Introduction
Background
Respiratory failure is a syndrome in which the respiratory system fails in one or both of its gas exchange functions: oxygenation and carbon dioxide elimination. In practice, respiratory failure is defined as a PaO2 value of less than 60 mm Hg while breathing air or a PaCO2 of more than 50 mm Hg. Furthermore, respiratory failure may be acute or chronic. Although acute respiratory failure is characterized by life-threatening derangements in arterial blood gases and acid-base status, the manifestations of chronic respiratory failure are less dramatic and may not be as readily apparent.
Bilateral airspace infiltrates on chest radiograph film secondary to acute respiratory distress syndrome that resulted in respiratory failure.
Classification of respiratory failure
Respiratory failure may be classified as hypoxemic or hypercapnic and may be either acute or chronic.
Hypoxemic respiratory failure (type I) is characterized by a PaO2 of less than 60 mm Hg with a normal or low PaCO2. This is the most common form of respiratory failure, and it can be associated with virtually all acute diseases of the lung, which generally involve fluid filling or collapse of alveolar units. Some examples of type I respiratory failure are cardiogenic or noncardiogenic pulmonary edema, pneumonia, and pulmonary hemorrhage.
Hypercapnic respiratory failure (type II) is characterized by a PaCO2 of more than 50 mm Hg. Hypoxemia is common in patients with hypercapnic respiratory failure who are breathing room air. The pH depends on the level of bicarbonate, which, in turn, is dependent on the duration of hypercapnia. Common etiologies include drug overdose, neuromuscular disease, chest wall abnormalities, and severe airway disorders (eg, asthma, chronic obstructive pulmonary disease [COPD]).
Distinctions between acute and chronic respiratory failure
Acute hypercapnic respiratory failure develops over minutes to hours; therefore, pH is less than 7.3. Chronic respiratory failure develops over several days or longer, allowing time for renal compensation and an increase in bicarbonate concentration. Therefore, the pH usually is only slightly decreased.
The distinction between acute and chronic hypoxemic respiratory failure cannot readily be made on the basis of arterial blood gases. The clinical markers of chronic hypoxemia, such as polycythemia or cor pulmonale, suggest a long-standing disorder.
Pathophysiology
Respiratory failure can arise from an abnormality in any of the components of the respiratory system, including the airways, alveoli, CNS, peripheral nervous system, respiratory muscles, and chest wall. Patients who have hypoperfusion secondary to cardiogenic, hypovolemic, or septic shock often present with respiratory failure.
Hypoxemic respiratory failure
The pathophysiologic mechanisms that account for the hypoxemia observed in a wide variety of diseases are ventilation-perfusion (V/Q) mismatch and shunt. These 2 mechanisms lead to widening of the alveolar-arterial oxygen difference, which normally is less than 15 mm Hg. With V/Q mismatch, the areas of low ventilation relative to perfusion (low V/Q units) contribute to hypoxemia. An intrapulmonary or intracardiac shunt causes mixed venous (deoxygenated) blood to bypass ventilated alveoli and results in venous admixture. The distinction between V/Q mismatch and shunt can be made by assessing the response to oxygen supplementation or calculating the shunt fraction following inhalation of 100% oxygen. In most patients with hypoxemic respiratory failure, these 2 mechanisms coexist.
Hypercapnic respiratory failure
At a constant rate of carbon dioxide production, PaCO2 is determined by the level of alveolar ventilation (Va), in which VCO2 is ventilation of carbon dioxide and K is a constant value (0.863).
(Va = K x VCO2)/PaCO2A decrease in alveolar ventilation can result from a reduction in overall (minute) ventilation or an increase in the proportion of dead space ventilation. A reduction in minute ventilation is observed primarily in the setting of neuromuscular disorders and CNS depression. In pure hypercapnic respiratory failure, the hypoxemia is easily corrected with oxygen therapy.
Ventilatory capacity versus demand
Ventilatory capacity is the maximal spontaneous ventilation that can be maintained without development of respiratory muscle fatigue. Ventilatory demand is the spontaneous minute ventilation that results in a stable PaCO2. Normally, ventilatory capacity greatly exceeds ventilatory demand. Respiratory failure may result from either a reduction in ventilatory capacity or an increase in ventilatory demand (or both). Ventilatory capacity can be decreased by a disease process involving any of the functional components of the respiratory system and its controller. Ventilatory demand is augmented by an increase in minute ventilation and/or an increase in the work of breathing.
Pathophysiologic Mechanisms in Acute Respiratory Failure
The act of respiration engages 3 processes: (1) transfer of oxygen across the alveolus, (2) transport of oxygen to the tissues, and (3) removal of carbon dioxide from blood into the alveolus and then into the environment. Respiratory failure may occur from malfunctioning of any of these processes. In order to understand the pathophysiologic basis of acute respiratory failure, an understanding of pulmonary gas exchange is essential.
Physiology of gas exchange
Respiration primarily occurs at the alveolar capillary units of the lungs, where exchange of oxygen and carbon dioxide between alveolar gas and blood takes place. Following diffusion into the blood, the oxygen molecules reversibly bind to the hemoglobin. Each molecule of hemoglobin contains 4 sites for combination with molecular oxygen, 1 g of hemoglobin combines with a maximum of 1.36 mL of oxygen. The quantity of oxygen combined with hemoglobin depends on the level of blood PaO2. This relationship, expressed as the oxygen hemoglobin dissociation curve, is not linear but has a sigmoid-shaped curve with a steep slope between a PaO2 of 10 and 50 mm Hg and a flat portion above a PaO2 of 70 mm Hg. The carbon dioxide is transported in 3 main forms: (1) in simple solution, (2) as bicarbonate, and (3) combined with protein of hemoglobin as a carbamino compound.
During ideal gas exchange, blood flow and ventilation would perfectly match each other, resulting in no alveolar-arterial PO2 difference. However, even in normal lungs, not all alveoli are ventilated and perfused perfectly. For a given perfusion, some alveoli are underventilated while others are overventilated. Similarly, for known alveolar ventilation, some units are underperfused while others are overperfused. The optimally ventilated alveoli that are not perfused well are called high V/Q units (acting like dead space), and alveoli that are optimally perfused but not adequately ventilated are called low V/Q units (acting like a shunt).
Alveolar ventilation
At steady state, the rate of carbon dioxide production by the tissues is constant and equals the rate of carbon dioxide elimination by the lung. This relationship is expressed as PaCO2 = VCO2 x 0.862/Va. This relationship signifies whether the alveolar ventilation is adequate for metabolic needs of the body.
The efficiency of lungs at carrying out of respiration can be further evaluated by measuring alveolar-to-arterial PaO2 difference. This difference is calculated by the following equation:
PA O2 = FIO2 x (PB – PH2 O) – PA CO2/RFor the above equation, PA O2 = alveolar PO2, FIO2 = fractional concentration of oxygen in inspired gas, PB = barometric pressure, PH2 O = water vapor pressure at 37°C, PA CO2 = alveolar PCO2, assumed to be equal to arterial PCO2, and R = respiratory exchange ratio. R depends on oxygen consumption and carbon dioxide production. At rest, VCO2/VO2 is approximately 0.8.
Even normal lungs have some degree of V/Q mismatching and a small quantity of right-to-left shunt, alveolar PO2 is slightly higher than arterial PO2. However, an increase in alveolar-to-arterial PO2 above 15-20 mm Hg indicates pulmonary disease as the cause of hypoxemia.
Pathophysiologic causes of acute respiratory failure
Hypoventilation, V/Q mismatch, and shunt are the most common pathophysiologic causes of acute respiratory failure. These are described in the following paragraphs.
Hypoventilation
Hyperventilation is an uncommon cause of respiratory failure and usually occurs from depression of the CNS from drugs or neuromuscular diseases affecting respiratory muscles. Hypoventilation is characterized by hypercapnia and hypoxemia. The relationship between PaCO2 and alveolar ventilation is hyperbolic. As ventilation decreases below 4-6 L/min, PaCO2 rises precipitously. Hypoventilation can be differentiated from other causes of hypoxemia by the presence of a normal alveolar-arterial PO2 gradient.
V/Q mismatch
V/Q mismatch is the most common cause of hypoxemia. V/Q units may vary from low to high ratios in the presence of a disease process. The low V/Q units contribute to hypoxemia and hypercapnia in contrast to high V/Q units, which waste ventilation but do not affect gas exchange unless quite severe. The low V/Q ratio may occur either from a decrease in ventilation secondary to airway or interstitial lung disease or from overperfusion in the presence of normal ventilation. The overperfusion may occur in case of pulmonary embolism, where the blood is diverted to normally ventilated units from regions of lungs that have blood flow obstruction secondary to embolism. Administration of 100% oxygen eliminates all of the low V/Q units, thus leading to correction of hypoxemia. Hypoxemia increases minute ventilation by chemoreceptor stimulation, but the PaCO2 level generally is not affected.
Shunt
Shunt is defined as the persistence of hypoxemia despite 100% oxygen inhalation. The deoxygenated blood (mixed venous blood) bypasses the ventilated alveoli and mixes with oxygenated blood that has flowed through the ventilated alveoli, consequently leading to a reduction in arterial blood content. The shunt is calculated by the following equation:
QS/QT = (CCO2 – CaO2)/CCO2 – CvO2)QS/QT is the shunt fraction, CCO2 (capillary oxygen content) is calculated from ideal alveolar PO2, CaO2 (arterial oxygen content) is derived from PaO2 using the oxygen dissociation curve, and CVO2 (mixed venous oxygen content) can be assumed or measured by drawing mixed venous blood from pulmonary arterial catheter.
Anatomical shunt exists in normal lungs because of the bronchial and thebesian circulations, accounting for 2-3% of shunt. A normal right-to-left shunt may occur from atrial septal defect, ventricular septal defect, patent ductus arteriosus, or arteriovenous malformation in the lung. Shunt as a cause of hypoxemia is observed primarily in pneumonia, atelectasis, and severe pulmonary edema of either cardiac or noncardiac origin. Hypercapnia generally does not develop unless the shunt is excessive (>60%). When compared with V/Q mismatch, hypoxemia produced by shunt is difficult to correct by oxygen administration.
Frequency
United States
Respiratory failure is a syndrome rather than a single disease process, and the overall frequency of respiratory failure is not well known. The estimates for individual diseases mentioned here can be found in the appropriate eMedicine article.
Mortality/Morbidity
The mortality rate associated with respiratory failure varies according to the etiology. For acute respiratory distress syndrome, the mortality rate is approximately 45% in most studies; this percentage has not changed over the years.1,2 Acute exacerbation of COPD carries a mortality rate of approximately 30%. The mortality rates for other causative disease processes have not been well described.Race
The relation between acute respiratory failure and race is still debated. A recent work by Khan et al suggested that no differences in mortality exist in patients of Asian and Native Indian descent with acute critical illness after adjusting for differences in case mix.3 Moss and Mannino, in a 2002 manuscript, showed worse outcome for African-Americans suffering from acute respiratory distress syndrome (ARDS) as compared with whites when adjusted for case mix.4
As more prospective association studies are performed, we will have a better knowledge of the impact of race on the outcome of respiratory failure.
Clinical
History
The diagnosis of acute or chronic respiratory failure begins with clinical suspicion of its presence. Confirmation of the diagnosis is based on arterial blood gas analysis. Evaluation of an underlying cause must be initiated early, frequently in the presence of concurrent treatment for acute respiratory failure.
- The cause of respiratory failure is often evident after a careful history and physical examination.
- Cardiogenic pulmonary edema usually develops in the context of a history of left ventricular dysfunction or valvular heart disease.
- A history of previous cardiac disease, recent symptoms of chest pain, paroxysmal nocturnal dyspnea, and orthopnea suggest cardiogenic pulmonary edema.
- Noncardiogenic edema (eg, acute respiratory distress syndrome [ARDS]) occurs in typical clinical contexts such as sepsis, trauma, aspiration, pneumonia, pancreatitis, drug toxicity, and multiple transfusions.
Physical
The signs and symptoms of acute respiratory failure reflect the underlying disease process and the associated hypoxemia or hypercapnia. Localized pulmonary findings reflecting the acute cause of hypoxemia, such as pneumonia, pulmonary edema, asthma, or COPD, may be readily apparent. In patients with acute respiratory distress syndrome (ARDS), the manifestations may be remote from the thorax, such as abdominal pain or long-bone fracture. Neurological manifestations include restlessness, anxiety, confusion, seizures, or coma.
- Asterixis may be observed with severe hypercapnia. Tachycardia and a variety of arrhythmias may result from hypoxemia and acidosis.
- Once respiratory failure is suspected on clinical grounds, arterial blood gas analysis should be performed to confirm the diagnosis and to assist in the distinction between acute and chronic forms. This helps assess the severity of respiratory failure and also helps guide management.
- Cyanosis, a bluish color of skin and mucous membranes, indicates hypoxemia. Visible cyanosis typically is present when the concentration of deoxygenated hemoglobin in the capillaries or tissues is at least 5 g/dL.
- Dyspnea, an uncomfortable sensation of breathing, often accompanies respiratory failure. Excessive respiratory effort, vagal receptors, and chemical stimuli (hypoxemia and/or hypercapnia) all may contribute to the sensation of dyspnea.
- Both confusion and somnolence may occur in respiratory failure. Myoclonus and seizures may occur with severe hypoxemia. Polycythemia is a complication of long-standing hypoxemia.
- Pulmonary hypertension frequently is present in chronic respiratory failure. Alveolar hypoxemia potentiated by hypercapnia causes pulmonary arteriolar constriction. If chronic, this is accompanied by hypertrophy and hyperplasia of the affected smooth muscles and narrowing of the pulmonary arterial bed. The increased pulmonary vascular resistance increases afterload of the right ventricle, which may induce right ventricular failure. This, in turn, causes enlargement of the liver and peripheral edema. The entire sequence is known as cor pulmonale.
- Criteria for the diagnosis of acute respiratory distress syndrome
- Clinical presentation - Tachypnea and dyspnea; crackles upon auscultation
- Clinical setting - Direct insult (aspiration) or systemic process causing lung injury (sepsis)
- Radiologic appearance - Three-quadrant or 4-quadrant alveolar flooding
- Lung mechanics - Diminished compliance (<40 mL/cm water)
- Gas exchange - Severe hypoxia refractory to oxygen therapy (PaO2/FIO2 <200)
- Normal pulmonary vascular properties - Pulmonary capillary wedge pressure of less than 18 mm Hg
Causes
These diseases can be grouped according to the primary abnormality and the individual components of the respiratory system, as follows:
- Central nervous system disorders
- A variety of pharmacological, structural, and metabolic disorders of the CNS are characterized by depression of the neural drive to breathe.
- This may lead to acute or chronic hypoventilation and hypercapnia.
- Examples include tumors or vascular abnormalities involving the brain stem, an overdose of a narcotic or sedative, and metabolic disorders such as myxedema or chronic metabolic alkalosis.
- Disorders of the peripheral nervous system, respiratory muscles, and chest wall
- These disorders lead to an inability to maintain a level of minute ventilation appropriate for the rate of carbon dioxide production.
- Concomitant hypoxemia and hypercapnia occur.
- Examples include Guillain-Barré syndrome, muscular dystrophy, myasthenia gravis, severe kyphoscoliosis, and morbid obesity.
- Abnormalities of the airways
- Severe airway obstruction is a common cause of acute and chronic hypercapnia.
- Examples of upper airway disorders are acute epiglottitis and tumors involving the trachea; lower airway disorders include COPD, asthma, and cystic fibrosis.
- Abnormalities of the alveoli
- The diseases are characterized by diffuse alveolar filling, frequently resulting in hypoxemic respiratory failure, although hypercapnia may complicate the clinical picture.
- Common examples are cardiogenic and noncardiogenic pulmonary edema, aspiration pneumonia, or extensive pulmonary hemorrhage. These disorders are associated with intrapulmonary shunt and an increased work of breathing.
- Common causes of type I (hypoxemic) respiratory failure
- Chronic bronchitis and emphysema (COPD)
- Pneumonia
- Pulmonary edema
- Pulmonary fibrosis
- Asthma
- Pneumothorax
- Pulmonary embolism
- Pulmonary arterial hypertension
- Pneumoconiosis
- Granulomatous lung diseases
- Cyanotic congenital heart disease
- Bronchiectasis
- Adult respiratory distress syndrome
- Fat embolism syndrome
- Kyphoscoliosis
- Obesity
- Common causes of type II (hypercapnic) respiratory failure
- Chronic bronchitis and emphysema (COPD)
- Severe asthma
- Drug overdose
- Poisonings
- Myasthenia gravis
- Polyneuropathy
- Poliomyelitis
- Primary muscle disorders
- Porphyria
- Cervical cordotomy
- Head and cervical cord injury
- Primary alveolar hypoventilation
- Obesity hypoventilation syndrome
- Pulmonary edema
- Adult respiratory distress syndrome
- Myxedema
- Tetanus
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Further Reading
Keywords
respiratory failure, hypoxemic respiratory failure, chronic respiratory failure, hypercapnic respiratory failure, type I respiratory failure, type II respiratory failure, chronic obstructive pulmonary disease, COPD, chronic obstructive pulmonary disease, respiratory system failure, respiratory system, system failure, PaO2 value, oxygenation, carbon dioxide elimination, hypoxemic, hypercapnic
