CHAPTER 13 Improving Oxygenation and Management of Acute Respiratory Distress Syndrome OUTLINE BASICS OF OXYGENATION USING FIO2, PEEP STUDIES, AND Transmission of Airway Pressure to Pleural Space PRESSURE–VOLUME CURVES FOR ESTABLISHING OPTIMAL PEEP Uses of PEEP for Problems Other Than ARDS Basics of Oxygen Delivery to the Tissues Weaning from PEEP Evaluating PaO2, SpO2, and FIO2 in Ventilator Patients ACUTE RESPIRATORY DISTRESS SYNDROME Adjusting FIO2 Pathophysiology Selection of FIO2 or Adjustment of Mean Airway Pressures Changes in Computed Tomogram With ARDS Introduction to Positive End-Expiratory Pressure and Continuous ARDS as an Inflammatory Process Positive Airway Pressure Two Categories of ARDS Pathophysiology of Atelectasis ARDS: A Heterogeneous Disorder—Normal Lung Versus ARDS Goals of PEEP and CPAP PEEP and the Vertical Gradient in ARDS Terminology Lung-Protective Strategies: Setting Tidal Volume and Pressures Technical Aspects of PEEP and CPAP Devices in ARDS Application of CPAP and PEEP to the Patient’s Airway Long-Term Follow-Up on ARDS Circuitry for Spontaneous CPAP With Freestanding Systems and Pressure–Volume Loops and Recruitment Maneuvers in Setting Mechanical Ventilators PEEP in ARDS Peep Ranges Patient Evaluation for Lung Recruitment Minimum or Low PEEP Pressure–Volume Loops in Setting PEEP Therapeutic PEEP Recruitment Maneuvers Optimal PEEP Derecruitment Maneuver Indications for PEEP and CPAP Summary of Recruitment Maneuvers in ARDS Initiating PEEP Therapy The Importance of Body Position During Positive Pressure Selecting the Appropriate PEEP/CPAP Level (Optimal PEEP) Ventilation Application of PEEP Above 5 cm H2O Positioning in a Patient With ARDS: Prone Positioning Optimal PEEP Study Patient Position in Unilateral Lung Disease Use of Pulmonary Vascular Pressure Monitoring With PEEP ADDITIONAL PATIENT CASES Contraindications and Physiological Effects of PEEP Summary Contraindications for PEEP Pulmonary Effects of PEEP KEY TERMS • Absorption atelectasis • Exudative • Prone positioning • Cytokines • Fibrosing alveolitis • Recruitment maneuver • Deflation point • Independent lung ventilation • Thrombotic mediators • Deflection point • Lower inflection point • Upper inflection point LEARNING OBJECTIVES On completion of this chapter, the reader will be able to accomplish the following: 1. Calculate a desired FIO2 required to achieve a desired PaO2, 6. Explain the effects of PEEP/CPAP therapy on a patient with a based on current ventilator settings and blood gases. unilateral lung disease. Describe the problems associated with 2. Calculate a patient’s pulmonary shunt fraction. initiating PEEP in a patient with an untreated pneumothorax. 3. Identify indications and contraindications for continuous 7. Recommend adjustments in PEEP and ventilator settings based positive airway pressure (CPAP) and positive end-expiratory on the physical assessment of the patient, arterial blood gases pressure (PEEP). (ABGs), and ventilator parameters. 4. List the primary goal of PEEP and the conditions in which high 8. Compare static compliance, hemodynamic data, and ABGs as levels of PEEP are most often used. indicators of an optimal PEEP. 5. Describe the most appropriate method for establishing an op- 9. Identify from patient assessment and ABGs when it is appropri- timum level of PEEP for a patient with acute respiratory distress ate to change from CPAP to mechanical ventilation with PEEP. syndrome (ARDS) using a recruitment–derecruitment maneuver 10. Identify the severity of ARDS using the PaO2/FIO2 ratio. and the deflection point (lower inflection point during deflation 11. Recommend an appropriate tidal volume (VT) setting for a or derecruitment). patient with ARDS. 234 Improving Oxygenation and Management of Acute Respiratory Distress Syndrome CHAPTER 13 235 12. Identify the maximum plateau pressure (Pplat) value to use for 15. Describe the procedure for prone positioning in ventilated patients with ARDS. patients with ARDS. 13. Identify the criteria that should be used to liberate a patient 16. List potential problems associated with placing the patient in a from PEEP or CPAP. prone position during mechanical ventilation. 14. Recommend a PEEP setting based on the inflection point on the 17. Discuss several theories that describe how prone positioning deflation curve using the pressure–volume loop for a patient improves ventilation-perfusion in ARDS. with ARDS. mproving the ventilatory status of a patient with hypercapnic treated with the administration of blood products, which in turn respiratory failure (i.e., reducing the partial pressure of carbon improves the patient’s O2-carrying capacity (i.e., hemoglobin). dioxide [PaCO2]) can be accomplished by improving alveolar Circulatory hypoxia occurs when the patient’s cardiac output is Iventilation, reducing physiological dead space, and reducing car- reduced. The treatment of this type of hypoxia typically involves bon dioxide (CO2) production. Improving oxygenation, on the fluid resuscitation and pharmacological interventions, which nor- other hand, involves using various patient management strategies, malize the patient’s cardiac output (e.g., administering drugs that such as administering supplemental oxygen (O2), applying posi- increase ventricular contractility or decrease vascular resistance) tive end-expiratory pressure (PEEP) or continuous positive airway and therefore improve O2 delivery (DO2) to the tissues. With his- pressure (CPAP), and patient positioning. totoxic hypoxia, cyanide interferes with a person’s ability to utilize Although the terms hypoxia and hypoxemia are often used O2 to produce energy (cellular respiration) by uncoupling oxidative interchangeably, it is important to recognize that hypoxia is defined phosphorylation (i.e., cytochrome oxidase). Treatment of cyanide as a reduction in O2 in the tissues, whereas hypoxemia refers to a poisoning involves administering a cyanide antidote (e.g., hydroxo- reduction in the partial pressure of O2 in the blood (i.e., PaO2 <80 cobalamin) and providing supportive care to maintain oxygenation mm Hg and SaO2 <95%). Box 13.1 provides a brief description of and acid–base balance. the four types of hypoxia, and Key Point 13.1 provides PaO2 and SaO2 values typically used to identify mild, moderate, and severe Case Study 13.1 hypoxemia. Myasthenia Gravis A patient with myasthenia gravis is placed on mechanical Key Point 13.1 ventilation. The chest radiograph is normal. Breath sounds are clear. Initial arterial blood gases (ABGs) on 0.25 FIO2 a Levels of Hypoxemia 20 minutes after beginning ventilation are as follows: pH PaO2 Value PaO2 Range Saturation = 7.31; PaCO2 = 62 mm Hg; bicarbonate = 31 mEq/L; and Level (mm Hg) (mm Hg) (SaO2) (%) PaO2 = 58 mm Hg. What change in ventilator setting might Mild hypoxemia <80 60–79 90–94 improve this patient’s ABG findings? Moderate hypoxemia <60 40–59 75–89 Severe hypoxemia <40 <40 <75 Improvement in oxygenation status may require time before a Values given are for a young adult breathing room air. (NOTE: The levels of hy- the response to treatment is evident. This is particularly evident in poxemia defined here may differ depending among clinicians and institutions.) cases involving hypoventilation, anemia, and circulatory hypoxia. In these cases, it is appropriate to administer supplemental O2 until the hypoxemia is relieved. The strategy used to treat hypoxia should focus on its cause. This chapter begins with a discussion of how to make simple For example, hypoxemic hypoxia, which occurs when a person adjustments of FIO2 to improve oxygenation. It is followed by a breathes rarefied air at a high altitude (i.e., reduced partial pres- discussion of techniques involving the use of PEEP to improve sure of inspiratory O2 [PIO2]) can be reversed by having the per- oxygenation. Achieving optimal PEEP requires close monitoring son breathe an enriched O2 mixture. When hypoventilation causes and the use of either static or dynamic pressure–volume loops. hypoxemia, increasing minute ventilation generally improves oxy- Methods used to set optimal PEEP are provided along with a genation (Case Study 13.1). Serious anemia, on the other hand, is review of pressure–volume loops. Additional uses of PEEP are also discussed in this chapter, along with a description of the effects, complications, and consequences associated with discon- BOX 13.1 Types of Hypoxia tinuation of PEEP. A discussion of the pathophysiology of acute respiratory dis- • Hypoxemic hypoxia (lower than normal P O , ascent to a 2 tress syndrome (ARDS) is included to provide the reader with an altitude, hypoventilation) • Anemic hypoxia (lower than normal red blood cell count understanding of the complexity of this disorder. Patients with [anemia], abnormal hemoglobin, carbon monoxide ARDS are among the most difficult to oxygenate and manage in poisoning) the critical care unit. The concept of lung-protective strategies and • Circulatory hypoxia (reduced cardiac output, decreased lung recruitment maneuvers (RMs) that are currently being used tissue perfusion) to improve oxygenation, particularly in patients with ARDS, are • Histotoxic hypoxia (cyanide poisoning) included, along with three clinical scenarios related to the topics discussed in this chapter. 236 CHAPTER 13 Improving Oxygenation and Management of Acute Respiratory Distress Syndrome The pS O2 can be used to titrate FIO2 once the relationship Basics of Oxygenation Using FIO2, PEEP Studies, and Pressure–Volume Curves for Establishing between PaO2 and SpO2 has