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Energy Requirements of Breathing

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Energy Requirements of Breathing The mass movement of gases into all intercostal muscles when ventilation and out of the lungs is accomplished requirements approach 50% of the pre­ by muscular work. This requires en­ dicted vital capacity. ergy expenditure, caloric consump­ The diaphragm and intercostals are tion, oxygen utilization, and carbon di­ the only respiratory muscles used by oxide production. Inspiration is always the normal individual under ordinary an active process, enlarging the vol­ circumstances. Other muscles are mo­ ume of the thorax, thereby increasing bilized when ventilatory demands are the negative pressure so that air flows increased, either because of increase into the lungs. Expiration is ordinarily in total ventilation in the normal indi­ passive but may require muscular vidual or because of altered physiology work. of respiration in disease states. The so­ The position of the chest wall at called accessory muscles of respiration rest is a mechanically neutral point include particularly the scalenes, the at which the tendency of the chest sternomastoid, and the trapezius. wall to expand is balanced by the The scalene muscles arise from the Energy Requirements tendency of the lungs to recoil. This is transverse processes of the cervical the end expiratory level or midposi­ vertebrae and insert in the 1st and of Breathing* tion, and the lung volume at this point 2nd ribs. They serve to stabilize and is termed the functional residual ca­ elevate the 1st and 2nd ribs. They are pacity. An increase in chest size can not ordinarily employed in quiet respir­ be achieved only by exerting pressure ation. W. T. THOMPSON, JR. greater than the elastic forces of the Although a well trained subject lungs. In addition, airway resistance was able to breathe 60 L per minute to airflow and nonelastic tissue re­ without electromyographic evidence of sistance must be overcome. In the scalene activity, these muscles are ordi­ Department of Medicine normal individual, 60 to 70% of the narily brought into play when an in­ total ventilatory work is required for trathoracic pressure of -5 to -6 cm of Medical College of Virginia, Richmond overcoming elastic forces, the re­ H20 is exerted and chest volume is mainder for overcoming nonelastic re­ increased by 800 to 900 ml of air. sistance, largely due to airflow. On rapid inspiration they are brought Expiration is passive in the normal into action at the very beginning of individual during quiet breathing, the inspiration (Thompson, Patterson, and energy stored in the elastic lung tissue Shapiro, 1964). during inspiration being sufficient. While many muscles of the thorax The diaphragm is the major muscle may be used in severe dyspnea, only of inspiration and is probably always the sternomastoid and the trapezius active even with so-called thoracic will be considered. The former arises breathing. It arises from the xiphoid from the manubrium and the sternal process, from the inner surface of the end of the clavicle and inserts at the last six ribs and their cartliages, and mastoid process and lateral half of the from the lumbar vertebrae. By virtue nuchal line. It can assist in lifting the of its position and attachments, it not thorax. Campbell (1955) showed that only increases the vertical dimension there was never any detectable activity of the chest by a downward movement, of this muscle in any subject during but also increases the transverse di­ quiet respiration, but that increasing ameter by flaring the lower ribs. intensity of contraction was exhibited The intercostal muscles are second as intrathoracic pressures progressed in importance to the diaphragm. The from -10 to -50 cm of H,O. external intercostal fibers run down­ The trapezius arises from the occi­ ward and forward and the internal put, the ligamentum nuchae, and the intercostal fibers are directed upward spines of the last cervical and thoracic and forward to the next rib, so that vertebrae. It inserts into the acromial contraction elevates the rib with re­ 3rd of the clavicle, and the scapula. sulting increase in the anteroposterior The upper fibers tend to lift the chest. diameter, transverse diameter, or both. These muscles do not function with Studies by Koepke et al. (1958) showed equal effectiveness as muscles of res­ that the 1st intercostals are active in piration, and they are described in quiet breathing of most individuals. order of decreasing efficiency. The dia­ With increased ventilation, there is phragm and the intercostals are pe­ increased utilization of the intercostals, culiarly suited for their task of increas­ so that the majority of individuals use ing the chest volume. The scalene * Presented at the 1964 Stoneburner muscles have a direct pull to lift the Symposium. 1st rib. The sternomastoid and trape- 48 zius are large muscles that have a poor to their contribution to effective alveo­ status of the patient. In the past we mechanical advantage in lifting the lar ventilation. have repeatedly observed such patients thorax. The diaphragm is a much less to exhibit rapid and progressive loss The normal individual at rest has effective inspiratory muscle when it of weight with concomitant deteriora­ an oxygen requirement of about 300 is flattened by overdistended lungs. Its tion in their condition. Two factors ml that must be furnished, and pro­ contraction no longer increases the appeared to be responsible primarily duces about 240 ml of carbon dioxide vertical dimension of the chest. Fur­ for the loss of weight in these patients: that must be eliminated. This is thermore, it acts like a flat sheet of (1) loss of appetite, which seems, at achieved by a minute volume of ap­ muscle, draws the lower ribs together, least in part, to be related to infec­ proximately 7 L. About 2 % of the and thus reduces rather than increases tion and to gastric distention, and (2) total oxygen consumption at rest, or the transverse diameter. patients with advanced pulmonary in­ some 6 ml, is used by the respiratory Expiration which is normally pas­ sufficiency frequently develop severe muscles. sive may require muscle activity. This dyspnea while eating. The distress re­ An individual with normal heart and is because of loss of elastic recoil of sulting from shortness of breath thus lungs is able to increase his ventila­ lung and because of obstruction to limits the time that these patients are tion considerably without much in­ expiratory airflow. Unanswered is the willing to spend in chewing and swal­ crease in total oxygen consumption. question of whether there is structural lowing their food. As a result they As respiratory effort increases, the or biochemical change in the respira­ take smaller meals and their caloric oxygen cost of breathing becomes ex­ tory muscles so that they require more intake may be severely compromised. cessive in everyone, whether sick or oxygen and do less work. This chain of events and the interac­ well. Eventually the point is reached Many patients with chronic lung tion of the different factors described where the respiratory muscles con­ disease are alive only because their above are graphically represented in sume so much oxygen that further in­ respiratory muscles are able to per­ figure l. crease in ventilation does not supply form increased work day in and day additional oxygen for utilization by out. Greater force must be exerted if References other tissues of the body. With this effective alveolar ventilation is to be CAMPBELL, E. J. M. Tbe role of the there is a greatly increased production maintained and blood gases kept at scalene and sternomastoid muscles of carbon dioxide, and carbon dioxide tolerable limits. This means that ca­ in breathing in normal subjects. An electromyographic study. J. Anat. retention may occur because alveolar loric intake must be adequate, nutri­ 89: 378-386, 1955. ventilation cannot keep pace with the tion must be good, and muscle tone KOEPKE, G. H., E. M. SMITH, A. J. carbon dioxide produced by the re­ and strength must be maintained. The MURPHY, AND D. G. DICKINSON. Se­ spiratory muscles. This point is esti­ nutritional care, particularly of pa­ quence of action of the diaphragm mated to take place in the normal sub­ tients with chronic pulmonary emphy­ and intercostal muscles during res­ ject at ventilatory levels of about 140 sema, has in the past received little piration. 1. Inspiration. Arch. Phys. L per minute. attention. There is a tendency for at­ Med. Rehabil. 39: 426-430, 1958. THOMPSON, W. T., J. L. PATTERSON, AND In patients with lung disease, the tention to be centered on the more W. SHAPIRO. Observations on the ventilatory work is often increased, so obvious pulmonary abnormalities and that there may be a disproportionate scalene respiratory muscles. A. M . to disregard measures designed to im­ A . Arch. Internal Med. 113: 856- oxygen utilization and carbon dioxide prove and maintain the nutritional 865, 1964. production by respiratory muscles even at rest. This is particularly apt to oc­ Fig. 1-Circular deterioration of pulmonary emphysema cur in patients with airway obstruction, especially in those with chronic ob­ structive pulmonary emphysema. The effective ventilatory level may be reached at 15 to 20 L per minute in such patients. Any efforts to increase ventilation further only add oxygen which is used by the respiratory mus­ cles. Carbon dioxide is produced in excess of the ability to excrete it, and respiratory acidosis may be wors­ ened. The energy problems of patients with lung disease, particularly those with emphysema, are multiple. They often must use the less efficient ac­ cessory muscles at rest, a breathing pattern the normal subject uses only on extreme exertion. The mechanical effi­ ciency of these muscles is poor, their oxygen consumption and carbon diox­ ide production being out of proportion 49 .
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