Chapter 32
Acute Respiratory Failure and Acute Respiratory Distress Syndrome
KEY POINTS
Acute Respiratory Failure
• Acute respiratory failure (ARF) results from a failure of oxygenation and/or ventilation or both, which occurs
when the exchange of O2 and CO2 between the atmosphere, alveoli, and the blood, is inadequate.
• Respiratory failure is not a disease but a symptom of underlying pathology affecting lung function. The major
threat of ARF is the inability of the lungs to meet the O2 demands of the tissues.
• ARF is classified as hypoxemic or hypercapnic.
• Hypoxemic respiratory failure is defined as a PaO2 less than or equal to 60 mm Hg with normal or slightly
suboptimal PaCO2 levels.
• Common causes include pneumonia, pulmonary edema, pulmonary emboli, heart failure, and shock.
• Four main physiologic mechanisms may cause hypoxemia and hypoxemic respiratory failure: V/Q mismatch,
shunt, diffusion impairment, and alveolar hypoventilation.
• In hypercapnic respiratory failure, the lungs are often normal, and the main problem is ventilatory failure
(insufficient CO2 removal).
• It is defined as a PaCO2 greater than 50 mm Hg, which may or may not be accompanied by hypoxemia and/or
acidemia (arterial pH less than 7.35).
• Disorders that compromise CO2 removal include drug overdoses, central nervous system (CNS) depressants,
neuromuscular diseases, acute asthma, exacerbation of chronic obstructive pulmonary disease (COPD), and
spinal cord injury.
• Hypoxemia occurs when the amount of O2 in arterial blood is less than normal. Hypoxia occurs when there is
a decrease I supply of oxygen at the cellular level. Hypoxemia can lead to hypoxia if not corrected.
• Cyanosis is an unreliable indicator of hypoxemia and the severity of ARF.
Clinical Manifestations
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2023 by Elsevier, Inc. All rights reserved.
, • Signs of respiratory failure are related to the extent of change in PaO 2 and/or PaCO2, the rapidity of change
(acute versus chronic), and the ability of the body to compensate.
• One of the first signs of acute hypoxemic failure is a change in the patient’s mental status. Other early signs
include tachycardia, tachypnea, pallor, and a mild increase in the patient’s work of breathing (WOB).
• A morning headache and slow respiratory rate suggest hypercapnia.
• Other signs and symptoms that provide information about the patient’s respiratory status and work of
breathing include:
o Position the patient assumes
o Patient’s ability to speak
o Use of pursed-lip breathing
o Use of the accessory muscles, including the presence of muscle retractions of the intercostal spaces or
the supraclavicular area
o Work of breathing (WOB)
• Auscultate breath sounds. Crackles may indicate pulmonary edema and pneumonia. Absent or decreased
breath sounds may occur with atelectasis or pleural effusion. Bronchial breath sounds over the lung periphery
result from lung consolidation due to pneumonia.
Diagnostic Studies
• Arterial blood gases (ABGs) are done to identify oxygenation (PaO2) and ventilation (PaCO2) status, and give
information about acid-base balance.
• A chest x-ray can help identify possible causes of respiratory failure.
• Other diagnostic studies include a complete blood cell count, serum electrolytes, sputum and blood cultures,
urinalysis, and electrocardiogram.
2 Copyright ©
2023 by Elsevier, Inc. All rights reserved.
Acute Respiratory Failure and Acute Respiratory Distress Syndrome
KEY POINTS
Acute Respiratory Failure
• Acute respiratory failure (ARF) results from a failure of oxygenation and/or ventilation or both, which occurs
when the exchange of O2 and CO2 between the atmosphere, alveoli, and the blood, is inadequate.
• Respiratory failure is not a disease but a symptom of underlying pathology affecting lung function. The major
threat of ARF is the inability of the lungs to meet the O2 demands of the tissues.
• ARF is classified as hypoxemic or hypercapnic.
• Hypoxemic respiratory failure is defined as a PaO2 less than or equal to 60 mm Hg with normal or slightly
suboptimal PaCO2 levels.
• Common causes include pneumonia, pulmonary edema, pulmonary emboli, heart failure, and shock.
• Four main physiologic mechanisms may cause hypoxemia and hypoxemic respiratory failure: V/Q mismatch,
shunt, diffusion impairment, and alveolar hypoventilation.
• In hypercapnic respiratory failure, the lungs are often normal, and the main problem is ventilatory failure
(insufficient CO2 removal).
• It is defined as a PaCO2 greater than 50 mm Hg, which may or may not be accompanied by hypoxemia and/or
acidemia (arterial pH less than 7.35).
• Disorders that compromise CO2 removal include drug overdoses, central nervous system (CNS) depressants,
neuromuscular diseases, acute asthma, exacerbation of chronic obstructive pulmonary disease (COPD), and
spinal cord injury.
• Hypoxemia occurs when the amount of O2 in arterial blood is less than normal. Hypoxia occurs when there is
a decrease I supply of oxygen at the cellular level. Hypoxemia can lead to hypoxia if not corrected.
• Cyanosis is an unreliable indicator of hypoxemia and the severity of ARF.
Clinical Manifestations
1 Copyright ©
2023 by Elsevier, Inc. All rights reserved.
, • Signs of respiratory failure are related to the extent of change in PaO 2 and/or PaCO2, the rapidity of change
(acute versus chronic), and the ability of the body to compensate.
• One of the first signs of acute hypoxemic failure is a change in the patient’s mental status. Other early signs
include tachycardia, tachypnea, pallor, and a mild increase in the patient’s work of breathing (WOB).
• A morning headache and slow respiratory rate suggest hypercapnia.
• Other signs and symptoms that provide information about the patient’s respiratory status and work of
breathing include:
o Position the patient assumes
o Patient’s ability to speak
o Use of pursed-lip breathing
o Use of the accessory muscles, including the presence of muscle retractions of the intercostal spaces or
the supraclavicular area
o Work of breathing (WOB)
• Auscultate breath sounds. Crackles may indicate pulmonary edema and pneumonia. Absent or decreased
breath sounds may occur with atelectasis or pleural effusion. Bronchial breath sounds over the lung periphery
result from lung consolidation due to pneumonia.
Diagnostic Studies
• Arterial blood gases (ABGs) are done to identify oxygenation (PaO2) and ventilation (PaCO2) status, and give
information about acid-base balance.
• A chest x-ray can help identify possible causes of respiratory failure.
• Other diagnostic studies include a complete blood cell count, serum electrolytes, sputum and blood cultures,
urinalysis, and electrocardiogram.
2 Copyright ©
2023 by Elsevier, Inc. All rights reserved.
