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Mechanics of the respiratory system in the newborn tammar wallaby

P. M. MacFarlane¹, P. B. Frappell^1,* and J. P. Mortola²

¹ Department of Zoology, La Trobe University, Melbourne, Victoria 3086, Australia and
² Department of Physiology, McGill University, Montreal, Quebec, Canada H3G 1Y6

View larger version (15K): [in a new window]   Fig. 1. Schematic diagram of the apparatus used to measure the mechanics of the respiratory system. The joey fitted with a facemask was placed in a water-jacketed chamber to maintain temperature and humidity; the facemask opened to room air. A pneumotachograph (P) was inserted directly into the open end of the facemask and the corresponding flow trace electronically integrated for volume. A vacuum was applied to the compartment enclosing the body to produce negative pressures of 981 Pa, adjusted by altering the size of a leak and monitored by a water-filled manometer. Abdominal displacement was measured using a force transducer with a wire arm resting laterally on the abdominal surface just below the final rib. A moist swab maintained humidity in the chamber.

View larger version (12K): [in a new window]   Fig. 2. Determination of respiratory system compliance (Crs) in the newborn tammar wallaby. Crs was determined from the change in volume (V) associated with the change in pressure (P) induced by applying a negative pressure to the body compartment. Note that the breathing pattern is characterized by an end-inspiratory pause, probably due to laryngeal closure (Farber, 1978). Inspiration is upwards.

View larger version (20K): [in a new window]   Fig. 3. Breathing pattern (flow and volume) and abdominal motion in newborn tammar wallabies at 1 and 6 days of age. At the end of inspiration, the respiratory muscles are contracting (active), whereas they are relaxed (passive) during the end-inspiratory pause. The distortion index is calculated from the degree of abdominal displacement (A) during active and passive conditions, 1–(Ap/Aa), with a value of 1 implying maximum distortion and a value of 0 no distortion.

View larger version (25K): [in a new window]   Fig. 4. Respiratory system compliance (Crs) (A) and resistance (Rrs) (B) as a function of body mass (W) in newborn mammals [open symbols (Mortola, 2001); solid symbols, this study]. The exponents of the allometric equations for all newborns and for eutherians only are indicated. Values for tammar wallabies are means ±1 S.E.M. (day 1, N=7; day 6, N=6).

View larger version (12K): [in a new window]   Fig. 5. Mass-specific ventilation (E), tidal volume (O2), breathing frequency (f), convection requirement (E/O2), relative importance of skin O2 (O2 skin:total) and chest wall distortion index in newborn tammar wallabies at 1 (open columns) and 6 (filled columns) days. Values are means +1 S.E.M. (day 1, N=7; day 6, N=6); an asterisk indicates a statistically significant difference (P<0.05) between the two age groups.