Functional residual capacity and ventilation homogeneity in mechanically ventilated small neonates

J Appl Physiol (1985). 1992 Jul;73(1):276-83. doi: 10.1152/jappl.1992.73.1.276.


A modification of a computerized tracer gas (SF6) washout method was designed for serial measurements of functional residual capacity (FRC) and ventilation homogeneity in mechanically ventilated very-low-birth-weight infants with tidal volumes down to 4 ml. The method, which can be used regardless of the inspired O2 concentration, gave accurate and reproducible results in a lung model and good agreement compared with He dilution in rabbits. FRC was measured during 2-4 cmH2O of positive end-expiratory pressure (PEEP) in 15 neonates (700-1,950 g), most of them with mild-to-moderate respiratory distress syndrome. FRC increased with body weight and decreased (P less than 0.05) with increasing O2 requirement. Change to zero end-expiratory pressure caused an immediate decrease in FRC by 29% (P less than 0.01) and gave FRC (ml) = -1.4 + 17 x weight (kg) (r = 0.83). Five minutes after PEEP was discontinued (n = 12), FRC had decreased by a further 16% (P less than 0.01). The washout curves indicated a near-normal ventilation homogeneity not related to changes in PEEP. This was interpreted as evidence against the presence of large volumes of trapped alveolar gas.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Body Weight / physiology
  • Functional Residual Capacity / physiology
  • Gestational Age
  • Helium
  • Humans
  • Infant, Newborn / physiology*
  • Models, Biological
  • Positive-Pressure Respiration
  • Respiration / physiology*
  • Respiration, Artificial*
  • Respiratory Distress Syndrome, Newborn / physiopathology
  • Sulfur Hexafluoride


  • Helium
  • Sulfur Hexafluoride