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1 Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Brussels, Belgium; Chest Service, Erasme University Hospital, Brussels, Belgium
2 Laboratory of Cardiorespiratory Physiology, Brussels School of Medicine, Brussels, Belgium; Department of Radiology, Erasme University Hospital, Brussels, Belgium
* To whom correspondence should be addressed. E-mail: a_detroyer{at}yahoo.fr.
At resting end-expiration (FRC), the actions of the left and right hemidiaphragms on the lung are synergistic. However, the synergism decreases in magnitude as muscle tension decreases. The hypothesis was therefore tested in anesthetized dogs that the degree of synergism between the two hemidiaphragms also decreases with increasing lung volume. In a first experiment, the changes in airway opening pressure (
Pao) and abdominal pressure
( Pab) obtained during simultaneous stimulation of the left and right phrenic nerves (measured P) at different lung volumes were compared to the sum of the pressure changes produced by their separate stimulation (predicted P). Although the pressure changes decreased markedly with increasing lung volume, the measured Pao and Pab were
substantially greater than the predicted values at all lung volumes. The ratio of the measured to the predicted Pao, in fact, remained constant. In a second experiment, radiographic measurements showed that the fractional shortening of the muscle during bilateral contraction at high lung volumes was similar to that during unilateral contraction. During unilateral
contraction at high lung volumes, however, the passive hemidiaphragm moved in the cranial direction, whereas during unilateral contraction at FRC, it moved in the caudal direction. These observations indicate that 1) for a given muscle tension, the synergism between the two
halves of the diaphragm is greater at high lung volumes than at FRC; and 2) this difference is primarily related to the greater distortion of the muscle configuration.
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