Children younger than 5 have a much higher risk of developing pediatric respiratory failure due to their anatomy and physiology. They have a higher risk of hypoxia, an immature cardiovascular system, and inefficient mechanics of breathing.
Pediatric respiratory infections have hit particularly hard this year. Early influenza (flu) and Respiratory Syncycial Virus (RSV) infections, combined with the ongoing SARS CoV 2 (COVID-19) pandemic, are driving hospitalizations. As of December 2022, many US pediatric hospitals were at or near capacity. With triage tents in the parking lots, fear is growing that they could be overwhelmed.
The high hospitalization rate has multiple causes. Dramatic changes in community behavior experienced during the pandemic has promoted a lack of immunity. There is co-infection with multiple viruses. However, children younger than 5 years have an anatomy and physiology that puts them at particular risk for respiratory failure.
Babies and Small Children Get Hypoxic Very Easily
Pediatric respiratory failure can develop quickly in infants and young children because they have little respiratory reserve when ill. The differences are most marked in the infant.
An Infant Can’t Hold His Breath as Long as an Adult
Metabolic Rate Is Higher
Infants needs more oxygen than an adult, with a metabolic rate is roughly double the adult rate. Their CO2 production is also higher.
Lung Structure Immature
An infant only has about 10% of the 200 million to 500 million alveoli found in the adult—giving them about 26 times less area for gas exchange. Surface area varies from 2.8 m2 in the infant (about the size of a baby blanket) to 75 m2 in the adult (about half a tennis court). In addition, functional residual capacity (FRC), the amount of air left in the lung after a normal exhalation, effectively acts as the lung’s oxygen tank. The larger the FRC, the bigger the oxygen tank and the more respiratory reserve. FRC in an adult is about 70 mL/kg and is about 18 mL/kg in the infant. With a 75% smaller oxygen tank, the infant can’t hold his breath as long as an adult.
Dead space, the area of the lung not participating in gas exchange, is also higher in the infant (3 mL compared with 2 mL/kg in the adult). A larger dead space means less of the tidal volume ventilates alveoli and more of each breath is wasted.
Regardless of age, a normal resting tidal volume equals about 8 mL/kg of lean body weight. A 2.7-kg infant’s tidal volume is only about 22 mL. It doesn’t take much reduction in tidal volume to produce severe hypoventilation. See the illustration below.
Less Respiratory Reserve Can Lead to Pediatric Respiratory Failure
Therefore, the infant or toddler must have a higher respiratory rate and heart rate to compensate for smaller tidal volumes, larger dead space, higher oxygen consumption, and higher carbon dioxide production. Slow respiratory or heart rate can rapidly lead to cardiorespiratory failure. The smaller tidal volumes of an infant and a toddler also increase the risk for barotrauma when they are exposed to large tidal volumes or high inflation pressures.
Infant’s Chest Wall Shape Increases Work of Breathing
Pediatric respiratory failure can also develop due to exhaustion from work of breathing. The pediatric chest wall is mechanically less efficient and limits potential lung expansion. Babies “belly breathe.” To take a deep breath, the infant’s chest can only expand a little because of the more horizontal angles of the rib cage. The abdomen therefore expands a lot as the diaphragm descends, pushing abdominal contents down and out of the way. Anything that interferes with descent of the diaphragm, such as a stomach or intestines distended with air or liquid, can seriously impair an infant’s breathing.
The infant’s chest wall is also more compliant than an adult’s, with an elastic recoil close to zero because of the lack of rib cage ossification. When the infant takes a breath against resistance, such as with airway obstruction or poor pulmonary compliance from pneumonia, the chest wall actually moves inward as the belly moves outward. The inward movement of the chest wall decreases the amount of air that enters. A rocking chest wall motion is very common in children who have even partial airway obstruction.
Because chest wall structure and belly breathing limit the ability to increase tidal volume, the infant must rely on increases in respiratory rate to compensate for respiratory distress. The harder a child tries to breathe, the less efficient and more labored breathing becomes.
To read more about the inefficient mechanics of pediatric breathing see:
Pediatric Airway Risks: Inefficient Mechanics of Breathing
Hypoxia in a Child Causes Bradycardia!
Consider bradycardia in a child to be caused by hypoxia until proven otherwise! Hypoxia in an adult or older child usually triggers a sympathetic response, producing a tachycardia that improves oxygen delivery. Bradycardia is a late and very dangerous sign of imminent cardiovascular collapse.
In contrast, bradycardia in babies and young children may be the first sign of hypoxia—not the last. The small child’s immature nervous system has a better developed parasympathetic nervous system. Babies spend a lot of time eating and sleeping, and not so much time engaged in fight-or-flight activities. As a result, hypoxia triggers a vagal response and slows the heart.
Infants and small children have a cardiac output that is rate dependent. Bradycardia significantly lowers cardiac output and oxygen delivery; hypoxia and hypercarbia worsen. Acidosis develops, further depressing the myocardium. Cardiac arrest can occur very quickly in the hypoxic child and must be treated urgently with oxygen, ventilation, and if needed, atropine.
Pediatric Hypovolemia Can Worsen Hypoxia
Sick children commonly develop hypovolemia. Pediatric cardiac output is highly dependent on blood volume and heart rate for a variety of reasons. (Pediatric Hypotension: Think Hypovolemia). Hypovolemia and bradycardia can both precipitate respiratory failure by lowering cardiac output and decreasing oxygen delivery. When hypoxemia or hypoxia develop, hypotension increases which can worsen hypoxia in a vicious cycle. Hypovolemia can result in cardiac arrest more easily in a child.
Precautions and Prevention
We should all encourage vaccination against the flu and administration of the COVID-19 boosters for our pediatric population, as well as their adult care givers. During a period of time when mask wearing and social distancing is falling out of favor, we need to continue to protect our little ones. Hand washing is still one of the best protections we have for preventing RSV and other respiratory infections.
Pediatric respiratory failure can develop quickly in young children. Be vigilant for signs of decompensation and act quickly. Rapid respiratory rate, shortness of breath, and labored breathing in a child need to be taken very seriously.
See my review article in Airway Management on common problems in pediatric airway management caused by their anatomy and physiology, and how to fix them:
10 Common Pediatric Airway Problems— And Their Solutions
