Chapter 36: Oxygenation (1)

What Are the Main Points in This Chapter?

-The structures of the airways, lungs, chest cavity, heart, and blood vessels function together to supply oxygen to the tissues; thus, abnormalities in any of these structures can interfere with tissue oxygenation. -Developmental stage, environment, lifestyle, smoking, medications, and pathological factors can interfere with ventilation, circulation, or gas exchange, leading to problems with tissue oxygenation. -A health assessment related to oxygenation includes assessment of ventilation (breathing), circulation (blood flow), and gas exchange (exchange of oxygen and carbon dioxide). The length and focus of the assessment varies with the purpose and urgency of the clinical situation. -A physical examination related to adequacy of oxygenation includes observations of adequacy of breathing, circulation, and gas exchange. -Pulse oximetry and arterial blood gases are used to monitor oxygen saturation. -Small changes in oxygen saturation are associated with large shifts in the amount of oxygen available to the tissues and organs. -Interventions to promote optimal respiratory function include deep regular breathing, supporting smoking cessation, flu and pneumonia immunizations for at-risk individuals, frequent position changes, incentive spirometry, and preventing aspiration. -Deep breathing, coughing exercises, and hydration promote deep inhalation and forceful expulsion of secretions. -Supplemental oxygen is used to prevent hypoxemia. It may be delivered by a variety of methods. -Artificial airways provide an open airway for patients who cannot maintain their own airway. The most common artificial airways are oropharyngeal, nasopharyngeal, and endotracheal. -Airways can be suctioned to remove secretions and maintain patency. Signs that indicate the need for suctioning include gurgling sounds during respiration, restlessness, labored respirations, decreased oxygen saturation, increased heart and respiratory rates, and the presence of adventitious breath sounds during auscultation. -Mechanical ventilation with a positive pressure ventilator requires the patient to have an artificial airway. -Chest tubes remove air or fluid from the pleural space so that the lungs can fully expand. Chest tubes should be clamped only for changing the drainage system; clamping can lead to tension pneumothorax. -Respiratory medications promote ventilation and oxygenation by their effects on the respiratory system. The major types of respiratory medicines include bronchodilators, corticosteroids, cough preparations, decongestants, antihistamines, and mucolytics.

MEDICATIONS THAT CAN INTERFERE WITH OXYGENATION

A number of medications can interfere with oxygenation by depressing respirations or cardiovascular function. Respiratory depressants generally act either by depressing CNS control of breathing or by weakening the muscles of breathing. Respiratory depressants include general anesthetics, opioids, anti-anxiety and sedative/hypnotic drugs, neuromuscular blocking agents, and magnesium sulfate: -General anesthetics are used to produce unconsciousness and analgesia during surgical procedures. The loss of the protective gag reflex creates a high risk for aspiration of fluids and particles into the airways, while respiratory depression produces hypoventilation. Patients receiving general anesthesia must be closely monitored. -Opioids depress respiration through their action on brain receptors. In therapeutic doses, they may result in a decreased respiratory rate or hypoventilation. Children, older adults, and those with respiratory disease are more likely to experience respiratory depression. Risk increases when opioids are taken along with other CNS depressants, such as alcohol. Chronic opioid use does produce tolerance of the medicine and limits the likelihood of respiratory depression from opioids. Patients who have chronic pain can tolerate large doses of opioids without fear of respiratory depression. -Anti-anxiety and sedative/hypnotic drugs are used to reduce anxiety states and to induce sleep. Many new medications are available to reduce anxiety that do not produce respiratory depression, such as selective serotonin reuptake inhibitors (SSRIs, e.g., paroxetine [Paxil], fluoxetine hydrochloride [Prozac]). However, a variety of medications are still in use for anxiety and insomnia that do produce respiratory depression. Barbiturates (e.g., secobarbital [Seconal]) are powerful respiratory depressants and thus are popular drugs for suicide. Benzodiazepines (e.g., diazepam [Valium]) may cause respiratory depression, especially when they are mixed with alcohol. -Neuromuscular blocking agents are used therapeutically to relax skeletal muscles during surgical procedures, endotracheal intubation, mechanical ventilation, and other procedures in which muscle relaxation is important. Neuromuscular blocking agents produce muscle paralysis by preventing acetylcholine from activating muscle contraction. The diaphragm and intercostal muscles become paralyzed and cannot contract for breathing. The airway must be protected, ventilation must be supported, and oxygenation must be continuously monitored when neuromuscular blocking agents are given. Neuromuscular blocking agents do not depress brain function; thus, a fully paralyzed person is still conscious, can feel pain, and knows what is happening, unless sedative drugs are administered as well. -Magnesium sulfate IV infusions are administered to replace magnesium and to stop preterm labor by relaxing the uterus. Successful therapy, however, does produce widespread skeletal muscle weakness and requires close monitoring of oxygenation and breathing until labor has stopped, and the infusion can be gradually discontinued.

Pleural Effusion

A pleural effusion is the collection of fluid (blood, lymph, or pus) in the pleural space. The fluid usually comes either from blood or lymphatic vessels near the pleura or from an abscess draining into the pleural space. Because there is no injury to the pleura itself, pleural pressure remains negative and the lung does not collapse. Instead, lung tissue and alveoli are compressed by the fluid accumulation. The degree to which a pleural effusion causes hypoventilation and hypoxemia depends on how quickly and how much fluid accumulates. A large, rapid hemorrhage into the pleural space (hemothorax) may cause acute respiratory distress and hypoxemia, whereas a small, slow accumulation of fluid due to inflammation may cause no immediate problem. Treatment for a large effusion that causes impaired respiratory function is to drain the fluid through a needle placed directly into the accumulation (needle aspiration thoracentesis).

Pneumothorax

A pneumothorax occurs when air enters the pleural space through a tear or hole in the pleural membrane. Air leakage can come from within the air-filled lung tissue itself, from outside the lung through the chest wall, or from both sources. Air leakage destroys the normal vacuum that keeps the lungs pulled into contact with the chest wall, causing the lung to recoil and collapse. A collapsed lung (or portion of a lung) can dramatically reduce ventilation, causing dyspnea, hypoventilation, and hypoxemia. Treatment of pneumothorax includes chest tube insertion into the pleural space to reestablish negative pressure, so that breathing can gradually reinflate the lung. The chest tube is connected to a water-sealed drainage system that allows air in the pleural space to escape through the tube with every exhalation. Once the lung is fully reexpanded and there are no further air leaks into the pleural space, the chest tube can be removed. An open pneumothorax (or traumatic pneumothorax) is an injury to the chest wall that results in a hole or "opening" in the chest wall (e.g., gunshot wound, stab, or deep laceration), allowing air to enter the pleural space from the atmosphere with each inhalation (ESG Fig. 36-1). If the wound in the chest wall remains open, large amounts of air enter the pleural space, making ventilation difficult. Initial emergency treatment for such a wound is to cover it with an occlusive (airtight) dressing to keep air from entering into the pleural space. An open chest injury can lead to a tension pneumothorax (ESG Fig. 36-2). In this case, air is entering the pleural space, the lung is collapsed, and the air does not escape with exhalation. Pressure accumulates and pushes on the heart, aorta, and vena cava, lowering venous return and cardiac output. This is a life-threatening emergency. Treatment is aimed at reducing pressure. Special needles with a one-way valve are made for this purpose. These needles allow air to escape from the pleural space during expiration without allowing air to enter during inhalation. A spontaneous pneumothorax (ESG Fig. 36-3) occurs unexpectedly as a result of the rupture of a bleb (blister-like formation) on the surface of the lung. Air from the lung enters the pleural space, causing a pneumothorax. This usually occurs in young healthy men and often does not collapse the entire lung. When the pneumothorax is small enough, it may heal on its own without invasive treatment. However, if the pleural rupture acts like a one-way valve, then a tension pneumothorax can develop, requiring immediate treatment.

Pulmonary Embolus

A pulmonary embolus is an obstruction in pulmonary arterial circulation by a foreign substance. Although the most common embolus is a thrombus (blood clot), air (injected through an IV line), fat (mobilized from the marrow of a bone by injury or surgery), or amniotic fluid (which enters the mother's circulation during childbirth) can become a pulmonary embolus. A pulmonary embolus causes mechanical obstruction of the pulmonary circulation. Obstruction triggers vasoconstriction and bronchoconstriction in the affected area, leading to impaired gas exchange. The clinical effects of a pulmonary embolus depend on its size and location. Small microemboli can lodge in pulmonary vessels and cause no signs and symptoms. However, multiple microemboli over time can result in elevated pressure in the vessels. Moderate-sized emboli lodge in larger vessels, causing breathlessness, pleuritic pain, anxiety, and cough with blood-tinged sputum. A massive embolus lodges in a main pulmonary artery, causing crushing chest pain, distended neck veins, rapid and weak pulse, hypotension, and sudden collapse. Emergency treatment is aimed at supporting vital functions while restoring pulmonary circulation. Deep vein thrombosis in the leg is the most common cause of pulmonary emboli.

Alveolar-Capillary Membrane Disorders

Alveolar-capillary membrane disorders are characterized by a change in the consistency of the lung tissue, especially at the alveolar level. The alveoli become stiff and difficult to ventilate, and gas exchange is impaired. Pulmonary edema, acute respiratory distress syndrome, pulmonary fibrosis, and atelectasis are examples.

AIR QUALITY AFFECTS OXYGENATION

Air pollution triggers acute respiratory problems that interfere with oxygenation. Even healthy people may experience headache, coughing, and other symptoms when exposed to air pollution. People with existing respiratory disease may become unable to function. Pollutants in outdoor and indoor air are most harmful to infants and toddlers, older adults, and those with heart or lung disease.

Airway Inflammation and Obstruction

Allergic reactions (e.g., asthma) or irritation from smoke or other irritants may cause airway inflammation. Obstruction may be mechanical (caused by a foreign object or bolus of food) or may be caused by spasm.

PATHOPHYSIOLOGICAL CONDITIONS THAT INFLUENCE GAS EXCHANGE

Alterations in gas exchange are caused by a number of disorders that affect the structure and function of the pulmonary and cardiovascular systems, as well as the regulation of these systems. The next sections examine pulmonary system abnormalities, pulmonary circulation abnormalities, neuromuscular abnormalities, cardiovascular abnormalities, peripheral vascular abnormalities, oxygen transport abnormalities, and metabolism extremes and oxygen demand.

Cough Suppressants

Although coughing is a protective mechanism to remove secretions from the airway, an involuntary, nonproductive cough can cause respiratory discomfort and interfere with sleep. Cough suppressants are effective in reducing the frequency of such a cough. They do not, however, interfere with the ability to voluntarily cough to remove secretions. Cough suppressants should not be given to children under age 4 years. Cough suppressants may be opioid or nonopioid. Opioid. Opioid cough suppressants depress the cough center in the medulla of the brainstem and are considered the most effective cough-suppressing agents. They are relatively safe but can cause dependence, bronchial constriction, CNS depression, and constipation. The major opioid cough suppressants are codeine and hydrocodone. Nonopioid. The most commonly used nonopioid cough suppressant is dextromethorphan, a derivative of opioids but without dependence or other side effects. It is found in most over-the-counter cough suppressant preparations. Benzonatate (Tessalon) is a cough suppressant that has an anesthetic effect on the stretch receptors in the respiratory tract, thus reducing reflexive coughing. It has a number of side effects that make it less desirable than dextromethorphan.

Central Nervous System Abnormalities

Any condition that causes injury to the CNS or alters CNS function can interfere with the regulation of breathing. Trauma and stroke (cerebrovascular accident) are the most commonly seen problems. -Significant head trauma may cause swelling and increase pressure within the skull, resulting in abnormal function of the respiratory control centers of the brainstem. The respiratory pattern may be altered, or breathing may cease. -Spinal cord injuries impede nerve transmission from the brain to the area below the level of the injury. The location of the injury level is crucial to determining its effect on breathing. The phrenic nerve, located above the level of the 3rd cervical vertebra, carries messages controlling diaphragm contraction from the respiratory center in the brainstem to the diaphragm. An injury between the 1st and 3rd cervical vertebrae frequently damages the phrenic nerve, resulting in severe hypoventilation or total inability to breathe. An injury between C3 and C5 may limit diaphragm function. An injury above the 7th thoracic vertebra may result in hypoventilation because of the loss of intercostal muscle function. -A cerebrovascular accident (CVA) may also cause loss of muscle function. The location of the CVA determines what effect, if any, it has on oxygenation. Abnormal breathing patterns or limited muscular control may be seen. Medications that depress the CNS can produce similar results.

Chest Wall Restriction

Anything that restricts or limits the free movement of the chest wall can lead to hypoventilation and eventually hypoxemia. Common causes are fractured ribs, musculoskeletal deformities, and obesity. -Fractured ribs, usually the result of trauma, may be limited (simple linear crack with no bone displacement) to severe (multiple rib fractures with bone displacement). Even limited fractures are quite painful, causing cautious, shallow breathing that limits ventilation. Multiple rib fractures can become displaced and create an unstable section of chest wall, called a flail chest. The unstable portion moves inward during inhalation (instead of outward) and outward during exhalation (instead of inward), which compromises ventilation. -Musculoskeletal deformities (e.g., kyphosis, scoliosis, rheumatoid arthritis of the spine, pectus excavatum) can also limit ventilation by impairing chest wall movement. -Obesity limits diaphragm movement downward with inhalation, frequently causing dyspnea with little or no exercise. All these conditions place a person at high risk for respiratory complications (e.g., retained secretions and pneumonia), especially in the presence of other illnesses or when undergoing surgery.

Aspiration

Aspiration is the presence of fluid or particles in the airways. The most common causes of aspiration are swallowing difficulties or depressed cough reflex secondary to anesthesia, sedation, neurological abnormalities, or gastric reflux. The effect of aspiration on respiratory function and gas exchange depends on the amount and type of substance aspirated. Aspiration of acidic gastric fluid causes a chemical inflammation in the airways, resulting in impaired cilia function, accumulation of pulmonary secretions, bronchospasm, and decreased airflow. Gastric fluid that reaches the alveoli damages the alveolar-capillary membrane, causing alveolar collapse and disruption in surfactant production. To prevent aspiration, follow these tips: -Elevate the head of the bed when a nasogastric feeding is in progress or in the presence of chronic gastric reflux. -Do not offer food or fluids when the patient is heavily sedated or during the initial recovery phase of anesthesia. -Position the unconscious patient on his side to protect his airway. -Follow special feeding techniques when the patient's swallowing is impaired.

Foreign Body Obstruction

Aspiration of a foreign body can cause partial or complete airway obstruction. Children younger than 3 years are at greatest risk for aspirating foreign bodies. Common culprits are pieces of hot dog, peanuts, marbles, coins, candy, and buttons. Adults are more likely to aspirate large bites of food when talking or laughing while eating. The American Heart Association's basic cardiopulmonary resuscitation guidelines include protocols for assessment of and interventions for partial and complete obstructed airways in the adult, child, and infant. For further information, See ESG Boxes 23-4 and 23-5, Rescue Maneuver for Choking, in Chapter 24 of this Electronic Study Guide.

Asthma

Asthma is a chronic inflammatory disease of the airways characterized by recurrent inflammation, bronchospasm, and airway obstruction. Asthma attacks can be triggered by stimuli that do not cause symptoms in nonasthmatic people. The person with asthma has hyperresponsive airways. Inflammatory triggers are inhaled allergens or irritants that trigger swelling, increased mucus production, and bronchospasm. Bronchospasm triggers, such as cold air, exercise, emotional upset, or bronchial irritants, cause airway obstruction, dyspnea, and poor oxygenation. Signs and symptoms of an asthma attack include wheezing, coughing, chest tightness, and airway obstruction. Treatment is aimed at supporting ventilation and oxygenation, reversing bronchospasm, and reducing inflammation through medication and lifestyle adjustment.

Atelectasis

Atelectasis is alveolar collapse. It may be caused by compression from a tumor, fluid accumulation, abdominal distention, or obstructed or hypoventilated alveoli. The larger the area of atelectasis, the greater the risk of hypoxemia. Anything that reduces ventilation can cause atelectasis. Deep-breathing and coughing exercises, frequent position changes, and early ambulation after surgery help prevent atelectasis by increasing secretion removal, keeping surfactant distributed among alveoli, and inflating obstructed alveoli through the pores between them.

Beta-2 Adrenergic Agonists

Beta-2 adrenergic agonists relax smooth muscle in the walls of the airways, relieving bronchospasm with minimal elevation of heart rate or blood pressure. Side effects of these medications are dose related. At low doses, increased heart rate and blood pressure are minimal, but at higher doses or in patients with underlying cardiac conditions, the side effects become more prevalent. Older adults and patients with heart disease should be monitored for tachycardia and hypertension when taking these medications. Also, beta-2 agonists suppress histamine release in the lung and increase ciliary motility, which helps reduce inflammation and more effectively remove secretions. Inhaled short-acting beta-2 agonists are used for rapid relief of asthma or acute bronchospasm and are commonly available as metered-dose inhalers (MDI). Longer acting inhaled and oral beta-2 agonists are used on a regular basis to control persistent symptoms. Inhaled short-acting beta-2 agonists used before exercise can control exercise-induced asthma. Tachycardia and muscular tremor, especially in the hands and arms, are the most common side effects.

Acute Bronchitis

Bronchitis is an infection of the bronchi, causing bronchial irritation and inflammation and leading to coughing and mild airway obstruction. Acute bronchitis is inflammation of the bronchi, but with no evidence of pneumonia, common cold, or asthma. It may be viral or bacterial. Bronchitis may occur after viral infection in healthy individuals or in those with chronic lung conditions. Acute bronchitis results in fever, cough, chills, malaise, and chest wall pain from coughing. In bacterial infections, the cough is productive, with yellow to green sputum. In viral infections, the cough is nonproductive and aggravated by cold, dry, or dusty air and may cause coughing paroxysms (prolonged bouts of continuous coughing) that are difficult to stop. Treatment is aimed at controlling the symptoms.

Bronchodilators

Bronchodilators relax the smooth muscles lining the airways. They can be administered as oral or inhaled medicines. The main types of bronchodilators are beta-2 adrenergic agonists, anticholinergics, and methylxanthines.

Corticosteroids

Corticosteroids reduce airway inflammation. They can be administered by inhalation, orally, or intravenously. With inhaled dosing, corticosteroids have minimal systemic side effects. When they are taken orally for long periods of time, numerous adverse effects are likely to occur, including bone loss, immune suppression, salt and water retention, and elevated blood sugar. IV use is associated with the greatest risk for side effects.

Cough Preparations

Cough preparations include expectorants, which help make coughing more productive, and antitussives (cough suppressants), which reduce the frequency of an involuntary hacking nonproductive cough. These agents are often found mixed together in one preparation to achieve both desirable effects with one medication. The goal is to reduce the frequency of dry, unproductive coughing while making voluntary coughing more productive. Over-the-counter cold remedies should not be used for children under age 6. There is little evidence that they are effective, and they can be dangerous. One study (Cohen, Rozen, Kristal, et al., 2012) found honey to be more effective for children's cough than dextromethorphan.

Cromolyn

Cromolyn (Intal) and nedocromil (Tilade) suppress inflammation but are not bronchodilators. They stabilize mast cells and prevent release of histamine and other mediators when administered before the onset of an asthma attack, but they do not relieve acute bronchospasm. These drugs are administered by inhalation, have no systemic effects, and produce adequate control in 60% to 70% of patients with moderate asthma. Cromolyn can prevent exercise-induced bronchospasm when inhaled 15 minutes before exercise. A nasal preparation (Nasalcrom) can prevent symptoms of seasonal nasal allergies.

Acute Respiratory Distress Syndrome

Diffuse injury to the alveolar-capillary membrane, known as acute respiratory distress syndrome (ARDS), may be caused by various serious disorders (e.g., aspiration of gastric contents, near drowning, major trauma, and reactions to drugs and toxins). ARDS increases the permeability of the alveolar-capillary membrane so that fluid, blood cells, and proteins can easily leak from the capillaries into the alveoli. The result is widespread pulmonary edema. Treatment often requires tracheal intubation, mechanical ventilation, and very high levels of oxygen therapy to overcome hypoxemia until the cause can be reversed and the lungs have had a chance to heal themselves.

Expectorants

Expectorants increase the movement of secretions in the respiratory tract by reducing the viscosity of secretions. Increased fluid intake is often as effective as expectorants in making pulmonary secretions easier to remove. Expectorants, however, continue to be widely used. Guaifenesin is the most widely used expectorant and is found in many over-the-counter cough medicines.

Pulmonary Circulation Abnormalities

For gas exchange to occur across the alveolar-capillary membrane, there must be adequate blood flow through the pulmonary circulation. Altered pulmonary circulation can disrupt gas exchange. The most common causes of altered pulmonary circulation are pulmonary embolus and pulmonary hypertension.

Antihistamines

Histamine is a naturally occurring body substance that is released in response to tissue damage, microorganisms, and allergens. Histamine triggers smooth muscle constriction (especially in stomach and lungs), increased body secretions (especially salivary, gastric, lacrima, and bronchial secretions), and vasodilation with increased capillary permeability (causing fluid to leak from vessels into tissues, leading to edema and low blood pressure). Antihistamines are drugs that prevent the effects of histamine release. In addition to their antihistamine action, they also have varying degrees of anticholinergic and sedative activities. There are two types of histamine receptors: H-1 receptors, which mediate smooth muscle contraction and capillary dilation, and H-2 receptors, which mediate gastric acid secretion. H-1 receptor blockers are used to treat upper respiratory and nasal allergy symptoms. They include such drugs as diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), brompheniramine (Dimetane), loratadine (Claritin), fexofenadine (Allegra), and cetirizine (Zyrtec). Each of these medications has sedating qualities. Warn patients of this effect before their use. The last three medications (Claritin, Allegra, and Zyrtec) are newer drugs with less sedating qualities. They are very effective for relieving seasonal allergy and hay fever symptoms without undesirable side effects. Antihistamines can cause dry mouth, constipation, blurred vision, and urinary retention, especially in older adults.

Indoor Air

Indoor air pollutants include carbon monoxide, nitrogen oxides, radon, suspended particles (dust, soot, ash, and aerosols), asbestos, and tobacco smoke. -Carbon monoxide. Unvented or poorly vented stoves and heaters, cigarettes, and car exhaust release carbon monoxide, which can be lethal even in people with healthy lungs. Carbon monoxide attaches tightly to hemoglobin at the oxygen-receptor sites, preventing oxygen from combining with hemoglobin, leading to profound hypoxemia and tissue hypoxia. -Nitrogen oxides. Nitrogen oxides are released from the burning of fossil fuels and are mucous membrane irritants. -Radon. Radon, which causes lung cancer, is a natural radioactive gas produced by uranium 238 decay that seeps into basements through cracked floors, bricks, and concrete materials. Homes in high-risk geographical areas can be tested and repaired to prevent seepage. -Suspended dust. Soot, ash, and aerosols irritate mucous membranes and can lead to respiratory problems. -Asbestos. Asbestos (found in older insulation, shingles, sidings, paints, and appliances) is now regulated, and its use is prohibited. Asbestos deteriorates and releases fibers into the air; these fibers are a cause of lung cancer when inhaled over time. The rate of lung cancer is 5 times greater in people with asbestosis and 50 times greater when those people with asbestosis also smoke one pack of cigarettes per day (Roy, Khanra, Mukherjee, et al., 2013). -Tobacco smoke. Tobacco smoke, either directly inhaled by the smoker or indirectly inhaled by people in the environment, contributes to chronic lung disease, lung cancer, asthma, respiratory infections, and cardiovascular disease. It is discussed further in a separate section of your textbook.

Influenza

Influenza, or flu, is a communicable viral disease of the respiratory tract. Symptoms include fever, headache, myalgia (muscle pain or tenderness), exhaustion, nasal inflammation and discharge, sore throat, and cough. The flu virus is highly contagious. New strains of the virus continually emerge, so it is difficult to develop immunity to the disease. Influenza immunizations are manufactured for each season in an attempt to keep pace with the changing virus. A major and dangerous complication of influenza is pneumonia.

Laryngospasm

Laryngospasm is constriction of the muscles of the larynx. Irritants such as smoke, noxious fumes, heat, or sudden immersion in water can trigger laryngospasm, which is life threatening and may require an emergency tracheostomy (surgical opening into the trachea below the level of the larynx) to establish an open airway.

Leukotriene Antagonists

Leukotriene antagonists (also called leukotriene receptor antagonists, or LTRAs) are a new class of respiratory anti-inflammatory agents. They are the first new drugs for treating asthma in more than 20 years. Given orally, LTRAs suppress leukotrienes, which are compounds that promote bronchoconstriction, mucus production, and airway edema. LTRAs are approved for maintenance therapy but should not be used for treatment of acute bronchospasm. Improvement in symptoms is usually seen within 1 week of beginning treatment.

MEDICATIONS USED TO IMPROVE OXYGENATION

Medications used to treat disorders that affect oxygenation include oral and inhaled respiratory medications, cardiovascular drugs, anti-anxiety agents, and analgesics.

Neuromuscular Disorders

Neuromuscular disorders that affect the nerves involved in breathing can depress respiratory function. Guillain-Barré syndrome, amyotrophic lateral sclerosis, and myasthenia gravis are examples. -Guillain-Barré syndrome results in an ascending paralysis that may involve breathing muscles and may require mechanical ventilatory support. -Amyotrophic lateral sclerosis (ALS) causes degeneration of motor neurons in the brain, spinal cord, and periphery. Death is often caused by respiratory failure when the disease destroys the nerves that control the muscles of breathing. -Myasthenia gravis is an autoimmune disease characterized by loss of acetylcholine receptors on the motor end-plate of muscle fibers. Without an adequate number of receptors, the muscle cannot contract. This results in muscle weakness, easy fatigability, and ultimately paralysis. People with myasthenia gravis are at risk for hypoventilation and hypoxemia related to weak and ineffective contractions of the muscles of breathing.

Methylxanthines

Methylxanthines are CNS stimulants that also dilate the bronchioles. These medications also act as cardiac stimulants, produce vasodilation, and have a diuretic effect. Historically, these drugs were used widely in the treatment of asthma, but their use has diminished because of their significant side effects, as well as the availability of more effective medications. Theophylline (e.g., Theospan-SR), aminophylline (e.g., Paladron), and caffeine (e.g., Vivarin) are the most common methylxanthines. Theophylline is administered orally, and aminophylline is administered intravenously. IV infusions are controlled with a volumetric pump. The patient requires monitoring for tachycardia and abnormal heart rhythms. Caffeine is a methylxanthine with common effects. Strong coffee or other caffeine-containing beverages tend to have a mild bronchodilation effect. All methylxanthines have a slow onset of action and thus are more useful for preventing bronchospasm than for relieving acute bronchospasm.

Mucolytic Agents

Mucolytic agents react directly with mucus to reduce viscosity and make secretions easier to remove from the airways. There are two preparations, concentrated saline and acetylcysteine (Mucomyst), both of which are administered by inhalation. Acetylcysteine can trigger bronchospasm and smells like rotten eggs; therefore, its use is limited. Saline is well tolerated and is frequently instilled into artificial airways to liquefy secretions.

Nasal Decongestants

Nasal decongestants relieve stuffy, blocked nasal passages by constricting local blood vessels through stimulation of alpha-1 adrenergic nerve receptors in the vessels. Although the desired effect is on the nasal mucosa, these medications can have systemic adrenergic effects, causing elevated blood pressure, tachycardia, and palpitations, especially in people with a history of cardiovascular conditions. Decongestants may be administered orally, by inhalation, or in topical nasal sprays. Pseudoephedrine is the most common decongestant administered orally. L-Desoxyephedrine (Vicks inhaler) and propylhexedrine (Benzedrex inhaler) are over-the-counter inhalers. Decongestant nasal sprays contain ephedrine, epinephrine, oxymetazoline (Afrin), phenylephrine (Neo-Synephrine), or xylometazoline (Otrivin). Overuse of topical sprays produces a side effect known as rebound nasal congestion. Excess use causes local mucosa ischemia and irritation that leads to secondary extensive vasodilation and congestion once the spray wears off. In 2000, the Food and Drug Administration (FDA) began removing phenylpropanolamine (PPA), an agent found in many over-the-counter cold and cough remedies, from all drug preparations because of evidence that it increased the risk of hemorrhagic stroke. Patients should be advised to avoid any products that might still contain this ingredient.

Neuromuscular Abnormalities

Neuromuscular abnormalities can affect gas exchange by interfering with the regulation of breathing or by limiting the movement of the muscles involved in breathing. Major causes are CNS abnormalities and neuromuscular disorders.

Pneumonia

Pneumonia is an infection of the lungs caused by bacteria, fungi, or viruses. It occurs more often during winter months and often follows a recent upper respiratory tract infection or influenza. Pneumonia-causing organisms gain entry into the lungs from being released into the air with coughing, sneezing, or talking; from contaminated respiratory therapy equipment; from the blood spreading to the lung; or from the nose and throat. The invading pathogen releases toxins that damage bronchial and alveolar-capillary membranes. A full-scale inflammatory response triggers edema in the small airways and deposits debris and exudate in the alveoli. Some toxins even cause lung tissue necrosis. The area of the lung affected becomes consolidated (solid rather than air filled). Symptoms of pneumonia include cough, malaise, pleural pain from coughing, discolored sputum, fever, chills, dyspnea, and elevated WBC counts. Treatment includes antipyretics for fever, expectorants to enhance mobilization of secretions, humidity to moisten inhaled air, hydration to thin secretions, pulmonary hygiene (deep breathing, coughing, and chest percussion and vibration) to move secretions out of the airways, rest to conserve body energy stores, and, if needed, oxygen therapy. Curative therapy includes appropriate anti-infective agents to kill the causative organisms. Immunizations are available and are discussed in your textbook.

Outdoor Air

Some sources of outdoor air pollution are natural (e.g., volcano eruptions and forest fires), but the most common and damaging sources are the result of human activities. Common pollutants found in the air are carbon monoxide, sulfur dioxides, nitrogen oxides, suspended particles, and hydrocarbons. Many of these contaminants result from incomplete combustion. The U.S. Environmental Protection Agency (EPA) developed the Air Quality Index (AQI), which converts pollutant concentration for a community into a number on a scale (ESG Table 36-1). When the AQI rating exceeds 100, those at greatest risk should avoid outdoor activities.

Pulmonary Edema

Pulmonary edema is the movement of fluid from pulmonary capillaries into the alveoli. The most common cause is heart failure, a condition in which the heart cannot pump effectively, causing blood to back up into the pulmonary circulation. The alveoli become stiff and difficult to ventilate, and gas exchange is impaired. The patient with pulmonary edema is gasping for air. Heart rate is fast, skin is moist and cool, and cyanosis may develop. The patient may cough up pink, frothy sputum. Fine crackles are audible with auscultation. Oxygen therapy, measures to improve the heart's effectiveness as a pump (drugs to strengthen heart contractility), and diuretics to reduce blood volume are part of the treatment.

Pulmonary Fibrosis

Pulmonary fibrosis is an excess of fibrous connective tissue in the lung that makes the lungs stiff and difficult to ventilate and reduces oxygen diffusion across the alveolar-capillary membrane. Causes include inhalation of harmful substances and scar formation. The long-term prognosis is poor.

Pulmonary Hypertension

Pulmonary hypertension is elevated blood pressure within the pulmonary arterial system. Normally, this is a low-pressure system with thin-walled compliant vessels. When pressure rises in the pulmonary circulation, it increases the workload of the heart. Over time, this causes right-sided heart failure, with less blood pumped into the pulmonary circulation. Pulmonary hypertension also damages pulmonary arterioles, resulting in destruction of these vessels and impaired gas exchange. Primary pulmonary hypertension is caused by genetic factors, exposure to certain drugs, HIV infection, and autoimmune disorders. It is uncommon but rapidly progressive. Secondary pulmonary hypertension is caused by a chronic increase in pulmonary arterial pressure associated with cardiac or pulmonary disease. It can be caused by chronic high pressures in the left atrium due to untreated blood pressure, stenotic heart valves, increased pulmonary blood flow, or chronic hypoxemia. (Hypoxemia causes constriction of pulmonary vessels but dilation of peripheral vessels.) As chronic lung disease progresses, more lung areas become underventilated, thus more pulmonary vessels become constricted. Eventually, overall pulmonary artery pressures rise.

Pulmonary System Abnormalities

Pulmonary system abnormalities include structural abnormalities, airway inflammation and obstruction, infections of the airways and lungs, and alveolar-capillary membrane disorders.

Inhalation Injuries

Smoke, noxious chemicals, and hot air inhalation are responsible for 60% to 70% of deaths in a fire. Typically, thermal injuries occur above the glottis because the vocal cords and glottis close as a protective mechanism that prevents the hot gases from entering the lower airways. Hot air, steam, or smoke causes swelling that can rapidly obstruct the upper airway. Clues to such inhalation burn injuries are singed nasal hairs, facial burns, hoarseness, painful swallowing, darkened oral mucous membranes, and coughing up black sputum. Injury to lower airway tissues (below the glottis) is caused by exposure to smoke or toxic fumes and may not produce signs or symptoms for 12 to 24 hours. It can, however, result in severe respiratory distress.

Respiratory Syncytial Virus (RSV)

RSV is a respiratory virus that can affect the upper respiratory tract and the lower airways. Healthy people usually recover from RSV infection in 1 or 2 weeks. However, RSV can be severe in infants, young children, and older adults. It is the most common cause of pneumonia in children under 1 year of age in the United States. It is also an important cause of respiratory illness in older adults, as well as potentially linked to reactive airway disease in children. Almost all children have had an RSV infection by the age of 2 years (CDC, 2010). It is spread by airborne droplets, although the droplets do linger only briefly in the air. It is also spread by direct and indirect contact with infected persons and can survive on hard surfaces for many hours.

Anti-inflammatory Medications

Respiratory anti-inflammatory agents combat inflammation in the airways. They are important in the treatment and control of respiratory conditions characterized by hypersensitive airways and airway inflammation (e.g., asthma). The major categories include corticosteroids, cromolyn, and leukotriene antagonists.

Infections of the Airways & Lungs

Respiratory infections are among the most common causes of short-term disability in the United States. Lower respiratory tract infections (acute bronchitis, pneumonia, and tuberculosis) occur more often in children, older adults, and those with impaired immunity or lung function.

Upper Respiratory Infections

Respiratory infections are among the most common causes of short-term disability in the United States. Upper respiratory infections (URIs) and influenza are both caused by a virus. Both start out with similar symptoms. It is important to distinguish between them because antiviral medications are available for the flu. Diagnosis is made more difficult because there are other conditions that start with cold-like symptoms (e.g., whooping cough, allergies, measles, and pneumonia). URI symptoms include stuffy nose, sore throat, cough, sneezing, tearing, and a mild fever. Colds are more common in children and tend to decline with age. They are rarely dangerous to healthy adults and children. The following are some URIs: The common cold: Nonspecific upper respiratory infections caused by viruses. Rhinosinusitis: Inflammation of the nasal mucosa and sinus cavities. Differentiating viral from bacterial sinusitis can't be done on the basis of clinical findings alone. Pharyngitis: Sore throat. Pharyngitis may be viral or bacterial. "Strep" throat, caused by Streptococcus pyogenes, is the most common cause of infectious pharyngitis. It cannot be differentiated from a viral sore throat by any one sign or symptom, so pharyngeal cultures or rapid antigen tests are conducted. Influenza: Flu is usually more severe than the common cold and often involves the lower airways—although some types may not. Symptoms often include—in addition to cold-like symptoms—headache, fatigue, weakness, exhaustion, and high fever. Most flu fatalities occur in children under age 2 and older adults, especially the frail elderly.

Respiratory Infections in Children

Respiratory infections in children are common. The small diameter of a child's airway can easily become obstructed with secretions or inflammation. Several respiratory infections occur more commonly in young children than in any other age group. These infections include viral croup (acute laryngotracheobronchitis), epiglottitis, and bronchiolitis. Table ESG 36-2 outlines these infections and their major characteristics.

Respiratory Medications

Respiratory medications promote ventilation and oxygenation by their effects on the respiratory system. The major types of respiratory medications include bronchodilators, anti-inflammatory agents, cough preparations, nasal decongestants, antihistamines, and mucolytic agents.

Pulmonary Structural Abnormalities

Structural abnormalities include anything that restricts or limits the free movement of the chest wall (e.g., fractured ribs or kyphosis), interruptions in the chest cavity that inhibit inflation of the lungs (e.g., pneumothorax), or a collection of fluid (e.g., blood, lymph, pus) in the pleural space that inhibits lung expansion.

The Airway

The airway consists of the nasal passages, mouth, pharynx, larynx, trachea, bronchi, and bronchioles (Fig. 36-1 in Volume 1 of your textbook). Air flows through these structures into and out of the lungs. In addition, the airway structures do the following: -Moisten the air—A moist mucous membrane lining adds water to inhaled air. -Warm the air—Blood flowing through the vascular airway walls transfers body heat to the inhaled air. -Filter the air—(1) Specialized cells in the lining of the airways secrete sticky mucus to trap foreign particles. (2) Cilia, tiny hair-like projections from the walls of the airways, move rhythmically to sweep trapped debris up and out of the airway.

The Lungs

The chest cavity is a closed compartment sealed with neck muscles at the top, ribs and intercostal muscles all around, and the large diaphragm muscle at the bottom. The lungs are soft, spongy, cone-shaped organs that lie on each side of the chest cavity, separated by the mediastinum, which contains the heart and great vessels. The right lung is composed of three lobes; the left lung has two lobes. The upper portion of each lung, the apex, extends upward above the clavicle. The lower portion of each lung, the base, rests on the diaphragm. Knowing the location of lung tissue beneath the chest wall helps you to perform a complete and accurate assessment of the lungs. The lungs are composed of millions of alveoli. The alveoli are tiny air sacs with thin walls surrounded by a fine network of capillaries. Gases easily pass back and forth between the alveoli and capillaries. It is in the alveolar-capillary membrane that inhaled air in the lungs comes in contact with the blood of the pulmonary circulation. See ESG Figure 37-1. Alveoli are composed of two types of cells (see ESG Fig. 36-1). Type I alveolar cells are the gas exchange cells. Type II alveolar cells produce surfactant, a lipoprotein that lowers the surface tension within alveoli. The moist, membranous inner walls of the alveoli tend to draw together because of the high surface tension inside the alveoli. Therefore, adequate surfactant levels are key to preventing the alveoli from adhering to each other and allowing them to inflate during breathing. The pleura is a thin, double-layered membrane. One layer lines the inside of the chest cavity, and the other covers the outside of the lungs. Between the two layers is the pleural space, a thin film of fluid that allows the two layers to remain in contact but glide over each other during breathing movements. Although the pleural membranes glide over each other, they also cling to each other because the negative pressure inside the pleural space creates a mild suction. This suction effect is important because the outside of the lungs must be in constant contact with the inside of the chest cavity for normal function to occur. As long as the pleural space is intact with negative pressure and only a thin film of fluid inside, the lungs expand and contract as the chest expands and contracts. If the pleural space fills with air or fluid, or the chest cavity is opened, the lungs will no longer be in contact with the inside of the chest wall, altering lung expansion and pulmonary function.

Anticholinergics

The only anticholinergic agent used in the treatment of bronchospasm is inhaled ipratropium bromide (Atrovent). Ipratropium bromide is not a strong bronchodilator when used alone, but it is potentiated when used with beta-2 agonists. Its actions are slow but prolonged, so it is more effective for the prevention rather than acute treatment of bronchospasm. Combivent is a commercially available combination of ipratropium bromide and albuterol (beta-adrenergic agonist).

The Lower Airway

The trachea and bronchial tree make up the lower airway. The trachea, sometimes called the "windpipe," is a tube supported by horseshoe-shaped rings of cartilage that keep it from collapsing during inhalation. The trachea lies just under the skin of the anterior neck, making it accessible for creation of an emergency surgical airway opening. The trachea extends from the larynx to the point at which it divides to form the right and left mainstem bronchi. The right mainstem bronchus is shorter and fatter than the left and leaves the trachea at a more vertical angle. Thus, it is more susceptible to aspiration of foreign bodies, food, and fluid than the left. The mainstem bronchi are also supported by cartilage rings. However, as the airways branch and become smaller, the cartilage becomes progressively thinner until it disappears in the smaller bronchioles. The walls of the bronchi and bronchioles contain layers of smooth muscles. Spasm of these muscles (bronchospasm) narrows the airway and obstructs airflow.

The Upper Airway

The upper airway, located above the larynx, includes the nasal passages, mouth, and pharynx. The lower airway, located below the larynx, includes the trachea, bronchi, and bronchioles. The lower airway is considered sterile. Normally, we inhale air through the nose. The nasal passages contain coarse hairs that filter air and vascular mucous membranes that warm and moisten the air. When nasal passages are blocked or we need increased airflow, we use mouth breathing. The mouth does not efficiently warm and moisten inhaled air and has no filtering capacity, so the air entering the lungs is cooler, drier, and less clean. The pharynx (throat) contains the openings to the esophagus and trachea. The trachea lies just in front of the esophagus; thus, the openings to both lie next to each other in the pharynx. The epiglottis, a small flap of tissue superior to the larynx, closes off the trachea during swallowing so that food and fluid do not enter the lower airway. The epiglottis opens during breathing to allow air to move through the airway. The larynx is the narrowest portion of the upper airway, so it is a common site for airway obstruction. It is composed of cartilage and muscles. Spasm of these muscles (laryngospasm) can close off the entire lower airway and create a respiratory emergency.

THE NERVOUS SYSTEM

This is merely a brief overview of the nervous system. For more complete information, consult anatomy and physiology textbooks. The nervous system has two divisions: 1. The central nervous system (CNS), which consists of the brain and the spinal cord 2. The peripheral nervous system (PNS), which consists of cranial nerves and spinal nerves and all their branches The PNS relays information to and from the CNS, where the brain integrates the information and initiates responses. The autonomic nervous system (ANS) is actually part of the PNS. However, the ANS has specialized functioning. The ANS has two divisions that attempt to balance each other to maintain homeostasis: 1. The sympathetic system dominates in stressful situations. It is responsible for the "fight-or-flight" response. For example, it increases heart rate, causes vasodilation in skeletal muscles, dilates the bronchioles to take in more air, and causes the liver to convert glycogen to glucose to supply energy. At the same time, it suppresses digestion and peristalsis, causes vasoconstriction in the skin and viscera, and shunts blood to vital organs, such as the heart and brain. 2. The parasympathetic system dominates in relaxed situations and promotes normal functioning—for example, digestion, peristalsis, defecation, urination, and normal heart rate.

Tuberculosis

Tuberculosis (TB) is an infection caused by the acid-fast bacillus Mycobacterium tuberculosis. Although TB is commonly thought of as a respiratory disease, infection may occur anywhere in the body. Historically, TB was a major cause of death and disability in North America. From 1950 to 1980, the incidence of TB declined with the introduction of effective antibiotics. However, the incidence is rising again because of the growing number of drug-resistant strains and the increasing number of people with compromised immune responses (related to aging and disease). Pulmonary TB is transmitted via airborne droplets, which may occur in overcrowded, poorly ventilated living conditions. Once inhaled into the lungs, the TB bacteria are usually walled off through the inflammatory response into granulomatous lesions. Immunity develops, and the bacteria either can remain walled off and dormant or can escape and cause active tuberculosis, especially in someone with impaired immunity. Common signs and symptoms include fatigue, weight loss, anorexia, night sweats, and blood-tinged sputum. The diagnosis of TB is made from sputum cultures or chest x-ray changes. Skin testing is used to screen for TB. A positive TB skin test (+PPD [purified protein derivative]) implies that a person has been exposed to the tubercle bacillus and has developed antibodies against it, but it does not indicate active disease.