing during moderate exercise. Evidence is provided that the upper airways can modulate expiratory flow but not expiratory time during exercise.     

A phrenic nerve-actuated electronically controlled positive-pressure ventilator

We have constructed an electronically controlled positive-pressure ventilator actuated by phrenic neural activity for use in open-chested or paralyzed experimental animals for the study of breathing pattern. A Bird Mark 14 positive-pressure ventilator was modified such that flow is a linear function of a command signal. Flow is delivered by advancing an air valve with a servo-motor that is controlled by one of three different operational modes. In two of the modes, the difference between the electronic average of inspiratory phrenic activity (moving average) and a feedback signal determines the inspiratory flow. The feedback signal is derived from either tracheal pressure or an electronic measure of inspired volume. In the third mode, the moving average is differentiated to provide control of inspiratory flow and volume. Physiological flow profiles were created using all three operational modes. Integration of an air-valve position signal provides an electronic measure of tidal volume. An additional feature of this ventilator allows inspiratory flow and duration to be predetermined for a given breath.     

Pressure-time product, flow, and oxygen cost of resistive breathing in humans

We examined the relationship between the pressure-time product (Pdt) of the inspiratory muscles and the O2 cost of breathing (VO2 resp) in five normal subjects breathing through an external inspiratory resistance with a tidal volume of 800 ml at a constant end-expiratory lung volume [functional residual capacity, (FRC)]. Each subject performed 30-40 runs, each of approximately 30 breaths, with inspiratory flow rates ranging from 0.26 +/- 0.01 to 0.89 +/- 0.04 l/s (means +/- SE) and inspiratory mouth pressures ranging from 10 +/- 1 to 68 +/- 4% of the maximum inspiratory pressure at FRC. In all subjects VO2 resp was linearly related to Pdt when mean inspiratory flow (VI) was constant, but the slope of this relationship increased with increasing VI. Therefore, Pdt is an accurate index of VO2 resp only when VI is constant. There was a linear relationship between the VO2 resp and the work rate across the external resistance (W) for all runs in each subject over the range of W 10 +/- 1 to 137 +/- 21 J/min. Thus, at a constant tidal volume the VO2 resp was related to the mean inspiratory pressure, independent of flow or inspiratory duration. If the VO2 resp were determined mainly during inspiration, then for a given rate of external work or O2 consumption, VI would be inversely related to mean inspiratory pressure. Efficiency (E) was 2.1 +/- 0.2% and constant over a large range of VI, pressure, work rate, or resistance and was not altered by the presence of a potentially fatiguing load. The constant E over such a wide range of conditions implies a complex integration of the recruitment, mechanical function, and energy consumption of the muscles utilized in breathing.     

Human breathing patterns on mouthpiece or face mask during air, CO2, or low O2

Steady-state breathing patterns on mouthpiece and noseclip (MP) and face mask (MASK) during air and chemostimulated breathing were obtained from pneumotachometer flow. On air, all 10 subjects decreased frequency (f) and increased tidal volume (VT) on MP relative to that on MASK without changing ventilation (VE), mean inspiratory flow (VT/TI), or mean expiratory flow (VT/TE). On elevated CO2 and low O2, MP exaggerated the increase in VE, f, and VT/TE due to profoundly shortened TE. On elevated CO2, MASK exaggerated VT increase with little change in f. Increased VE and VT/TI were thus due to increased VT. During low O2 on MASK, both VT and f increased. During isocapnia, shortened TE accounted for increased f; during hypocapnia, increased f was related primarily to shortened TI. Thus the choice of a mouthpiece or face mask differentially alters breathing pattern on air and all components of ventilatory responses to chemostimuli. In addition, breathing apparatus effects are not a simple consequence of a shift from oronasal to oral breathing, since a noseclip under the mask did not change breathing pattern from that on mask alone.     

Inspiratory-to-expiratory time ratio and alveolar ventilation during high-frequency ventilation in dogs

It has been suggested that the increase in inspiratory flow rate caused by a decrease in the inspiratory-to-expiratory time ratio (I:E) at a constant tidal volume (VT) could increase the efficiency of ventilation in high-frequency ventilation (HFV). To test this hypothesis, we studied the effect of changing I:E from 1:1 to 1:4 on steady-state alveolar ventilation (VA) at a given VT and frequency (f) and at a constant mean lung volume (VL). In nine anesthetized, paralyzed, supine dogs, HFV was performed at 3, 6, and 9 Hz with a ventilator that delivered constant inspiratory and expiratory flow rates. Mean airway pressure was adjusted so that VL was maintained at a level equivalent to that of resting FRC. At each f and one of the I:E chosen at random, VT was adjusted to obtain a eucapnic steady state [arterial pressure of CO2 (PaCO2) = 37 +/- 3 Torr]. After 10 min of each HFV, PaCO2, arterial pressure of O2 (PaO2), and CO2 production (VCO2) were measured, and I:E was changed before repeating the run with the same f and VT. VA was calculated from the ratio of VCO2 and PaCO2. We found that the change of I:E from 1:1 to 1:4 had no significant effects on PaCO2, PaO2, and VA at any of the frequencies studied. We conclude, therefore, that the mechanism or mechanisms responsible for gas transport during HFV must be insensitive to the changes in inspiratory and expiratory flow rates over the VT-f range covered in our experiments.     

The effect of aeration and agitation on Claviceps purpurea dimorphism and alkaloid synthesis during submerged fermentation

Summary Varying the air flow rate (vvm) in a fermentor under constant drive speed, Claviceps purpurea dimorphism as well as alkaloid biosynthesis were greatly influenced. At a high flow rate (2.5 vvm) sclerotial growth was favoured in seed and in production media, while at a low air flow rate (1.0 vvm) sphacelial growth dominated. When using high flow rates the oxygen uptake rate was small, but at low flow rates it increased markedly. In both cases the alkaloid production was lower than at the intermediate value of 1.5 vvm of air flow rate, which proved to be optimal. This could be explained by the difference in the air/water interface and two-phase oxygen uptake. At a high air/water interface direct oxygen uptake from the gaseous phase prevails, while at a low air/water interface uptake is due to the oxygen liquid-phase only. Thus for optimal fungal development and alkaloid production a compromise between uptake from the liquid and the gaseous phase has to be established by a defined ratio between aeration and agitation.     

Shadows Alter Facial Expressions of Noh Masks

Background

A Noh mask, worn by expert actors during performance on the Japanese traditional Noh drama, conveys various emotional expressions despite its fixed physical properties. How does the mask change its expressions? Shadows change subtly during the actual Noh drama, which plays a key role in creating elusive artistic enchantment. We here describe evidence from two experiments regarding how attached shadows of the Noh masks influence the observers’ recognition of the emotional expressions.

Methodology/Principal Findings

In Experiment 1, neutral-faced Noh masks having the attached shadows of the happy/sad masks were recognized as bearing happy/sad expressions, respectively. This was true for all four types of masks each of which represented a character differing in sex and age, even though the original characteristics of the masks also greatly influenced the evaluation of emotions. Experiment 2 further revealed that frontal Noh mask images having shadows of upward/downward tilted masks were evaluated as sad/happy, respectively. This was consistent with outcomes from preceding studies using actually tilted Noh mask images.

Conclusions/Significance

Results from the two experiments concur that purely manipulating attached shadows of the different types of Noh masks significantly alters the emotion recognition. These findings go in line with the mysterious facial expressions observed in Western paintings, such as the elusive qualities of Mona Lisa’s smile. They also agree with the aesthetic principle of Japanese traditional art “yugen (profound grace and subtlety)”, which highly appreciates subtle emotional expressions in the darkness.     

Experimental optimization of a step aeration waste treatment process

Evolutionary operation (EVOP) was used to experimentally investigate the optimum steady state operating conditions for a step aeration activated sludge waste treatment process. A laboratory scale two tank step aeration activated sludge unit with fixed total volume, total influent flow rate, recycle flow rate, and sludge wasting rate was employed. The volume ratio and flow rate ratio which minimized effluent chemical oxygen demand were determined. The results indicate that EVOP is a useful technique for improving the performance of biological processes.     

Identification of respiratory vagal feedback in awake normal subjects using pseudorandom unloading.

Evidence of the Hering-Breuer reflex has been found in humans during anesthesia and sleep but not during wakefulness. Cortical influences, present during wakefulness, may mask the effects of this reflex in awake humans. We hypothesized that, if lung volume were increased in awake subjects unaware of the stimulus, vagal feedback would modulate breathing on a breath-to-breath basis. To test this hypothesis, we employed proportional assist ventilation in a pseudorandom sequence to unload the respiratory system above and below the perceptual threshold in 17 normal subjects. Tidal volume, integrated respiratory muscle pressure per breath, and inspiratory time were recorded. Both sub- and suprathreshold stimulation evoked a significant increase in tidal volume and inspiratory flow rate, but a significant decrease in inspiratory time was present only during the application of a subthreshold stimulus. We conclude that vagal feedback modulates respiratory timing on a breath-by-breath basis in awake humans, as long as there is no awareness of the stimulus.     

Reflex control of inspiratory duration in newborn infants

We applied graded resistive and elastic loads and total airway occlusions to single inspirations in six full-term healthy infants on days 2-3 of life to investigate the effect on neural and mechanical inspiratory duration (TI). The infants breathed through a face mask and pneumotachograph, and flow, volume, airway pressure, and diaphragm electromyogram (EMG) were recorded. Loads were applied to the inspiratory outlet of a two-way respiratory valve using a manifold system. Application of all loads resulted in inspired volumes decreased from control (P less than 0.001), and changes were progressive with increasing loads. TI measured from the pattern of the diaphragm EMG (TIEMG) was prolonged from control by application of all elastic and resistive loads and by total airway occlusions, resulting in a single curvilinear relationship between inspired volume and TIEMG that was independent of inspired volume trajectory. In contrast, when TI was measured from the pattern of airflow, the effect of loading on the mechanical time constant of the respiratory system resulted in different inspired volume-TI relationships for elastic and resistive loads. Mechanical and neural inspired volume and duration of the following unloaded inspiration were unchanged from control values. These findings indicate that neural inspiratory timing in infants depends on magnitude of phasic volume change during inspiration. They are consistent with the hypothesis that termination of inspiration is accomplished by an "off-switch" mechanism and that inspired volume determines the level of vagally mediated inspiratory inhibition to trigger this mechanism.     

Significance of airway resistance for the pattern of breathing and lung volumes in exercising humans

The effects of increased airway resistance on lung volumes and pattern of breathing were studied in eight subjects performing leg exercise on a cycle ergometer. Airway resistance was changed 1) by increasing the density (D) of the respired gas by a factor of 4.2 and changing the inspired gas from O2 at 1.3 bar to air at 6 bar and 2) by increasing airway flow rates by exposing the subjects to incremental work loads of 0-200 W. Increased gas D caused a slower and deeper respiration at rest and during exercise and, at work loads greater than 120 W, depressed the responses of ventilation and mean inspiratory flow. Raised airway resistance induced by increases in D and/or airway flow rates altered respiratory timing by increasing the ratio of inspiratory time (TI) to total breath duration. Furthermore, analyses of the relationships between tidal volume and TI and between end-inspiratory volume and TI revealed elevation of Hering-Breuer inspiratory volume thresholds. We propose that this elevation, and hence exercise-induced increases of tidal volume, can largely be explained by previous observations that the threshold of the inspiratory off-switch mechanisms depends on central inspiratory activity (cf. C. von Euler, J. Appl. Physiol. 55: 1647-1659, 1983), which in turn increases with airway resistance (Acta Physiol. Scand. 120: 557-565, 1984).     

Changes in respiratory activity induced by mastication in humans.

We studied the influence of mastication on respiratory activity in nine healthy volunteers who were requested to masticate a 5-g chewing gum bolus at a spontaneous rate (SR) for 5 min and "at the maximum possible rate" (MPR) for 1 min. Significant increases in respiratory frequency were induced by SR mastication due to a decrease in both the inspiratory and expiratory time. Tidal volume displayed slight nonsignificant decreases, but minute ventilation and mean inspiratory flow significantly increased. The duty cycle (TI/TT) did not change significantly. Total airway resistance significantly increased. Both peak and rate of rise of the integrated electromyographic activity of inspiratory muscles presented marked increases, accompanied by the appearance of a low level of tonic muscular activity. Similar but more intense effects on respiratory activity were induced by MPR mastication; in addition, a significant decrease in tidal volume and a significant increase in TI/TT were observed. Rhythmic handgrip exercise performed at metabolic rates comparable to those attained during SR or MPR mastication induced similar changes in the drive and time components of the breathing pattern, although accompanied respectively by nonsignificant or significant increases in tidal volume. Furthermore, the frequency of SR mastication significantly entrained the respiratory rhythm. The results suggest that mastication-induced hyperpnea does not merely represent a ventilatory response to exercise but also reflects complex interactions between respiratory and nonrespiratory functions of the upper airway and chest wall muscles.