Arousal and cardiopulmonary responses to hyperoxic hypercapnia in lambs

Experiments were done to investigate the arousal and cardiopulmonary responses to hyperoxic hypercapnia in 8 lambs. Each lamb was anaesthetized and instrumented for recordings of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms and measurements of arterial blood pressure and haemoglobin oxygen saturation. No sooner than 3 days after surgery, measurements were made in quiet sleep and active sleep during control periods when the animal was breathing 21% oxygen and during experimental periods of hyperoxic hypercapnia when the animal was breathing 10% carbon dioxide and 30% oxygen. Hyperoxic hypercapnia was terminated during each epoch by returning the inspired gas mixture to 21% oxygen once the animal aroused from sleep. Arousal occurred from both sleep states during hyperoxic hypercapnia but was delayed in active sleep compared to quiet sleep (active sleep 58 +/- 17 s; quiet sleep 21 +/- 10 s; mean +/- 1SD). There were no significant changes in heart rate or blood pressure during hyperoxic hypercapnia before arousal. However, respiratory rate and diaphragm electrical activity did increase during hyperoxic hypercapnia before arousal. Thus, our data provide evidence that hypercapnia can initiate arousal from sleep in young lambs. The mechanisms responsible for this response are yet to be determined.     

Influence of repeated exposure to rapidly developing hypoxaemia on the arousal and cardiopulmonary response to rapidly developing hypoxaemia in lambs

Experiments were done on four lambs to determine if repeated exposure to rapidly developing hypoxaemia influences the cardiopulmonary and arousal response from sleep. Each lamb was anaesthetized and instrumented for sleep staging and measurements of arterial haemoglobin oxygen saturation. No sooner than three days after surgery, measurements were made in quiet sleep and active sleep during control periods when the animal was breathing 21% oxygen and during experimental periods of rapidly developing hypoxaemia when the animal was breathing 5% oxygen for approximately 100 epochs of sleep. Arousal occurred from both sleep states during rapidly developing hypoxaemia but was delayed in active sleep compared to quiet sleep. The time to arousal and the decrease in arterial haemoglobin oxygen saturation were significantly increased with repeated exposure to rapidly developing hypoxaemia during both quiet sleep and active sleep. Thus, our data provide evidence that repeated exposure to rapidly developing hypoxaemia produces an arousal response decrement in lambs. Since it is possible that alterations in the arousal response to respiratory stimuli play a role in sudden infant death, studies to investigate the mechanism of the arousal response decrement following repeated exposure to rapidly developing hypoxaemia are warranted.     

Ventilatory and arousal responses of sleeping lambs to respiratory challenges: effect of prenatal maternal anemia.

We have examined the effects of exposure to chronic maternal anemia, throughout the final one-third of gestation, on postnatal ventilatory and arousal responses to hypoxia, hypercapnia, and combined hypoxia-hypercapnia in sleeping lambs. While resting quietly awake, lambs from anemic ewes had higher arterial PCO(2) levels than control animals during the first 2-3 postnatal wk, but pH, arterial PO(2), and arterial O(2) saturation were not different. During active and quiet sleep lambs from anemic ewes had higher end-tidal CO(2) levels than control animals when breathing room air and at the time of spontaneous arousal or when aroused by progressive hypercapnia or by combined hypoxia-hypercapnia. Ventilation and arterial O(2) saturation during uninterrupted sleep and ventilatory responsiveness to hypoxia (inspiratory O(2) fraction, 10%), progressive hypercapnia, and combined hypoxia/hypercapnia were not significantly affected by exposure to maternal anemia. Our findings show that maternal anemia results in elevated PCO(2) levels in the offspring. This effect may be due, at least in part, to altered pulmonary function.     

The effect of short-term sleep fragmentation produced by intense auditory stimuli on the arousal response to upper airway obstruction in lambs

Upper airway obstruction is recognized to cause apnoea in newborns as well as in adults. However, very little is known about factors that influence the arousal response from sleep during upper airway obstruction in newborns. Experiments were therefore done to investigate the effect of short-term sleep fragmentation on the arousal response to upper airway obstruction in six lambs aged 8 to 14 days. Each lamb was anaesthetized and instrumented for recordings of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms and measurements of systemic arterial blood pressure and oxygen saturation (fiberoptic catheter oximeter). A tracheostomy was done and a fenestrated tracheostomy tube placed in the trachea. Experiments were not done before the third postoperative day. During a study, a 5F balloon tipped catheter was inserted into the tube so that airflow could be obstructed by inflating the balloon. Measurements were made during 30 s control periods and during experimental periods of upper airway obstruction in at least three epochs of quiet sleep and active sleep in each animal. These measurements were made following a period of uninterrupted sleep and repeated following a 36-42 h period of sleep fragmentation. Sleep fragmentation was produced by 30 s of noise separated by 2 min of quiet. Sleep fragmentation produced small but statistically significant increases in the time to arousal and decreases in the haemoglobin oxygen saturation at arousal during upper airway obstruction in quiet sleep but not active sleep. However, these changes, although consistent, were small and are of questionable biological significance. Therefore, I believe it is unlikely that short-term sleep fragmentation per se significantly impairs the arousal response to respiratory stimuli in newborns.     

Arousal response to upper airway obstruction in young lambs: comparison of nasal and tracheal occlusion

Experiments were done on seven lambs to determine if site of occlusion--nasal versus tracheal--influences the cardiopulmonary and arousal responses from sleep to upper airway obstruction. Each lamb was anesthetized and instrumented for sleep staging and measurements of heart rate and arterial hemoglobin oxygen saturation. A tracheostomy was also done and a fenestrated tracheostomy tube placed in the trachea. Prior to an experiment, A 5F balloon-tipped catheter was inserted through the decannulation cannula into the tracheostomy tube so that tracheal occlusions could be accomplished by inflating the balloon. In addition, a 5F balloon-tipped catheter was inserted into the inlet of a pre-formed silicone mask sealed to the animals snout with silicone rubber foam so that nasal occlusions could be accomplished by inflating the balloon. During an experiment, measurements were made in quiet sleep and in active sleep during control periods of tidal breathing and during experimental periods of nasal or tracheal occlusion. Upper airway obstruction was terminated by deflating the balloon once the animal aroused from sleep. Arousal occurred sooner following nasal occlusion than during tracheal occlusion in quiet sleep; 64 percent of arousals occurred within five seconds of nasal occlusion whereas only 14 percent of arousals occurred within five seconds of tracheal occlusion in quiet sleep. In addition, SaO2 and heart rate decreased more before arousal following tracheal occlusion than following nasal occlusion. However, there was not a significant effect of site of obstruction on time to arousal or the change in SaO2 before arousal in active sleep.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of the stimulation of carotid body chemoreceptors on the gastric mucosal blood flow in artificially ventilated and spontaneously breathing rats.

The cardiovascular effects of the stimulation of arterial chemoreceptors are different in spontaneously breathing and artificially ventilated animals. Respiratory failure and long term sojourn at high altitude coincide frequently with the occurrence of gastric ulceration. In both these situations a profound stimulation of arterial chemoreceptors is present. The purpose of the paper was to investigate the reflex effect of stimulation carotid chemoreceptors on gastric mucosal blood flow in the rat. Arterial chemoreceptors were stimulated by two methods (I) substitution gas mixture of 10% oxygen in nitrogen for room air and (II) direct injection of acid saline ( 0.05 ml, pH = 6.8) into the distal part of left common carotid artery. In artificially ventilated rats stimulation of arterial chemoreceptors caused significant increase in gastric mucosal vascular resistance, accompanied by marked decline in blood flow. This effect was mediated by adrenergic mechanism. On the contrary to artificially ventilated rats, decline of gastric mucosal vascular resistance with concomitant increase in blood flow was found in spontaneously breathing animals. This effect was not abolished either by phentolamine or atropine. As vasodilatatory effect of arterial chemoreceptors stimulation was abolished by bilateral vagotomy, we postulate that non adrenergic and non cholinergic vagal fibers mediate observed vascular changes in gastric mucosa in spontaneously breathing rats. We hypothesize that in artificially ventilated patients with respiratory failure stimulation of arterial chemoreceptors by hypoxemia and or acidosis may contribute to the development of gastric mucosal lesions.     

Chemoreceptor and vagal influences on thyroarytenoid muscle activity in awake lambs during hypoxia.

This study was designed to identify the various controllers of thyroarytenoid (TA) activity in lambs during resting breathing, hypocapnic hypoxia, and isocapnic hypoxia. The TA muscle is known as the major adductor of the laryngeal aperture. We assumed that both the chemoreceptors and vagal nerves would interact to inhibit TA activity during hypoxia and to favor the occurrence of hyperpnea as a defense against hypoxia. We recorded TA activity directly in 11 awake lambs, aged 11 to 22 days, and studied them in three groups: four normals, four carotid body denervated, and three vagotomized. To test the contribution of the chemoreceptors to TA activity, we used pure O2 tests (Dejours' test) to silence the effects of the peripheral arterial chemoreceptors on the larynx during resting breathing and during the course of two hypoxia tests (the first: hypocapnic hypoxia; the second: isocapnic hypoxia). Our results confirmed 1) that both the peripheral arterial chemoreceptors and the vagal nerves inhibit the TA activity of 15-day-old lambs, during both resting and hypocapnic hypoxia conditions, and 2) that their effects override the hypocapnic effects that would otherwise recruit the TA muscle and close the glottis during hypocapnic hypoxia. We also found that vagotomy, or the pure O2 test, causes major recruitment of TA activity. These findings confirm that 15-day-old lambs are capable of using sustained hyperventilation as a means of fighting hypoxia, and that, because of the control of both the vagus nerves and the chemoreceptors, the laryngeal dynamic is able to keep the glottis aperture actively open, thereby favoring the hyperpnea.     

Response time and sensitivity of the ventilatory response to CO2 in unanesthetized intact dogs: central vs. peripheral chemoreceptors.

We assessed the speed of the ventilatory response to square-wave changes in alveolar P(CO2) and the relative gains of the steady-state ventilatory response to CO2 of the central chemoreceptors vs. the carotid body chemoreceptors in intact, unanesthetized dogs. We used extracorporeal perfusion of the reversibly isolated carotid sinus to maintain normal tonic activity of the carotid body chemoreceptor while preventing it from sensing systemic changes in CO2, thereby allowing us to determine the response of the central chemoreceptors alone. We found the following. 1) The ventilatory response of the central chemoreceptors alone is 11.2 (SD = 3.6) s slower than when carotid bodies are allowed to sense CO2 changes. 2) On average, the central chemoreceptors contribute approximately 63% of the gain to steady-state increases in CO2. There was wide dog-to-dog variability in the relative contributions of central vs. carotid body chemoreceptors; the central exceeded the carotid body gain in four of six dogs, but in two dogs carotid body gain exceeded central CO2 gain. If humans respond similarly to dogs, we propose that the slower response of the central chemoreceptors vs. the carotid chemoreceptors prevents the central chemoreceptors from contributing significantly to ventilatory responses to rapid, transient changes in arterial P(CO2) such as those after periods of hypoventilation or hyperventilation ("ventilatory undershoots or overshoots") observed during sleep-disordered breathing. However, the greater average responsiveness of the central chemoreceptors to brain hypercapnia in the steady-state suggests that these receptors may contribute significantly to ventilatory overshoots once unstable/periodic breathing is fully established.     

Relationships between eupnoeic pattern of breathing and ventilatory control in man II. Early response to transient hypercapnia.

The individual importance of peripheral chemosensitive afferents was studied using a transient hypercapnia (inhalation of a 5% or a 10% CO2 in air gas mixture respectively during 4 or 2 breaths) in human conscious subjects chosen for their different eupnoeic ventilatory patterns. Calculation of the speed of change in end-tidal CO2 pressure in tracheal gas (sPETCO2) and of the rate of change in tidal volume (sVI) gave assessment for quantifying the sensitivity of arterial chemoreceptors to hypercapnia (sCO2=SVI/SPETCO2). Our results showed that, independently of any outside influence of the eupnoeic ventilatory pattern on the components of the chemical stimulus, sVI and sCO2 were found to be much smaller in subjects whose pattern of breathing was slow (i.e. having a large tidal volume). The possible causes of the weak importance of peripheral chemosensitive afferents in such subjects were discussed.     

Influence of the stimulation of central chemoreceptors on the gastric mucosal blood flow in artificially ventilated and spontaneously breathing rats.

Respiratory failure coincides frequently with the occurrence of gastric ulceration. In advanced respiratory insufficiency hypoxemia is often accompanied by hypercapnia, which is the stimulus for central chemoreceptors as well as for carotid body chemoreceptors. The purpose of the work was to investigate the reflex effect of stimulation of central chemoreceptors on gastric mucosal blood flow (GMBF) in the rat. Central chemoreceptors were stimulated by a gas mixture composed of 10% carbon dioxide, 50% oxide and 40% nitrogen. In artificially ventilated and spontaneously breathing animals, the stimulation of central chemoreceptors caused a significant increase in gastric mucosal vascular resistance, accompanied by a marked decline in blood flow. We hypothesize that in patients with respiratory insufficiency accompanied by hypercapnia, the reflex impairment of GMBF may contribute to gastric ulceration.     

Coordination between glottic adductor muscle and diaphragm EMG activity in fetal lambs in utero

It haspreviously been reported that active glottic adduction is presentduring prolonged apneas but absent during periods of breathingmovements in fetal lambs in utero. The present study was aimed atexamining the precise coordination between fetal breathing movements[diaphragm electromyographic (EMG) activity (Di EMG)] andglottic adduction [thyroarytenoid muscle EMG activity (TAEMG)]. Electrodes for electroencephalogram, eye movements, TAEMG, and Di EMG and an arterial catheter were surgically implanted infetal lambs 123-142 days postconception. Polygraphic recordings were performed without sedation while the ewe breathed room air (n = 11) or various gas mixtures(hypoxia, n = 5; hyperoxia,n = 4; hypercapnia,n = 5; hypercapnia+hyperoxia,n = 5). Tonic TA EMG was observedthroughout >90% of apneas (>6 s) in both non-rapid-eye-movement and rapid-eye-movement sleep, and when Di EMG frequency decreased inrapid-eye-movement sleep. In all but two fetuses, TA EMG was immediately inhibited when Di EMG appeared. Altering blood gases did not modify these results. In conclusion, Di EMG and TAEMG are well coordinated in late gestation in fetal lambs,except in a few cases. These findings may have consequencesfor understanding the pathogenesis of mixed/obstructiveapneas of prematurity.

     

Ventilatory responses to hypoxaemia during sleep in the newborn

Ventilatory responses to rapidly developing hypoxaemia during N2-rebreathing were compared for active and quite sleep in three newborn lambs and four puppies. In lambs, active sleep was associated with: (i) development of ribcage deflation during inspiration, which persisted during progressive hypoxaemia; (ii) depressed ventilatory response to hypoxaemia despite increments of respiratory rate; (iii) delayed arousal. In the puppies, inspiratory collapse of the ribcage did not occur in active sleep and the ventilatory responses during hypoxaemia were similar to those in quite sleep. While apparently defective when related to adults, these responses to hypoxaemia in the lamb are normal. This study illustrates the importance of considering behavioural state and species differences when studying the regulation of breathing, particularly during development.     

Effects of hypoxia and hypercapnia on nonnutritive swallowing in newborn lambs.

The aim of the present study was to investigate the effect of hypercapnia and hypoxia on apnea and nonnutritive swallowing (NNS) frequency, as well as on the coordination between NNS and phases of the respiratory cycle in newborn lambs, while taking into account the potential effects of states of alertness. Six lambs were chronically instrumented for recording electroencephalogram, eye movements, diaphragm and thyroarytenoid muscle (a glottal adductor) activity, nasal airflow, and electrocardiogram. Polysomnographic recordings were performed in nonsedated lambs exposed to air (control), 10% O(2), and 5% CO(2) in a random order at 3, 4, and 5 days of age. Although hypercapnia decreased apnea frequency in wakefulness and active sleep (P = 0.002 vs. air and hypoxia), hypoxia had no significant effect on apnea. In addition, although hypercapnia increased NNS frequency during wakefulness and quiet sleep (P < 0.005 vs. air and hypoxia), hypoxia tended to decrease NNS frequency. Finally, only hypercapnia altered NNS-breathing coordination by increasing NNS at the transition from inspiration to expiration (ie-type NNS; P < 0.001 vs. air and hypoxia). In conclusion, whereas hypercapnia increases overall NNS frequency by specifically increasing ie-type NNS, hypoxia has the inverse tendency. Results were identical in all three states of alertness.     

Calibration of respiratory inductive plethysmography during quiet and active sleep in lambs

Respiratory inductive plethysmography provides a noninvasive method of measuring breathing patterns. Calibration of respiratory inductive plethysmography requires calculation of gain factors for ribcage and abdomen transducers utilizing 2 breathing patterns with different ribcage and abdomen contributions and tidal volume measured by either spirometry or integrated pneumotachography. The purpose of this study was to determine if respiratory inductive plethysmography can be calibrated to provide accurate measurements during quiet and active sleep in lambs. We used a least squares linear regression calibration technique with breaths selected from quiet sleep and active sleep to calculate gain factors in 6 tracheostomized lambs. Validation of gain factors was performed by comparing tidal volumes obtained simultaneously by respiratory inductive plethysmography and pneumotachography during quiet sleep and active sleep. Tidal volume differences between respiratory inductive plethysmography and pneumotachography on validation runs of 15 consecutive breaths each revealed 90% of validation breaths within +/- 20% during quiet sleep and 82% of validation breaths within +/- 20% during active sleep. These data provide evidence that respiratory inductive plethysmography can be calibrated to allow breathing pattern measurement during sleep.     

Behavioural state influences the cardiovascular response to haemorrhage in lambs

We investigated the effect of behavioural state on the cardiovascular response to an acute venous haemorrhage in 7 lambs aged 13 to 19 days. Each lamb had previously been anaesthetized and instrumented for measurements of electrocorticogram, electro-oculogram, nuchal and diaphragm electromyograms, pulmonary blood flow (electromagnetic flow transducer), aortic and right atrial blood pressures. The lambs were allowed to recover from surgery at least three days before they were studied. Measurements were made during a 1-minute control period and during a 1-minute experimental period that followed a 10 ml/kg body weight haemorrhage during quiet wakefulness, quiet sleep and active sleep; the haemorrhage took approximately 30s. Haemorrhage produced similar decreases in right atrial pressure and pulmonary blood flow during the three behavioural states. However, mean aortic pressure decreased more following haemorrhage during active sleep than during quiet sleep or quiet wakefulness. These results provide evidence that reflex control of the peripheral circulation is altered during active sleep compared to quiet sleep and quiet wakefulness in lambs.     

Ventilatory response to hyperoxia in the chick embryo

In the avian embryo at term we measured the ventilatory response to hyperoxia, which lowers the chemoreceptor activity, to test the hypothesis that the peripheral chemoreceptors are tonically functional. Measurements of pulmonary ventilation (VE) were conducted in chicken embryos during the external pipping phase, at 38 degrees C, during air and hyperoxia, and during hypercapnia in air or in hyperoxia. Hyperoxia (95% O2) maintained for 30 min lowered VE by 15-20%, largely because of a reduction in breathing frequency (f). The oxygen consumption and carbon dioxide production of the embryo were not altered. The hyperoxic drop of VE was more marked in those embryos, which had higher values of normoxic VE. Hypercapnia, whether 2 or 5% CO2, increased VE, almost exclusively because of the increase in tidal volume (VT). The increase in VT was less pronounced when hypercapnia was associated with hyperoxia, and f slightly decreased. Hence, in hyperoxia, the VE response to CO2 was less than in air. The results are in support of the hypothesis that in the avian embryo, after the onset of breathing, the peripheral chemoreceptors exert a tonic facilitatory input on . This differs from neonatal mammals, where the chemoreceptors have minimal or no activity at birth, presumably because the increased arterial oxygenation with the onset of air breathing is a much more sudden phenomenon in mammals than it is in birds.     

Stimulation of fetal breathing activity by beta-adrenergic mechanisms

Experiments were done on chronically prepared fetal lambs, 125-135 days gestation, to test the effects of various catecholamines on fetal breathing (FB) as well as the influence of isoproterenol on the fetal respiratory response to hypoxemia. Bolus injections of epinephrine, norepinephrine, and isoproterenol (5-20 micrograms) were administered via the lingual artery or femoral or jugular vein during periods of FB activity or apnea. The effects of epinephrine and norepinephrine on FB were variable and not statistically significant. Isoproterenol produced a significant increase in FB, frequency of breathing, and mean inspiratory effort, when infused during rapid-eye-movement (REM) sleep but it failed to induce FB during non-rapid-eye-movement (NREM) sleep. The positive response during REM sleep was absent following pretreatment with 3-5 mg propranolol and after bilateral section of the sinus nerves. The effect of hypoxia on FB was tested before and during constant infusion of isoproterenol (1 microgram/min iv). A reduction of the fetal arterial PO2 by 3-10 Torr produced the characteristic depression of FB in either situation. These results indicate that the fetal carotid body chemoreceptors can reflexly stimulate FB under certain circumstances but that their effectiveness is limited by more powerful inhibitory mechanisms such as those operative during NREM sleep and hypoxemia.     

Denervation of peripheral chemoreceptors decreases breathing movements in fetal sheep

The role of the peripheral chemoreceptors in the control of fetal breathing movements has not been fully defined. To determine whether denervation of the peripheral chemoreceptors affects fetal breathing movements, we studied 14 chronically catheterized fetal sheep from 120 to 138 days of gestation. In seven fetuses the chemoreceptors were denervated by bilateral section of the vagus and carotid sinus nerves; in seven others, sham operations were performed. We compared several variables during two study periods: 0-5 and 6-13 days after operation. In the denervated fetuses there were significant decreases in the incidence and amplitude of fetal breathing movements during both study periods. There were no differences between the two groups in incidence of low-voltage electrocortical activity, arterial pH and blood gas tensions, fetal heart rate, mean arterial blood pressure, or duration of survival after operation or birth weight. We conclude that denervation of the peripheral chemoreceptors decreases fetal breathing movements. These results indicate that the peripheral chemoreceptors are active during fetal life and participate in the control of fetal breathing movements.     

Effect of hilar nerve denervation on breathing and arterial PCO2 during CO2 inhalation

We determined the effects of denervating the hilar branches (HND) of the vagus nerves on breathing and arterial PCO2 (PaCO2) in awake ponies during eupnea and when inspired PCO2 (PICO2) was increased to 14, 28, and 42 Torr. In five carotid chemoreceptor-intact ponies, breathing frequency (f) was less, whereas tidal volume (VT), inspiratory time (TI), and ratio of TI to total cycle time (TT) were greater 2-4 wk after HND than before HND. HND per se did not significantly affect PaCO2 at any level of PICO2, and the minute ventilation (VE)-PaCO2 response curve was not significantly altered by HND. Finally, the attenuation of a thermal tachypnea by elevated PICO2 was not altered by HND. Accordingly, in carotid chemoreceptor-intact ponies, the only HND effect on breathing was the change in pattern classically observed with attenuated lung volume feedback. There was no evidence suggestive of a PCO2-H+ sensory mechanism influencing VE, f, VT, or PaCO2. In ponies that had the carotid chemoreceptors denervated (CBD) 3 yr earlier, HND also decreased f, increased VT, TI, and TT, but did not alter the slope of the VE-PaCO2 response curve. However, at all levels of elevated PICO2, the arterial hypercapnia that had persistently been attenuated, since CBD was restored to normal by HND. The data suggest that during CO2 inhalation in CBD ponies a hilar-innervated mechanism influences PaCO2 by reducing physiological dead space to increase alveolar ventilation.     

Preterm birth in lambs: sleep patterns and cardio-respiratory changes.

Cardio-respiratory physiology in sleep was examined in eight preterm lambs born at 133-135 (134 +/- 1, mean SEM) days of gestation after 3-5 days of pulsatile ACTH/TRH infusion, and contrasted with eight lambs born at term (147 +/- 1 days). Lambs were instrumented with electrodes for recording electrocorticogram, electro-oculogram and nuchal electromyogram to define behavioural states, as well as carotid arterial catheters for determination of arterial pressure, heart rate and arterial blood gases. Compared to full-term lambs, the preterm lambs exhibited extended active sleep times, elevated PaCO2 and faster heart rate in all behavioural states than full-term lambs; with increasing postnatal age, sleep times and heart rate declined. As similar differences are found in preterm human infants, the preterm lamb will be a useful model to study the underlying physiology of these cardio-respiratory alterations.