Interaction between chemical factors and duration of apnea following lung inflation

Impact of cervical spinal cord contusion on the breathing pattern across the sleep-wake cycle in the rat

The present study was designed to investigate breathing patterns across the sleep-wake state following a high cervical spinal injury in rats. The breathing patterns (e.g., respiratory frequency, tidal volume, and minute ventilation), neck electromyogram, and electroencephalography of unanesthetized adult male rats were measured at the acute (i.e., 1 day), subchronic (i.e., 2 wk), and/or chronic (i.e., 6 wk) injured stages after unilateral contusion of the second cervical spinal cord. Cervical spinal cord injury caused a long-term reduction in the tidal volume but did not influence the sleep-wake cycle duration. The minute ventilation during sleep was usually lower than that during the wake period in uninjured animals due to a decrease in respiratory frequency. However, this sleep-induced reduction in respiratory frequency was not observed in contused animals at the acute injured stage. By contrast, the tidal volume was significantly lower during sleep in contused animals but not uninjured animals from the acute to the chronic injured stage. Moreover, the frequency of sigh and postsigh apnea was elevated in acutely contused animals. These results indicated that high cervical spinal contusion is associated with exacerbated sleep-induced attenuation of the tidal volume and higher occurrence of sleep apnea, which may be detrimental to respiratory functional recovery after cervical spinal cord injury.

NEW & NOTEWORTHY Cervical spinal injury is usually associated with sleep-disordered breathing. The present study investigated breathing patterns across sleep-wake state following cervical spinal injury in the rat. Unilateral cervical spinal contusion significantly impacted sleep-induced alteration of breathing patterns, showing a blunted frequency response and exacerbated attenuated tidal volume and occurrence of sleep apnea. The result enables us to investigate effects of cervical spinal injury on the pathogenesis of sleep-disordered breathing and evaluate potential therapies to improve respiration.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Impact of the vagal feedback on cardiorespiratory coupling in anesthetized rats

Cardiorespiratory coupling can be significantly influenced by both pontine and vagal modulation of medullary motor and premotor areas. We investigated influences of the pontine intertrigeminal region (ITR) and peripheral vagal pathways on the coupling between systolic blood pressure (SBP) and respiration in 9 anesthetized rats. Glutamate injection into the ITR perturbed both respiration and SBP and decreased SBP-respiratory coherence (0.95±0.01 vs 0.89±0.02; (p=0.01). Intravenous infusion of serotonin (5-HT) produced apnea and hypertension and also decreased SBP-respiratory coherence (0.95±0.01 vs 0.72±0.06; p=0.04). Bilateral vagotomy eliminated the cardiorespiratory coherence perturbations induced by central (glutamate injection into the ITR: 0.89±0.03 vs 0.86±0.03; p=0.63) and peripheral (5-HT infusion: 0.89±0.03 vs 0.88±0.02; p=0.98) pharmacologic manipulations. Glutamate stimulation of the ITR postvagotomy increased the relative spectral power density of SBP in the respiratory frequency range (0.25±0.08 vs 0.55±0.06; p=0.01). The data suggest that SBP-respiratory coupling is largely mediated within the central nervous system, with vagal systems acting in a way that disrupts coherence during transient cardiorespiratory disturbances. Although decreased cardiorespiratory coherence may increase cardiac work during perturbations, this may be physiologically advantageous in restoring homeostatic equilibrium of respiration and blood pressure.

Contrasting effects of estradiol and progesterone on respiratory pattern and hypoxic ventilatory response in newborn male rats

We tested the hypothesis that postnatal exposure to progesterone or estradiol exerts distinct effects on respiratory control, apnea frequency, and on hypoxic ventilatory response (HVR). To this aim, we assessed breathing pattern using whole body plethysmography in normoxia and during a sustained hypoxic exposure (10% O(2)-30min) in 10-day-old male rats raised by dams implanted with osmotic minipumps delivering either estradiol (E(2), 7.0microgday(-1)), estradiol+progesterone (E(2)+P, 7.0+70microgday(-1)) or vehicle (propylene glycol) at a regular flow rate throughout postnatal days 1-14. Compared to vehicle, E(2) and E(2)+P pups had a reduced ventilation, metabolic rate and rectal temperature. HVR was specifically increased in E(2)+P pups compared to controls and E(2) pups. On the contrary, both E(2) and E(2)+P pups did not reduced metabolism as much as controls during hypoxic exposure, and the decrease in rectal temperature was abolished. Surprisingly, E(2)+P pups showed a dramatic elevation of sigh frequency, while progesterone (in E(2)+P compared to E(2) and Veh pups) reduced apnea frequency. These findings are relevant to better understand the role of placental steroids on respiratory and metabolic control during early development in rats, and could ultimately contribute to a better understanding of specific respiratory control disorders in preterm neonates, which are chronically deprived from placental steroids exposure.

Progesterone increases hypoxic ventilatory response and reduces apneas in newborn rats

We hypothesized that progesterone may enhance the hypoxic ventilatory response and reduce the occurrence of apneas in newborn male rats. We studied 10-day-old rats chronically exposed to progesterone (Prog) or vehicle through the milk of lactating mothers. Respiratory and metabolic recordings were performed using whole body plethysmography under normoxia and during hypoxic exposure (10% O(2)--30 min). While progesterone did not alter baseline breathing and metabolic rate, it increased hypoxic ventilatory response particularly by limiting the magnitude of the ventilatory roll-off during the second phase of the hypoxic ventilatory response (i.e. following 5 min of exposure). In parallel, progesterone lowered the number of spontaneous apneas and drastically reduced the occurrence of post-sigh apneas during hypoxic exposure by limiting the time of the post-sigh expiratory pause. Following domperidone injection (used to block peripheral D2 dopamine receptor), minute ventilation increased in Veh pups and the number of spontaneous apneas decreased. These responses were not observed in Prog pups, suggesting that progesterone reduces peripheral dopaminergic inhibition on breathing. We conclude that progesterone is a potent stimulant of hypoxic ventilatory response in newborn rats and effectively reduces the occurrence of apneas.

Monoaminergic system lesions increase post-sigh respiratory pattern disturbance during sleep in rats

Monoamines are important regulators of behavioral state and respiratory pattern, and the impact of monoaminergic control during sleep is of particular interest for the stability of breathing regulation. The aim of this study was to test the effects of systemically induced chemical lesions to noradrenergic and serotonergic efferent systems, on the expression of sleep-wake states, pontine wave activity, and sleep-related respiratory pattern and its variability. In chronically instrumented male adult Sprague-Dawley rats we lesioned noradrenergic terminal axonal branches by a single intraperitoneal dose of DSP-4 (N-(2-chloroethyl)-N-ethyl-2-brombenzilamine; 50 mg/kg, i.p.), and serotonergic axonal terminals by two intraperitoneal doses, 24 h apart, of PCA (p-chloroamphetamine; 6 mg/kg, i.p.). In each animal, we recorded sleep, pontine waves (P-waves) and breathing at baseline, following sham injection, and every week for 5 weeks following injection of either systemic neurotoxin. Distinct responses were observed to the two lesions. DSP-4 lesions were associated with a trend toward increased NREM sleep (p < 0.06), decreased wakefulness (p < 0.05) and increased respiratory tidal volume during NREM (p = 0.0002) and REM (p = 0.0001) sleep with respect to baseline. None of these effects, however, were observed during the first 14 days after injection. No significant changes were observed in the frequency of apneas or sighs, nor in the coupling between these two, at any time after DSP-4 injection. Conversely, selective serotonergic lesion by PCA produced no change in the baseline respiratory frequency or tidal volume during sleep or wakefulness, nor was the expression of Wake, NREM or REM sleep affected. Instead, PCA injection resulted in a sustained increase in the frequency and duration of post-sigh apneas (PS) during NREM sleep (p = 0.002). This reflected increased coupling between sighs and apneas, because neither the frequency nor the amplitude of spontaneous sighs was altered by PCA.

The effects of positioning on the Hering-Breuer reflex in the preterm infant

The aim of this study was to determine the effect of positioning on the Hering-Breuer inflation reflex (HBIR) in the preterm infant. Seven, non-intubated, premature infants, ranging in birthweight from 732 to 1450 g with post-conceptual ages of 32-36 weeks were studied. In each infant, pulmonary function testing, including the HBIR was obtained using the SensorMedics 2600 during quiet sleep in the supine and prone position. The strength of the HBIR was quantified by the measurement of the percent prolongation of expiration after an occluded breath. Sleep states were categorized by the criteria of Prechtl. There was a significant difference in Hering-Breuer activity in the prone position versus the supine position with a consistently stronger reflex in the prone position. The mean percent prolongation of expiration was 237 +/- 108% in the prone position versus 95 +/- 32% in the supine position. Analysis of the data, using paired t-testing revealed a mean difference of 142 +/- 119% between prone and supine positions (P=0.028). Significant differences in the strength of the HBIR occur in relation to positioning in the preterm infant. Newborn positioning may affect pulmonary reflexes and may play a role in control of breathing.

Influence of temperature on metabolism and breathing during mammalian ontogenesis

A literature survey of the ventilatory responses to changes in ambient temperature (T) in neonatal mammals reveals that, as in adults, the metabolic response to T is the major determining factor. In fact, the newborn's metabolic response to changes in T determines not only the pulmonary ventilation and the breathing pattern, but also the magnitude of the ventilatory responses to chemical stimuli and the intensity of the pulmonary reflexes at different T. The important difference from the adult is that in many neonatal mammals the control of body temperature (T(b)) is poorly developed. Hence, the metabolic response can be more similar to that of an ectothermic, rather than endothermic, animal, and T(b) can vary substantially with T. When hypoxia occurs in cold, T(b) can decrease greatly, because of the hypoxic drop in the thermoregulatory set-point, and this lowers pulmonary ventilation. Hence, in addition to the metabolic response, also the changes in T(b) are a factor modulating the ventilatory responses to T. Artificial warming of the newborn during hypoxia causes heat-dissipation responses that can be counterproductive. During ontogenesis, with prolonged cold conditions, the sustained alterations in metabolic rate and body growth do not modify the postnatal development of the respiratory control mechanisms. Presumably, this indicates that respiratory regulation develops independently from the individual's metabolic history.

Hypothermia and hypoxia inhibit the Hering-Breüer reflex in the marsupial newborn

The effects of lowering body temperature (Tb) on metabolic rate, ventilation, and the strength of the Hering-Breüer expiratory promoting reflex (HB reflex; determined from an inhibitory ratio calculated from volumetric measurements of the respiratory rhythm) were examined in 18-day-old ectothermic pouch young of the tammar wallaby during normoxia or hypoxia (10% O2). Hypoxia and hypothermia, either singularly or combined, depressed metabolic rate. At all Tb, the hypoxic hyperventilation was associated with a significant hyperpnea. At pouch Tb (36.5°C) during normoxia, inflation of the lungs with -5 or -10 cmH2O extrathoracic pressure induced a significant HB reflex. Exposure to cold reduced the strength of the reflex, almost abolishing it at 28°C. For Tb above 28°C, the reflex in hypoxia was always less than the corresponding normoxic value. Taken in context with the changes in metabolic state that occurred, these data in the ectothermic marsupial newborn suggest that the decline in the HB reflex during moderate hypothermia is the result of a direct effect of Tb on vagal mechanisms rather than a temperature-driven decline in metabolic rate that should have acted to strengthen the HB reflex. Therefore, it seems that inputs inhibitory to breathing are more negatively affected during cold than those inputs that are excitatory.

Prolonged neural expiratory time induced by mechanical ventilation in infants

Mechanical ventilation may interfere with the spontaneous breathing pattern in infants because they have strong reflexes that play a large role in the control of breathing. This study aimed to answer the following questions: does a ventilator-assisted breath 1) reduce neural inspiratory time, 2) reduce the amplitude of the diaphragm electrical activity, and 3) prolong neural expiration, within the delivered breath? In 14 infants recovering from acute respiratory failure (mean age and weight were 2.3 +/- 1.3 mo and 3.95 +/- 0.82 kg, respectively), we measured 1) the electrical activity of the diaphragm with a multiple-array esophageal electrode, and 2) airway opening pressure, while patients breathed on synchronized intermittent mandatory ventilation (mean rate, 11.2 +/- 6.5 breaths/min). We compared neural inspiratory and expiratory times for the mandatory breaths and for the spontaneous breaths immediately preceding and following the mandatory breath. Although neural inspiratory time was not different between mandatory and spontaneous breaths, neural expiratory time was significantly increased (p < 0.001) for the mandatory breaths (953 +/- 449 ms) compared with the premandatory and postmandatory spontaneous breaths (607 +/- 268 ms and 560 +/- 227 ms, respectively). Delivery of the mandatory breath resulted in a reduction in neural respiratory frequency by 28.6 +/- 6.4% from the spontaneous premandatory frequency. The magnitude of inspiratory electrical activity of the diaphragm was similar for all three breath conditions. For the mandatory breaths, ventilatory assist persisted for 507 +/- 169 ms after the end of neural inspiratory time. Infant-ventilator asynchrony (both inspiratory and expiratory asynchrony) was present in every mandatory breath and constituted 53.4 +/- 26.2% of the total breath duration.

Hering-Breuer reflex and sleep state in the preterm infant

The aim of this study was to determine the effect of sleep state on the Hering-Breuer inflation reflex in the preterm infant. Seventeen nonintubated, premature infants, ranging in birth weight from 980-2,440 g with postconceptual ages of 30-36 weeks, were studied. In each infant, pulmonary function testing, including the Hering-Breuer inflation reflex, was obtained using the SensorMedics 2600 during active and quiet sleep states in supine position. The strength of the Hering-Breuer inflation reflex was quantified by the measurement of the percent prolongation of expiration after an occluded breath. Sleep states were categorized by the criteria of Prechtl. There was a significant difference in Hering-Breuer activity in active (REM) vs. quiet (non-REM) sleep, with a consistently stronger reflex in the active sleep state. The mean percent prolongation of expiration was 419% in active sleep vs. 87% in quiet sleep. Analysis of the data, using a paired t-test, revealed a mean difference of 331 +/- 185% between active and quiet sleep (P = 0.000). In conclusion, significant differences in the strength of the Hering-Breuer inflation reflex occur in relation to sleep state, and may explain the variability of the reflex described in previous studies. Measurement of the Hering-Breuer inflation reflex may be affected by pulmonary stretch receptors as well as chest wall afferents in the preterm infant.

Ventilation and metabolic rate in the platypus: insights into the evolution of the mammalian breathing pattern

The platypus (Ornithorhyncus anatinus) is characterized by a rate of oxygen consumption (V(O2))that is higher than that reported for other similar sized monotremes, similar to marsupials and somewhat lower than eutherians. The platypus is also characterized by a breathing pattern, more typical of a diving mammal, with a high 'inspiratory drive' and a post-inspiratory pause. Further, the platypus reveals an attenuated hyperventilatory response to hypoxia and a reduced hyperpnoea to hypercapnia; such a response to these chemical stimuli is commonly observed in semi-fossorial and diving mammals. Nevertheless, under conditions of normoxia, ventilation (V(E))is matched to (V(O(2)) such that the convection requirement (V(E)/V(O2)) is similar to that reported for other mammals (approx. 37). The apparent consistency of the convection requirement in mammals suggests the blueprint for the design of the mammalian respiratory system has remained an interspecies constant in the three divergent extant sub-classes of mammals.

Selective vagal afferent dysfunction in dogs with congenital idiopathic megaoesophagus

Congenital idiopathic megaoesophagus (CIM) is a rare, naturally occurring disorder of the dog that is characterised by deficient motility and dilatation of the oesophagus. Recent studies indicate that the vagal sensory system mediating reflexes induced by oesophageal distension is defective in, and may underlie the pathomechanism of this disorder. We sought to establish whether other distension sensitive vagal afferent systems were impaired in CIM, or whether the vagal afferent dysfunction was selective. Thus, we examined the Hering-Breuer lung inflation reflex (HBR), which is subserved by a contiguous and physiologically similar vagal afferent system, in five dogs with CIM in which oesophageal vagal afferent dysfunction had been demonstrated. At varying levels of lung inflation, we found the HBR to be normally graded and of normal strength in affected dogs and that this result was unlikely to be influenced by other factors known to alter the strength of the reflex. These observations provide evidence for an organ specific, selective vagal afferent dysfunction in dogs with CIM. It is possible that similar processes may be active in disorders of visceral organ systems subserved by vagal afferents in other species, including man.

Response of ventilator-dependent patients to delayed opening of exhalation valve

In conventional mechanical ventilation, the inflation cycle often extends into neural expiration (TEN), potentially exacerbating dynamic hyperinflation (DH). We wished to determine the extent to which patients defend against DH when this happens. Such defense may include prolongation of TEN (timing response) and/or expiratory muscle recruitment (neuromuscular response). Fifty patients were ventilated in the Proportional Assist mode, allowing us to infer these responses noninvasively. At random intervals, exhalation of single breaths was delayed by briefly delaying the opening of exhalation valve (occlusion) (T(occ) = 0.78 +/- 0.34 seconds). Timing response was assessed from the change in TEN. Neuromuscular response was assessed from the difference between volume exhaled after release of occlusion and volume exhaled in unoccluded breaths over a similar expiratory flow duration (DeltaV(iso-time)). There was no evidence of an acute neuromuscular response; DeltaV(iso-time) averaged 0.005 +/- 0.023 L (NS). Forty-five of 50 patients significantly lengthened TEN. However, the timing response offset only 36 +/- 20% of the delay in expiration. Because of absent neuromuscular responses and weak timing responses, DH increased in most patients in postocclusion breaths (DeltaDH = 0.10 +/- 0.08 L, p = 2E-10). We conclude that acute compensatory responses to delays in opening of exhalation value are weak in ventilator-dependent patients. As a result, such nonsynchrony tends to exacerbate DH.

A method for measuring passive elastance during proportional assist ventilation

There are currently no reliable, noninvasive ways to monitor respiratory elastance (E) during assisted ventilation. We describe a method that is suited for proportional assist ventilation (PAV). In this mode, the end of the ventilator's inflation phase occurs during the declining phase of inspiratory effort (Pmus). If the opening of the exhalation valve is delayed, airway pressure (Paw) should initially rise as Pmus continues its decline. When Pmus declines to zero, a Paw plateau should appear. Paw at this point should reflect passive recoil at the prevailing volume. A cohort of 74 ventilator-dependent patients, ventilated in the PAV mode, were studied. Brief end-inspiratory occlusions were applied at random intervals. The magnitude of early change in Paw during the occlusion was inversely related to level of assist (r = 0.7, p < 0.00001). At high assist (> 75%), Paw was nearly flat or declined slightly, indicating minimal residual Pmus at the onset of occlusion. At lower assist levels, Paw increased exponentially in most patients with an average time constant of 0.21 +/- 0.06 s. Extraneous events that may corrupt the measurement (e.g., behavioral responses) were extremely rare (< 0.5%) in the first 0.25 s. From these findings, we concluded that Paw measured 0.25 s from occlusion onset (P0.25) includes little inspiratory Pmus and is free of extraneous events. E, estimated from P0.25 during PAV (EPAV), agreed well (r = 0.92) with passive E measured during controlled ventilation (ECMV); the average difference (EPAV - ECMV) was (+/- SD) -0.3 +/- 4.9 cm H2O x L(-1), corresponding to 0.9 +/- 16.4% of average E. We conclude that Paw measured at 0.25 s from the onset of end-inspiratory occlusion in the PAV mode provides a reliable estimate of passive elastic recoil.

Effects of the inspiratory pressure waveform during patient-triggered ventilation on pulmonary stretch receptor and phrenic nerve activity in cats

OBJECTIVE

To examine the effects of square wave, sinusoidal, and linear inspiratory pressure waveforms during pressure-controlled assist/control ventilation on the firing pattern of pulmonary stretch receptors and phrenic nerve activity.

DESIGN

Experimental, comparative study.

SETTING

Research laboratory at a university biomedical center.

SUBJECTS

Nine anesthetized, endotracheally intubated young cats (2.5-3.4 kg).

INTERVENTION

With interposed periods of continuous positive airway pressure (0.2 kPa), each cat was exposed to periods of assist/control ventilation with three different pressure waveforms, where the peak inspiratory pressure (0.74 +/- 0.13 kPa), end-expiratory pressure (0.2 +/- 0.02 kPa), and tidal volume (14.9 +/- 5.22 mL/kg) were kept constant. Preset controlled ventilator rate was set below the rate of spontaneous breathing, and the mechanical inflation time equaled the inspiratory time during spontaneous breathing on continuous positive airway pressure.

MEASUREMENTS AND MAIN RESULTS

Respiratory rate and arterial blood gases did not change between the three pressure waveforms during assist/control ventilation. Peak pulmonary stretch receptor activity was lower and mean phrenic nerve activity higher during continuous positive airway pressure than during assist/control ventilation (p <.05). Peak inspiratory pulmonary stretch receptor activity was the same with all three pressure waveforms (82 +/- 17 impulses.sec-1) but occurred earlier with square wave than with sinusoidal or linear pressure waveforms (p <.05). The total number of impulses in the phrenic nerve activity burst was smaller with square wave than with the other two pressure waveforms (0.21 +/- 0.17 vs. 0.33 +/- 0.27 and 0.42 +/- 0.30 arbitrary units; p <.05), and the phrenic nerve activity burst duration was shorter with square wave (1.10 +/- 0.45 vs. 1.54 +/- 0.36 and 1.64 +/- 0.25 secs; p <.05).

CONCLUSION

Square wave pressure waveform during pressure-controlled assist/control ventilation strongly inhibits spontaneous inspiratory activity in cats. One mechanism for this inhibition is earlier and sustained peak pulmonary stretch receptor activity during inspiration. These findings show that differences in inspiratory pressure waveforms influence the spontaneous breathing effort during assist/control ventilation in cats.

Volume of activation of the Hering-Breuer inflation reflex in the newborn infant

Although the Hering-Breuer inflation reflex (HBIR) is active within tidal breathing range in the neonatal period, there is no information regarding whether a critical volume has to be exceeded before any effect can be observed. To explore this, effects of multiple airway occlusions on inspiratory and expiratory timing were measured throughout tidal breathing range using a face mask and shutter system. In 20 of the 22 healthy infants studied, there was significant shortening of inspiration because the volume at which occlusion occurred rose from functional residual capacity (FRC) to end-inspiratory volume [14.9% reduction in inspiratory time (per ml/kg increase in lung volume at occlusion)]. All infants showed a significant increase in expiratory time [17.1% increase (per ml/kg increase in lung volume at occlusion)]. Polynomial regression analyses revealed a progressive increase in strength of HBIR from FRC to approximately 4 ml/kg above FRC. Eighteen infants showed no further shortening of inspiratory time and 10 infants no further lengthening of expiratory time with increasing occlusion volumes, indicating maximal stimulation of the reflex had been achieved. There was a significant relationship between strength of HBIR and respiratory rate, suggesting that HBIR modifies the breathing pattern in the neonatal period.

Hering-Breuer reflex in conscious newborn rats: effects of changes in ambient temperature during hypoxia

In a previous study in conscious normoxic newborn rats, we found that the strength of the Hering-Breuer reflex (HB reflex) was greater (188%) at high (36 degrees C) than at low (24 degrees C) ambient temperature (T(a); D. Merazzi and J. P. Mortola. Pediatr. Res. 45: 370-376, 1999). We now asked what the effect would be of changes in T(a) during hypoxia. Rat pups at 3-4 days of age were studied in a double-chamber airflow plethysmograph. The HB reflex was induced by negative body surface pressures of 5 or 10 cmH(2)O and quantified from the inhibition of breathing during maintained lung inflation. Rats were first studied at T(a) = 32 degrees C in normoxia, followed by hypoxia (10% O(2) breathing). During hypoxia, oxygen consumption (VO(2)) averaged 47%, and HB reflex 115%, of the corresponding normoxic values, confirming that in the newborn, differently from the adult, hypoxia does not decrease the strength of the HB reflex. As hypoxia was maintained, lowering T(a) to 24 degrees C or increasing it to 36 degrees C, on average, had no significant effects on VO(2) and the HB reflex. However, with 5-cmH(2)O inflations, the HB reflex during the combined hypoxia and hyperthermia was significantly stronger than in normoxia. We conclude that in conscious newborn rats during normoxia the T(a) sensitivity of the HB reflex is largely mediated by the effects of T(a) on thermogenesis and VO(2); in hypoxia, because thermogenesis is depressed and VO(2) varies little with T(a), the HB reflex is T(a) independent. The observation that the reflex response to lung inflations during hypoxic hyperthermia can be greater than in normoxia may be of importance in the pathophysiology of apneas during the neonatal period.

Influence of ventilator settings in determining respiratory frequency during mechanical ventilation

During mechanical ventilation, changes in inspiratory flow and tidal volume (VT) have been shown to alter respiratory frequency (f ). However, the changes in flow and VT have been accompanied by alteration in ventilator inspiratory time (TI,(vent)), and it is not clear which variable is the primary determinant. To address this issue, we employed four protocols in 15 healthy volunteers receiving assist-control ventilation. When VT was fixed and flow was delivered at 30, 60, and 90 L/min, f increased as a function of the increase in flow and the decrease in TI,(vent). When flow was held constant and VT was changed among 0.5, 1.0, and 1.5 L, f increased as a function of the decreases in VT and TI,(vent). When flow was increased from 60 to 90 L/min and these changes were balanced with VT settings of 1.0 and 1.5 L to maintain a constant TI,(vent), f did not change. When flow and VT were held constant and TI,(vent) was varied by the application of inspiratory pauses (0 to 2 s), f decreased as a function of the increase in TI,(vent) (p < 0.001). In conclusion, the imposed ventilator inspiratory time during mechanical ventilation can determine f independently of delivered inspiratory flow and VT.

Repeated acute hypoxia temporarily attenuates the ventilatory respiratory response to hypoxia in conscious newborn rats

We asked whether repeated hypoxic exposures during the early neonatal periods could affect the ventilatory control, such as the lung volume-dependent ventilatory inhibition (HBR), pulmonary ventilation (VE), and CO2 production (VCO2). Within each litter of rats, one group of pups (experimental group H) was exposed to 6% O2 (30-min duration twice a day from postnatal d 1 to 4). The other group (control group C) was exposed to air. At 5 d after birth, the HBR was triggered by lung inflation via negative body surface pressure (10 cm H2O). Measurements of VE and VCO2 were done by plethysmography and the inflow-outflow CO2 difference, respectively. At 2 wk of age, VE and VCO2 measurements were repeated by the barometric technique and the inflow-outflow CO2 difference, respectively. Each conscious pup was breathing normoxia (21% O2) and then hypoxia (10% O2). Results were as follows: 1) during normoxia, HBR was stronger and both VE and VCO2 were higher in H pups than in C pups; 2) during hypoxia, the HBR of C was as in normoxia, whereas that of H was increased above the normoxic value; 3) during hypoxia, C maintained VE, whereas H decreased it; 4) in hypoxia, VCO2 was reduced significantly in both groups; 5) at 2 wk of age, VE and VCO2 did not differ between H and C during normoxia or in response to 10% hypoxia. We conclude that in rat pups, repeated hypoxic episodes can modify the HBR and, at least temporarily, reduce the VE response to hypoxia with a decrease in VCO2. The findings are in agreement with the view that repeated hypoxic exposures in the neonatal period could interfere with the development of respiratory control and could possibly be involved in the mechanisms of neonatal apnea or sudden infant death syndrome.

Effects of changes in ambient temperature on the Hering-Breüer reflex of the conscious newborn rat

We questioned to what extent changes in temperature could affect the newborn's ventilatory inhibition provoked by lung inflation, or Hering-Breüer (HB) inflation reflex. Conscious newborn rats (3-4 d old) were studied in a double chamber airflow plethysmograph at ambient temperatures of 32 degrees C (slightly below their thermoneutrality), 24 degrees C (cold), and 36 degrees C (warm). At these ambient temperatures, the corresponding body temperatures averaged 35.4, 31.0, and 37 degrees C. The HB reflex was triggered by negative body surface pressures of 5 or 10 cm H2O, and quantified as the duration of the expiratory time during the maintained inflation, either in absolute values or in relation to the control expiratory time. In cold the HB reflex decreased to 80%, and in warm it increased to 150%, of the values measured at 32 degrees C. Oxygen consumption, measured by an open flow system, averaged 59, 47, and 29 mL x kg(-1) x min(-1) at, respectively, 24, 32, and 36 degrees C. In cold, the ventilatory response to hypoxia (10% O2) was almost absent, that to hypercapnia (5% CO2) was greater, and that to hypoxia and hypercapnia combined was less than in warm. We conclude that in newborn rats the strength of the vagal inhibition on breathing, evaluated in the form of the HB reflex, is sensitive to temperature, becoming stronger as temperature increases. One contributing factor is the temperature-induced change in metabolic rate, whereas the role of temperature-induced changes in ventilatory chemosensitivity is variable. The strong temperature-dependency of the neonatal HB reflex implies that in newborns exposed to a warm environment breathing is more susceptible to inhibitory inputs.

Influence of volume dependency and timing of airway occlusions on the Hering-Breuer reflex in infants

Both end-inspiratory (EIO) and end-expiratory (EEO) airway occlusions are used to calculate the strength of the Hering-Breuer inflation reflex (HBIR) in infants. However, the influence of the timing of such occlusions is unknown, as is the extent to which changes in volume within and above the tidal range affect this reflex. The purpose of this study was to compare both techniques and to evaluate the volume dependency of the HBIR in healthy, sleeping infants up to 1 yr of age. The strength of the HBIR was expressed as the ratio of expiratory or inspiratory time during EIO or EEO, respectively, to that recorded during spontaneous breathing, i.e., as the "inhibitory ratio" (IR). Paired measurements of the EIO and EEO in 26 naturally sleeping newborn and 15 lightly sedated infants at approximately 1 yr showed no statistically significant differences in the IR according to technique: mean (95% CI) of the difference (EIO - EEO) being -0.02 (-0.17, 0.13) during the first week of life and 0.04 (-0.14, 0.22) at 1 yr. During tidal breathing, a volume threshold of approximately 4 ml/kg was required to evoke the HBIR. Marked volume and age dependency were observed. In newborn infants, occlusions at approximately 10 ml/kg during sighs always resulted in an IR > 4, whereas a similar response was only evoked at 25 ml/kg in older infants. Age-related changes in the volume threshold may reflect maturational changes in the control of breathing and respiratory mechanics throughout the first year of life.

The Hering-Breuer reflex in ventilated children

The authors tested the hypothesis that, in the paediatric population, the Hering-Breuer reflex would be provoked by the positive pressure inflation of mechanical ventilation (IPPV), and that the strength of the reflex would relate to the child's age and compliance of the respiratory system (CRS). During IPPV, changes in air flow, volume and airway pressure were recorded simultaneously. From the traces, expiratory times and CRS were calculated. The mean expiratory time (Te1) prior to each inflation and the mean expiratory time (Te2) immediately following an inflation were determined. The presence of the reflex was indicated by Te2 being longer than Te1, and the strength of the reflex was calculated as the difference between Te2 and Te1 expressed as a percentage of Te1. Twenty children were studied who had a mean age of 0.7 years (range 0.1-9.8 years). The reflex was provoked in 19 of the 20 patients and was not seen in the oldest patient. The strength of the reflex varied from 22 to 144% and was inversely related to postnatal age significantly (r = -0.73) but not CRS or inflation volume. It is concluded that the Hering-Breuer reflex is provoked during IPPV in young children; whether, as in neonates, this influences the efficacy of ventilation deserves investigation.

Autonomic response patterns during voluntary facial action

The effects of six voluntarily adopted emotional facial expressions on heart rate, respiration, emotional report, and effort ratings were examined. The results indicate that the facial expressions that were most difficult to produce (anger, fear, and sadness) showed larger cardiac accelerations than those that were easy to produce (disgust and surprise); the happiness expression fell somewhere in between. Emotional self-report revealed that in the majority of the facial configuration trials subjects experienced only moderate or no emotion at all. In addition to the emotional configurations, a nonemotional expression was examined. Post hoc comparisons showed that the heart rate data of the nonemotional face were not significantly different from any of the other emotional expressions. With respect to respiration, the production of facial expressions induced an increase in functional residual capacity, and decrease in tidal volume, shortened inspiratory and expiratory phase duration, and an increase in inspiratory pause duration. These effects were most pronounced during the facial expressions that were difficult to produce. We conclude that changes in heart rate are not the consequence of the capacity of facial activity to recruit emotion-specific autonomic activity but instead are modulated by effort-related changes in respiration.

High frequency oscillation, respiratory activity and changes in blood gases

Spontaneous respiratory activity during high frequency oscillation (HFO) and its relationship to changes in blood gases on transfer to HFO has been assessed. Eighteen infants were studied, median gestational age 27 weeks and postnatal age 1 day. Simultaneous measurements of changes in oesophageal and airway pressure, flow and volume were made during a period of conventional ventilation and then during HFO. From these recordings, the infants' spontaneous respiratory rate during the two ventilatory techniques were calculated. Arterial blood gases were measured immediately before and after a 30-min period of HFO. All the infants were breathing during conventional ventilation (median rate of 55 breaths/min). On transfer to HFO, the respiratory rate of the whole group decreased to a median of 23 breaths/min (P < 0.001), but only five infants became apnoeic. The changes in respiratory rate did not relate significantly to changes in PaCO2. Oxygenation deteriorated in four of the five apnoeic infants and in the two infants who became agitated during HFO. In the remaining 11 infants, whose median respiratory rate was 28 breaths/min (range 15-77) during HFO, oxygenation improved by a median of 12 mmHg (range 4-42). We conclude that, in the majority of infants, spontaneous respiratory activity during HFO is compatible with improvements in blood gases.

High frequency oscillation

During high frequency oscillation (HFO) small volumes are delivered at frequencies up to 40 Hz; both inspiration and expiration are active. Oxygenation is controlled by the mean airway pressure level and carbon dioxide elimination by the delivered volume and, less so, by frequency. In some infants the delivered volume can be 50%-100% of the dead space. Measurements during HFO demonstrate oscillator performance can be impaired at high frequencies. HFO may be a useful alternative to extra corporeal membrane oxygenation in infants with severe respiratory failure, but its role in reducing bronchopulmonary dysplasia remains controversial.

Effect of sleep on changes in breathing pattern accompanying sigh breaths

We studied the effect of sleep on the characteristics of sigh breaths and the associated changes in breathing pattern in breaths following spontaneous sighs in 4 unrestrained dogs with an intact upper airway. The sigh breath was characterized by its large tidal volume (VT), long TI and TE in comparison with the control breath. The volume of the sigh breath was larger in awake sighs than in those recorded during non-REM (NREM) and REM sleep. The strength of Hering-Breuer reflex as determined by duration of the post-sigh apnea was similar in NREM and REM sleep. Sighs occurring during wakefulness, NREM and REM sleep were associated with augmented activity of the parasternal muscles during inspiration, and a persistent tonic abdominal muscle activity during the expiratory period. Breathing pattern in the post-sigh period was characterized by a smaller VT and longer TE in the first post-sigh breath in all sleep states (compared with the control breath), but the pattern returned to control level within the second or third post-sigh breath in both NREM and REM sleep. Sighs did not precipitate periodic breathing or other forms of abnormal breathing patterns in either wakefulness or sleep. We conclude that the respiratory control mechanisms stabilizing breathing after a sigh in the awake dog are intact in NREM and REM sleep.

Phrenic nerve responses to lung inflation and hypercapnia in decerebrate dogs

The effects of changes in static airway pressure (Paw) and arterial PCO2 (PaCO2) on phrenic nerve activity were studied in unanesthetized, decerebrate dogs and compared with previous results from chloralose/urethane anesthetized dogs using the same experimental preparation (Mitchell et al. 1982; Mitchell and Selby 1987). In ten mid-collicular decerebrate dogs, the lungs were independently ventilated while the left pulmonary artery was occluded and the right vagus nerve was transected. Changes in left lung Paw, therefore, exerted effects on pulmonary stretch receptors without altering blood gases; changes in the inspired gas ventilating the right lung controlled blood gas composition, without altering lung volume feedback. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cmH2O at various constant levels of PaCO2 between 31 and 69 mmHg. The major findings of this study are: (1) hypercapnia decreases the slope of the relationship between expiratory duration (tE) and Paw in both decerebrated and anesthetized dogs; (2) hypercapnia increases the inspiratory duration (tI) in decerebrated, but not anesthetized dogs; and (3) hypercapnia decreases the slope of the relationship between f and Paw due to these effects on tE and tI. These results support previous studies indicating that vagal and suprapontine mechanisms exert independent effects on respiratory timing. It is concluded that neither suprapontine influences nor anesthesia are necessary in the mechanism underlying interactions between stretch receptors and CO2-chemoreceptors in modulating tE. Furthermore, decerebration reveals a unique effect of CO2-chemoreceptors on tI, an effect found in anesthetized dogs only after carotid denervation.

Breathing pattern, lung inflation reflex and airway tone in acrylamide neuropathy

Slowly adapting pulmonary stretch receptors (SARs) participate in numerous respiratory reflexes, including the Hering-Breuer lung inflation reflex (HBR), and reflex control of respiratory rate and depth. In addition, SAR discharge may modulate airway tone. We studied the effect of acrylamide neuropathy, which causes reversible dysfunction of SARs and their myelinated vagal afferents, on the breathing pattern, HBR and airway tone in conscious dogs. As neuropathy evolved, breathing became slow and deep, and both the apnoeic and bronchodilatory responses to lung inflation were markedly reduced, findings consistent with SAR dysfunction. Recovery of clinical neuropathy and respiratory reflexes followed acrylamide withdrawal. Despite the obvious abnormalities of SAR-mediated reflexes in acrylamide-affected dogs, airway tone remained normal and sinus arrhythmia prominent in the presence of neuropathy, suggesting that small diameter vagomotor efferents are relatively resistant to acrylamide. Acrylamide neuropathy provides a useful preparation for the study of vagomotor function in animals in which feedback from myelinated pulmonary afferents is attenuated.

The effect of lung inflation on breathing in man during wakefulness and sleep

To determine whether the Hering-Breuer inflation reflex could be demonstrated in the presence and absence of behavioral influences on breathing, lung inflation was performed via the tracheal stoma in eleven laryngectomized subjects: seven seated at rest with their eyes closed and six recumbent during their deepest non-rapid eye movement sleep. Laryngectomized subjects were chosen for study since their permanent tracheal stoma and absence of a glottis abolished the sensation of inflation in the upper airways, avoided problems with glottic closure and allowed simple, airtight connection to respiratory apparatus. At rest awake, inflation volumes of 500-3050 ml caused no reproducible apnea. During EEG-documented sleep, the inflation reflex was tested on 75 occasions using inflation volumes of 540-2100 ml. On the 66 occasions not associated with subject arousal inflations exceeding approximately 1 L produced apnea terminated by inspiration; greater volumes gave longer apneas. At the end of the study the subjects were woken and retested; no reproducible apnea was found. We conclude that the Hering-Breuer inflation reflex can be demonstrated above the resting tidal volume range in adult man in the absence of the behavioral control of breathing.

Mechanisms mediating the heart rate response to hypoxemia

We studied the effect of phasic pulmonary afferent information on heart rate (HR) during a progressive reduction in oxygen saturation (SaO2). The Hering-Breuer reflex was evaluated with the use of the ratio of apnea duration after lung inflation to the preceding expiratory time (dT). Phasic afferent activity was stopped in anesthetized, paralyzed dogs by constant-flow ventilation (CFV), a technique that removes cyclic changes in lung volume. During normocapnic (PaCO2 = 36.4 +/- 1.1 mm Hg) spontaneous breathing, there was a wide variability in HR response, with a mean delta HR/delta SaO2 (+/- SE) of 0.62 +/- 0.27 beats/min/% (values greater than 0 indicate a tachycardiac response). There was a good correlation between delta HR/delta SaO2 and dT (r = .79). Mean delta HR/delta SaO2 for the combined normocapnic and hypercapnic studies during CFV was lower (-1.32 +/- 0.19 bpm/%) than that during spontaneous breathing (0.23 +/- 0.19, p less than .0001). We suggest that the HR response to hypoxemia is strongly related to the strength of the Hering-Breuer reflex, which may explain the large interdog variability in HR responses.

Respiratory reflexes in the anesthetized miniature swine

To assess the suitability of the miniature swine for studies of the control of breathing we evaluated the response of these animals to commonly used respiratory stimuli. Hanford miniature pigs were anesthetized with alpha chloralose and allowed to breathe spontaneously. Rapid lung inflations induced a prolonged expiratory pause proportional to load. Mechanical stimulation of the upper airways induced coughing. Central venous injections of C-fiber stimulants produced bradycardia, hypotension with apnea and/or rapid shallow breathing. CO2 rebreathing increased ventilation primarily through an increase in tidal volume; inspiratory time was not changed. Bilateral vagotomy caused a slower, deeper pattern of breathing, and significantly attenuated the ventilatory response to CO2; all other reflexes were abolished by vagotomy. Cooling the vagus nerves caused reversible blockade of the cough, inflation and C-fiber mediated reflexes in that order. We conclude that the pig can serve as a useful animal in which to study the control of breathing.

Differences in respiratory patterns after acute and chronic pulmonary denervation

The role of pulmonary vagal information in the control of respiratory patterns was assessed in awake and anaesthetised rats in which pulmonary denervation was effected by bilateral cervical vagotomy or by right cervical vagotomy combined with left pneumonectomy or left intrathoracic vagotomy. Acute denervation led to increases of tidal volume (VT), inspiratory duration (TI) and expiratory duration (TE) in both awake and halothane anaesthetised animals; in awake rats the increase of TE rapidly subsided. Chronic pulmonary denervation produced markedly smaller increases of VT and TI and no change of TE from control values. In hypercapnia, awake animals with combined pneumonectomy and vagotomy consistently increased respiratory frequency by reductions in TI and TE; awake animals with combined intrathoracic and cervical vagotomy showed no increase in f because decreases in TI offset increases in TE; in anaesthetised rats with acute bilateral cervical vagotomy there was a consistent fall in respiratory frequency due to an expiratory pause. The results demonstrate that (1) the role of vagal activity in the production of respiratory patterns is unlikely to be accounted for solely in terms of influences arising from pulmonary stretch receptors; (2) vagal influences of TE are transitory; (3) under halothane anaesthesia hypercapnia induces an expiratory pause; and (4) the combination of pneumonectomy with contralateral vagotomy makes possible studies in awake rats although pulmonary denervation is less complete than with bilateral intrathoracic vagotomy.

Effects of hypercapnia on phrenic and stretch receptor responses to lung inflation

To determine if hypercapnia and reflex bronchoconstriction attenuate lung inflation effects on ventilatory activity by indirect effects on intrapulmonary stretch receptors (PSR), phrenic nerve activity and single unit PSR were monitored at controlled levels of static airway pressure (Paw) and arterial PCO2 in 15 anesthetized dogs. Paw in a vascularly isolated lung was varied between 2 and 14 cm H2O at levels of PaCO2 between 35 and 85 mm Hg. PSR activity (n = 38) in fine strands dissected from an otherwise intact vagus nerve and the integrated phrenic neurogram were recorded. The response to Paw varied from one PSR to another, but was consistent in a given unit; PaCO2 had no consistent effect on individual responses. Selected PSR (n = 15) were averaged to yield a population response to Paw; the selection criteria were: phrenic activity responded briskly to Paw and measurements were made at three levels of PaCO2. Average PSR discharge increased linearly with Paw but was unaffected by PaCO2. On the other hand, phrenic burst frequency decreased as Paw increased and hypercapnia attenuated the slope of this relationship. These results suggest that effects on the relationship between PSR activity and Paw cannot account for attenuation of the relationship between phrenic frequency and Paw in hypercapnia. The effect of PaCO2 on the phrenic frequency vs Paw relationship probably arises from integrative mechanisms in the central nervous system.

Effects of carotid denervation on interactions between lung inflation and PaCO2 in modulating phrenic activity

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration. We examined the role of carotid chemoreceptors in this response using an experimental preparation that allowed independent control of lung inflation and CO2 reflexes. Experiments were conducted in intact (n = 6) and carotid denervated (CBX; n = 12) chloralose/urethane anesthetized dogs. Integrated phrenic amplitude (Phr), f, and the inspiratory (TI) and expiratory durations (TE) were measured as a function of Paw (2-12 cm H2O) at levels of PaCO2 between 30 and 80 mm Hg. In intact dogs: (1) f decreased as Paw increased, and elevated PaCO2 decreased the slope of this relationship; (2) neither PaCO2 nor Paw affected TI; and (3) TE increased hyperbolically with Paw, and elevated PaCO2 attenuated this relationship. In CBX dogs: (1) f decreased as Paw increased, but this relationship was not affected by PaCO2; (2) TI increased as PaCO2 increased but was unaffected by Paw; and (3) TE increased as Paw increased but was unaffected by PaCO2. The results indicate that carotid chemoreceptors are necessary in the mechanism whereby hypercapnia attenuates the effects of Paw on f and TE. Furthermore, carotid denervation reveals an effect of hypercapnia on TI, an effect that is not evident in dogs with functional carotid chemoreceptors.

Improved accuracy of the occlusion technique for assessing total respiratory compliance in infants

The occlusion technique to measure total respiratory system compliance (Cocc) was used in 28 sedated infants with a variety of cardiopulmonary diseases and 14 anaesthetised infants during the first 2 years of life. In this report, we describe some of the potential problems in the technique and how to avoid them. Invalidation of the occlusion technique because of consistent failure to relax during brief airway occlusions occurred in only four of 42 infants despite the fact that only four infants were studied during the first month of life and 17 were between 12 and 26 months old. The technique was invalidated in two intubated infants because of persistent leaks around the endotracheal tube. In the remaining 36 infants, data sometimes had to be excluded as a result of instability and end-expiratory volume, intermittent leaks, or failure to relax during occlusions performed at low lung volumes or during inspiration. However, by performing 15-25 occlusions per infant, it was possible to obtain sufficient reliable data for accurate analysis of Cocc in all these infants. Providing that the potential errors and limitations of the occlusion technique are recognized, it appears to be applicable to a wide range of healthy and sick infants.

Reflex partitioning of inputs from stretch receptors of bronchi and thoracic trachea

A selective block of slowly adapting stretch receptors in anesthetized rabbits was induced by exposing to SO2 all thoracic airways (T) or the carina and bronchi alone (B). Increment of inspiratory time (TI) relative to control was 61% greater under B than T. The reverse would have happened if input responsible for Breuer-Hering inflation reflex originated from both bronchi and trachea. Hence, bronchial input activates inspiratory off-switch, while tracheal input delays its activation. During single inspiratory efforts with airways closed at end expiration diaphragm activity (Adi) decreased and TI0 increased relative to control equally under B and T. Hence, the input facilitating central inspiratory activity at end expiratory volume does not stem from trachea. At end of inspiratory ramp Adi stopped within 43 msec at control and 57 msec under B and T. Hence, bronchial input speeds up off-switching of inspiration. Postinspiratory Adi was greater under B than T, and nearly nil at control. Hence, bronchial input inhibits postinspiratory Adi, while tracheal input facilitates it. Inspiratory and expiratory flows were more damped under B than T, and under T than at control.

Posthyperventilation apnea associated with severe hypoxemia

We studied a 14-year-old girl who suffered fractures of her mandible and tegmen following a fall from a balance beam. Thirteen days after hospitalization, she developed severe, protracted, recurrent episodes of hyperventilation; subsequently, she suffered posthyperventilation apnea, which at times was prolonged and association with severe hypoxemia with an arterial oxygen pressure as low as 25 mm Hg. The patient was treated with added dead space and chlorpromazine hydrochloride (Thorazine). Postulated mechanisms for her disorder are discussed. The importance of close clinical and laboratory observation in similar cases is stressed.

Effect of premature delivery on the maturation of the Hering-Breuer inspiratory inhibitory reflex in human infants

The Hering-Breuer inspiratory inhibitory reflex was studied serially in a group of premature infants and in a group of term infants in the immediate postnatal period. The premature infants had a stronger inspiratory inhibitory reflex than did the term infants at birth; this reflex decreased with maturation of the premature. Development in the extrauterine environment significantly delayed the rate of disappearance of this reflex. This may indicate that premature delivery retards the neurologic maturation of the human infant. The term infant showed no change in the activity of the inspiratory inhibitory reflex in the first five days of life.

Respiratory regulation after elastic loading and CO2 rebreathing in normal term infants

Studies of airway pressure, tidal volume, respiratory duration, and total breath duration before and after elastic loading airway occlusions were carried out on ten full-term, normal infants on two occassions in the first week of life. Using these noninvasive techniques we infer that static compliance and the Hering-Breuer reflex are unchanged during the first week; that infants may increase sensitivity to chemical drive toward the end of the first week; and that there may be a volume related threshold for vagal inhibition of inspiration in some infants.

Immediate response to expiratory threshold load

In anesthetized rabbits, cats and dogs the end-inspiratory volume referred to normal FRC (deltaVe.i.) of the first breath during expiratory threshold load (ETL) was greater than control deltaVe.i. The increase of deltaVe.i. was roughly proportional to the increase of end-expiratory volume relative to normal FRC (deltaVe.e.) caused by ETL. During squeezing of the rib cage or of the abdomen or after cordotomy at T1 the phenomenon occurred nearly to the same extent. This suggests that afferent impulses from the chest wall are not essential in determining the phenomenon. The first inspiration during ETL occurred after a delay that was roughly proportional to the increase of deltaVe.e: the arterial Pco2, however, was not significantly increased at the time of the first inspiration during ETL. After bilateral vagotomy the first inspiration during ETL was no longer increased. An explantation of the increased deltaVe.i. of the first inspiration during ETL is provided on the basis of the vagal dependent tidal volume-inspiratory duration characteristic of the respiratory center.

Abnormal breathing patterns

In health, breathing is regular and the respiratory rate is sufficiency constant to be useful as a vital sign of health and disease. This regularity depends on a complex interplay of chemical and neural control systems that operate automatically to reset the rate and depth of breathing as changes occur in posture and activity, to adjust the level of ventilation so that changes in gas tensions and pH in the blood and in the brain intersitial fluid are exceedingly modest despite wide swings in metabolic rate and in environmental conditions, and to coordinate ventilation and circulation so that the requirements of individual tissues for O2 delivery and CO2 removal are satisfied. Two broad categories of disorders can result from malfunction of these systems (Table 1): (1) disproportionate ventilation (too high or too low) for the level of metabolic activity, thereby producing severe abnormalities in blood gas tensions or in acid-base balance, and (2) an irregular breathing pattern without eliciting gross changes in blood gas tensions or in acid-base balance. Because of the complexity of the control system, each of these categories represents a final common pathway that can be produced in different ways. In this presentation, we will attempt to describe the general features that characterize the operation of the control system and some new technics that make it possible to trouble-shoot the malfunctioning system in order to identify the mechanism(s) responsible for the abnormality in breathing pattern.