Anatomic Correlates of Physiologic Function

The Muscle-Powered Air Pump

The work of breathing is supplied by two groups of muscles (three sets are inspiratory and three are expiratory) that act on skeletal and soft tissue structures of the trunk (chest plus abdomen or, in the classic Greek sense, the thorax). The thorax is bounded by the sternum, ribs, spine, intercostal muscles, abdominal wall muscles, clavicle and strap muscles superiorly, and the pelvic floor inferiorly. The actions of the muscles on the skeletal and soft tissue organs provide active inspiration, store energy in the parenchyma and thorax for passive exhalation, and provide active exhalation when required (see Table 2.1 ).

The inspiratory muscles consist of the diaphragm; the external intercostals; and the accessory muscles, including the strap muscles of the neck and the erector spini ( Fig. 2.1 ). ^,

The diaphragm is a dome-shaped structure consisting of a central tendon with muscle fibers radiating outward to attach on the xiphoid, on the seventh to twelfth ribs, and on the vertebral bodies. Contraction under phrenic nerve control flattens the dome shape, increasing the cephalocaudal (CC) dimension of the lung. Simultaneously, it displaces the liver, spleen, and stomach both anteriorly and laterally, increasing the anterior-posterior (AP) and the side-to-side (S-S) or lateral dimension of the chest through passive movement of the rib cage ( Fig. 2.2 ).

The rib cage volume is also increased by the action of the external (oblique) intercostal muscles. The fibers of these muscles run diagonally upward and backward from the top of one rib to the bottom of the next above. Three types of motion result from the slightly different articulation of: (1) the first two or three ribs, (2) the middle ribs, and (3) the lowermost two pairs of ribs: pump handle, bucket handle, and caliper motion. The first three or four ribs lie in planes that slope primarily from back to front. When they move upward, they increase the AP and CC diameters. This movement is like a pump handle. The next six or seven ribs lie in planes that are increasingly tilted to the side. When they move, the S-S diameter increases by the bucket handle motion induced by the articulation of these ribs at the sternum, as well as at the vertebral column. The eleventh and twelfth ribs move primarily outward, aided by pressure of the viscera, as well as upward, like the jaws of a caliper or tongs, primarily increasing the S-S diameter.

Although the diaphragm or the external intercostals are independently able to supply the tidal volume and about half of the vital capacity, the accessory muscles most often play an accessory role. The paravertebral muscles can straighten the spinal kyphosis, and the strap muscles of the neck lift the thoracic inlet. The strap muscles of the neck, including most importantly the sternocleidomastoid, and also those muscles from tongue to hyoid to thyroid to cricoid to sternum, generally referred to as the accessory muscles of inspiration, are capable of adding perhaps 10% to the inspiratory capacity on their own. Normally, during inspiration, they have a phasic increase in tone with each breath (i.e., isometric contraction), which serves to stabilize the thoracic inlet. This permits the external intercostals to increase the S-S diameter, each lifting the rib below. When the clavicle and first rib can be seen to move up and out, in either spontaneous or mechanical ventilation, one can be sure it represents an augmented tidal volume.

The inspiratory muscles have expiratory functions as well. First, by stretching the expiratory muscles, they increase their contractility when activated. Second, they stretch the lung, and with large tidal volumes the rib cage as well, storing elastic energy for exhalation. Finally, their tone is decreased slowly and progressively during expiration, providing a braking effect on expiratory flow, minimizing expiratory flow problems, and tending to increase average lung volume ( Fig. 2.3 ).

The expiratory muscles consist of the muscles of the abdominal wall, the internal intercostals muscles, and a number of other muscles of the upper limb and thorax. The expiratory muscles are not ordinarily involved in quiet expiration. They markedly increase expiratory flow in the sneeze or cough, and can decrease lung volume below functional residual capacity, to residual volume. The muscles involved in active expiration are, most importantly, those of the abdominal wall (external obliques, internal obliques, rectus abdominis, and transversus abdominis); the internal intercostals (whose fibers run more vertically than the external obliques); and to a very small degree, the muscles of the thoracic girdle and spine, which pull the shoulders forward and flex the vertebral column (and might be called the accessory muscles of expiration). These muscles ordinarily have little tone during anesthesia but come into play with cough. On emergence or in very light anesthesia, the abdominal components may be activated at the end of expiration. This end-expiratory tightening of the oblique abdominals thrusts the abdominal wall forward and may be mistaken for an inspiratory effort. Attendants trying to assist breathing with resuscitation bags (Ambu-bags or equivalent) may thus be out of synchronization, and their efforts may be counterproductive.

Lung Volumes: Anatomic Determinants

The nomenclature of the lung volumes was originally based on four independent volumes: residual volume (RV), expiratory reserves volume (ERV), tidal volume (V _T ), and inspiratory reserve volume (IRV). A fifth, overlapping lung volume closing volume (CV) has been added. Two or more volumes may be added to obtain a capacity.

• Functional residual capacity (FRC) = RV + ERV

• Inspiratory capacity (IC) = V _T + IRV

• Vital capacity (VC) = ERV + V _T + IRV

• Total lung capacity (TLC) = RV + ERV + V _T + IRV

• Closing capacity (CC) = CV + RV

When the lungs and viscera are removed from the body and all muscles are relaxed or paralyzed, the volume of the thoracic cage is several hundred milliliters larger than the FRC. Similarly, when the lungs are removed from the thoracic cavity and opened to the atmosphere, they decrease their volume by up to several hundred milliliters.

The resting position, the FRC, is set by equating the forces of the parenchyma to further collapse, with the force of the thorax tending to reexpand. At this point, the expiratory muscles are stretched slightly beyond their rest length and can contract to decrease the gas volume in the chest from the FRC to the RV, but the limit of this contraction differs somewhat in children and youths from older adults (see later) ( Fig. 2.4 ).

The VC is determined by the maximal excursion of the thoracic girdle, rib cage, spine, and diaphragm. From the FRC, the IC is limited by muscle shortening and rib excursion, not by lung compliance. The ERV and the RV are limited differently at different ages, however. In adults, the RV represents the volume of gas in the lung when all small airways have closed because of loss of tethering effect (see later). In children, the RV of the excised lung is somewhat smaller than the pleural cavity volume during maximum expiratory effort. Consequently, the pleural pressure is always negative. With increasing age, the increase of the closing capacity of lung tissue makes it higher than the minimal volume of the bony thorax at maximum expiration. Now expiratory effort produces a positive pleural pressure. ^,