To investigate the action of the inspiratory intercostals, we studied the patterns of rib cage and abdominal motion during tidal breathing in eight dogs before and after bilateral phrenicotomy. Hooks screwed into the sternum and the ribs were used to monitor the anteroposterior and transverse diameters of the rib cage and the axial displacements of the sternum and the ribs. In all animals, we found that during tidal inspirations performed with the inspiratory intercostals alone, 1) the rib cage moved outward while the abdomen moved inward; 2) the rib cage was displaced near its relaxation curve (defined by a plot of its transverse vs. anteroposterior diameter) but expanded more in its transverse than in its anteroposterior dimension; and 3) the ribs in the midaxillary line moved cephalad, whereas the sternum was displaced caudally. Additional experiments performed on four dogs demonstrated that contraction of the parasternal intercostals was responsible for the inspiratory caudal displacement of the sternum. These findings indicate that in the dog, 1) the inspiratory intercostals act essentially on the lateral walls of the rib cage, and 2) those of the parasternal area produce a caudal displacement of the sternum. In addition, they suggest strongly that the axial displacement of the sternum constitutes an additional degree of freedom of motion of the rib cage and that contraction of the neck accessory muscles is mandatory for the sternum to be displaced in a cephalad direction during inspiration.
It is conventionally considered that because of their fiber orientations, the external intercostal muscles elevate the ribs, whereas the internal interosseous intercostals lower the ribs. The mechanical action of the intercostal muscles, however, has never been studied directly, and the electromyographic observations supporting this conventional thinking must be interpreted with caution. In the present studies, the external and internal interosseous intercostal muscles have been separately stimulated in different interspaces at, above, and below end-expiratory rib cage volume in anesthetized dogs. The axial (cephalo-caudal) displacements of the ribs were measured using linear displacement transducers. The results indicate that when contracting in a single interspace and other muscles are relaxed, both the external and internal intercostals have a net rib elevating action at end-expiratory rib cage volume. This action increases as rib cage volume decreases, but it progressively decreases as rib cage volume increases such that at high rib cage volumes, both the external and internal intercostals lower the ribs. Stimulating the intercostal muscles in three adjacent intercostal spaces simultaneously produced similar directional rib motion results. We conclude that (a) in contrast with the conventional thinking, the external and internal interosseous intercostals acting alone have by and large a similar effect on the ribs into which they insert; (b) this effect is very much dependent on rib cage (lung) volume; and (c) intercostal muscle action is primarily determined by the resistance of the upper ribs to caudad displacement relative to the resistance of the lower ribs to cephalad displacement. The lateral intercostals, however, might be more involved in postural movements than in respiration. Their primary involvement in rotations of the trunk might account for the presence of two differently oriented muscle layers between the ribs.
We measured tracheal flow from tracheal sounds to estimate tidal volume, minute ventilation (VI), respiratory frequency, mean inspiratory flow (VT/TI), and duty cycle (TI/Ttot). In 11 normal subjects, 3 patients with unstable airway obstruction, and 3 stable asthmatic patients, we measured tracheal sounds and flow twice: first to derive flow-sound relationships and second to obtain flow-volume relationships from the sound signal. The flow-volume relationship was compared with pneumotach-derived volume. When subjects were seated, facing forward and with neck rotation, flexion, and standing, flow-volume relationship was within 15% of pneumotach-derived volume. Error increased with neck extension and while supine. We then measured ventilation without mouthpiece or nose clip from tracheal sounds during quiet breathing for up to 30 min. Normal results +/- SD revealed tidal volume = 0.37 +/- 0.065 liter, respiratory frequency = 19.3 +/- 3.5 breaths/min, VI = 6.9 +/- 1.2 l/min, VT/TI = 0.31 +/- 0.06 l/s, and TI/Ttot = 0.37 +/- 0.04. Unstable airway obstruction had large VI due to increased VT/TI. With the exception of TI/Ttot, variations in ventilatory parameters were closer to log normal than normal distributions and tended to be greater in patients. We conclude that phonospirometry measures ventilation reasonably accurately without mouthpiece, nose clip, or rigid postural constraints.
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