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Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Brussels; and Chest Service and Department of Radiology, Erasme University Hospital, 1070 Brussels, Belgium
Expansion of the lung during
inspiration results from the coordinated contraction of the diaphragm
and several groups of rib cage muscles, and we have previously shown
that the changes in intrathoracic pressure generated by the latter are
essentially additive. In the present studies, we have assessed the
interaction between the right and left hemidiaphragms in anesthetized
dogs by comparing the changes in airway opening pressure (
Pao)
obtained during simultaneous stimulation of the two phrenic nerves
(measured Pao) to the sum of the Pao values produced by their
separate stimulation (predicted Pao). The measured Pao was
invariably greater than the predicted Pao, and the ratio between
these two values increased gradually as the stimulation frequency was
increased; the ratio was 1.10 ± 0.01 (P < 0.05)
for a frequency of 10 Hz, whereas for a frequency of 50 Hz it amounted
to 1.49 ± 0.05 (P < 0.001). This interaction
remained unchanged after the rib cage was stiffened and its compliance
was made linear, thus indicating that the load against which the
diaphragm works is not a major determinant. However, radiographic
measurements showed that stimulation of one phrenic nerve extends the
inactive hemidiaphragm toward the sagittal midplane and reduces the
caudal displacement of the central portion of the diaphragmatic dome.
As a result, the volume swept by the contracting hemidiaphragm is
smaller than the volume it displaces when the contralateral
hemidiaphragm also contracts. These observations indicate that
1) the left and right hemidiaphragms have a synergistic,
rather than additive, interaction on the lung; 2) this
synergism operates already during quiet breathing and increases in
magnitude when respiratory drive is greater; and 3) this
synergism is primarily related to the configuration of the muscle.
respiratory muscles; mechanics of breathing
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