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1 Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Erasme University Hospital, Brussels, Belgium
* To whom correspondence should be addressed. E-mail: a_detroyer{at}yahoo.fr.
The changes in intrathoracic pressure produced by the various inspiratory intercostals are essentially additive, but the interaction between these muscles and the diaphragm remains uncertain. In the present study, this interaction was assessed by measuring the changes in airway opening (Pao) or transpulmonary (Ptp) pressure in vagotomized, phrenicotomized dogs during spontaneous inspiration (isolated intercostal contraction), during isolated rectangular or ramp stimulation of the peripheral ends of the transected C5 phrenic nerve roots (isolated diaphragm contraction), and during spontaneous inspiration with superimposed phrenic nerve stimulation (combined diaphragm-intercostal contraction). With the endotracheal tube occluded at FRC, 
Pao during combined diaphragm-intercostal contraction was nearly equal to the sum of the Pao's produced by the two muscle groups contracting individually. However, when the endotracheal tube was kept open, Ptp during combined contraction was 123 % of the sum of the individual Ptp's (P<0.001). The increase in lung volume during combined contraction was also 109 % of the sum of the individual volume increases (P<0.02). Abdominal pressure during combined contraction was invariably lower than during isolated diaphragm contraction. It is concluded, therefore, that the canine diaphragm and intercostal muscles act synergistically during lung expansion and that this synergism is primarily due to the fact that the intercostal muscles reduce shortening of the diaphragm. When the lung is maintained at FRC, however, the synergism is obscured because the greater stiffness of the rib cage during diaphragm contraction enhances the Pao produced by the isolated diaphragm and reduces the Pao produced by the intercostal muscles.
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