Respiratory effects of progressive asphyxia in rabbit pups and adult rabbits

Characterization of successful and failed autoresuscitation in human infants, including those dying of SIDS

Our purpose was to identify and further characterize physiologic mechanisms relevant to autoresuscitation from hypoxic apnea in infants dying suddenly and unexpectedly. We studied cardiorespiratory recordings of 24 infants (age range, 0.8-21 months) who died suddenly while being monitored at home. These recordings were analyzed for features indicated by studies in animal models to be characteristic of hypoxic gasping, and of recovery from bradycardia and apnea associated with gasping (e.g., autoresuscitation). Findings in 5 infants diagnosed as having sudden infant death syndrome were compared with 6 non-SIDS infants whose deaths resulted from other conditions. Additionally, we studied 15 healthy infants during sleep, using home monitor and other respiratory recording techniques, in order to obtain comparison data. We found in recordings from 23 of 24 subjects that hypoxic gasps with characteristic features occurred immediately preceding death. A unique pattern of complex, closely spaced gasps ("double" or "triple" gasps) was present in many subjects. Evidence of partially successful autoresuscitation closely following one or more gasps occurred in 11 subjects, while another 4 had evidence of complete autoresuscitation with return of normal heart rate and resolution of apnea on one or more occasions. Significant differences between SIDS infants and those dying from other causes included increased occurrence of complex gasps and decreased occurrence of partial or complete autoresuscitation in the SIDS infants. The non-SIDS cases were different from the SIDS cases in that only one had "double" gasps (n = 7), while none had "triple" gasps, as compared with 4 out of 5 SIDS cases with these patterns (P < 0.05, chi-square). Also, in contrast with the SIDS cases, more of the cases with specific postmortem diagnoses had evidence of partial (5 out of 6 cases) or complete (1 out of 6 cases) autoresuscitation (P < 0.05, chi-square). We conclude that partial or complete autoresuscitation by gasping is not uncommon in moribund infants during the first year of life. Failure of autoresuscitation mechanisms other than failure to initiate gasping may be characteristic of infants dying of SIDS. Some SIDS infants appear to be different from infants dying with other diagnoses with respect to efficacy and characteristics of hypoxic gasping.

Immunocytochemical detection of serotonin content in raphe neurons of newborn and young adult rabbits before and after acute hypoxia

The present immunocytochemical study demonstrates serotonin (5-HT) depletion in the dorsal raphe nucleus (DRN) of 3- and 21-day-old rabbits following exposure to mild (10% ambient partial pressure of oxygen) and severe hypoxia (5% ambient oxygen). Under the mild hypoxic condition, 5-HT immunoreactivity in cells and fibers of the DRN was decreased in 3-day-old as well as 21-day-old rabbits, as indicated by decreased intensity of the staining compared to age-matched controls. Although this decrease was more pronounced in the younger animals, recovery from mild hypoxia was seen in both age groups. Hypoxic effects were more striking in 3-day-old animals under the severe hypoxic condition, indicating a greater depletion of 5-HT than in the mildly hypoxic condition. However, little additional effect on the older age group was seen. Further, a decreased ability of the 3-day-old rabbits to recover following severe hypoxia suggests that protracted effects on the developing serotonergic system occur following severe hypoxia during the neonatal period. This was demonstrated by the long-lasting decrease in the number of stained cells and fibers of the DRN 4-hr after return to normal conditions (21% O2). We conclude that newborns have a decreased rate of 5-HT synthesis and/or metabolic turnover that results in rapid depletion of intracellular stores and protracted time to recover from a hypoxic challenge. Similar effects could occur in human fetuses, newborns or infants following birth trauma, apnea or other events associated with severe hypoxia.

Laryngeal motoneurone activity in the rabbit during asphyxic gasping

Unitary activity was recorded from 44 recurrent laryngeal nerve (RLN) and 18 external laryngeal nerve (ELN) motoneurones in anaesthetized, paralysed rabbits during the development of periods of severe asphyxia caused by temporarily interrupting artificial ventilation. Ventilation was only recommenced when a simultaneously recorded phrenic neurogram showed the animal had attempted several gasps. Inspiratory-phased fibres (17 RLN, 14 ELN) displayed brief high frequency discharges with each gasp. Some inspiratory-phased ELN motoneurones (6/14) also displayed longer tonic discharges after each of the first few gasps. In the expiratory intervals immediately following each gasp many expiratory-phased fibres (13/21 RLN, 2/4 ELN) were active and 7 RLN fibres discharged a long train of impulses with a decrementing discharge pattern. In 3 anaesthetized rabbits, breathing spontaneously through a tracheostomy, a constant airflow was directed up through the larynx; successive gasps were associated with wide swings in translaryngeal pressure which were confirmed visually as reflecting rapid dilation of the glottis during gasps and intense constriction in the intervals between gasps.

Inspiratory augmentation during asphyxic hyperpnoea and gasping: proprioceptive influences

The relative inspiratory augmentations of sternomastoid, scalene, external intercostal, interchondral and diaphragmatic electromyographic activities were examined during the progressive asphyxia induced by rebreathing in pentobarbitone/urethane anaesthetized rabbits. Diaphragmatic activity augmented significantly less than that of the scalene, intercostal and interchondral muscles in response to the asphyxic increase in inspiratory drive (hyperpnoea). Cervical vagotomy significantly increased the levels of inspiratory activity during the asphyxic hyperpnoea but did not abolish these relations. Dorsal rhizotomy at levels appropriate for the respective recording sites significantly limited the intercostal and interchondral augmentations, and depressed diaphragmatic activity to a less pronounced extent, but did not affect scalene activity. The asphyxic apnoea succeeding the hyperpnoea was terminated by gasping respiration. Excepting the interchondrals each muscle exhibited significantly greater electromyographic amplitudes during gasping than during the terminal hyperpnoeic efforts. Only during gasping were the sternomastoid muscles recruited. The intercostal and scalene gasps were significantly greater than those exhibited by either the interchondrals or the diaphragm. Such disproportionate responses were not affected by vagotomy or dorsal rhizotomy. The disparate inspiratory contributions during progressive asphyxia can therefore only be partially accounted for by differences in proprioceptive control.

Costal and crural diaphragm and intercostal muscle activity during augmented breaths in cats

The pattern of electrical activity of the costal and crural diaphragm and the external intercostal muscles was ascertained in ten anesthetized spontaneously breathing cats during eupnea and during augmented breaths. All muscles studied manifested a biphasic activity pattern during augmented breaths. The increase in activity during the latter portion (phase II) of the augmented breaths was greater for the crural than the costal diaphragm (P less than 0.05), and greater for cranially located intercostal muscles than for the costal diaphragm (P less than 0.02) and more caudally located intercostal muscles (P less than 0.02). These results suggest that during augmented breaths, activity of different thoracic muscles is modulated in a non-uniform manner.