Effect of compression pressure on forced expiratory flow in infants

The effect of the force of compression on expiratory flow was evaluated in 19 infants (2-13 mo of age) with respiratory illnesses of varying severity. An inflatable cuff was used to compress the chest and abdomen. Expiratory flow and volume, airway occlusion pressure, cuff pressure (Pc), and functional residual capacity were measured. Transmission of pressure from cuff to pleural space was assessed by a noninvasive occlusion technique. Close correlations (P less than 0.001) were found between Pc and the change in pleural pressure with cuff inflation (delta Ppl,c). Pressure transmission was found to vary between two cuffs of different design and between infants. Several forced expirations were then performed on each infant at various levels of delta Ppl,c. Infants with low maximal expiratory flows at low lung volumes required relatively gentle compression to achieve flow limitation and showed decreased flow for firmer compressions. Flow-volume curves in each infant tended to become more concave as delta Ppl,c increased. These findings underline the importance of knowledge of delta Ppl,c in interpreting expiratory flow-volume curves in infants.