CO2 sensitivity of the complexity of phrenic neurograms in the piglet during early maturation

In this paper, we investigate the influence of hypercapnia on the dynamics of the phrenic neurogram in the piglet in two different age groups: 3-7 days (n = 11) and 10-16 days (n = 9). The phrenic neurogram was recorded from 17 piglets (3-16 days old) during control (40% O-2 with 3-5% end-tidal CO2), mild hypercapnia (40% O-2 with 7% CO2) and severe hypercapnia (40% O-2 with 15% CO2) and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy (complexity) of phrenic neurograms during the first 7 days of the postnatal age were 1.56 +/- 0.1 (standard deviation) during normal breathing, 1.51 +/- 0.1 during mild hypercapnia and 1.37 +/- 0.08 during severe hypercapnia. These values for the 10-16 days age group were 1.51 +/- 0.1 during control, 1.49 +/- 0.11 during mild hypercapnia and 1.38 +/- 0.05 during severe hypercapnia. The mean values of phrenic neurogram durations during the first 7 days of the postnatal age were 0.82 +/- 0.03 (standard deviation) s during normal breathing, 0.85 +/- 0.007 s during mild hypercapnia and 0.65 +/- 0.05 s during severe hypercapnia. These values for the 10-16 days age group were 0.97 +/- 0.09 s during control, 1.10 +/- 0.05 during mild hypercapnia and 0.78 +/- 0.05 s during severe hypercapnia. Our results show that the complexity values of the phrenic neurogram were significantly decreased when the CO2 concentration was shifted from control or mild to severe hypercapnia (p < 0.05) for both the 3-7 days old and the 10-16 days old groups. In addition, the duration of the phrenic neurogram decreased when the concentration was shifted from control or mild to severe hypercapnia (p < 0.05). But no significant changes in the duration of the phrenic neurogram were observed between control and mild hypercapnia concentration. These results suggest that severe hypercapnia can be characterized with a significant decrease of the complexity values and durations of the phrenic neurogram during inspiration during early maturation.