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Breath-to-breath hypercapnic response in neonatal rats: temperature dependency of the chemoreflexes and potential implications for breathing stability

Department of Zoology, La Trobe University, Melbourne, Victoria, Australia

Submitted 15 December 2008 ; accepted in final form 29 April 2009

The breathing of newborns is destabilized by warm temperatures. We hypothesized that in unanesthetized, intact newborn rats, body temperature (T_B) influences the peripheral chemoreflex response (PCR response) to hypercapnia. To test this, we delivered square-wave challenges of 8% CO₂ in air to postnatal day 4–5 (P4–P5) rats held at a T_B of 30°C (Cold group, n = 11), 33°C (Cool group, n = 10), and 35°C thermoneutral zone group [thermoneutral zone (TNZ) group, n = 11], while measuring ventilation (E) directly with a pneumotach and mask. Cool animals were challenged with 8% CO₂ balanced in either air or hyperoxia (n = 10) to identify the PCR response. Breath-to-breath analysis was performed on 30 room air breaths and every breath of the 1-min CO₂ challenge. As expected, warmer T_B was associated with an unstable breathing pattern in room air: TNZ animals had a coefficient of variation in E (E CV%) that was double that of animals held at cooler T_B (P < 0.001). Hyperoxia markedly suppressed the hypercapnic ventilatory response over the first 10 breaths (or 4 s), suggesting that this domain is dominated by the PCR response. The PCR response (P = 0.03) and total response (P = 0.04) were significantly greater in TNZ animals compared with hypothermic animals. The total response had a significant, negative relationship with CO₂ (R² = 0.53; P < 0.001). Breathing stability was positively related to the total response (R² = 0.36; P < 0.001) and to a lesser extent, the PCR response (R² = 0.19; P = 0.01) and was negatively related to CO₂ (R² = 0.34; P < 0.001). ANCOVA confirmed a significant effect of T_B alone on breathing stability (P < 0.01), with no independent effects of CO₂ (P = 0.41), the PCR response (P = 0.82), or the total E response (P = 0.08). Our data suggest that in early postnatal life, the chemoreflex responses to CO₂ are highly influenced by T_B, and while related to breathing stability, are not predictors of stability after accounting for the independent effect of T_B.

ventilation; carotid body; newborn; apnea; CO₂ response