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UNIT 5 Gas Exchange and Respiratory Function PREVENTING READMISSIONS IN PATIENTS WITH COPD Case Study A 55-year-old woman is admitted to the medical unit for an exacerbation of chronic obstructive pulmonary disease (COPD). The nurse notes that this patient was on the...

UNIT 5 Gas Exchange and Respiratory Function PREVENTING READMISSIONS IN PATIENTS WITH COPD Case Study A 55-year-old woman is admitted to the medical unit for an exacerbation of chronic obstructive pulmonary disease (COPD). The nurse notes that this patient was on the unit two weeks ago and has experienced several hospital admissions in the past year for exacerbations. The nurse manager observes that recently the unit has had an increase in 30-day hospital readmissions of patients with exacerbations of COPD. The manager then asks the nurse to chair a task force to examine how to decrease the rate of 30-day hospital readmissions among patients with COPD. 1360 1361  20 Assessment of Respiratory Function LEARNING OBJECTIVES On completion of this chapter, the learner will be able to: 1. Describe the structures and functions of the upper and lower respiratory tracts. 2. Describe ventilation, diffusion, perfusion, and ventilation–perfusion imbalances. 3. Explain proper techniques utilized to perform a comprehensive respiratory assessment. 4. Discriminate between normal and abnormal assessment findings identified by inspection, palpation, percussion, and auscultation of the respiratory system. 5. Recognize and evaluate the major symptoms of respiratory dysfunction by applying concepts from the patient’s health history and physical assessment findings. 6. Identify the diagnostic tests used to evaluate respiratory function and related nursing implications. GLOSSARY apnea: temporary cessation of breathing bronchophony: abnormal increase in clarity of transmitted voice sounds heard when auscultating the lungs bronchoscopy: direct examination of the larynx, trachea, and bronchi using an endoscope 1362 cilia: short, fine hairs that provide a constant whipping motion that serves to propel mucus and foreign substances away from the lung toward the larynx compliance: measure of the force required to expand or inflate the lungs crackles: soft, high-pitched, discontinuous popping sounds during inspiration caused by delayed reopening of the airways dyspnea: subjective experience that describes difficulty breathing; shortness of breath egophony: abnormal change in tone of voice that is heard when auscultating the lungs fremitus: vibrations of speech felt as tremors of the chest wall during palpation hemoptysis: expectoration of blood from the respiratory tract hypoxemia: decrease in arterial oxygen tension in the blood hypoxia: decrease in oxygen supply to the tissues and cells obstructive sleep apnea: temporary absence of breathing during sleep secondary to transient upper airway obstruction orthopnea: shortness of breath when lying flat; relieved by sitting or standing oxygen saturation: percentage of hemoglobin that is bound to oxygen physiologic dead space: portion of the tracheobronchial tree that does not participate in gas exchange pulmonary diffusion: exchange of gas molecules (oxygen and carbon dioxide) from areas of high concentration to areas of low concentration pulmonary perfusion: blood flow through the pulmonary vasculature respiration: gas exchange between atmospheric air and the blood and between the blood and cells of the body rhonchi: low-pitched wheezing or snoring sound associated with partial airway obstruction, heard on chest auscultation stridor: harsh high-pitched sound heard on inspiration, usually without need of a stethoscope, secondary to upper airway obstruction tachypnea: abnormally rapid respirations tidal volume: volume of air inspired and expired with each breath during normal breathing ventilation: movement of air in and out of the airways wheezes: continuous musical sounds associated with airway narrowing 1363  or partial obstruction whispered pectoriloquy: whispered sounds heard loudly and clearly upon thoracic auscultation Disorders of the respiratory system are common and are encountered by nurses in every setting, from the community to the intensive care unit. Expert assessment skills must be developed and used to provide the best care for patients with acute and chronic respiratory problems. Alterations in respiratory status have been identified as important predictors of clinical deterioration in hospitalized patients (Helling, Martin, Martin, et al., 2014; Kirkland, Malinchoc, O’Byrne, et al., 2013). To differentiate between normal and abnormal assessment findings and recognize subtle changes that may negatively impact patient outcomes, nurses require an understanding of respiratory function and the significance of abnormal diagnostic test results. Anatomic and Physiologic Overview The respiratory system is composed of the upper and lower respiratory tracts. Together, the two tracts are responsible for ventilation (movement of air in and out of the airways). The upper respiratory tract, known as the upper airway, warms and filters inspired air so that the lower respiratory tract (the lungs) can accomplish gas exchange or diffusion. Gas exchange involves delivering oxygen to the tissues through the bloodstream and expelling waste gases, such as carbon dioxide, during expiration. The respiratory system depends on the cardiovascular system for perfusion, or blood flow through the pulmonary system (Porth, 2015). Anatomy of the Respiratory System Upper Respiratory Tract Upper airway structures consist of the nose; paranasal sinuses; pharynx, tonsils, and adenoids; larynx; and trachea. Nose The nose serves as a passageway for air to pass to and from the lungs. It filters impurities and humidifies and warms the air as it is inhaled. The nose is composed of an external and an internal portion. The external portion protrudes from the face and is supported by the nasal bones and 1364 cartilage. The anterior nares (nostrils) are the external openings of the nasal cavities. The internal portion of the nose is a hollow cavity separated into the right and left nasal cavities by a narrow vertical divider, the septum. Each nasal cavity is divided into three passageways by the projection of the turbinates from the lateral walls. The turbinate bones are also called conchae (the name suggested by their shell-like appearance). Because of their curves, these bones increase the mucous membrane surface of the nasal passages and slightly obstruct the air flowing through them (see Fig. 20-1). 1365 Figure 20-1 Cross-section of nasal cavity. Figure 20-2 The paranasal sinuses. Air entering the nostrils is deflected upward to the roof of the nose, and it follows a circuitous route before it reaches the nasopharynx. It comes into contact with a large surface of moist, warm, highly vascular, ciliated mucous membrane (called nasal mucosa) that traps practically all of the dust and organisms in the inhaled air. The air is moistened, warmed to body temperature, and brought into contact with sensitive nerves. Some of these nerves detect odors; others provoke sneezing to expel irritating dust. Mucus, secreted continuously by goblet cells, covers the surface of the nasal mucosa and is moved back to the nasopharynx by the action of the cilia (short, fine hairs). Paranasal Sinuses The paranasal sinuses include four pairs of bony cavities that are lined with nasal mucosa and ciliated pseudostratified columnar epithelium. These air spaces are connected by a series of ducts that drain into the nasal cavity. The sinuses are named by their location: frontal, ethmoid, sphenoid, and maxillary (see Fig. 20-2). A prominent function of the sinuses is to serve as a resonating chamber in speech. The sinuses are a common site of infection. Pharynx, Tonsils, and Adenoids The pharynx, or throat, is a tubelike structure that connects the nasal and oral cavities to the larynx. It is divided into three regions: nasal, oral, and 1366 laryngeal. The nasopharynx is located posterior to the nose and above the soft palate. The oropharynx houses the faucial, or palatine, tonsils. The laryngopharynx extends from the hyoid bone to the cricoid cartilage. The epiglottis forms the entrance to the larynx. The adenoids, or pharyngeal tonsils, are located in the roof of the nasopharynx. The tonsils, the adenoids, and other lymphoid tissue encircle the throat. These structures are important links in the chain of lymph nodes guarding the body from invasion by organisms entering the nose and the throat. The pharynx functions as a passageway for the respiratory and digestive tracts. Larynx The larynx, or voice box, is a cartilaginous epithelium-lined organ that connects the pharynx and the trachea and consists of the following: Epiglottis: a valve flap of cartilage that covers the opening to the larynx during swallowing Glottis: the opening between the vocal cords in the larynx Thyroid cartilage: the largest of the cartilage structures; part of it forms the Adam’s apple Cricoid cartilage: the only complete cartilaginous ring in the larynx (located below the thyroid cartilage) Arytenoid cartilages: used in vocal cord movement with the thyroid cartilage Vocal cords: ligaments controlled by muscular movements that produce sounds; located in the lumen of the larynx Although the major function of the larynx is vocalization, it also protects the lower airway from foreign substances and facilitates coughing; it is, therefore, sometimes referred to as the “watchdog of the lungs” (Porth, 2015). Trachea The trachea, or windpipe, is composed of smooth muscle with C-shaped rings of cartilage at regular intervals. The cartilaginous rings are incomplete on the posterior surface and give firmness to the wall of the trachea, preventing it from collapsing. The trachea serves as the passage between the larynx and the right and left main stem bronchi, which enter the lungs through an opening called the hilus. Lower Respiratory Tract 1367 The lower respiratory tract consists of the lungs, which contain the bronchial and alveolar structures needed for gas exchange. Lungs The lungs are paired elastic structures enclosed in the thoracic cage, which is an airtight chamber with distensible walls (see Fig. 20-3). Each lung is divided into lobes. The right lung has upper, middle, and lower lobes, whereas the left lung consists of upper and lower lobes (see Fig. 20-4). Each lobe is further subdivided into two to five segments separated by fissures, which are extensions of the pleura. Pleura The lungs and wall of the thoracic cavity are lined with a serous membrane called the pleura. The visceral pleura covers the lungs; the parietal pleura lines the thoracic cavity, lateral wall of the mediastinum, diaphragm, and inner aspects of the ribs. The visceral and parietal pleura and the small amount of pleural fluid between these two membranes serve to lubricate the thorax and the lungs and permit smooth motion of the lungs within the thoracic cavity during inspiration and expiration. Mediastinum The mediastinum is in the middle of the thorax, between the pleural sacs that contain the two lungs. It extends from the sternum to the vertebral column and contains all of the thoracic tissue outside the lungs (heart, thymus, the aorta and vena cava, and esophagus). 1368 Figure 20-3 The respiratory system. A. Upper respiratory structures and the structures of the thorax. B. Alveoli. C. A horizontal cross-section of the lungs. Figure 20-4 Anterior view of the lungs. 1369 The lungs consist of five lobes. The right lung has three lobes (upper, middle, lower); the left has two (upper and lower). The lobes are further subdivided by fissures. The bronchial tree, another lung structure, inflates with air to fill the lobes. Bronchi and Bronchioles There are several divisions of the bronchi within each lobe of the lung. First are the lobar bronchi (three in the right lung and two in the left lung). Lobar bronchi divide into segmental bronchi (10 on the right and 8 on the left); these structures facilitate effective postural drainage in the patient. Segmental bronchi then divide into subsegmental bronchi. These bronchi are surrounded by connective tissue that contains arteries, lymphatics, and nerves. The subsegmental bronchi then branch into bronchioles, which have no cartilage in their walls. Their patency depends entirely on the elastic recoil of the surrounding smooth muscle and on the alveolar pressure. The bronchioles contain submucosal glands, which produce mucus that covers the inside lining of the airways. The bronchi and bronchioles are also lined with cells that have surfaces covered with cilia. These cilia create a constant whipping motion that propels mucus and foreign substances away from the lungs toward the larynx. The bronchioles branch into terminal bronchioles, which do not have mucous glands or cilia. Terminal bronchioles become respiratory bronchioles, which are considered to be the transitional passageways between the conducting airways and the gas exchange airways. Up to this point, the conducting airways contain about 150 mL of air in the tracheobronchial tree that does not participate in gas exchange, known as physiologic dead space. The respiratory bronchioles then lead into alveolar ducts and sacs and then alveoli (see Fig. 20-3). Oxygen and carbon dioxide exchange takes place in the alveoli. Alveoli The lung is made up of about 300 million alveoli, constituting a total surface area between 50 and 100 m2 (Porth, 2015). There are three types of alveolar cells. Type I and type II cells make up the alveolar epithelium. Type I cells account for 95% of the alveolar surface area and serve as a barrier between the air and the alveolar surface; type II cells account for 1370 only 5% of this area but are responsible for producing type I cells and surfactant. Surfactant reduces surface tension, thereby improving overall lung function. Alveolar macrophages, the third type of alveolar cells, are phagocytic cells that ingest foreign matter and, as a result, provide an important defense mechanism. Function of the Respiratory System The cells of the body derive the energy they need from the oxidation of carbohydrates, fats, and proteins. This process requires oxygen. Vital tissues, like the brain and the heart, cannot survive long without a continuous supply of oxygen. As a result of oxidation, carbon dioxide is produced and must be removed from the cells to prevent the buildup of acid waste products. The respiratory system performs this function by facilitating life-sustaining processes such as oxygen transport, respiration, ventilation, and gas exchange. Oxygen Transport Oxygen is supplied to, and carbon dioxide is removed from, cells by way of the circulating blood through the thin walls of the capillaries. Oxygen diffuses from the capillary through the capillary wall to the interstitial fluid. At this point, it diffuses through the membrane of tissue cells, where it is used by mitochondria for cellular respiration. The movement of carbon dioxide occurs by diffusion in the opposite direction—from cell to blood. Respiration After these tissue capillary exchanges, blood enters the systemic venous circulation and travels to the pulmonary circulation. The oxygen concentration in blood within the capillaries of the lungs is lower than that in the lungs’ alveoli. Because of this concentration gradient, oxygen diffuses from the alveoli to the blood. Carbon dioxide, which has a higher concentration in the blood than in the alveoli, diffuses from the blood into the alveoli. Movement of air in and out of the airways continually replenishes the oxygen and removes the carbon dioxide from the airways and the lungs. This whole process of gas exchange between the atmospheric air and the blood and between the blood and cells of the body is called respiration. 1371 Ventilation Ventilation requires movement of the walls of the thoracic cage and of its floor, the diaphragm. The effect of these movements is alternately to increase and decrease the capacity of the chest. When the capacity of the chest is increased, air enters through the trachea (inspiration) and moves into the bronchi, bronchioles, and alveoli, and inflates the lungs. When the chest wall and the diaphragm return to their previous positions (expiration), the lungs recoil and force the air out through the bronchi and the trachea. Inspiration occurs during the first third of the respiratory cycle; expiration occurs during the latter two thirds. The inspiratory phase of respiration normally requires energy; the expiratory phase is normally passive, requiring very little energy. Physical factors that govern airflow in and out of the lungs are collectively referred to as the mechanics of ventilation and include air pressure variances, resistance to airflow, and lung compliance. Air Pressure Variances Air flows from a region of higher pressure to a region of lower pressure. During inspiration, movements of the diaphragm and intercostal muscles enlarge the thoracic cavity and thereby lower the pressure inside the thorax to a level below that of atmospheric pressure. As a result, air is drawn through the trachea and the bronchi into the alveoli. During expiration, the diaphragm relaxes and the lungs recoil, resulting in a decrease in the size of the thoracic cavity. The alveolar pressure then exceeds atmospheric pressure, and air flows from the lungs into the atmosphere. Airway Resistance Resistance is determined by the radius, or size of the airway through which the air is flowing, as well as by lung volumes and airflow velocity. Any process that changes the bronchial diameter or width affects airway resistance and alters the rate of airflow for a given pressure gradient during respiration (see Chart 20-1). With increased resistance, greater-than- normal respiratory effort is required to achieve normal levels of ventilation. Chart 20-1 Causes of Increased Airway Resistance 1372  Common phenomena that may alter bronchial diameter, which affects airway resistance, include the following: Contraction of bronchial smooth muscle—as in asthma Thickening of bronchial mucosa—as in chronic bronchitis Obstruction of the airway—by mucus, a tumor, or a foreign body Loss of lung elasticity—as in emphysema, which is characterized by connective tissue encircling the airways, thereby keeping them open during both inspiration and expiration Compliance Compliance is the elasticity and expandability of the lungs and thoracic structures. Compliance allows the lung volume to increase when the difference in pressure between the atmosphere and the thoracic cavity (pressure gradient) causes air to flow in. Factors that determine lung compliance are the surface tension of the alveoli, the connective tissue and water content of the lungs, and the compliance of the thoracic cavity. Compliance is determined by examining the volume–pressure relationship in the lungs and the thorax. Compliance is normal (1 L/cm H2O) if the lungs and the thorax easily stretch and distend when pressure is applied. Increased compliance occurs if the lungs have lost their elastic recoil and become overdistended (e.g., in emphysema). Decreased compliance occurs if the lungs and the thorax are “stiff.” Conditions associated with decreased compliance include morbid obesity, pneumothorax, hemothorax, pleural effusion, pulmonary edema, atelectasis, pulmonary fibrosis, and acute respiratory distress syndrome (ARDS). Lungs with decreased compliance require greater-than-normal energy expenditure by the patient to achieve normal levels of ventilation. Lung Volumes and Capacities Lung function, which reflects the mechanics of ventilation, is viewed in terms of lung volumes and lung capacities. Lung volumes are categorized as tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Lung capacity is evaluated in terms of vital capacity, inspiratory capacity, functional residual capacity, and total lung capacity. These terms are explained in Table 20-1. Pulmonary Diffusion and Perfusion Pulmonary diffusion is the process by which oxygen and carbon dioxide 1373 are exchanged from areas of high concentration to areas of low concentration at the air–blood interface. The alveolar–capillary membrane is ideal for diffusion because of its thinness and large surface area. In the normal healthy adult, oxygen and carbon dioxide travel across the alveolar–capillary membrane without difficulty as a result of differences in gas concentrations in the alveoli and capillaries. Pulmonary perfusion is the actual blood flow through the pulmonary vasculature. The blood is pumped into the lungs by the right ventricle through the pulmonary artery. The pulmonary artery divides into the right and left branches to supply both lungs. Normally, about 2% of the blood pumped by the right ventricle does not perfuse the alveolar capillaries. This shunted blood drains into the left side of the heart without participating in alveolar gas exchange. Bronchial arteries extending from the thoracic aorta also support perfusion but do not participate in gas exchange, further diluting oxygenated blood exiting through the pulmonary vein (Porth, 2015). The pulmonary circulation is considered a low-pressure system because the systolic blood pressure in the pulmonary artery is 20 to 30 mm Hg and the diastolic pressure is 5 to 15 mm Hg. Because of these low pressures, the pulmonary vasculature normally can vary its capacity to accommodate the blood flow it receives. However, when a person is in an upright position, the pulmonary artery pressure is not great enough to supply blood to the apex of the lung against the force of gravity. Thus, when a person is upright, the lung may be considered to be divided into three sections: an upper part with poor blood supply, a lower part with maximal blood supply, and a section between the two with an intermediate supply of blood. When a person who is lying down turns to one side, more blood passes to the dependent lung. TABLE 20-1 Lung Volumes and Lung Capacities 1374  Perfusion is also influenced by alveolar pressure. The pulmonary capillaries are sandwiched between adjacent alveoli. If the alveolar pressure is sufficiently high, the capillaries are squeezed. Depending on the pressure, some capillaries completely collapse, whereas others narrow. Pulmonary artery pressure, gravity, and alveolar pressure determine the patterns of perfusion. In lung disease, these factors vary, and the perfusion of the lung may become abnormal. Ventilation and Perfusion Balance and Imbalance Adequate gas exchange depends on an adequate ventilation–perfusion (V./Q.) ratio. In different areas of the lung, the (V./Q.) ratio varies. Airway blockages, local changes in compliance, and gravity may alter ventilation. Alterations in perfusion may occur with a change in the pulmonary artery pressure, alveolar pressure, or gravity. V./Q. imbalance occurs as a result of inadequate ventilation, inadequate perfusion, or both. There are four possible (V./Q.) states in the lung: normal (V./Q.) ratio, low (V./Q.) ratio (shunt), high (V./Q.) ratio (dead space), and absence of ventilation and perfusion (silent unit) (see Chart 20- 2). V./Q. imbalance causes shunting of blood, resulting in hypoxia (low level of cellular oxygen). Shunting appears to be the main cause of hypoxia after thoracic or abdominal surgery and most types of respiratory failure. Severe hypoxia results when the amount of shunting exceeds 20%. Supplemental oxygen may eliminate hypoxia, depending on the type of 1375 (V./Q.) imbalance. Gas Exchange Partial Pressure of Gases The air we breathe is a gaseous mixture consisting mainly of nitrogen (78.6%) and oxygen (20.8%), with traces of carbon dioxide (0.04%), water vapor (0.05%), helium, and argon. The atmospheric pressure at sea level is about 760 mm Hg. Partial pressure is the pressure exerted by each type of gas in a mixture of gases. The partial pressure of a gas is proportional to the concentration of that gas in the mixture. The total pressure exerted by the gaseous mixture, whether in the atmosphere or in the lungs, is equal to the sum of the partial pressures. Based on these facts, the partial pressures of nitrogen and oxygen can be calculated. The partial pressure of nitrogen in the atmosphere at sea level is 78.6% of 760, or 597 mm Hg; that of oxygen is 20.8% of 760, or 158 mm Hg (Grossman & Porth, 2014). Chart 20-3 identifies and defines terms and abbreviations related to partial pressure of gases. Once the air enters the trachea, it becomes fully saturated with water vapor, which displaces some of the other gases. Water vapor exerts a pressure of 47 mm Hg when it fully saturates a mixture of gases at the body temperature of 37°C (98.6°F). Nitrogen and oxygen are responsible for almost all of the remaining 713 mm Hg pressure. Once this mixture enters the alveoli, it is further diluted by carbon dioxide. In the alveoli, the water vapor continues to exert a pressure of 47 mm Hg. The remaining 713 mm Hg pressure is now exerted as follows: nitrogen, 569 mm Hg (74.9%); oxygen, 104 mm Hg (13.6%); and carbon dioxide, 40 mm Hg (5.3%). When a gas is exposed to a liquid, the gas dissolves in the liquid until equilibrium is reached. The dissolved gas also exerts a partial pressure. At equilibrium, the partial pressure of the gas in the liquid is the same as the partial pressure of the gas in the gaseous mixture. Oxygenation of venous blood in the lung illustrates this point. In the lung, venous blood and alveolar oxygen are separated by a very thin alveolar membrane. Oxygen diffuses across this membrane to dissolve in the blood until the partial pressure of oxygen in the blood is the same as that in the alveoli (104 mm Hg). However, because carbon dioxide is a by-product of oxidation in the cells, venous blood contains carbon dioxide at a higher partial pressure than that in the alveolar gas. In the lung, carbon dioxide diffuses out of venous blood into the alveolar gas. At equilibrium, the partial pressure of 1376 carbon dioxide in the blood and in alveolar gas is the same (40 mm Hg). The changes in partial pressure are shown in Figure 20-5. Chart 20-2 Ventilation–Perfusion Ratios Normal Ratio (A) In the healthy lung, a given amount of blood passes an alveolus and is matched with an equal amount of gas (A). The ratio is 1:1 (ventilation matches perfusion). Low Ventilation–Perfusion Ratio: Shunts (B) Low ventilation–perfusion states may be called shunt-producing disorders. When perfusion exceeds ventilation, a shunt exists (B). Blood bypasses the alveoli without gas exchange occurring. This is seen with obstruction of the distal airways, such as with pneumonia, atelectasis, tumor, or a mucus plug. 1377 High Ventilation–Perfusion Ratio: Dead Space (C) When ventilation exceeds perfusion, dead space results (C). The alveoli do not have an adequate blood supply for gas exchange to occur. This is characteristic of a variety of disorders, including pulmonary emboli, pulmonary infarction, and cardiogenic shock. Silent Unit (D) In the absence of both ventilation and perfusion or with limited ventilation and perfusion, a condition known as a silent unit occurs (D). This is seen with pneumothorax and severe acute respiratory distress syndrome. 1378 Chart 20-3 Partial Pressure Abbreviations P = Pressure PO2 = Partial pressure of oxygen PCO2 = Partial pressure of carbon dioxide PAO2 = Partial pressure of alveolar oxygen PACO2 = Partial pressure of alveolar carbon dioxide PaO2 = Partial pressure of arterial oxygen 1379  PaCO2 = Partial pressure of arterial carbon dioxide PvO2 = Partial pressure of venous oxygen PvCO2 = Partial pressure of venous carbon dioxide P50 = Partial pressure of oxygen when the hemoglobin is 50% saturated Effects of Pressure on Oxygen Transport Oxygen and carbon dioxide are transported simultaneously, either dissolved in blood or combined with hemoglobin in red blood cells. Each 100 mL of normal arterial blood carries 0.3 mL of oxygen physically dissolved in the plasma and 20 mL of oxygen in combination with hemoglobin. Large amounts of oxygen can be transported in the blood because oxygen combines easily with hemoglobin to form oxyhemoglobin: Figure 20-5 Changes occur in the partial pressure of gases during respiration. These values vary as a result of the exchange of 1380 oxygen and carbon dioxide and the changes that occur in their partial pressures as venous blood flows through the lungs. The volume of oxygen physically dissolved in the plasma is measured by the partial pressure of oxygen in the arteries (PaO2). The higher the PaO2, the greater the amount of oxygen dissolved. For example, at a PaO2 of 10 mm Hg, 0.03 mL of oxygen is dissolved in 100 mL of plasma. At PaO2 of 20 mm Hg, twice this amount is dissolved in plasma, and at PaO2 of 100 mm Hg, 10 times this amount is dissolved. Therefore, the amount of dissolved oxygen is directly proportional to the partial pressure, regardless of how high the oxygen pressure becomes. The amount of oxygen that combines with hemoglobin depends on both the amount of hemoglobin in the blood and on PaO2, although only up to a PaO2 of about 150 mm Hg. This is measured as O2 saturation (SaO2), the percentage of the O2 that could be carried if all the hemoglobin held the maximum possible amount of O2. When the PaO2 is 150 mm Hg, hemoglobin is 100% saturated and does not combine with any additional oxygen. When hemoglobin is 100% saturated, 1 g of hemoglobin combines with 1.34 mL of oxygen. Therefore, in a person with 14 g/dL of hemoglobin, each 100 mL of blood contains about 19 mL of oxygen associated with hemoglobin. If the PaO2 is less than 150 mm Hg, the percentage of hemoglobin saturated with oxygen decreases. For example, at a PaO2 of 100 mm Hg (normal value), saturation is 97%; at a PaO2 of 40 mm Hg, saturation is 70%. Oxyhemoglobin Dissociation Curve The oxyhemoglobin dissociation curve (see Chart 20-4) shows the relationship between the partial pressure of oxygen (PaO2) and the percentage of saturation of oxygen (SaO2). The percentage of saturation can be affected by carbon dioxide, hydrogen ion concentration, temperature, and 2,3-diphosphoglycerate. An increase in these factors shifts the curve to the right, thus less oxygen is picked up in the lungs, but more oxygen is released to the tissues, if PaO2 is unchanged. A decrease in these factors causes the curve to shift to the left, making the bond between oxygen and hemoglobin stronger. If the PaO2 is still unchanged, more 1381 oxygen is picked up in the lungs, but less oxygen is given up to the tissues. The unusual shape of the oxyhemoglobin dissociation curve is a distinct advantage to the patient for two reasons: 1. If the PaO2 decreases from 100 to 80 mm Hg as a result of lung disease or heart disease, the hemoglobin of the arterial blood remains almost maximally saturated (94%), and the tissues do not suffer from hypoxia. 2. When the arterial blood passes into tissue capillaries and is exposed to the tissue tension of oxygen (about 40 mm Hg), hemoglobin gives up large quantities of oxygen for use by the tissues. Chart 20-4 Oxyhemoglobin Dissociation Curve The oxyhemoglobin dissociation curve is marked to show three oxygen levels: 1. Normal levels—PaO2 >70 mm Hg 2. Relatively safe levels—PaO2 45–70 mm Hg 3. Dangerous levels—PaO2