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).^1,2

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.³

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.⁴

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

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.^5,6

The Upper Airway

The conducting passages from the nares and lips to the larynxserve not only as a simple conducting function, but as an important defensive function as well. They filter, warm, and humidify inspired gas and serve as a buffer against entry of irritating material to the more delicate lung parenchyma. During quiet breathing, maximal conservation of heat and humidity are obtained by nasal breathing. With hyperpnea, minimal work of breathing dictates mouth breathing, sacrificing supralaryngeal air conditioning. Bony, muscular, and mucosal structures provide for these optimizations and for the switch-over.

The nasal portion of the airway has a framework of cartilage and bone covered internally with hair-bearing, squamous epithelium in the funnel-shaped vestibule and ciliated, pseudostratified respiratory epithelium in the deeper cavum. The entrance is a pair of oval openings framed by the alar cartilages and the anterior border of the septal cartilage. Hairs called vibrissae form a coarse net across the openings to filter large particles. They can elicit a sneeze when moved lightly. Slips of striated muscle (the nasalis muscle under facial, cranial nerve VII control) dilate the entrance somewhat in hyperpnea. The nares can be narrowed, as in a sniff, when the decreasing pressure attendant on increased inspiratory air flow causes the lateral (cartilaginous) walls to move inward, narrowing the passageway and creating a jet directing the gas flow into the roof of the cavum in greater part. This has several functions: improving olfaction, clearing secretions into the pharynx, and directing dry cold gas over moist, warm surfaces (Fig. 2.5).

Beyond the vestibule, the cavum opens up into two bilaterally symmetric chambers with a floor provided by the hard palate, a medial wall by the nasal septum and a lateral wall by the maxilla with three curved, bony protuberances called turbinates or conchae. In the cavum the direction of the airstream is bent, and the flow further broken up by the turbinates. The medial-lateral surfaces, about 150 square centimeters in area, are rarely more than a few millimeters apart, except along the floor of the cavum, where there may be as much as a centimeter between medial, lateral, and inferior borders. Thus the floor is the obvious route of choice for advancing fiberoptic endoscopes and airways. The submucosa is so rich in blood vessels that it resembles erectile tissue. These vessels provide the heat and water necessary for the air-conditioning function of the nose. The nasolacrimal duct and orifices to the paranasal sinuses are found under the turbinates.

Innervation for olfaction is provided in the attic of the cavum by bipolar neurons in the epithelium, whose axons pass through the cribriform plate to the olfactory bulb and synapse there with axons of the olfactory cranial nerve I. Sensation is provided by the first two branches of the trigeminal cranial nerve V, which have broadly arborized before entering the mucosa. Motor innervation for facial expression and emotion comes from the facial cranial nerve VII. Autonomic innervation comes from the cervical (sympathetic) ganglia following the arterial supply (branches of the external carotid and parasympathetics) from cranial nerve V. Through autonomic reflexes, cold, dry inspirate increases both the flow and volume of blood in the mucosa, supplying calories and water to gas.

The nasopharynx begins at the posterior choanae with another sharp bend in the air­ stream. Inertia carries suspended particles into the posterior mucosal blanket where the rich supply of lymphoid tissue (Waldeyer’s ring) promotes defense. The eustachian tubes draining the middle ear open into the lateral wall. The nasopharynx is bounded superiorly by the base of the skull and posteriorly by the vertebral column. When the soft palate and tongue are relaxed, the nasopharynx is a widely patent part of the airway with a cross-sectional area 2 to 3 times as large as the trachea.

The resistance to air flow provided by the nose is nearly half of the resting resistance of the total airway. Major components are the turbulent flow pattern and narrow spaces created by the conchae. Resistance provided by the vestibule is variable, as noted earlier, however, with decreased air flow as the result of valving collapse of the lateral nasal cartilages during a sniff. Alar flaring opposes this collapse, decreasing resistance, as well as conveying emotion. In contrast, the air flow resistance from the choanae through the nasopharynx is either negligible (nose breathing) or infinite (i.e., closed). This valve-like function is caused by the effect of the soft palate: when elevated it seals the nasopharynx, and gas transverses the oral cavity and pharynx with very little resistance to flow.

The oropharynx is simply the vertical continuation of the nasopharynx when the soft palate is relaxed in quiet breathing. With the switch to oral breathing and anterior movement of the base of the tongue, it becomes the segment of the airway with the lowest resistance and the largest cross-sectional area. It acts as a gentle curve directing gas flow into the larynx.

The hypopharynx consists of two funnel-shaped cavities, the pyriform sinuses, on either side of the larynx. The entrance to the esophagus is normally closed by the cricopharyngeal muscle. The superior laryngeal nerves are just submucosal in the anterolateral wall of the sinuses and may be blocked by two local anesthetic-soaked pledgets held at this spot. This is in contrast to the rest of the sensory innervation: no one other anatomically identifiable spot serves as a landmark for a nerve supplying a large area of mucosa. Hence topical anesthesia with several local anesthetic-soaked cotton swabs placed in the nose or throat do not produce satisfactorily topical anesthesia.⁷

Secondary functions of the nasal, oral, and pharyngeal anatomy are to give rich quality and variety to speech and song and to seal off the airway during deglutition. The details of these structure-function relations may be found elsewhere. With regret, the author notes that the newborn can do both simultaneously, that is, to suck and sing, or at least breathe and swallow. The neonate has a very curved and relatively long epiglottis that extends up to the soft palate to provide a sealed transit for gas to and from the nose through the oropharynx and to deflect liquid laterally around the larynx to the esophagus. This ability is lost in infancy. Worse still, throughout the rest of life, the loss of those reflexes that make the separation of inspirate and alimentation less and less certain slowly accelerates (Fig. 2.6).

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