Respiratory rate and body mass in the first three years of life

The correlation between respiratory rate and body mass in the first three years of life was examined in 635 infants and children, aged between 15 days and 3.5 years. Assessments were made during quiet sleep and when awake and calm. Their body weight (2.8-20.5 kg) was measured at the same time. Respiratory rate decreased exponentially with increasing body weight. After log transformation of the data, a linear correlation was found between log(respiratory rate) and log(body weight) both during the sleeping and the wakeful states. For each body weight group, boys had about a 6.5% higher respiratory rate than girls, but both the boys’ and the girls’ lines were parallel. The slope of the line was -0.43 during sleep and -0.41 during wakefulness, indicating that the respiratory rate: body weight ratio is not a constant in either state, and that it decreases as body weight increases. Adding age to the model did not significantly increase the precision of the fitting. A simple equation of the form respiratory rate=avbody weight, can accurately describe the decrease in respiratory rate with growth. Smaller (hence younger) infants and children have a higher respiratory rate per unit body weight than larger (hence older) children. (Arch Dis Child 1997;76:151–154)