Postnatal changes in transdiaphragmatic pressure in piglets.

We examined diaphragmatic force output in 25 anesthetized piglets ranging in postnatal age from 4 to 21 days (weight 1.3-4.0 kg) in order to determine whether the diaphragm produces greater force output with maturation for a given level of neural input. Transdiaphragmatic pressure (Pdi) served as our index of diaphragmatic force output and was measured during "supramaximal" transvenous phrenic nerve stimulation at 100 Hz in order to control neural drive. Mean Pdi was 53 +/- 17 cm H2O and ranged from a minimum of 29 cm H2O to a maximum of 83 cm H2O. A significant positive correlation between Pdi and postnatal age was observed (r = 0.79, p less than 0.001). In addition, positive correlations were noted between Pdi and total body weight (r = 0.73, p less than 0.001) and Pdi and diaphragmatic wet weight (r = 0.77, p less than 0.001). The voltage needed to stimulate the phrenic nerves "supramaximally" did not correlate with postnatal age (r = 0.02, p = 0.16). We conclude that a developmental pattern of increasing Pdi with increasing postnatal age, total body weight, and diaphragmatic wet weight exists in piglets and occurs within the context of a controlled level of neural drive.