Variability in tracheal mucociliary transport is not controlled by beating cilia in lambs in vivo during ventilation with humidified and nonhumidified air.

Mucociliary transport in the respiratory epithelium depends on beating of cilia to move a mucus layer containing trapped inhaled particles toward the mouth. Little is known about the relationship between cilia beat frequency (CBF) and mucus transport velocity (MTV) in vivo under normal physiological conditions and when inspired air is dry or not fully humidified. This study was designed to use video-microscopy to simultaneously measure CBF and MTV in the tracheal epithelium through an implanted optical window in mechanically ventilated lambs. The inspired air in 6 animals was heated to body temperature and fully saturated with water for 4 hours as a baseline. In another series of experiments, 5 lambs were ventilated with air at different temperatures and humidities and the mucosal surface temperature was monitored with infrared macro-imaging. In the baseline experiments, during ventilation with fully humidified air at body temperature, CBF remained constant, mean 13.9 ± 1.6 Hz but MTV varied considerably between 0.1 and 26.1 mm/min with mean 11.0 ± 3.9 mm/min, resulting in a maximum mucus displacement of 34.2 µm/cilia beat. Fully humidified air at body temperature prevented fluctuations in the surface temperature during breathing indicating a thermodynamic balance in the airways. When lambs were ventilated with dryer air, the mucosal surface temperature and MTV dropped without a significant change in CBF. When inspired air was dry, mainly latent heat (92%) was transferred to air in the trachea, reducing the surface temperature by 5 °C. Reduced humidity of the inspired air lowered the surface temperature and reduced MTV in the epithelium during ventilation.

Unusual respiratory response to oxygen in an infant with repetitive cyanotic episodes.

High inspired oxygen concentrations have recently been recommended to control Cheyne-Stokes respiration in adults, with the intention of averting periodic apnea and its attendant arterial desaturation. We report a case study on an infant presenting with recurrent apnea and cyanosis in which oxygen treatment led to a gross form of respiratory instability we call episodic breathing, in which a breathing phase of 60 to 90 s alternated with an apnea lasting up to 60 s. When oxygen was discontinued, a profound arterial desaturation developed before breathing recommenced and restored oxygen levels. We propose that episodic breathing is an unusual respiratory pattern that involves the central chemoreceptors and results from the ventilatory threshold (the central PCO(2) at which breathing starts) lying considerably above the apneic threshold (the central PCO(2 )at which breathing stops). This feature predisposes to lengthy periods of hyperpnea alternating with lengthy periods of apnea. We suggest that when the case infant returned to air during episodic breathing, termination of apnea was entirely dependent upon carotid body activity, which reached a sufficient level to restart breathing only when arterial desaturation was severe. We conclude that oxygen therapy involves potential risks when employed to treat respiratory disorders involving unstable breathing patterns in the infant.

Paradoxical effect of oxygen administration on breathing stability following post-hyperventilation apnoea in lambs.

1. Oxygen administration is thought to suppress periodic breathing (PB) by reducing carotid body activity, and yet earlier experiments in neonates have shown that PB incidence may be increased following the application of hyperoxia. To clarify this paradox, we studied the changes in the pattern of PB that occur following administration of oxygen in a lamb model of PB. 2. PB was induced in eleven of seventeen anaesthetized lambs following passive hyperventilation with air. When oxygen was administered during PB, the pattern was first enhanced, as evidenced by a sudden decrease in the ratio of the ventilatory duration to the apnoeic pause duration, and then suppressed, as evidenced by a progressive return to stable breathing which was associated with an increase in minute ventilation. 3. Five of the six lambs that did not show PB following passive hyperventilation with air could be made to do so if oxygen was substituted for air as the inspired gas following passive hyperventilation. 4. Five of the eleven lambs that showed PB following hyperventilation with air responded to the application of oxygen during PB by switching to a gross form of episodic breathing consisting of long apnoeic pauses followed by equally long periods of breathing during which minute ventilation fell progressively with time. 5. We conclude that when applied against a background of arterial hypoxaemia, oxygen has a destabilizing influence on ventilation in that (a) it accentuates the unstable breathing that occurs during PB, (b) it induces PB in lambs that exhibited stable breathing in air, and (c) it may precipitate episodic breathing.

Source of respiratory drive during periodic breathing in lambs.

In order to investigate the mechanisms underlying periodic breathing (PB), we studied the initiation of breathing after passive hyperventilation in 14 anaesthetised 10-20 day old lambs. Eight of the lambs exhibited PB following post-hyperventilation apnea (PHA), with an epoch duration of 82.4 +/- 14.2 sec (mean +/- SEM), a cycle duration of 9.7 +/- 0.7 sec and a ratio of ventilatory duration to apnea duration (V-A ratio) of 1.24 +/- 0.32. The remaining lambs showed stable breathing patterns following PHA. The ventilatory response to isocapnic hypoxia was significantly greater in the group that had PB (-7.2 +/- 1.0 ml min-1% Sao2-1 kg-1) than in the animals that did not (-2.5 +/- 1.0 ml min-1%Sao2-1 kg-1). Using experimentally determined ventilatory response curves to O2 and CO2 we calculated that the swings in Sao2 and Paco2 during PB generated chemical drive that accounted for only 16.2% of the ventilatory oscillations observed during PB. Much of the remaining drive appeared to originate in the 'switch-on' characteristics of the respiratory controller, in lambs that exhibited periodic breathing, when breathing began after PHA ventilation jumped abruptly from zero to 55.1% of the eupneic ventilation. The magnitude of this jump in ventilation accounted for 51.9% of the amplitude of ventilatory oscillations that occur during PB. We speculate that this previously unrecognised feature of the respiratory controller, together with an elevated sensitivity to hypoxaemia, play crucial roles in generating PB in the infant.

Effect of venous oxygenation on arterial desaturation rate during repetitive apneas in lambs.

While arterial oxygen desaturation during apnea is a common occurrence in adults and infants, the factors determining the rate of desaturation are poorly understood. We describe a theoretical model which suggests that arterial desaturation during an apneic episode occurs in two stages. In the initial stage (stage 1) the oxygen store in the lung is depleted, while in the second phase of the desaturation process (stage 2) tissue oxygen needs are met predominantly by depletion of the blood store. Our model predicts that preapneic venous oxygenation (SvO2) will strongly influence the rate of the desaturation in stage 1 but not in stage 2. We therefore examined the effect of changing preapneic SvO2 on the rate of arterial oxygen desaturation (SaO2) during stage 1 and stage 2 apnea in anaesthetised 10-20 day-old lambs. Preapneic arterial oxygen saturation was maintained constant. In agreement with the model's prediction there were two stages to the desaturation process and during stage 1 a significant increase in SaO2 was observed when preapneic SvO2 was lowered; SaO2 was -3.1 +/- 0.4%.sec-1 when SvO2 = 47.4 +/- 2.1% increasing to -5.8 +/- 0.7%.sec-1 when SvO2 = 28.3 +/- 1.2%. During stage 2, SaO2 was -1.62 +/- 0.07%.sec-1 and was independent of preapneic SvO2, also in accord with the model's prediction. In order to assess whether the accelerated desaturation rate we observed in stage 1 could have resulted from a decline in lung volume during apnea rather than lower levels of SvO2, we repeated the experiment with CPAP applied. Under these conditions SaO2 continued to be greater at lower preapneic SvO2 levels. In summary, lowered preapneic SvO2 has a potent influence on SaO2 during stage 1 of the desaturation process but not during stage 2.

High-frequency oscillatory ventilation compared with conventional mechanical ventilation in newborn lambs: effects of increasing airway pressure on intracranial pressures.

We tested the hypothesis that intracranial pressures and cerebral perfusion pressure in the newborn are more seriously affected by increasing airway pressure during high-frequency oscillatory ventilation (HFOV) than during conventional mechanical ventilation (CMV). Mean airway pressure was acutely elevated in stepwise fashion to 25 cm H2O in six anesthetized, paralyzed newborn lambs. Pressure (mean +/- SE) increased similarly during HFOV and CMV in the jugular vein (7 +/- 1 and 8 +/- 1 cm H2O, respectively), the sagittal sinus (6 +/- 1 and 7 +/- 1 cm H2O), and the cerebrospinal fluid of the lateral ventricle (4 +/- 1 and 6 +/- 1 cm H2O). Decreases in arterial blood pressure (-13 +/- 2 and -10 +/- 2 cm H2O) and cerebral perfusion pressure (-17 +/- 2 and -16 +/- 2 cm H2O) were also similar during HFOV and CMV. Intracranial pressure-volume curves were generated by incrementing cerebrospinal fluid volume in eight lambs. Curves generated during HFOV and CMV were similar, reflecting a similar intracranial compliance during the two ventilatory modes. These data indicate that intracranial compliance and the effects of increasing airway pressure upon intracranial pressures are not significantly different between HFOV and CMV.

Easily-implantable electrodes for chronic recording of electromyogram activity in small fetuses.

A method is described for the construction of simple, robust electrodes suitable for obtaining long-term chronic recordings of electromyogram activity from delicate muscles of small experimental animals. The electrodes are equipped with small, gold-plated barbed tips which have the form of harpoons that may be pushed directly into a muscle, where they remain without being sutured into place. These features greatly facilitate the instrumentation of muscles which are very thin or of limited accessibility. The electrodes have been used successfully to obtain chronic recordings of respiratory muscle activity from early gestation fetal lambs in utero.

The effect of high frequency oscillatory ventilation on extravascular lung water in the rabbit.

High frequency ventilation (HFV) systems have the theoretical advantage of producing less barotrauma and lung damage because of the associated smaller oscillations in alveolar pressure compared to conventional ventilators. An index of lung damage is an increase in extravascular lung water but previous studies using HFV have produced conflicting results. Lung wet-to-dry ratio, extravascular lung water and blood mass were measured in 30 rabbits divided into three groups (n = 10); spontaneously breathing (SV), conventional positive pressure ventilation (CV) and high frequency oscillatory ventilation (HFV). HFV maintained normal blood gases and produced a significant reduction in the wet-to-dry weight ratio compared to SV and CV (P less than 0.05). Extravascular lung water and blood mass were significantly reduced in HFV compared to SV (P less than 0.05). It is concluded that the system of HFV used in this study has clinical potential in the treatment of lung injury.

Phasic respiratory activity in the fetal lamb during late gestation and labour.

We quantified the respiratory activity of 9 fetal lambs using computer-analysis of the diaphragmatic electromyogram (EMG) obtained during 2 h recording sessions interspersed over the last 13 days of gestation. The fetuses delivered unassisted at an average gestational age of 145 days (term = 147 days). During the last 2 h of labour the number of phasic EMG bursts (breaths) averaged 3% of the peak recorded earlier in the study. This decline in breathing began at least 2 days before labour and resulted predominantly from the fetus spending an increasing proportion of time in apnoea. Respiratory rate within epochs of breathing also fell significantly 1 day before labour, and the proportion of time spent in the low voltage electrocortical state declined once labour commenced. No significant change occurred in arterial PO2, PCO2 or pH over the study period. We conclude that fetal respiratory activity falls well before the onset of labour, largely as a result of increased apnoea, and that the decline does not result from the development of a progressive hypoxaemia associated with labour.

Function of the future respiratory system in the growth retarded fetal sheep.

The development of the future air breathing respiratory system was investigated in eight carunclectomised fetal sheep and five control fetal animals over the last third of gestation (approximately day 100-day 145). Carunclectomy resulted in the test fetal animals being hypoxaemic, hypercapnic, hypoglycaemic and acidotic. All fetal animals were chronically instrumented with diaphragmatic electromyographic leads, fetal jugular vein and carotid artery catheters, an amniotic fluid catheter and small electrocardiographic leads placed on the chest wall. There was evidence of a significant, but small, reduced relative lung size in the carunclectomised animals by day 140 without corresponding relative changes in the weight of the brain, liver, or heart. Diaphragmatic electromyograph (EMG) was analysed in detail over the last third of gestation. In both groups of animals, the normal reduction in fetal 'breathing' rate, and the normal increase in fetal apnoea was noted with increasing periods of diaphragmatic silence as fetal apnoea was noted with increasing periods of diaphragmatic silence as gestation advanced. Linear regression analyses revealed that fetal breathing rate was significantly reduced over the last third of gestation, whilst fetal apnoea significantly increased in the carunclectomised animals. Average total integrated diaphragmatic activity was reduced prior to 120 days. This reduction was evident in the strength of each diaphragmatic contraction before 110 days. In three test and three control animals, histochemical analyses were undertaken to fibre type the muscles of the diaphragm. Fatigue resistant fibres were significantly reduced in the carunclectomised fetuses, and fibre diameters were diminished. Centroid analyses of the power spectrum of the diaphragmatic EMG did not however support the concept of increased muscle fatigue in the test animals.

Hypercapnic stimulation of respiratory activity in unanesthetized fetal sheep in utero.

Respiratory responses to hypercapnia were studied in seven chronic in utero fetal lambs between 105 and 138 days' gestation (16 expts). Fetal arterial CO2 tension was raised (mean increase 9.9 Torr) by altering maternal inspired gas concentrations. Diaphragmatic electromyogram (EMG) was recorded as the index of respiratory activity. Electrocortical and electroocular activity (3 fetuses) were monitored in an attempt to define sleep state. Average respiratory rate increased (90%) and fetal apnea decreased (60%) during hypercapnia. Mean respiratory rate during "on" periods (greater than 6 EMG bursts/min) increased significantly during hypercapnia throughout the gestational epoch studied. Mean duration of the inspiratory time (TI) showed no significant change. Variability in both rate and TI decreased in response to CO2 at all gestations. Integrated EMG activity per burst divided by TI increased significantly at all gestations; however, no gestational increase in responsiveness to CO2 was seen. Sleep states were not able to be consistently identified, and a quantifiable electrocortical response to CO2 was not observed. These results indicate a relatively early functional maturation of fetal respiratory responses to CO2.

Influence of the compound NA872 on cardiopulmonary maturation in the unanaesthetised fetal lamb.

15 fetal lambs were instrumented on approximately day 100 (term 147 days) with small catheters in their jugular veins, carotid arteries and tracheas and fine electrodes in their diaphragms and on the lateral surfaces of their thoraces. Measurements were made intermittently of carotid arterial pressure, jugular venous pressure, amniotic fluid pressure, ECG and electrical activity in the diaphragm for 2-hour periods up to day 120. From day 120, two hourly recordings were made daily and samples of lung liquid taken until day 130. On this day, studies of lung mechanics were made on selected fetuses. From day 120, 8 animals were treated with the compound NA872 and 7 animals were given a control infusion of normal saline. In comparison to control animals the lecithin-sphingomyelin (L:S) ratio increased in the test fetuses from days 125--130; respiratory activity (diaphragmatic EMG) was suppressed in the test animals suggesting a more mature respiratory system; but resting lung volume on the test animals was smaller. Heart rate decreased as rapidly in the test animals while changes in systemic vascular pressures were similar. These results indicate that while the lungs are more 'mature' in that L:S ratio is increased, resting lung volumes are smaller, and respiratory activity is decreased. They raise the interesting questions of the influence of fetal respiratory activity on lung and thoracic cage development and the possible direct action of the drug NA872 on the respiratory pattern generator and the lung.

A technique for the direct observation of the unanesthetized fetal sheep.

A method is described for the chronic implantation of a lucite window in the uterus of a pregnant ewe which allows direct access to the amniotic fluid compartment in the unanesthetized state. The window can be directly connected to an extrauterine viewing compartment allowing repeated observations and measurements of physiologic function. The compartment also provides an easy method of changing the fetal environment.

Modification of respiratory center output in the unanesthetized fetal sheep "in utero".

Diaphragmatic electromyographic activity, tracheal and amniotic fluid pressures, lung liquid flow, and carotid and jugular venous pressures were measured on eight fetal lambs who survived for periods of 9-43 days postoperatively. The fetal gestational age ranged from 98 to 113 days at operation. Respiratory center output of the fetus as indicated by electromyographic activity was modified by the following stimuli. It was suppressed by anesthesia and fetal hypoxia (Pao2 = 12 mmHg), tonically reduced by lung inflation, and stimulated by cyanide injections (150-600 mug) into the fetal jugular vein. Neuromuscular transmission to the diaphragm was blocked with d-tubocurarine (0.2-0.6 mg). These experiments indicate that central and motor pathways to the diaphragm are sufficiently mature by 101 days in the fetal sheep to permit their output to be modified by chemical and mechanical stimuli.

Lung liquid production and composition in the "in utero" foetal lamb.

Experiments were carried out on foetal lambs in utero from 103 to 133 days' gestation, to measure the daily flow and solute composition of liquid from the foetal lung. The mean daily flow from foetuses with a tracheal fistula was 231 ml/day (9.6 ml/h), while the mean hourly flow in foetuses with a tracheal loop was 5.1 ml/h. The pattern of flow of lung liquid was intermittent and irregular, with short periods of rapid flow interspersed with longer periods of slow and no flow. The Na+, Ca2+, Mg2+, Cl- and urea concentrations showed no significant change with increasing gestational age, but the K+ concentration showed a significant rise. It is concluded that the lung of the foetal lamb behaves as an exocrine gland, secreting a substantial volume of liquid of a relatively constant composition.