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.

The contribution of the carotid body chemoreceptor to postnatal maturation of breathing was evaluated in lambs from 7 to 70 days of age. The study was conducted by comparing the eupneic ventilation and resting pneumograms in intact conscious lambs with those of lambs that were carotid body chemodenervated (CBD) at birth. In comparison to the 1-wk-old intact lambs, the CBD lambs had significant decreases in minute ventilation (VE, 313 vs. 517 ml/kg), tidal volume (VT, 7.2 vs. 10.5 ml/kg), respiratory rate (f, 44 vs. 51 breaths/min), and occlusion pressure (P0.1, 2.8 vs. 7.2 cmH2O). Arterial PO2's were 59 vs. 75 Torr (P less than 0.05) and arterial PCO2's 47 vs. 36 Torr (P less than 0.05), respectively, in CBD and intact lambs. In intact lambs from 7 to 70 days, resting VE decreased progressively from 517 to 274 ml/kg (P less than 0.01) due to a fall in VT, mean inspiratory flow (VT/TI), and f, whereas the ratio of inspiratory time to total breath duration remained constant. P0.1 decreased from 7.2 to 3.9 cmH2O from 7 to 42 days. In contrast the CBD lambs experienced only minimal changes in VE, VT, VT/TI, and f during the same period. VE only decreased from 313 to 218 and P0.1 from 2.8 to 2.4 cmH2O. In contrast to that of intact lambs the resting pneumogram of CBD lambs remained relatively fixed from 7 to 70 days. Three CBD lambs died unexpectedly, without apparent cause, in the 4th and 5th wk of life.(ABSTRACT TRUNCATED AT 250 WORDS)

The ventilatory response of newborn lambs to hypoxemia was evaluated in two groups of seven awake lambs studied at 2 and 7 days of life. Minute ventilation (VE) and airway occlusion pressure (P0.1) were monitored as the animals were exposed in sequence to room air, 12% O2 (15 min), 7% O2 (15 min), and room air. On 12 and 7% O2, 2-day-old lambs experienced a brisk hyperventilation followed by a VE depression, previously described in newborns of other species (diphasic response). The 7-day-old lambs had a clear diphasic VE response only on 7% O2 breathing. In the 2-day-old lambs, at the time of the relative VE depression to 12% O2, the respiratory centers showed a persisting responsiveness to further hypoxia; switching to 7% O2 caused a brisk increase in VE and P0.1 of 70 and 130%, respectively, which was followed again by a VE depression. The magnitude of the immediate VE response to hypoxia, taken as an index of the chemoreceptor strength, was inversely related to the magnitude of the VE depression (R = 0.81, P less than 0.001). It was concluded that 1) lambs as well as other neonates have an age-related diphasic VE response to hypoxia; 2) at the time of the VE depression, the respiratory centers maintain their responsiveness to further acute hypoxia; and 3) the weakness of the chemoreceptors in the newborn is a major determinant of the diphasic response.

Summary. The Sherbrooke School of Medicine, Quebec, has restructured its entire curriculum to make problem‐based learning (PBL) the main instructional format. This complete reform is explained both in terms of process and content. The curriculum problems were clearly identified and overcome by a major structural shift‐over following the stages of a strategic planning of change. Implementation over a period of 7 years is described according to a four‐stage framework: need for change; selection of the PBL solution; planning for implementation; and the full‐scale adoption of the PBL method. The programme is described in relation to the congruence of goals, learning and evaluation activities. Initial impact on student learning and evaluation, attracting better quality students, academic staff roles, and on financing the operation are discussed. Changing the undergraduate programme has become an institutional project directed by the Office of the Dean.

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