Pediatric Respiratory Failure 

Pediatric 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’s respiratory system loses the ability to provide sufficient oxygen to the blood, and hypoxemia develops, or when the patient is unable to adequately ventilate, and hypercarbia and hypoxemia develop.

Management of acute respiratory failure begins with supporting the patient, followed by determining and treating the underlying etiology. While supporting the respiratory system and ensuring adequate gas exchange in the blood, the clinician should initiate an intervention specifically defined to correct the underlying condition. (See Treatment.)

For patient education information, see the Lung and Airway Center, as well as Acute Respiratory Distress Syndrome.

Hypoxemia, defined as a decreased level of oxygen in the blood, is caused by one of the following abnormalities:

• Mismatch between alveolar ventilation (V) and pulmonary perfusion (Q)
• 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

Hypoxemia is to be distinguished from hypoxia, defined as a decreased level of oxygen in the tissues. These 2 conditions may be closely related and may or may not coexist, but they are not synonymous.

Ventilation-perfusion 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 (V=0). This condition is intrapulmonary shunting and is calculated by comparing the oxygen contents in arterial blood, mixed venous blood, and pulmonary capillary blood (see Workup).

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 (Q=0). 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 Workup).

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 Workup).

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.

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. In contrast, 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. Neonates present a unique susceptibility to respiratory failure, both resulting from and/or complicated by issues related to prematurity and transition from intrauterine to extrauterine life.

Extrathoracic airway differences

The area extending from the nose through the nasopharynx, oropharynx, and larynx to the subglottic region of the trachea constitutes the extrathoracic airway. This area differs in pediatric versus adult patients in 8 respects, as follows:

1. 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.
2. The airway is small; this is one of the primary differences in infants and children younger than 8 years compared with older patients.
3. Infants and young children have a large tongue that fills a small oropharynx.
4. Infants and young children have a cephalic larynx. The larynx is opposite vertebrae C3-4 in children versus C6-7 in adults.
5. 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.
6. 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. Adolescents and adults have a cylindrical airway that is narrowest at the glottic opening.
7. In slightly older children, adenoidal and tonsillar lymphoid tissue is prominent and can contribute to airway obstruction.
8. Uncorrected congenital anatomic abnormalities (eg, cleft palate, Pierre Robin sequence) or acquired abnormalities (eg, subglottic stenosis, laryngomalacia/tracheomalacia) may cause inspiratory obstruction.

Intrathoracic airway differences

The intrathoracic airways and lung include the conducting airways and alveoli, the interstitia, the pleura, the lung lymphatics, and the pulmonary circulation. There are 6 noteworthy differences between children and adults in this area, as follows:

1. Infants and young children have fewer alveoli than do adults. The number dramatically increases during childhood, from approximately 20 million at birth to 300 million by 8 years of age. Therefore, infants and young children have a relatively small area for gas exchange.
2. The alveolus is small. Alveolar size increases from 150-180 to 250-300 µm during childhood.
3. 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.
4. Smaller intrathoracic airways are more easily obstructed than larger ones. With age, the airways enlarge in diameter and length.
5. 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.
6. Residual alveolar damage from chronic lung disease of prematurity or bronchopulmonary dysplasia decreases pulmonary compliance.

Respiratory pump differences

The respiratory pump includes the nervous system with central control (ie, cerebrum, brainstem, spinal cord, peripheral nerves), respiratory muscles, and chest wall. The following 5 features mark the difference between the pediatric and adult population:

1. The respiratory center is immature in infants and young children and leads to irregular respirations and an increased risk of apnea.
2. 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.
3. The small surface area for the interaction between the diaphragm and thorax limits displacing volume in the vertical direction.
4. The musculature is not fully developed. The slow-twitch fatigue-resistant muscle fibers in the infant are underdeveloped.
5. 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.

The most common reasons for respiratory failure in the pediatric population can be divided by anatomic compartments, as follows.

Acquired extrathoracic airway causes include the following:

• Infections (eg, retropharyngeal abscess, Ludwig angina, laryngotracheobronchitis, bacterial tracheitis, peritonsillar abscess)
• Trauma (eg, postextubation croup, thermal burns, foreign-body aspiration)
• Other (eg, hypertrophic tonsils and adenoid)

Congenital extrathoracic airway causes include the following:

• Subglottic stenosis
• Subglottic web or cyst
• Laryngomalacia
• Tracheomalacia
• Vascular ring
• Cystic hygroma
• Craniofacial anomalies

Intrathoracic airway and lung causes include the following:

• 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 causes include the following:

• Diaphragm eventration
• Diaphragmatic hernia
• Flail chest
• Kyphoscoliosis
• Duchenne muscular dystrophy
• Guillain-Barré syndrome
• Infant botulism
• Myasthenia gravis
• Spinal cord trauma
• Spinal muscular atrophy (SMA)

Central control causes include the following:

• CNS infection
• Drug overdose
• Sleep apnea
• Stroke
• Traumatic brain injury

The prognosis depends on the underlying etiology leading to acute respiratory failure. It can be good when the respiratory failure is an acute event that is not associated with prolonged hypoxemia (eg, in the case of a seizure or intoxication). It may be fair to poor when a new process is associated with chronic respiratory failure secondary to a neuromuscular disease or thoracic deformity or in the case of warm hypoxia exceeding 10-20 minutes. This may herald the need for long-term mechanical ventilation.

The prognosis can vary when respiratory failure is associated with a chronic disease with acute exacerbations. 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. Respiratory failure may be the sign of an irreversible progressive disease that leads to death (eg, idiopathic pulmonary hypertension).