Chapter 10 Oxygen Therapy Vincent J. Thawley, Philadelphia, Pennsylvania Kenneth J. Drobatz, Philadelphia, Pennsylvania Inadequate oxygen delivery to the tissues is commonly encountered in critically ill patients and results in a shift to anaerobic metabolism to support cellular functions. Cellular energy production declines and lactic acid accumulates, which may exacerbate tissue injury and can lead to organ dysfunction or even death. Ensuring adequate tissue oxygenation is a principal goal in the critical care setting. This chapter explores the major determinants of tissue oxygen delivery as well as several methods that may be employed in an effort to improve blood oxygen content. Indications for Oxygen Therapy Oxygen therapy is indicated in situations in which there is inadequate oxygen delivery to the tissues (hypoxia). Oxygen delivery depends on cardiac output, hemoglobin concentration, oxygen saturation of hemoglobin, and the amount of oxygen dissolved in plasma. It can be compromised by a number of pathophysiologic conditions including hypoxemia (poorly oxygenated arterial blood); decreased blood flow from cardiovascular shock, congestive heart failure, or vascular obstruction; anemia; or decreased hemoglobin affinity for oxygen such as in carbon monoxide intoxication or methemoglobinemia. Clinical signs of hypoxemia include tachypnea, orthopnea, and open-mouth breathing. Patients may appear anxious and stand with their head and neck extended and elbows abducted. Severely hypoxemic patients may be syncopal, obtunded, or comatose. Cyanosis is an insensitive marker for hypoxemia because it is only detected when the arterial partial pressure of oxygen is less than 50 mm Hg and there is greater than 5 g/dl of desaturated hemoglobin. Furthermore, cyanosis is not apparent in patients with severe anemia or mucous membranes that are pale due to hypoperfusion. There are five principal causes of hypoxemia. Low inspired partial pressure of oxygen, which may occur in intubated patients breathing an oxygen-poor gas mixture, patients rebreathing dead space gas, or patients living at a high altitude, can result in poorly oxygenated arterial blood. Hypoventilation occurs most frequently as a result of airway obstruction, neuromuscular disease, pleural space disease, or pain or in conditions that result in dysfunction of the chest wall or diaphragm. Impaired diffusion of oxygen across the alveolar-capillary membrane is an uncommon cause of hypoxemia. Diseases that result in pneumocyte proliferation or accumulation of cellular infiltrates or interstitial fibrin deposits may lead to thickening of the alveolar membrane and diffusion barrier. The most common cause of hypoxemia seen clinically is ventilation/perfusion (V/Q) mismatch, which occurs when pulmonary blood passes by poorly ventilated alveoli. V/Q mismatch is seen with pneumonia, pulmonary edema, hemorrhage, pulmonary thromboembolism, neoplasia, and acute respiratory distress syndrome (ARDS). Right-to-left vascular shunt represents an extreme case of V/Q mismatch and may be seen in cases of lung lobe collapse or in some congenital cardiac defects. This is the only cause of hypoxemia that does not respond to oxygen supplementation. Measuring blood oxygenation can be accomplished via either pulse oximetry or arterial blood gas analysis. Pulse oximetry estimates the percent oxygen saturation of hemoglobin (SpO2), whereas arterial blood gas analysis measures the amount of dissolved oxygen in plasma (PaO2). The relationship between SpO2 and PaO2 is demonstrated by the oxygen-hemoglobin dissociation curve. A shift in the curve to the right indicates improved oxygen delivery to the tissues, with a decrease in pH or an increase in temperature or in concentration of 2,3-diphosphoglycerate. An increase in pH or a decrease in temperature or in concentration of 2,3-diphosphoglycerate shifts the curve to the left, indicating an increased binding affinity of hemoglobin for oxygen. A PaO2 less than 80 mm Hg or SpO2 less than 93% indicates hypoxemia, and oxygen supplementation is warranted. When pulse oximetry and arterial blood gas analysis are unavailable, the decision to supplement oxygen should be based on physical examination findings that suggest hypoxemia or decreased tissue oxygen delivery. However, not all patients with clinical signs referable to the respiratory system are hypoxemic. For example, tachypnea may be noted with anxiety, pain, increased temperature, or metabolic acidosis. If there is any doubt, supplemental oxygen should be provided. Techniques for Oxygen Supplementation There are several methods available to provide supplemental oxygen, including flow-by, face mask, nasal or nasopharyngeal catheter, tracheal catheter, oxygen cage, and mechanical ventilation. The method selected depends on the severity of hypoxemia, the desired fraction of inspired oxygen (FiO2), the expected duration of therapy, equipment availability, patient compliance, and underlying disease process(es). Sedation is often administered to patients with respiratory distress and in some situations may facilitate the delivery of supplemental oxygen. Flow-By Oxygen Flow-by oxygen, administered by holding the oxygen hose a few centimeters from the nose or mouth if the patient is panting, is a quick and easy method to provide short-term supplemental oxygen. Flow-by also may be useful in determining whether a patient is responsive to oxygen therapy before initiating long-term treatment. An FiO2 of 30% to 40% may be achieved by using oxygen flow rates of 2 to 5 L/min. Constant nursing attention is required because many patients turn away from the oxygen source. This method is also somewhat wasteful because a significant amount of oxygen may bypass the patient.< div class='tao-gold-member'> Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Drugs Used to Treat Toxicoses Rodenticide Toxicoses Chapter 8: Additional Considerations Related to Legal Claims Insulin Resistance Stay updated, free articles. Join our Telegram channel Join Tags: Kirks Current Veterinary Therapy XV Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Oxygen Therapy Full access? Get Clinical Tree
Chapter 10 Oxygen Therapy Vincent J. Thawley, Philadelphia, Pennsylvania Kenneth J. Drobatz, Philadelphia, Pennsylvania Inadequate oxygen delivery to the tissues is commonly encountered in critically ill patients and results in a shift to anaerobic metabolism to support cellular functions. Cellular energy production declines and lactic acid accumulates, which may exacerbate tissue injury and can lead to organ dysfunction or even death. Ensuring adequate tissue oxygenation is a principal goal in the critical care setting. This chapter explores the major determinants of tissue oxygen delivery as well as several methods that may be employed in an effort to improve blood oxygen content. Indications for Oxygen Therapy Oxygen therapy is indicated in situations in which there is inadequate oxygen delivery to the tissues (hypoxia). Oxygen delivery depends on cardiac output, hemoglobin concentration, oxygen saturation of hemoglobin, and the amount of oxygen dissolved in plasma. It can be compromised by a number of pathophysiologic conditions including hypoxemia (poorly oxygenated arterial blood); decreased blood flow from cardiovascular shock, congestive heart failure, or vascular obstruction; anemia; or decreased hemoglobin affinity for oxygen such as in carbon monoxide intoxication or methemoglobinemia. Clinical signs of hypoxemia include tachypnea, orthopnea, and open-mouth breathing. Patients may appear anxious and stand with their head and neck extended and elbows abducted. Severely hypoxemic patients may be syncopal, obtunded, or comatose. Cyanosis is an insensitive marker for hypoxemia because it is only detected when the arterial partial pressure of oxygen is less than 50 mm Hg and there is greater than 5 g/dl of desaturated hemoglobin. Furthermore, cyanosis is not apparent in patients with severe anemia or mucous membranes that are pale due to hypoperfusion. There are five principal causes of hypoxemia. Low inspired partial pressure of oxygen, which may occur in intubated patients breathing an oxygen-poor gas mixture, patients rebreathing dead space gas, or patients living at a high altitude, can result in poorly oxygenated arterial blood. Hypoventilation occurs most frequently as a result of airway obstruction, neuromuscular disease, pleural space disease, or pain or in conditions that result in dysfunction of the chest wall or diaphragm. Impaired diffusion of oxygen across the alveolar-capillary membrane is an uncommon cause of hypoxemia. Diseases that result in pneumocyte proliferation or accumulation of cellular infiltrates or interstitial fibrin deposits may lead to thickening of the alveolar membrane and diffusion barrier. The most common cause of hypoxemia seen clinically is ventilation/perfusion (V/Q) mismatch, which occurs when pulmonary blood passes by poorly ventilated alveoli. V/Q mismatch is seen with pneumonia, pulmonary edema, hemorrhage, pulmonary thromboembolism, neoplasia, and acute respiratory distress syndrome (ARDS). Right-to-left vascular shunt represents an extreme case of V/Q mismatch and may be seen in cases of lung lobe collapse or in some congenital cardiac defects. This is the only cause of hypoxemia that does not respond to oxygen supplementation. Measuring blood oxygenation can be accomplished via either pulse oximetry or arterial blood gas analysis. Pulse oximetry estimates the percent oxygen saturation of hemoglobin (SpO2), whereas arterial blood gas analysis measures the amount of dissolved oxygen in plasma (PaO2). The relationship between SpO2 and PaO2 is demonstrated by the oxygen-hemoglobin dissociation curve. A shift in the curve to the right indicates improved oxygen delivery to the tissues, with a decrease in pH or an increase in temperature or in concentration of 2,3-diphosphoglycerate. An increase in pH or a decrease in temperature or in concentration of 2,3-diphosphoglycerate shifts the curve to the left, indicating an increased binding affinity of hemoglobin for oxygen. A PaO2 less than 80 mm Hg or SpO2 less than 93% indicates hypoxemia, and oxygen supplementation is warranted. When pulse oximetry and arterial blood gas analysis are unavailable, the decision to supplement oxygen should be based on physical examination findings that suggest hypoxemia or decreased tissue oxygen delivery. However, not all patients with clinical signs referable to the respiratory system are hypoxemic. For example, tachypnea may be noted with anxiety, pain, increased temperature, or metabolic acidosis. If there is any doubt, supplemental oxygen should be provided. Techniques for Oxygen Supplementation There are several methods available to provide supplemental oxygen, including flow-by, face mask, nasal or nasopharyngeal catheter, tracheal catheter, oxygen cage, and mechanical ventilation. The method selected depends on the severity of hypoxemia, the desired fraction of inspired oxygen (FiO2), the expected duration of therapy, equipment availability, patient compliance, and underlying disease process(es). Sedation is often administered to patients with respiratory distress and in some situations may facilitate the delivery of supplemental oxygen. Flow-By Oxygen Flow-by oxygen, administered by holding the oxygen hose a few centimeters from the nose or mouth if the patient is panting, is a quick and easy method to provide short-term supplemental oxygen. Flow-by also may be useful in determining whether a patient is responsive to oxygen therapy before initiating long-term treatment. An FiO2 of 30% to 40% may be achieved by using oxygen flow rates of 2 to 5 L/min. Constant nursing attention is required because many patients turn away from the oxygen source. This method is also somewhat wasteful because a significant amount of oxygen may bypass the patient.< div class='tao-gold-member'> Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Drugs Used to Treat Toxicoses Rodenticide Toxicoses Chapter 8: Additional Considerations Related to Legal Claims Insulin Resistance Stay updated, free articles. Join our Telegram channel Join
