History of high frequency oscillation

The article describes the evolution of high frequency oscillation since its first use by Lunkenheimer through the initial failed NIH trial and subsequent more successful trials to its current widespread use in the neonatal population. The importance of oscillating at an optimal lung volume, achieved through a volume recruitment manoeuvre, is emphasised as is the efficacy with which oscillation clears CO2. The lack of adequate control of these two factors in the initial NIH trial is suggested as a possible cause of the trial's failure. Comment is made on optimising oscillator settings as well on elementary mechanics of high frequency oscillation and the effect of high frequency oscillation on surfactant degradation. Given the difficulty of recruiting lung volume in late RDS, a suggestion is made to combine high frequency oscillation with perfluorocarbon. The former as a mechanism for maintaining lung volume which has been recruited by the perfluorocarbon. The authors speculate that the use of high frequency oscillation will increase in both the paediatric and adult population.

The open lung during small tidal volume ventilation: concepts of recruitment and "optimal" positive end-expiratory pressure

OBJECTIVES

To test the hypotheses that during small tidal volume ventilation (5 mL/kg) deliberate volume recruitment maneuvers allow expansion of atelectatic lung units and that a high positive end-expiratory pressure (PEEP) above the lower inflection point of the pressure/volume (PV) curve is not necessarily required to maintain recruited lung volume in acute lung injury.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

An animal laboratory in a university setting.

SUBJECTS

Adult New-Zealand rabbits.

INTERVENTIONS

We studied a) the relationship of dynamic loops during intermittent positive pressure ventilation to the quasi-static PV curve, and b) the effect of lung recruitment on oxygenation, end-expiratory lung volume (EELV), and dynamic compliance in two groups (n = 4 per group) of lung-injured animals (lung lavage model): 1) the sustained inflation group, which received ventilation after a recruitment maneuver (sustained inflation); and 2) the control group, which received ventilation without any lung recruitment.

MEASUREMENTS AND MAIN RESULTS

In the presence of PV hysteresis, a single sustained inflation to 30 cm H2O boosted the ventilatory cycle onto the deflation limb of the PV curve. This resulted in a significant increase in EELV, oxygenation, and dynamic compliance despite equal PEEP levels used before and after the recruitment maneuver. Furthermore, after a single sustained inflation, oxygenation remained high over 4 hrs of ventilation when a PEEP above the critical closing pressure of the lungs, defined as "optimal" PEEP, was used and was significantly higher compared with that in the control group ventilated at equal PEEP without preceding lung recruitment.

CONCLUSIONS

The observation that ventilation occurs on the deflation limb of the tidal cycle-specific PV curve allows placement of the ventilatory cycle, by means of a recruitment maneuver, onto the deflation limb of the PV envelope of the optimally recruited lung. This strategy ensures sufficient lung volume recruitment to maintain the lungs during the tidal cycle while using relatively low airway pressures.

Concealed air leak associated with large tidal volumes in partial liquid ventilation

Current ventilator strategies aim at maintaining an open lung and limiting both peak inspiratory pressures and tidal volumes to avoid alveolar distension. Perfluorocarbons, as well as being excellent solvents for oxygen and carbon dioxide, have the unique properties of being able to recruit dependent lung regions and improve pulmonary mechanics. Optimal ventilator strategies for partial liquid ventilation (PLV) have not yet been clearly defined. In the surfactant-depleted rabbit model, an approach involving a large tidal volume (VT) (15 ml/kg) and lung filled to FRC with perfluorocarbon (PFC) was compared with strategies involving a moderate VT (9 ml/kg) and partially filled lung (6 ml/kg), a moderate VT (9 ml/kg) and lung filled to FRC with PFC, and a large VT (15 ml/kg) and partially filled lung (6 ml/kg). PEEP was maintained at 5 cm H2O except in the moderate VT, partial-filling group, in which a PEEP of 9 cm H2O was used to maintain the rabbits for the duration of the experiment. Oxygenation was satisfactory in all groups, and peak inspiratory pressures were not significantly different. However, five of the 13 animals in the large-VT, PFC-filled lung group died of a pneumothorax prior to completion of the experiment. Of the eight animals in this group surviving the experiment, two had radiographic evidence of pneumothoraces, with an additional three animals having autopsy evidence of air leak. Of the 22 animals in the other groups, all survived with the exception of a single rabbit in the large VT, partial-filling group, which had both radiographic and autopsy evidence of air leak. We conclude that there is a significant risk of barotrauma in a PLV strategy in which a large VT is used in association with a lung filled to FRC with perfluorocarbon. Adequate gas exchange can be achieved with alternative ventilation strategies in combination with PLV.

The effect of single-dose and continuous skeletal muscle paralysis on respiratory system compliance in paediatric intensive care patients

OBJECTIVE

To investigate the effect of single dose and continuous skeletal muscle paralysis on respiratory system compliance in 53 paediatric intensive care patients.

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary paediatric intensive care unit.

PATIENTS

Twenty-three children ventilated for acute pulmonary pathology, and 30 ventilated for isolated intracranial pathology, who initially had normal lungs.

INTERVENTIONS

The 23 patients with acute pulmonary pathology received a single dose of muscle relaxant to facilitate diagnostic procedures. Fifteen patients with isolated intracranial pathology received continuous skeletal muscle paralysis for longer than 24 h, and the other 15 received no paralysis.

MEASUREMENTS AND RESULTS

Respiratory system compliance deteriorated by 14% from 0.519 +/- 0.2 to 0.445 +/- 0.18 ml cmH2O-1 kg-1 (p < 0.001) following a single dose of muscle relaxant in the 23 patients with acute pulmonary pathology. In the 15 with isolated intracranial pathology who received continuous skeletal muscle paralysis there was a progressive deterioration in compliance, which reached 50% of the initial compliance by day 4 of paralysis (p < 0.001) and improved back to normal following discontinuation of paralysis. There were no changes in compliance in the 15 patients with isolated intracranial pathology who were ventilated but not paralysed. The paralysed patients required mechanical ventilation longer than the non-paralysed patients (p < 0.001), and 26% of these patients developed nosocomial pneumonia (p = 0.03), a complication that was not seen in the non-paralysed patients.

CONCLUSIONS

Skeletal muscle paralysis results in immediate and progressive deterioration of respiratory system compliance and increased incidence of nosocomial pneumonia. The benefits of paralysis should be balanced against the risks of deteriorating pulmonary function.

Increased respiratory system resistance and bronchial smooth muscle hypertrophy in children with acute postoperative pulmonary hypertension

Following surgery for congenital heart disease, there is often an increased reactivity of the pulmonary vasculature to stimuli, resulting in rapid increases in pulmonary artery pressure and a clinical impression of stiff lungs. Lung mechanics were measured in 30 children, mean age 6.7 +/- 4.1 mo, who were ventilated and had pulmonary artery pressure monitoring following surgery for congenital heart disease. A group of 15 patients developed postoperative pulmonary hypertension. In these patients, respiratory system resistance was 43% higher (p = 0.001) and compliance 11% lower (p = 0.004) during acute pulmonary hypertension compared with baseline pulmonary artery pressure. No changes in resistance or compliance were seen in the 15 patients who did not develop pulmonary hypertension. The changes in lung mechanics interfered with mechanical ventilation, resulting in a 9.4% rise in PaCO2 during pulmonary hypertension. The bronchial smooth muscle was found to be increased by 68%, and the vascular smooth muscle was more than twice normal in lung biopsies from 9 pulmonary hypertension patients compared with 6 age-matched postmortem controls patients who had no cardiac or pulmonary disease. The bombesin-immunoreactive pulmonary neuroendocrine cells (PNEC) were also increased in the pulmonary hypertension patients. These findings suggest a coconstriction and cohypertrophy of bronchial and vascular smooth muscle during pulmonary hypertension. Mediators, such as bombesin, endothelin-1, and serotonin, are known to be produced by PNEC and may be involved in the observed vasoconstriction, increased respiratory system resistance, and smooth muscle hypertrophy.

Maximal expiratory flows generated by rapid chest compression following end-inspiratory occlusion or expiratory clamping in young children

Partial forced expiratory flow-volume curves obtained by the rapid chest compression technique are being widely used to assess pulmonary function in infants and young children. The aim of this study is to assess whether in this age group flow limitation is achieved with the partial forced expiratory flow-volume curve with rapid chest compression. In eight infants and young children sedated with chloral hydrate, flow-volume curves were obtained by regular rapid chest compression technique, end-inspiratory airway occlusion prior to rapid chest compression, and expiratory clamping prior to rapid chest compression. In each technique, beginning with a cuff pressure of 20 cmH2O, the cuff pressure was increased by 10 cmH2O increments until the compression pressure reached 90 cmH2O. Maximal flow-volume curves were generated by each technique. End-inspiratory occlusion prior to rapid chest compression caused higher flows over the entire phase of expiration than the regular rapid chest compression. This increase could be observed over the entire phase of expiration. Forced expiratory flow at 50% and at 75% of vital capacity (V50 and V75) with regular rapid chest compression were 207 +/- 44 ml.s-1 (mean +/- SD) and 138 +/- 59 ml.sec-1, respectively. When end-inspiratory occlusion preceded rapid chest compression, V50 and V75 increased to 283 +/- 114 and 206 +/- 61 ml.sec-1 respectively, with a mean increase in V50 of 34% and in V75 of 31%. When expiratory clamping preceded the compression, even higher expiratory flows resulted.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between PaO2 and lung volume during high frequency oscillatory ventilation

The relationship between oxygenation and lung volume during high frequency oscillatory ventilation (HFOV) was studied. We ventilated anesthetized, tracheostomized adult rabbits that were rendered surfactant-deficient by lung lavage. Lung volume was measured by the 'disconnection technique'. In the first experiment, HFOV was commenced after conventional mechanical ventilation (CMV) for 1 hr. In the absence of sustained inflation (SI), oxygenation improved with time during HFOV. In the second experiment, HFOV was instituted after CMV for 4 hr. In the absence of SI, all animals expired during the experimental period. In the third experiment we ventilated rabbits for 4 hr and then switched to HFOV. We applied SI first and increased mean airway pressure (MAP) by increments of 2 cmH2O every 15 min. However, there was little improvement in PaO2 despite the use of repeated SI and the increase in MAP. We conclude that oxygenation has a linear relationship to lung volume during HFOV, and that secondary lung injury due to long-term CMV impairs the response to HFOV. Therefore, it is important to minimize the risk of such secondary injuries before instituting HFOV.

Pulmonary hypertension after postlavage lung injury in rabbits: possible role of polymorphonuclear leukocytes

Previous studies showed that repeated lung lavage leads to a severe lung injury with very poor gas exchange, a substantial protein leak into the alveoli with hyaline membrane formation, pulmonary hypertension, and migration of granulocytes (PMN) into the alveolar spaces. Depletion of PMN leads to a better gas exchange and a markedly decreased protein leak with only scanty hyaline membranes. In this study we show that there is sustained pulmonary hypertension after the lung lavage, but in PMN-depleted rabbits there is no postlavage increase in pulmonary arterial pressure. Changing the shunt fraction by manipulating mean airway pressure still leads to a hypoxic vasoconstriction with increase of pulmonary arterial pressure. Thus, after lung lavage, pulmonary reactivity to hypoxia is still preserved. Comparisons between high-frequency ventilation and conventional mechanical ventilation at the same mean airway pressures showed that equal mean airway pressure in these two very different modes of ventilation do not translate into the same mean functional lung volumes.

Lung injury in a surfactant-deficient lung is modified by indomethacin

Repetitive total lung lavage in adult rabbits leads to a reproducible severe surfactant-deficient lung injury. Hypoxemia requiring mechanical ventilation occurs, accompanied by a substantial pulmonary hypertension, a large intra-alveolar protein leak, peripheral neutropenia, and pathological features of marked neutrophil infiltration with extensive hyaline membrane formation. Pretreatment with indomethacin abolishes postlavage pulmonary hypertension, preserves a slightly better lung function with higher arterial PO2, and prevents the postlavage peripheral neutropenia found in untreated animals. Pretreatment with a thromboxane A2 receptor blocker (L 655,240, Merck Frosst, Canada) also completely attenuated pulmonary hypertension, providing evidence that thromboxane A2 mediates pulmonary arterial hypertension after lung lavage. However, specific thromboxane receptor blockade had no other long-lasting beneficial effects on the ongoing injury in this model.

Sequential pulmonary function measurements during treatment of infantile chronic interstitial pneumonitis

Three infants with histologically confirmed chronic interstitial pneumonitis were treated with monthly intravenously administered high doses of methylprednisolone with or without daily hydroxychloroquine therapy. We applied the multiple occlusion technique to measure the static respiratory system compliance, and the end-inspiratory occlusion technique to measure passive respiratory system compliance, resistance, and time constant. When assessed by clinical criteria and pulmonary function measurements, all three patients showed improvement with this treatment. Clinical improvement was associated with an increase in respiratory system compliance as measured by both techniques (60% to 100% increase in all patients). The passive respiratory resistance and the time constant did not closely reflect the clinical course. We conclude (1) that high doses (pulses) of methylprednisolone and daily oral doses of hydroxychloroquine are effective in the treatment of infantile chronic interstitial pneumonitis and (2) that the respiratory system compliance, measured by both pulmonary function techniques, correlates well with the response to treatment and change in clinical status.

A model of the maturation of respiratory control in the newborn infant

A model of automatic neonatal respiratory control has been constructed as an aid in the investigation of a possible maturation in respiratory control loops during the newborn period. The primary objective was to provide a framework for investigating this hypothesis without the need for external stimuli or invasive measurements. Spontaneous sighs provide a physiological disturbance to the respiratory system by transiently altering the levels of the blood gases. The dynamic ventilatory response following such a disturbance was modeled. A change from a highly damped to less damped pattern was found when model parameter values were varied to mimic maturation in the neonatal period. A perturbation model analysis demonstrated the dynamic ventilatory response is most sensitive to factors affecting the gain of the peripheral chemoreflex loop. It is concluded that the model provides valuable insight into the hypothesis that the peripheral chemoreflex matures during the neonatal period and provides a viable method for testing this in the human infant.

Comparison of diaphragmatic fatigue in newborn and older rabbits

The ability to maintain occlusion pressure (i.e., fatigability) during activation of the diaphragm via phrenic nerve stimulation was compared in newborn (less than 14 days old) and older (greater than 30 days old) rabbits. The younger animals had lower maximum inspiratory pressures (MIP) and markedly greater falls in pressure during sustained diaphragmatic contractions at greater than 40% MIP than did the older animals. Histological analysis showed a paucity of high-oxidative type I fibers in the diaphragms of the young animals. We therefore conclude that the newborn rabbit diaphragm is extremely susceptible to fatigue and that this susceptibility correlates with the distribution of muscle fiber types.

Effect of surgical repair on respiratory mechanics in congenital diaphragmatic hernia

To determine whether surgical repair of congenital diaphragmatic hernia (CHD) results in improvement in respiratory mechanics, we measured respiratory system compliance in nine patients (five survivors and four nonsurvivors) before and after operation. In all nine infants, CHD was diagnosed within 6 hours of life, and surgical repair was through an abdominal approach after a period of stabilization. Measurements were made noninvasively, using the passive expiratory flow-volume technique. In only one of the nine infants did compliance immediately improve after surgical repair, and in another it showed no change. Both of these infants survived, with an uneventful postoperative course. In the remaining seven infants, however, postoperative compliance immediately decreased to 10% to 77% from the preoperative value. The four infants with more than 50% decrease in compliance died with increasing hypoxemia and acidosis. These results suggest that respiratory mechanics in CHD, far from improving, frequently deteriorate as a result of repair of the hernia. The role of urgent surgery in this malformation should be reevaluated.

Reduction of platelet counts induced by mechanical ventilation in newborn infants

The association between platelet counts and mechanical ventilation was assessed in 61 newborn infants with respiratory distress syndrome, 10 infants with congenital diaphragmatic hernia, and 10 infants with tracheoesophageal fistula. A significant decrease in platelet counts was observed during mechanical ventilation: (mean +/- SD) reduction of 39% +/- 5%, 42% +/- 5.6%, and 11.9% +/- 5.4% in the three groups, respectively, independent of other causes. In the group with congenital diaphragmatic hernia, there was a significant correlation between mean airway pressure and the reduction in platelet counts. In a subsequent series of experiments, platelet counts were recorded before and during ventilation in rabbits. A significant mean decrease of 37.3% in platelet counts was associated with ventilation with either air or pure oxygen. Results of these studies indicate that mechanical ventilation itself may cause a major decrease in platelet count in newborn infants.

Comparison of methods of measurement of compliance of the respiratory system in children

Compliance of the respiratory system (Crs) was compared using 2 previously described methods, namely, the passive expiratory flow and multiple occlusion techniques. These 2 methods have the advantage of being totally noninvasive, but both have limitations precluding their use in some circumstances. The results of this study of 10 intubated patients, who varied widely in age (premature newborn to adolescence) and diseases, showed no significant difference between the Crs obtained by the 2 methods. The Crs measurements can therefore be used interchangeably, the choice of technique being dependent on the subjects' clinical state.

Tidal volumes required to maintain isocapnia at frequencies from 3 to 30 Hz in the dog

We ventilated seven mongrel dogs with high frequency oscillatory ventilation (HFO) at frequencies from 3 to 30 Hz. At each frequency, the tidal volume required to achieve isocapnia (VTiso) was measured by plethysmography. In an individual dog, VTiso could be related to frequency by an equation of the form VTiso = KfA. A was similar for the seven dogs, A = -0.54 +/- 0.03. K varied from 179 to 325 cc- Hz-A, reflecting differences between the dogs in dead space, CO2 production and gradient for gas transport. This relation is consistent with that predicted from the data of previous investigators who used different ventilators, circuits and methodology. From 3 to 30 Hz, VTiso decreased from 1.2 to 0.4 times anatomic plus equipment dead space, but minute volumes required to maintain isocapnia increased from 6 to 18 times those required during conventional ventilation. We conclude that low tidal volume ventilation is also high minute volume ventilation.

Diaphragmatic work of breathing in premature human infants

Present methods of assessing the work of breathing in human infants do not account for the added load when intercostal muscle activity is lost and rib cage distortion occurs. We have developed a technique for assessing diaphragmatic work in this circumstance utilizing measurements of transdiaphragmatic pressure and abdominal volume displacement. Eleven preterm infants without evidence of lung disease were studied. During periods of minimal rib cage distortion, inspiratory diaphragmatic work averaged 5.9 g X cm X ml-1, increasing to an average of 12.4 g X cm X ml-1 with periods of paradoxical rib cage motion (P less than 0.01). Inspiratory work was strongly correlated with the electrical activity of the diaphragm as measured from its moving time average (P less than 0.05). Assuming a mechanical efficiency of 4% in these infants, the caloric cost of diaphragmatic work may reach 10% of their basal metabolic rate in periods with rib cage distortion. When lung disease is superimposed, the increased metabolic demands of the diaphragm may predispose preterm infants to fatigue and may contribute to a failure to grow.

Effect of granulocyte depletion in a ventilated surfactant-depleted lung

In a previous paper Cutz, Bryan et al. showed that in rabbits after repetitive lung lavage high-frequency oscillatory ventilation maintained excellent gas exchange and did not cause hyaline membrane formation (J. Appl. Physiol. 55: 131-138, 1983). In contrast, conventional mechanical ventilation had poor gas exchange and extensive hyaline membrane formation and we attributed these differences to mechanical barotrauma. However, we completely overlooked the large number of granulocytes in the damaged lung. To investigate this using the same model we have used mechanical ventilation on two groups of rabbits, one with normal granulocytes, the other depleted of granulocytes by pretreatment with nitrogen mustard. The nondepleted rabbits had poor gas exchange, a substantial protein leak into the lung and extensive hyaline membranes. The depleted animals had good gas exchange, a very small protein leak and no hyaline membranes. Repletion of granulocytes from donor rabbits lead to poor gas exchange and hyaline membrane formation. It is concluded that lung lavage causes prompt margination of granulocytes which become activated by the ongoing epithelial barotrauma of conventional ventilation.

Long volume during high frequency oscillation

Previous investigators have reported alarming gas trapping during high frequency oscillation. We have assessed trapping by measuring the difference between occlusion pressure (alveolar pressure) and measured maximal mean airway circuit pressure. Under normal operating conditions, we were unable to demonstrate trapping in normal or lung-lavaged rabbits at 15 or at 25 Hz, or in infants with infant respiratory distress syndrome, but we could produce trapping by operating at unusually low mean airway pressure. At these low pressures, lung volume is low, and we believe that choke points develop that limit maximal expiratory flow.

High-frequency oscillatory ventilation and pulmonary extravascular water

The lungs of anesthetized rabbits were embolized by intravenous injection of 75 mg/kg of starch particles in divided doses. One group received high-frequency oscillatory ventilation (HFOV) at a rate of 15 Hz. Both groups were ventilated with an FIO2 of 1.0. After 3 hr of ventilation, the animals were sacrificed, and the extravascular lung water (EVLW) was determined gravimetrically. With conventional mechanical ventilation (CMV), the EVLW/body weight ratio was 4.1 +/- 1.4 g/kg (mean +/- SD) and was significantly higher than the ratio of 2.8 +/- 0.6 in animals given HFOV (P less than 0.05). In a second series of animals, 20 muCi of 125I-labeled bovine albumin was injected intravenously prior to embolization and followed by the same two types of ventilation. After the rabbits were sacrificed, the lungs were lavaged with 0.9 N saline, and the radioactivity of the fluid was measured to determine the protein leak. After CMV, 2.76 +/- 2.05% of the injected radioactivity was recovered from the fluid, significantly higher (P less than 0.01) than the amount of radioactivity recovered after HFOV, 0.3 +/- 0.28%. There also was significantly more labeled protein in the lung tissue that had been on CMV. We conclude that in this model of pulmonary edema, HFOV significantly reduces both the amount of fluid and protein leaking into the lung.

State-related incidence of spontaneous breathing during high frequency ventilation

Previous reports indicate that apnea is induced by high frequency ventilation (HFV) in anesthetized animals. We have studied conscious dogs ventilated with HFV during sleep and wakefulness to determine if this apnea is state related. Eucapnic apnea or very low breathing frequencies were observed during quiet wakefulness, quiet sleep and tonic REM sleep. However, spontaneous breathing at nearly normal rates was invariably seen during phasic REM sleep. We conclude that HFV as administered induces apnea or near apnea in conscious animals and that the inhibitory pathways causing this apnea are probably disabled during phasic REM sleep.

Neuroendocrine cells in the developing human lung: morphologic and functional considerations

The structure, distribution, and frequency of neuroendocrine (NE) cells in human fetal lung from early stages of development to term are described. Neuroendocrine cells were studied by electron microscopy and immunostaining for serotonin and bombesin, recently identified markers of these cells in human lung. The differentiation of NE cells within the airway epithelium proceeded centrifugally and followed the development of the bronchial tree. The first NE cells, identified at 8 weeks' gestation, appeared well-differentiated compared with adjacent epithelial cells, and were immunoreactive for serotonin. The first bombesin-immunoreactive cells were detected at 10 weeks' gestation. Fetal lungs from midgestation contained several ultrastructurally distinct NE cell types, distributed singly and in groups. Serotonin-immunoreactive cells were more frequent during early stages of development and were predominantly located in larger airways. Bombesin-immunoreactive cells became more numerous towards term and were concentrated in small peripheral airways. The well-differentiated appearance and large number of NE cells in fetal lung, and their increase in number towards term, suggest an important role for these cells during intrauterine life and neonatal adaptation. Whether this role involves neurohormonal regulation of fetal-neonatal pulmonary circulation or local (paracrine) or endocrine function requires further investigation.

Sudden infant death syndrome: increased carotid-body dopamine and noradrenaline content

Carotid bodies from infants dying of sudden infant death syndrome contained significantly higher concentrations of dopamine (10-fold) and noradrenaline (3-fold) than those from age-matched control infants. Administration of dopamine inhibits respiration by direct action on the carotid body, and it is suggested that the elevated levels of endogenous catecholamines found in victims of sudden infant death syndrome may compromise the normal function of the carotid body, particularly the ventilatory response to hypoxia.

Historic and future development of high-frequency ventilation

High-frequency ventilation (HFV) has not one but several histories. Proceeding along largely independent pathways are techniques using frequencies of 1-5 Hz and techniques using 10-40 Hz, the former being introduced by Sjostrand in 1971, the latter by Lunkenheimer in 1972. It was nearly ten years before it was recognized that these techniques must radically alter our concepts of gas transport within the lung. There has also been an unfortunate series of clinical studies purporting to show that HFV is superior to conventional ventilation in patients with lung disease. There is no doubt that nearly all reports show that HFV controls PaCO2 very easily, and that the mechanisms by which this is achieved is the primary topic of the symposium. What has not been proved is that HFV is better than conventional ventilation in increasing PaO2. The mechanisms of oxygen exchange when the lung has extensive shunts are quite different from those for CO2 exchange, and this problem has not been rigorously addressed.

Ultrastructure of carotid bodies in sudden infant death syndrome

Recent studies have implicated an abnormality in carotid body structure and function in the pathogenesis of sudden infant death syndrome (SIDS). In the present investigation, the light and electron microscopic findings in carotid bodies from ten victims of SIDS were compared with those in six control infants and five infants dying of congenital heart disease. The cross-sectional area of carotid body chemoreceptor cells and the frequency, distribution, and size of neurosecretory granules were assessed morphometrically. The area of carotid body occupied by chemoreceptor cells (the functional area) was comparable in SIDS victims, control infants, and infants with congenital heart disease. By electron microscopy, the carotid body chief cells from all groups contained numerous electron-dense neurosecretory granules. Distribution, frequency, and size of neurosecretory granules in SIDS victims and control infants did not differ significantly. Morphology of carotid bodies from SIDS victims was found to be normal. The presence of neurosecretory granules in chemoreceptor cells of SIDS victims suggests that the cellular mechanism of neurotransmitter synthesis and storage is not altered.

Rib cage and abdominal contributions to ventilatory response to CO2 in infants

We have measured the ventilatory response to inhaled CO2 of six newborn infants in rapid-eye-movement (REM) and non-REM (NREM) sleep. Ventilatory responses were measured using the Read rebreathing technique. The response was further partitioned into the volume contributions of the rib cage and abdominal compartment using the respiratory inductance plethysmograph. Sleep state was defined by electroencephalogram, electrooculogram, and behavioral criteria. In NREM sleep, there was a highly significant linear correlation between both tidal volume (VT) and instantaneous minute ventilation (VI) with CO2. Among infants, the slope of VT varied from 1.0 to 0.34 ml X Torr-1 X kg-1. However, these differences were largely due to differences in rib cage contribution, which varied from 0.56 to -0.08 ml X Torr-1 X kg-1. The abdominal contribution was similar among infants (0.41-0.56 ml X Torr-1 X kg-1). In REM, the slopes of VI were less steep than in NREM, with greater breath-to-breath variability. Slopes of VT also tended to be lower. The abdominal responses were similar to those in NREM, whereas the rib cage response was low and negative in three studies. We conclude that the slope of the CO2 response curve is primarily determined by the extent of rib cage recruitment.

Passive respiratory mechanics in newborns and children

When the Hering-Breuer reflex is used to relax the respiratory muscles, the passive compliance (Crs), resistance (Rrs), and time constant (Trs) of the respiratory system can be measured from the subsequent expiration. We used this method to assess 22 newborns with respiratory illness. Passive expirations were also recorded in 6 paralyzed, ventilated children. Using a simple slide valve, airway occlusion pressure was measured from a face mask or endotracheal tube, and expiratory flow was measured from a pneumotachygraph. In all subjects, there was a linear function of expiratory flow versus its integral, volume; by extrapolating the linear function to zero flow and zero volume (i.e., initial volume at time zero) Crs, Rrs, and Trs could be calculated; Crs was significantly reduced in mechanically ventilated versus spontaneously breathing infants, and Rrs was significantly higher in intubated versus extubated infants. During passive expiration in newborns, inspiration occurred at a volume above passive functional residual capacity. The passive expiratory flow-volume technique is simple, noninvasive, and appears to provide accurate measurements of respiratory mechanics in the newborn and in paralyzed children.

Upper airway morphology in patients with idiopathic obstructive sleep apnea

Fundamental to the pathogenesis of obstructive sleep apnea (OSA) is the interaction of physiologic and anatomic alterations of the upper airway. However, many patients with OSA have no identifiable abnormality of the upper airway, and they have been termed idiopathic. In an attempt to find a structural deviation in upper airway anatomy, we performed acoustic echography and cephalometric roentgenograms in 9 male patients with OSA and no clinical evidence of upper airway abnormality. Mean cross-sectional area of the pharynx by acoustic reflection was less in these patients (3.7 +/- 0.8 cm2) than in subjects in a control group (5.3 +/- 0.6 cm2) (p less than 0.001). Mean glottic cross-sectional area was less in the patient group (1.5 +/- 0.5 cm2) than in the control group (2.7 +/- 0.5) (p less than 0.001). There was a significant correlation between the number of apneas per sleep hour and pharyngeal cross-sectional area (r = 0.87, p less than 0.01). Cephalometric analysis indicated that the patients had smaller mandibles by a mean of 5.4 +/- 6.6 mm (p less than 0.05). The overall posterior displacement of the mandibular symphysis, which is representative of the skeletal support of the anterior pharyngeal wall and is dependent on both mandibular size and position, was highly significant (6.4 +/- 4.7 mm) (p less than 0.01). Furthermore, there was a significant correlation between the number of apnea episodes per sleep hour and the total posterior displacement (r = 0.67, p less than 0.05). This study indicates that patients with so-called idiopathic OSA may have an anatomic predisposition to the development of upper airway occlusion that may not be detectable on clinical examination.

Total resistance of the respiratory system in preterm infants with and without an endotracheal tube

The passive compliance and resistance of the respiratory system were measured in 12 spontaneously breathing newborn infants before and after endotracheal extubation. End-inspiratory airway occlusions were used to relax the respiratory muscles, allowing occlusion pressure to be measured and respiratory system compliance and resistance to be calculated from the flow volume relationship of the subsequent passive expiration. Airway pressure was measured from an endotracheal tube or a face mask, expiratory flow from a pneumotachograph, and expiratory volume from the integrated flow signal. In six of the infants, diaphragmatic electromyography was also performed before and after extubation. Resistance and EMG findings were both decreased by extubation (mean decrease 43.9%, P less than 0.001 and 27.3%, P less than 0.05, respectively), but compliance was unchanged. Thus, by substantially increasing resistance, an endotracheal tube causes the diaphragm to increase its activity to maintain ventilation.

Neuromuscular blockade enhances phrenic nerve activity during high-frequency ventilation

Phrenic nerve activity was monitored in anesthetized cats during high-frequency ventilation (HFV). Rhythmic phrenic discharge disappeared during HFV in all animals at normal arterial PCO2 levels. Rhythmic activity returned after neuromuscular blockade in the vagally intact animal. Although vagotomy alone also restored phrenic discharge, this activity was further enhanced by subsequent neuromuscular blockade. Therefore we suggest that apnea during HFV results from inspiratory inhibition mediated by both chest wall and vagal afferent mechanisms.

Effect of chest wall distortion on occlusion pressure and the preterm diaphragm

We studied the effect of chest wall distortion (CWD) on transdiaphragmatic pressure (Pdi) and/or mouth pressure during end-expiratory airway occlusions in seven preterm infants. We measured mouth occlusion pressure (Pmo) with a face mask and pressure transducer, gastric pressure (Pga) with a fluid-filled catheter, diaphragmatic electromyogram (Edi) using surface electrodes, and rib cage and abdominal motion using magnetometers. We reasoned that Pdi = Pmo - Pga on airway occlusion. Periods with maximal and periods with minimal CWD were compared. We found that 1) when CWD was minimal, an increase in Edi produced an increase in Pmo and Pdi in all infants; when CWD was greatest, large increases in Edi produced no increase in Pmo or Pdi in four infants; 2) when breaths with the same Pmo or Pdi from each period in each infant were compared, those from the period with greatest CWD had an increased Edi (mean increase 76%, P less than 0.005, and 144%, P less than 0.01, for Pmo and Pdi, respectively). We conclude that in preterm infants, Pmo can be a poor indicator of respiratory drive, and CWD markedly limits the effectiveness of the diaphragm as a force generator.

Influence of chest wall distortion on esophageal pressure

The caudocephalad profile of esophageal pressure swings was studied in 10 preterm and 5 full-term infants, and the effect of chest wall distortion on esophageal pressure swings was analyzed in 12 preterm infants. Esophageal pressure was measured with a fluid-filled catheter, tidal volume with a pneumotachograph, mouth pressure with a face mask and pressure transducer, and rib cage and abdominal motion with magnetometers. In preterm infants the profile of esophageal pressure swings fell very steeply in the caudocephalad direction. In full-term infants it was flat during quiet sleep and steep during rapid-eye-movement sleep. When breaths, standardized for pleural pressure, were compared between a period with maximal and a period with minimal chest wall distortion, esophageal pressure swings for both spontaneous and occluded breaths were higher in the former period. We conclude that the complaint preterm rib cage results in an uneven distribution of pleural pressure and that this distribution varies with changes in chest wall distortion. Esophageal pressure measurements are therefore an unreliable estimate of mean pleural pressure in the preterm infant and can be unreliable in the term infant.

Myelination of the human vagus nerve from 24 weeks postconceptional age to adolescence

Significant changes in respiratory reflexes occur with maturation. The vagus nerve, the pathway for the Hering-Breuer and irritant-receptor reflexes, was studied quantitatively in 33 infants and 5 adolescents. In the infants, total myelinated vagus fibers increased linearly (r m0.682, p less than 0.001) with postconceptional age (PCA), and by 40 weeks after conception, total counts were comparable to those of adolescent group. Counts of total myelinated vagus fibers in 16 term infants (greater than 41 weeks PCA) were comparable to those in the adolescent group (p less than 0.40), whereas 17 preterm infants (less than 38 weeks PCA) showed significantly fewer total myelinated vagus fibers than term or adolescent groups (p less than 0.001). Smaller-diameter (less than 2 micrometer) myelinated vagus fibers depended upon PCA in the preterm group (p less than 0.005), but were independent of PCA in the term group (p less than 0.5). Preterm infants have a higher percentage of small to total myelinated vagus fibers than term infants (p less than 0.1).

Oxygenation during high-frequency ventilation compared with conventional mechanical ventilation in two models of lung injury

Oxygenation and mean lung volume were investigated during high frequency oscillation (HFO) and conventional mechanical ventilation (CMV) in two models of lung disease and related to the lung mechanics of the lesions. Oleic acid (n = 10) or lung lavage (n = 12) pulmonary injury was induced in a series of rabbits. Each animal was alternately ventilated with HFO (15 Hz sinusoidal wave form) and CMV (flow generator I:E, 1:2; f, 30 breaths/min; VT, 10 to 15 ml/kg) at matched mean airway pressure. Pao2 was measured 5 minutes after onset of ventilation. In the lung lavage model Pao2 was significantly greater during HFO than CMV (Pao2 228 +/- 116 torr vs 71 +/- 42 torr) provided that mean airway pressure was greater than the distinct opening pressure characteristic of this lesion. In the oleic acid model oxygenation was again superior during HFO (Pao2 269 +/- 116 torr vs 110 +/- 83 torr), but only if HFO was preceded by a sustained inflation. Plethysmography in a subset of six rabbits from each group revealed that the improvements in oxygenation were associated with significantly higher mean lung volumes during HFO than CMV (58 +/- 30 ml vs 29 +/- 14 ml lung lavage model, 45 +/- 15 ml vs 30.9 +/- 13 ml on the oleic acid model). The importance of a sustained inflation in rapidly optimizing gas exchange during HFO but not CMV was demonstrated. A sustained inflation resulted in immediate and sustained increases in Pao2 (from 134 +/- 102 torr to 274 +/0 124 torr in the oleic acid model; from 115 +/- 105 torr to 291 +/- 143 torr in the lung lavage model) and mean lung volume (41.8 +/- 11 to 53.8 +/- 9.7 ml in the oleic acid model, 30.9 +/- 7.7 ml to 42.8 +/- 5 ml in the lung lavage model). It is suggested that in these two particular models of lung disease, HFO, when combined with a sustained inflation (to provide opening forces), can more fully exploit the pressure volume hysteresis of unstable lung units than CMV, thereby resulting in the larger mean lung volumes and better oxygenation observed during HFO.

High-frequency oscillation compared with standard ventilation in pulmonary injury model

Hemorrhagic pulmonary edema was induced by intra-atrial infusion of 0.04--0.1 ml/kg of oleic acid into six anesthetized dogs. Gas exchange and cardiac outputs were then compared at identical mean airway pressures during randomized ventilation with either a volume-cycled ventilator with positive end-expiratory pressure (conventional positive-pressure ventilation, tidal volume 16--21 ml/kg, frequency 15--20 cycles/min) or a variable volume piston pump operating at 15 Hz (high-frequency oscillation). The fractional inspired oxygen concentration was maintained at 0.5 throughout. During 17 data sets matched for intratracheal mean airway pressures over a range of 7.5--27 cmH2O, measurements of systemic arterial pressure, arterial blood gas tensions, thermodilution cardiac outputs, and pulmonary arterial and capillary wedge pressures were identical (P less than 0.05) during ventilation with conventional positive-pressure ventilation and high-frequency oscillation. With both forms of ventilation, arterial oxygen tension progressively improved as mean airway pressure increased. In a shunt model of acute lung injury we were unable to show significant differences in oxygenation or cardiac output when high-frequency oscillation was compared with conventional positive-pressure ventilation with positive end-expiratory pressure at equivalent mean airway pressures.

Vagotomy reverses apnea induced by high-frequency oscillatory ventilation

Apnea has been observed in both animals and patients during high-frequency oscillatory ventilation. The effects of vagotomy were studied during periods of oscillator-induced apnea in 11 pentobarbital-anesthetized dogs. The animals were intubated and breathing spontaneously. An arterial cannula was inserted for monitoring blood pressure and blood gases. Intratracheal airway pressure was measured, and respiratory activity was assessed using either an intrapleural catheter or esophageal balloon. The dogs then underwent high-frequency ventilation at 15 Hz. Apnea was induced by appropriate selection of volume displacement of the piston pump and the distal airway pressure in eucapnic animals. Segments of right and left vagus nerves were exposed in the neck, bathed in local anesthetic, and transected. Spontaneous ventilation resumed immediately in nine animals and could not be suppressed at the same CO2 partial pressure despite continuation of oscillation. We conclude that the apnea observed during high-frequency ventilation is mediated by active vagal inhibition of central respiratory activity and is usually reversed by vagotomy.

Importance of inspiratory muscle tone in maintenance of FRC in the newborn

The importance of inspiratory muscle tone in the maintenance of functional residual capacity (FRC) in newborns was studied in eight premature infants with birth weights of 1,166 +/- 217 g and gestational age 29 +/- 1.9 wk (mean +/- SD). Rib cage and abdominal anteroposterior diameters were monitored with magnetometers, and electromyograms of the diaphragm and intercostal muscles were recorded with surface electrodes. Sleep state was monitored using electrooculogram and behavioral criteria. We assessed the decrease in tonic activity of the inspiratory muscles and the fall in end-expiratory lung volume during apnea compared with the period just preceding apnea. A total of 98 apneas were analyzed. In all instances a decrease in diaphragmatic and intercostal tone was associated with a decrease in the anteroposterior diameter of both rib cage and abdomen, indicating a fall in FRC. These changes were more marked during quiet sleep than during rapid-eye-movement sleep (P less than 0.01). Our results suggest that inspiratory muscle tone is a major determinant of FRC in the newborn.