Respiratory Acidosis

The respiratory system is responsible for eliminating the volatile acid carbon dioxide (CO2), which is produced via aerobic metabolism. The body produces approximately 15,000 mmol of CO2 daily, which is the majority of daily acid production; the remainder of the daily acid load (only about 70 mmol of nonvolatile acids) is excreted through the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys. In the setting of hypoventilation, this acid load is not adequately blown off, and respiratory acidosis occurs. Renal compensation occurs after 3–5 days, as the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys attempt to increase the serum bicarbonate levels. Patients are often asymptomatic, or they may present with neuropsychiatric manifestations or mild dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea. Diagnosis is made with arterial blood gas measurement. Management involves treating the underlying etiology, stabilizing the patient, and avoiding respiratory sedatives.

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Overview

Definition

Respiratory acidosis is the process that results in an accumulation of carbon dioxide (CO2) due to abnormal gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange in the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs. In primary respiratory acidosis, the arterial blood gas will show:

  • pH < 7.4 
  • PCO2 (partial pressure of carbon dioxide) > 40 mm Hg (i.e., hypercapnia)

Epidemiology

  • Incidence: varies based on the etiology
  • More common in:
    • End-stage chronic obstructive pulmonary disease Chronic obstructive pulmonary disease Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD) ( COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD))
    • Surgical patients

Acid–Base Review

Acid–base disorders are classified according to the primary disturbance (respiratory or metabolic) and the presence or absence of compensation.

Identifying the primary disturbance

Look at the pH, PCO2 (partial pressure of CO2), and HCO3 (bicarbonate) to determine the primary disturbance. 

  • Normal values:
    • pH: 7.35–7.45
    • PCO2:  35–45 mmHg
    • HCO3: 22–28 mEq/L
  • “-emia” versus “-osis”:
    • “-emia” refers to “in the blood”:
      • Acidemia: more hydrogen ions (H+) in the blood = pH < 7.35
      • Alkalemia: more hydroxide ions (OH) in the blood = pH > 7.45 
    • “-osis” refers to a process:  
      • Acidosis and alkalosis refer the processes that cause acidemia and alkalemia. 
      • pH may be normal in acidosis and alkalosis.
  • Primary (uncompensated) respiratory disorders: 
    • Disorders caused by abnormalities in PCO2
    • Both the pH and PCO2 are abnormal, in opposite directions.
    • Primary respiratory acidosis:  pH < 7.35 and PCO2 > 45 
    • Primary respiratory alkalosis Respiratory alkalosis The respiratory system is responsible for eliminating the volatile acid carbon dioxide (CO2), which is produced via aerobic metabolism. When hypoventilation occurs, excess carbon dioxide is blown off and respiratory alkalosis develops. The kidneys respond by decreasing serum bicarbonate (HCO3-) through increased HCO3- excretion or decreased excretion of H+. Respiratory Alkalosis: pH > 7.45 and PCO2 < 35
  • Primary (uncompensated) metabolic disorders: 
    • Disorders caused by abnormalities in HCO3 
    • Both the pH and PCO2 are abnormal, in the same direction. 
    • Primary uncompensated metabolic acidosis Metabolic acidosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic acidosis occurs when there is an increase in the levels of new non-volatile acids (e.g., lactic acid), renal loss of HCO3-, or ingestion of toxic alcohols. Metabolic Acidosis:  
      • pH < 7.35 and PCO2 < 40 
      • Think: “So the acidosis is not due to ↑ CO2 … it must be due to ↓ serum HCO3 metabolic acidosis Metabolic acidosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic acidosis occurs when there is an increase in the levels of new non-volatile acids (e.g., lactic acid), renal loss of HCO3-, or ingestion of toxic alcohols. Metabolic Acidosis
      • Confirm by looking at HCO3: will be low (< 22 mEq/L)
    • Primary uncompensated metabolic alkalosis Metabolic alkalosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic alkalosis also occurs when there is an increased loss of acid, either renally or through the upper GI tract (e.g., vomiting), increased intake of HCO3-, or a reduced ability to secrete HCO3- when needed. Metabolic Alkalosis
      • pH > 7.45 and PCO2 > 40
      • Think: “So the alkalosis is not due to ↓ CO2… it must be due to ↑ serum HCO3 metabolic alkalosis Metabolic alkalosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic alkalosis also occurs when there is an increased loss of acid, either renally or through the upper GI tract (e.g., vomiting), increased intake of HCO3-, or a reduced ability to secrete HCO3- when needed. Metabolic Alkalosis
      • Confirm by looking at HCO3: will be high (> 28 mEq/L)
  • Simple disorders:
    • The presence of one of the above disorders with appropriate compensation
    • Respiratory disorders are compensated by renal mechanisms.
    • Metabolic disorders are compensated by respiratory mechanisms
  • Mixed disorders: two primary disorders present

Compensation

When acidosis or alkalosis develops, the body will try to compensate. Often, compensation will result in a normal pH.

  • In primary respiratory acid–base disorders, the kidney may try to compensate in an attempt to normalize the pH.
    • Kidneys respond to respiratory acidosis by increasing serum HCO3 through ↑ secretion of H+.
    • Kidneys respond to respiratory alkalosis Respiratory alkalosis The respiratory system is responsible for eliminating the volatile acid carbon dioxide (CO2), which is produced via aerobic metabolism. When hypoventilation occurs, excess carbon dioxide is blown off and respiratory alkalosis develops. The kidneys respond by decreasing serum bicarbonate (HCO3-) through increased HCO3- excretion or decreased excretion of H+. Respiratory Alkalosis by decreasing serum HCO3 through:
      • ↓ Secretion of H+
      • Urinary excretion of HCO3 (normally bicarbonate is 100% absorbed)
  • In primary metabolic acid–base disorders, the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs may try to compensate in an attempt to normalize the pH.
    • Lungs respond to metabolic acidosis Metabolic acidosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic acidosis occurs when there is an increase in the levels of new non-volatile acids (e.g., lactic acid), renal loss of HCO3-, or ingestion of toxic alcohols. Metabolic Acidosis by ↑ ventilation.
    • Lungs respond to metabolic alkalosis Metabolic alkalosis The renal system is responsible for eliminating the daily load of non-volatile acids, which is approximately 70 millimoles per day. Metabolic alkalosis also occurs when there is an increased loss of acid, either renally or through the upper GI tract (e.g., vomiting), increased intake of HCO3-, or a reduced ability to secrete HCO3- when needed. Metabolic Alkalosis by ↓ ventilation.
  • Interpreting the serum HCO3:
    • Normal range: 22–28 mEq/L
    • ↑ HCO3 is due to either:
      • Metabolic alkalosis, or
      • Compensated chronic respiratory acidosis
    • ↓ HCO3 is due to either:
      • Metabolic acidosis, or
      • Compensated chronic respiratory alkalosis Respiratory alkalosis The respiratory system is responsible for eliminating the volatile acid carbon dioxide (CO2), which is produced via aerobic metabolism. When hypoventilation occurs, excess carbon dioxide is blown off and respiratory alkalosis develops. The kidneys respond by decreasing serum bicarbonate (HCO3-) through increased HCO3- excretion or decreased excretion of H+. Respiratory Alkalosis

Pathophysiology and Etiology

Review of relevant pulmonary concepts

  • Tidal volume (TV): volume of air moved into and out of the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs per breath
  • Hypercapnia: elevated levels of CO2 in the blood
  • Dead space: air in the respiratory tree that does not participate in gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange
    • Anatomic dead space:
      • Upper airway down to the terminal bronchioles
      • Fixed volume of air
    • Alveolar dead space:
      • Refers to certain alveoli that are ventilated but not perfused
      • Normally very small total volume, unless there is a pathologic process
    • Physiologic dead space = anatomic dead space + alveolar dead space 
  • Effects of deep versus shallow breaths:
    • Deep breath:  ↑ TV + fixed volume of dead space
      • Dead space is a smaller fraction of total ventilation.
      • Better gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange → no hypercapnia
    • Shallow breath = ↓ TV + fixed volume of dead space
      • Dead space is a higher fraction of total ventilation.
      • Worse gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange → risk for hypercapnia
  • Minute ventilation:
    • Volume of air moved into and out of the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs per minute
    • Minute ventilation = tidal volume × respiratory rate
  • Alveolar ventilation (VA):
    • The fraction of the minute ventilation that participates in gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange 
    • VA is inversely related to PaCO2 
      • As ventilation ↑ → PaCO2 ↓ 
      • As ventilation ↓ → PaCO2 ↑ 
    • VA = (tidal volume – dead space) × respiratory rate
Alveolar ventilation

Relationship between alveolar ventilation and PCO2

Image by Lecturio.

Pathophysiology

Respiratory acidosis occurs when the PCO2 is elevated. 

  • Causes of respiratory acidosis:
    • ↓ Alveolar ventilation (VA):
      • ↓ Respiratory rate
      • ↓ Tidal volume
      • ↑ Dead space
    • Severe ↓ lung diffusion capacity 
    • Severe ventilation–perfusion mismatch
  • These conditions may occur because of abnormalities in the respiratory pathway affecting CO2 elimination:
    • CNS
    • Peripheral nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System
    • Respiratory muscles and the chest wall Chest wall The chest wall consists of skin, fat, muscles, bones, and cartilage. The bony structure of the chest wall is composed of the ribs, sternum, and thoracic vertebrae. The chest wall serves as armor for the vital intrathoracic organs and provides the stability necessary for the movement of the shoulders and arms. Chest Wall
    • Upper airways 
    • Lungs
Table: Respiratory pathway affecting CO2 elimination
How is CO2 elimination affected? Disruption of CO2 elimination is caused by disorders of the:
“Won’t breathe” CNS
“Can’t breathe”
  • Peripheral nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System
  • Respiratory muscles and chest wall Chest wall The chest wall consists of skin, fat, muscles, bones, and cartilage. The bony structure of the chest wall is composed of the ribs, sternum, and thoracic vertebrae. The chest wall serves as armor for the vital intrathoracic organs and provides the stability necessary for the movement of the shoulders and arms. Chest Wall
  • Upper airways
Abnormal gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange: “can’t breathe enough” Lungs

Etiology

Table: Etiologies of respiratory acidosis
Etiology Examples
↓ Respiratory rate: ↓ respiratory drive
  • Medications:
    • Opiates
    • Benzodiazepines Benzodiazepines Benzodiazepines work on the gamma-aminobutyric acid type A (GABAA) receptor to produce inhibitory effects on the CNS. Benzodiazepines do not mimic GABA, the main inhibitory neurotransmitter in humans, but instead potentiate GABA activity. Benzodiazepines
  • Primary brain disorders:
    • Stroke
    • Encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis
    • Brainstem disease
  • Obstructive sleep apnea Obstructive sleep apnea Obstructive sleep apnea (OSA) is a disorder characterized by recurrent obstruction of the upper airway during sleep, causing hypoxia and fragmented sleep. Obstructive sleep apnea is due to a partial or complete collapse of the upper airway and is associated with snoring, restlessness, sleep interruption, and daytime somnolence. Obstructive Sleep Apnea
↓ Tidal volume: impaired ability to fully expand the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs
  • Respiratory muscle weakness:
    • Guillain–Barré syndrome
    • Myasthenia gravis Myasthenia Gravis Myasthenia gravis (MG) is an autoimmune neuromuscular disorder characterized by weakness and fatigability of skeletal muscles caused by dysfunction/destruction of acetylcholine receptors at the neuromuscular junction. MG presents with fatigue, ptosis, diplopia, dysphagia, respiratory difficulties, and progressive weakness in the limbs, leading to difficulty in movement. Myasthenia Gravis
    • ALS ALS Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a sporadic or inherited neurodegenerative disease of upper motor neurons (UMNs) and lower motor neurons (LMNs). ALS is the most common progressive motor neuron disease in North America, primarily affecting men and individuals of Caucasian ethnicity. Amyotrophic Lateral Sclerosis
    • Muscular dystrophy
    • Cervical spine injury above C3
    • Metabolic disorders: hypophosphatemia, hypomagnesemia, hypokalemia Hypokalemia Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5-5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. Hypokalemia
  • Decreased chest wall Chest wall The chest wall consists of skin, fat, muscles, bones, and cartilage. The bony structure of the chest wall is composed of the ribs, sternum, and thoracic vertebrae. The chest wall serves as armor for the vital intrathoracic organs and provides the stability necessary for the movement of the shoulders and arms. Chest Wall compliance:
    • Obesity Obesity Obesity is a condition associated with excess body weight, specifically with the deposition of excessive adipose tissue. Obesity is considered a global epidemic. Major influences come from the western diet and sedentary lifestyles, but the exact mechanisms likely include a mixture of genetic and environmental factors. Obesity
    • Kyphoscoliosis
    • Ankylosing spondylitis Ankylosing spondylitis Ankylosing spondylitis (also known as Bechterew's disease or Marie-Strümpell disease) is a seronegative spondyloarthropathy characterized by chronic and indolent inflammation of the axial skeleton. Severe disease can lead to fusion and rigidity of the spine. Ankylosing Spondylitis
    • Pectus excavatum
↑ Alveolar dead space: impaired ability for gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange
  • Pulmonary fibrosis Pulmonary Fibrosis Idiopathic pulmonary fibrosis is a specific entity of the major idiopathic interstitial pneumonia classification of interstitial lung diseases. As implied by the name, the exact causes are poorly understood. Patients often present in the moderate to advanced stage with progressive dyspnea and nonproductive cough. Pulmonary Fibrosis: Scar tissue prevents gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange.
  • Pulmonary edema Pulmonary edema Pulmonary edema is a condition caused by excess fluid within the lung parenchyma and alveoli as a consequence of a disease process. Based on etiology, pulmonary edema is classified as cardiogenic or noncardiogenic. Patients may present with progressive dyspnea, orthopnea, cough, or respiratory failure. Pulmonary Edema: Fluid in alveoli prevents gas exchange Gas exchange Human cells are primarily reliant on aerobic metabolism. The respiratory system is involved in pulmonary ventilation and external respiration, while the circulatory system is responsible for transport and internal respiration. Pulmonary ventilation (breathing) represents movement of air into and out of the lungs. External respiration, or gas exchange, is represented by the O2 and CO2 exchange between the lungs and the blood. Gas Exchange.
  • Pulmonary embolus: Blood clot prevents perfusion of alveoli.
  • Chronic obstructive pulmonary disease ( COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD)): alveoli destruction
  • Asthma Asthma Asthma is a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. The disease is believed to result from the complex interaction of host and environmental factors that increase disease predisposition, with inflammation causing symptoms and structural changes. Patients typically present with wheezing, cough, and dyspnea. Asthma: airway obstruction Airway obstruction Airway obstruction is a partial or complete blockage of the airways that impedes airflow. An airway obstruction can be classified as upper, central, or lower depending on location. Lower airway obstruction (LAO) is usually a manifestation of chronic disease, such as asthma or chronic obstructive pulmonary disease (COPD). Airway Obstruction preventing effective alveolar ventilation

Renal compensation

The kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys respond to respiratory acidosis by increasing serum HCO3.

  • Mechanism of ↑ HCO3: ↑ renal secretion of H+ 
    • H+ exists in the nephrons as H2CO3 (carbonic acid).
    • For each H+ secreted, one HCO3 is left over (i.e., regenerated).
    • This HCO3 is then reabsorbed into circulation → ↑ serum HCO3  
  • Level of HCO3 increases:
    • Initially by approximately 1 mEq/L per 10 mmHg ↑ in PaCO2
    • By approximately 4 mEq/L per 10 mm Hg ↑ in the partial pressure of arterial CO2 (PaCO2) in patients with chronic respiratory acidosis
  • Process takes 3–5 days to complete:
    • Cells must undergo physical changes to allow increased H+ secretion.
    • Serum HCO3 and pH rise slowly during this time.
    • pH improves.

Acute versus chronic respiratory acidosis

Acute versus chronic respiratory acidosis is defined by the degree of renal compensation.

  • Acute respiratory acidosis is uncompensated:
    • Not enough time for renal compensation to occur
    • More likely to be symptomatic because of hypercapnia
    • High risk for acute respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure within minutes to hours
  • Chronic respiratory acidosis is compensated:
    • Renal compensation is complete.
    • Usually asymptomatic, despite chronic hypercapnia
    • Low risk for acute respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure within the next minutes to hours
    • Significant long-term risk for acute respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure if exposed to additional insults

Clinical Presentation, Diagnosis, and Management

Clinical presentation and diagnosis

Diagnosing a respiratory acidosis typically requires an arterial blood gas.

  • Clinical presentation of hypercapnia:
    • Neurologic:
      • Anxiety/paranoia
      • Headaches
      • Somnolence
      • Delirium Delirium Delirium is a medical condition characterized by acute disturbances in attention and awareness. Symptoms may fluctuate during the course of a day and involve memory deficits and disorientation. Delirium
      • Coma Coma Coma is defined as a deep state of unarousable unresponsiveness, characterized by a score of 3 points on the GCS. A comatose state can be caused by a multitude of conditions, making the precise epidemiology and prognosis of coma difficult to determine. Coma
    • Pulmonary: dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea (usually mild)
  • Diagnosis: primarily with an arterial blood gas (ABG):
    • Acute respiratory acidosis: 
      • pH < 7.35
      • PaCO2 > 45 mmHg
      • Normal HCO3
    • Chronic respiratory acidosis (compensated):
      • pH < 7.4 (low or near-normal)
      • PaCO2 > 45 mmHg
      • HCO3 elevated

Management

  • Assess the ABCs:
    • Ensure that the airway is secure.
    • Administer supplemental O2.
    • Ventilatory support as needed
  • Treat the underlying etiology; examples include:
    • COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD) exacerbation: bronchodilators and corticosteroids
    • Pneumonia in neuromuscular disorders: antibiotics
  • Avoid respiratory sedatives.

Clinical Relevance

  • Myasthenia gravis Myasthenia Gravis Myasthenia gravis (MG) is an autoimmune neuromuscular disorder characterized by weakness and fatigability of skeletal muscles caused by dysfunction/destruction of acetylcholine receptors at the neuromuscular junction. MG presents with fatigue, ptosis, diplopia, dysphagia, respiratory difficulties, and progressive weakness in the limbs, leading to difficulty in movement. Myasthenia Gravis: an autoimmune disorder characterized by abnormalities in neuromuscular conduction that results in fluctuating weakness and can lead to acute hypercapnic respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure 
  • Guillain–Barré syndrome: a postinfectious acute, immune-mediated polyneuropathy Polyneuropathy Polyneuropathy is any disease process affecting the function of or causing damage to multiple nerves of the peripheral nervous system. There are numerous etiologies of polyneuropathy, most of which are systemic and the most common of which is diabetic neuropathy. Polyneuropathy of the peripheral nerve roots characterized by progressive, symmetrical, and ascending paralysis that ultimately affects the patient’s ability to breath 
  • Chronic obstructive pulmonary disease ( COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD)): a spectrum of conditions characterized by irreversible airflow limitation due to chronic inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of small airways: Exacerbations of COPD COPD Chronic obstructive pulmonary disease (COPD) is a lung disease characterized by progressive, largely irreversible airflow obstruction. The condition usually presents in middle-aged or elderly persons with a history of cigarette smoking. Signs and symptoms include prolonged expiration, wheezing, diminished breath sounds, progressive dyspnea, and chronic cough. Chronic Obstructive Pulmonary Disease (COPD) can impair alveolar ventilation, raise PaCO2, and induce an acidotic state.
  • Asthma Asthma Asthma is a chronic inflammatory respiratory condition characterized by bronchial hyperresponsiveness and airflow obstruction. The disease is believed to result from the complex interaction of host and environmental factors that increase disease predisposition, with inflammation causing symptoms and structural changes. Patients typically present with wheezing, cough, and dyspnea. Asthma: a chronic inflammatory condition of the airways, characterized by bronchial hyperreactivity, which presents as wheezing Wheezing Wheezing is an abnormal breath sound characterized by a whistling noise that can be relatively high-pitched and shrill (more common) or coarse. Wheezing is produced by the movement of air through narrowed or compressed small (intrathoracic) airways. Wheezing, cough, and dyspnea Dyspnea Dyspnea is the subjective sensation of breathing discomfort. Dyspnea is a normal manifestation of heavy physical or psychological exertion, but also may be caused by underlying conditions (both pulmonary and extrapulmonary). Dyspnea: Acute exacerbations of asthma cause a sudden impairment of alveolar ventilation and can lead to respiratory acidosis.

References

  1. Emmett, M., Palmer B. F. (2020). Simple and mixed acid-base disorders. UpToDate. Retrieved April 1, 2021, from https://www.uptodate.com/contents/simple-and-mixed-acid-base-disorders 
  2. Feller-Kopman, D. J., Schwartzstein, R. M. (2021). Mechanisms, causes, and effects of hypercapnia. UpToDate. Retrieved April 1, 2021, from https://www.uptodate.com/contents/mechanisms-causes-and-effects-of-hypercapnia
  3. Theodore, A. C. (2020). Arterial blood gases. UpToDate. Retrieved April 1, 2021, from https://www.uptodate.com/contents/arterial-blood-gases
  4. Feller-Kopman, D. J., Schwartzstein, R. M. (2021). The evaluation, diagnosis, and treatment of the adult patient with acute hypercapnic respiratory failure Respiratory failure Respiratory failure is a syndrome that develops when the respiratory system is unable to maintain oxygenation and/or ventilation. Respiratory failure may be acute or chronic and is classified as hypoxemic, hypercapnic, or a combination of the two. Respiratory Failure. UpToDate. Retrieved April 8, 2021, from https://www.uptodate.com/contents/the-evaluation-diagnosis-and-treatment-of-the-adult-patient-with-acute-hypercapnic-respiratory-failure

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