Ventilatory and timing parameters in normal horses at rest up to age one year

The purpose of the study was to document the developmental changes in the ventilatory and timing parameters associated with quiet breathing at rest in awake, standing horses during the first year post partum. Tidal volume (VT), breathing frequency, airflow, mechanical timing intervals and minute ventilation (VE) were measured serially in foals age 24 h-1 year. In the growing foal, VE increased due to a progressive rise in VT, in spite of a pronounced decrease in respiratory frequency. When normalised to body weight (bwt), VE/kg declined with maturation in a curvilinear fashion, from mean +/- s.d. 848 +/- 231 ml/min/kg in the 24 h-old foal, to 155 +/- 15 ml/kg/min in the 1-year-old foal. Tidal volume normalised to bwt remained relatively constant during the study period, with the exception that at age 3 weeks and from 2-6 months, VT/kg was significantly lower than the value recorded at age 1 week. The decrease in frequency resulted from prolongation of both inspiratory (TI) and expiratory (TE) time but there was a disproportionately larger increase in TE compared to TI, which resulted in a significantly lower ratio of TI/TE in older foals. The allometric equation relating VT to bwt suggested that lung growth in the horse is dysanaptic, with increases in overall body size exceeding lung growth in the maturing foal during the first year post partum.