Spontaneous variability in minute ventilation oxygen consumption and heart rate of low birth weight infants

Continuous measurements of minute ventilation (V̇(I)), oxygen consumption (V̇(O2)), heart rate (HR), activity, and temperature were made in eleven low birth weight infants during the interval between feedings. Significant increases in V̇(I), V̇(O2), and HR were noted between quiet and active sleep. (V̇(I Active) - V̇(I Quiet)/V̇(I Quiet)) X 100 = 18.4% V̇(O2 Active) - V̇(O2 Quiet)/V̇(O2 Quiet)) X 100 = 10.1% and HR(Active) - HR(Quiet)/HR(Quiet)) X 100 = 6.4%. Significant differences were also noted within epochs of the same state of sleep: mean slope V̇(I) versus time in epoch (t) = -156 ml/kg X min/hr, V̇(O2) versus t. = 1.49 ml/kg X min/hr and HR versus t = -15.0 beats/min/hr. Differences between successive epochs of the same state of sleep were also observed V̇(I), +5.9 to 46.6%; V̇(O2), 4.7 to 24.6%; HR, 1.0 to 9.7%. These differences were related to the length of time after feeding. These data indicate that steady state conditions do not occur in growing low birth weight infants and that the design of studies of respiration and metabolism in these infants should include continuous assessment of the state of sleep or activity and time after feeding to ensure that experimental and control periods are truly comparable.