Action of the Inspiratory Muscles of the Rib Cage during Breathing in Newborns

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Abstract
To determine whether the rib cage muscles actively contribute to tidal volume change in infancy, we measured tidal volume (VT), using a pneumotachograph, respiratory gastric pressure swings (Pga), using a liquid-filled gastric catheter, and rib cage and abdominal volume, using respiratory inductive plethysmography in 15 newborns, both before and during 2% CO2-induced hyperventilation. Active rib cage expansion produced by phasic contraction of the inspiratory muscles of the rib cage should reduce respiratory abdominal pressure fluctuations by moving the anterior abdominal wall outward and cephalad, thereby having an expanding influence on the abdominal cavity. During quiet sleep (n = 13), CO2-induced hyperventilation was associated with significant increases in VT, Pga, rib cage volume (Vrc), and abdominal volume (Vab). Increments in Pga were small relative to VT, as shown by an increase in the slope of the VT versus Pga respiratory loop (VT/Pga) in all subjects (p < 0.001, paired t test). CO2 breathing increased mean VT by 72%, whereas mean Pga increased by only 23%. During quiet sleep, CO2 breathing was associated with an increase in the contribution of the rib cage compartment to total volume change (Vrc/Vrc+Vab) in all infants studied (p < 0.001, paired t test), and the total volume response to hyperventilation was more strongly related to changes in rib cage volume (slope = 0.62, r = 0.90) than to abdominal volume (slope = 0.31, r = 0.60). During REM sleep (n = 6), mean VT/Pga did not change significantly, and the rib cage contribution to tidal breathing decreased in three of six infants. We conclude, in newborns during quiet sleep, that active rib cage expansion prevents large swings in abdominal pressure during hyperventilation, and that rib cage expansion during CO2-breathing is most likely the result of phasic contraction of the inspiratory muscles of the rib cage.