eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Critical Care

Respiratory Failure

Author: Margaret A Priestley, MD, Assistant Professor of Anesthesia and Pediatrics, Department of Anesthesiology and Critical Care Medicine, University of Pennsylvania; Associate Director of Trauma, The Children's Hospital of Philadelphia
Coauthor(s): Jimmy Huh, MD, Assistant Professor of Anesthesia and Pediatrics, Department of Anesthesia and Critical Care Medicine, The Children's Hospital of Philadelphia
Contributor Information and Disclosures

Updated: Feb 12, 2008

Introduction

Background

The respiratory system supplies the body with adequate oxygen for aerobic metabolism and simultaneously removes carbon dioxide, the major metabolic waste. Respiratory failure develops when the rate of gas exchange between the atmosphere and blood is unable to match the body's metabolic demands. It is diagnosed when the patient loses the ability to provide sufficient oxygen to the blood and develops hypoxemia or when the patient is unable to adequately ventilate and develops hypercarbia and hypoxemia.

Pathophysiology

Hypoxemia is caused by one of the following abnormalities:

• Alveolar ventilation (V) and pulmonary perfusion (Q) mismatch
• Intrapulmonary shunt
• Hypoventilation
• Abnormal diffusion of gases at the alveolar-capillary interface
• Reduction in inspired oxygen concentration
• Increased venous desaturation with cardiac dysfunction plus one or more of the above 5 factors

V/Q mismatch, intrapulmonary shunt, and hypoventilation

The 3 most important abnormalities in gas exchange that lead to respiratory failure are V/Q mismatch, intrapulmonary shunt, and hypoventilation.

The V/Q ratio determines the adequacy of gas exchange in the lung. When alveolar ventilation matches pulmonary blood flow, CO₂ is eliminated and the blood becomes fully saturated with oxygen. In the normal lung, gravitational forces affect the V/Q ratio. When a person stands, the V/Q is greater than 1 at the apex of the lung (ventilation exceeds perfusion) and less than 1 at the base (less ventilation with more perfusion). In the overall healthy lung, the V/Q ratio is assumed to be ideal and equals 1.

A mismatch between ventilation and perfusion is the most common cause of hypoxemia. When the V/Q ratio is less than 1 throughout the lung, arterial hypoxemia results. As V/Q mismatch worsens, the minute ventilation increases producing either a low or normal arterial partial pressure of CO₂ (PaCO₂). The hypoxemia caused by low V/Q areas is responsive to supplemental oxygen administration. The more severe the V/Q imbalance, the higher the concentration of inspired oxygen is needed to raise the arterial partial pressure of oxygen (PaO₂).

In the extreme case when the V/Q ratio equals 0, pulmonary blood flow does not participate in gas exchange because the perfused lung unit receives no ventilation. This condition is intrapulmonary shunting and is calculated by comparing the oxygen contents in arterial blood, mixed venous blood, and pulmonary capillary blood (see Other Tests). In healthy people, the percentage of intrapulmonary shunt is less than 10%. When the intrapulmonary shunt is greater than 30%, resultant hypoxemia does not improve with supplemental oxygenation because the shunted blood does not come in contact with the high oxygen content in the alveoli. Instead, treatment consists of recruiting and maximizing lung volume with positive pressure. PaO₂ continues to fall proportionately as the shunt increases.

In contrast, PaCO₂ remains constant because of a compensatory increase in minute ventilation until the shunt fraction exceeds 50%. The protective reflex that reduces the degree of intrapulmonary shunting is hypoxic pulmonary vasoconstriction (HPV); alveolar hypoxia leads to vasoconstriction of the perfusing vessel. This partially corrects the regional V/Q mismatch by improving PaO₂ at the expense of increasing pulmonary vascular resistance.

When ventilation is in excess of capillary blood flow, the V/Q ratio is greater than 1. At the extreme, areas of ventilated lung receive no perfusion, and the V/Q ratio approaches infinity. This extreme condition is referred to as alveolar dead-space ventilation. In addition to alveolar dead space, anatomic dead space represents the volume of air in conducting airways that cannot participate in gas exchange.

Combined, the alveolar and anatomic dead-space volumes are referred to as physiologic dead space, which normally accounts for 30% of total ventilation. Increased dead-space ventilation results in hypoxemia and hypercapnia. This increase can be caused by decreased pulmonary perfusion due to hypotension, pulmonary embolus, or alveolar overdistention during mechanical ventilation. The ratio of dead-space to tidal-gas volume can be calculated on the basis of the difference between CO₂ in arterial blood and in exhaled gas (see Other Tests).

Under steady-state conditions, PaCO₂ is directly proportional to CO₂ production (VCO₂) and inversely proportional to alveolar ventilation (V_A), as follows: PaCO₂ = VCO₂ X (k/V_A), where k is a constant = 0.863.

Therefore, when V_A decreases or VCO₂ increases, PaCO₂ increases. With alveolar hypoventilation, hypoxemia is predicted by using the alveolar gas equation, but the alveolar-arterial gradient remains normal (see Other Tests).

Another way to approach respiratory failure is based on 2 patterns of blood-gas abnormalities. Type I respiratory failure results from poor matching of pulmonary ventilation to perfusion; this leads to noncardiac mixing of venous blood with arterial blood. As a result, type I respiratory failure is characterized by arterial hypoxemia with normal or low arterial CO₂. As an alternative, type II respiratory failure results from inadequate alveolar ventilation in relation to physiologic needs and is characterized by arterial hypercarbia and hypoxemia. Type II respiratory failure occurs when a disease or injury imposes a load on a child's respiratory system that is greater than the power available to do the respiratory work. In this scenario, the hypoxemia is proportional to the hypercarbia.

A wide array of diseases can cause respiratory failure. Therefore, the physician must identify the affected area in the respiratory system that contributes to the respiratory failure. Identification can be achieved by dividing the respiratory system into 3 anatomic parts: (1) the extrathoracic airway, (2) the lungs responsible for gas exchange, and (3) the respiratory pump that ventilates the lung and that includes the nervous system, thorax, and respiratory muscles. In general, diseases that affect the anatomic components of the lung result in regions of low or absent V/Q ratios, initially leading to type I (or hypoxemic) respiratory failure. As an alternative, diseases of the extrathoracic airway and respiratory pump result in a respiratory power-load imbalance and type II respiratory failure. Hypercarbia due to alveolar hypoventilation is the hallmark of diseases involving the respiratory pump.

Pediatric considerations

The frequency of acute respiratory failure is higher in infants and young children than in adults for several reasons. This difference can be explained by defining anatomic compartments and their developmental differences in pediatric patients that influence susceptibility to acute respiratory failure.

The extrathoracic airway comprises the area extending from the nose through the nasopharynx, oropharynx, and larynx to the subglottic region of the trachea. Differences in pediatric versus adult patients include the following:

• Neonates and infants are obligate nasal breathers until the age of 2-6 months because of the proximity of the epiglottis to the nasopharynx. Nasal congestion can lead to clinically significant distress in this age group.
• The small size of the airway is one of the primary differences in infants and children younger than 8 years compared with older patients.
• Infants and young children have a large tongue that fills a small oropharynx.
• Infants and young children have a cephalic larynx. The larynx is opposite vertebrae C3-4 in children versus C6-7 in adults.
• The epiglottis is larger and more horizontal to the pharyngeal wall in children than in adults. The cephalic larynx and large epiglottis can make laryngoscopy challenging.
• Infants and young children have a narrow subglottic area. In children, the subglottic area is cone shaped, with the narrowest area at the cricoid ring. A small amount of subglottic edema can lead to clinically significant narrowing, increased airway resistance, and increased work of breathing. Older patients and adults have a cylindrical airway that is narrowest at the glottic opening.
• In slightly older children, adenoidal and tonsillar lymphoid tissue is prominent and can contribute to airway obstruction.

The intrathoracic airways and lung include the conducting airways and alveoli, the interstitia, the pleura, the lung lymphatics, and the pulmonary circulation. Noteworthy differences among pediatric children include the following:

• Infants and young children have fewer alveoli than do adults. The number dramatically increases during childhood, from approximately 20 million after birth to 300 million by 8 years of age. Therefore, infants and young children have a relatively small area for gas exchange.
• The alveolus is small. Alveolar size increases from 150-180 to 250-300 µm during childhood.
• Collateral ventilation is not fully developed; therefore, atelectasis is more common in children than in adults. During childhood, anatomic channels form to provide collateral ventilation to alveoli. These pathways are between adjacent alveoli (pores of Kohn), bronchiole and alveoli (Lambert channel), and adjacent bronchioles. This important feature allows alveoli to participate in gas exchange even in the presence of an obstructed distal airway.
• Smaller intrathoracic airways are more easily obstructed than larger ones. With age, the airways enlarge in diameter and length.
• Infants and young children have relatively little cartilaginous support of the airways. As cartilaginous support increases, dynamic compression during high expiratory flow rates is prevented.

The respiratory pump includes the nervous system with central control (ie, cerebrum, brainstem, spinal cord, peripheral nerves), respiratory muscles, and chest wall. Features of note in pediatric patients include the following:

• The respiratory center is immature in infants and young children and leads to irregular respirations and an increased risk of apnea.
• The ribs are horizontally oriented. During inspiration, a decreased volume is displaced, and the capacity to increase tidal volume is limited compared with that in older individuals.
• The small surface area for the interaction between the diaphragm and thorax limits displacing volume in the vertical direction.
• The musculature is not fully developed. The slow-twitch fatigue-resistant muscle fibers in the infant are underdeveloped.
• The soft compliant chest wall provides little opposition to the deflating tendency of the lungs. This leads to a lower functional residual capacity in pediatric patients than in adults, a volume that approaches the pediatric alveolus critical closing volume.

Mortality/Morbidity

Acute respiratory failure remains an important cause of morbidity and mortality in children. Cardiac arrests in children frequently result from respiratory failure. In 2000, data from the National Center for Health Statistics listed respiratory illnesses as one of the top 10 causes of pediatric mortality.

Clinical

History

• Does the patient have factors that increase the risk for respiratory failure? Factors may include young age; history of prematurity; immunodeficiency; and chronic pulmonary, cardiac, or neuromuscular disease (eg, cystic fibrosis, bronchopulmonary dysplasia, unrepaired congenital heart disease, or spinal muscular atrophy [SMA]).
• Does the patient have a cough, rhinorrhea, or other symptoms of an upper respiratory tract infection to define an etiology?
• Does the patient have a fever or signs of sepsis? Several infections can lead to respiratory failure because of a systemic inflammatory response, pulmonary edema, or acute respiratory distress syndrome (ARDS) or because it can produce a power-load imbalance secondary to increased oxygen consumption.
• How long have the symptoms been present? The natural course of a respiratory illness must be considered. Respiratory syncytial virus (RSV) infections frequently worsen over the initial 3-5 days before improvement occurs.
• Does the patient have any pain? Pain can suggest pleuritis or foreign-body aspiration.
• Does the patient have a possible or known exposure to sedatives (eg, benzodiazepines, tricyclic antidepressants, narcotics) or has he or she recently undergone a procedure that used general anesthesia? This could suggest hypoventilation.
• Does the patient have symptoms of neuromuscular weakness or paralysis? What is the distribution of weakness and duration of symptoms to narrow the differential diagnosis?
  • Bulbar dysfunction suggests myasthenia gravis.
  • Distal weakness that progresses upward suggests Guillain-Barré syndrome.
  • Apnea associated with a traumatic injury suggests a cervical spinal cord injury.
• Does the patient have a history suggestive of a stroke or seizure?
• Does the patient have a history of headaches? With chronic hypercapnia, headaches typically occur at nighttime or upon the patient's awakening in the morning.

Physical

During physical examination, clinicians should avoid interfering with the patient's mechanisms for compensation. Children often find the most advantageous position for breathing. Children with respiratory distress commonly sit up and lean forward to improve leverage for the accessory muscles and to allow for easy diaphragmatic movement. Children with epiglottitis sit upright with their necks extended and heads forward while drooling and breathing through their mouths. Making a child lie down or making him or her cry during an otoscopic examination can precipitate acute respiratory failure.

• General appearance
  • Does the patient appear well or sick?
  • Is the patient cyanotic?
• Respiratory rate, quality, and effort
  • Bradypnea is most often observed in central control abnormalities. Slow and large tidal volume breaths also minimize turbulence and resistance in significant extrathoracic airway obstruction (eg, epiglottitis).
  • The fast and shallow breathing of tachypnea is most efficient in intrathoracic airway obstruction. It decreases dynamic compliance of the lung.
  • Auscultation provides information about the symmetry and quality of air movement. Evaluate the patient for stridor, wheezing, crackles, and decreased breath sounds (eg, alveolar consolidation, pleural effusion).
  • Grunting is an expiratory sound made by infants as they exhale against a closed glottis. It is an attempt to increase functional residual capacity and lung volume. This attempt is made in order to raise functional residual capacity above the critical closing volume and to avoid alveolar collapse. This is a concerning physical finding.
  • Assess for accessory muscle use and nasal flaring. Suprasternal and intercostal retractions are present when highly negative pleural pressures are required to overcome airway obstruction or stiff lungs.
• Chest wall findings: Evaluate for paradoxical movement of the chest wall. In the presence of abnormalities of the pulmonary pump, paradoxical chest-wall movement occurs because of instability of the chest wall associated with absent intercostal muscle use. As the diaphragm contracts and pushes abdominal contents out, the chest wall retracts inward instead of expanding normally.
• Cardiovascular findings
  • Tachycardia and hypertension may occur secondary to increased circulatory catecholamine levels.
  • A gallop is suggestive of myocardial dysfunction leading to respiratory failure.
  • Peripheral vasoconstriction may develop secondary to respiratory acidosis.
• Neurologic findings
  • Patients may be lethargic, irritable, anxious, or unable to concentrate.
  • The inability to breathe comfortably creates anxiety, and superimposed hypoxemia and hypercapnia accentuates any restlessness and agitation. Increased agitation may indicate a general worsening of the patient's condition.

Causes

The most common reasons for respiratory failure in the pediatric population are divided into anatomic compartments, as follows:

• Extrathoracic airway
  • Acquired lesions
    • Infections (eg, retropharyngeal abscess, Ludwig angina, laryngotracheobronchitis, bacterial tracheitis, peritonsillar abscess)
    • Traumatic causes (eg, postextubation croup, thermal burns, foreign-body aspiration)
    • Other (eg, hypertrophic tonsils and adenoid)
  • Congenital lesions
    • Subglottic stenosis
    • Subglottic web or cyst
    • Laryngomalacia
    • Tracheomalacia
    • Vascular ring
    • Cystic hygroma
    • Craniofacial anomalies
• Intrathoracic airway and lung
  • Acute respiratory distress syndrome (ARDS)
  • Asthma
  • Aspiration
  • Bronchiolitis
  • Bronchomalacia
  • Left-sided valvular abnormalities
  • Pulmonary contusion
  • Near drowning
  • Pneumonia
  • Pulmonary edema
  • Pulmonary embolus
  • Sepsis
• Respiratory pump
  • Chest wall
    • Diaphragm eventration
    • Diaphragmatic hernia
    • Flail chest
    • Kyphoscoliosis
  • Respiratory muscles
    • Duchenne muscular dystrophy
    • Guillain-Barré syndrome
    • Infant botulism
    • Myasthenia gravis
    • Spinal cord trauma
    • SMA
• Central control
  • CNS infection
  • Drug overdose
  • Sleep apnea
  • Stroke
  • Traumatic brain injury

• Search for CME/CE on This Topic »

• Respiratory Failure (Critical Care)
• Intraosseous Cannulation (Pediatrics: Cardiac Disease and Critical Care Medicine)
• Alveolar Proteinosis (Pediatrics: General Medicine)
• Acute Respiratory Distress Syndrome (Critical Care)

• Lung and Airway Center
• Acute Respiratory Distress Syndrome Causes
• Acute Respiratory Distress Syndrome Treatment
• Acute Respiratory Distress Syndrome Overview
• Acute Respiratory Distress Syndrome Symptoms

• Extracorporeal Membrane Oxygenation May Improve Outcomes in Severe Acute Respiratory Failure
• ECMO Can Improve Survival in Some Older Patients With Acute Respiratory Failure
• Extracorporeal Membrane Oxygenation Improves Survival in H1N1 Patients With Respiratory Failure

• Survival for Patients With HIV Admitted to the ICU Continues to Improve in the Current Era of Combination Antiretroviral Therapy
• Role of the Pulmonary Epithelium and Inflammatory Signals in Acute Lung Injury
• Positive Pressure Ventilation: What is the Real Cost?