When airway resistance increases in a pediatric patient, the child must work dramatically harder to move air in and out of the lungs. In PALS (Pediatric Advanced Life Support), this is one of the most critical changes to recognize early because it sets off a chain of events that can progress from respiratory distress to respiratory failure to cardiac arrest. Children are especially vulnerable because their airways are smaller, meaning even minor swelling creates exponentially greater resistance than it would in an adult.
Why Small Airways Create Big Problems
Airway resistance follows an inverse relationship with the radius of the airway, raised to the fourth power. That means if the airway radius is cut in half, resistance doesn’t just double. It increases 16-fold. When a child is crying, coughing, or struggling to breathe, airflow becomes turbulent rather than smooth, and the relationship worsens to the fifth power.
This physics matters enormously in pediatrics. An infant’s trachea is roughly 4 mm in diameter. Just 1 mm of circumferential swelling from croup, an allergic reaction, or infection reduces that radius by half and increases resistance 16-fold under calm breathing. If the child becomes agitated and airflow turns turbulent, that same 1 mm of swelling increases resistance 32-fold. An adult with the same amount of swelling would barely notice the difference. This is the core reason PALS emphasizes keeping distressed children as calm as possible.
What You’ll See: Signs of Increased Resistance
The body compensates for increased airway resistance by recruiting extra muscles and breathing faster. The AHA identifies several key signs of respiratory distress tied to this compensation: nasal flaring, intercostal and subcostal retractions, seesaw breathing (where the chest and abdomen move in opposite directions), and grunting. Tachypnea, a faster-than-normal breathing rate, is typically the earliest sign.
The sounds you hear help localize where resistance is highest. Stridor is a loud, musical sound of constant pitch caused by obstruction at or above the trachea and larynx. It’s most prominent during inspiration because negative pressure generated during inhalation tends to collapse the extrathoracic (above the chest) airway inward. Wheezing, by contrast, is a high-pitched continuous sound associated with narrowing in the lower, intrathoracic airways. It’s typically loudest during expiration because the positive pressure generated inside the chest during exhalation compresses those smaller airways further. A prolonged expiratory phase is a classic finding when lower airway resistance is elevated, as seen in asthma or bronchiolitis.
The location of the obstruction also determines how breathing phases are affected. Extrathoracic obstructions (like croup or a foreign body in the throat) worsen on inspiration. Intrathoracic obstructions (like asthma or bronchiolitis) worsen on expiration. In normal subjects breathing through the mouth, the upper airway accounts for roughly 45% of total airway resistance, with the larynx contributing the largest share.
Effects on Gas Exchange
As resistance rises, less fresh air reaches the alveoli with each breath. The immediate consequence is a mismatch between how much air the lungs need and how much they actually receive. Oxygen levels in the blood begin to drop, and carbon dioxide starts to accumulate because the child can’t exhale effectively enough to clear it.
This CO2 retention, called hypercapnia, can itself worsen the problem. Elevated carbon dioxide levels in the lungs can increase airway constriction and further limit ventilation to already-compromised areas of the lung. This creates a feedback loop: higher resistance leads to CO2 buildup, which increases constriction, which raises resistance further. Left uncorrected, this cycle can culminate in respiratory failure.
In bronchiolitis, a common cause of increased airway resistance in infants, researchers have measured resistance values exceeding 140 cmH2O/L/s in half of mechanically ventilated infants, compared to normal values of 30 to 50 cmH2O/L/s. That’s roughly three to five times normal, which illustrates just how dramatically resistance can climb in sick children.
Air Trapping and Its Consequences
When expiratory resistance is high, the child can’t fully exhale before the next breath begins. Air gets trapped in the lungs, progressively inflating them beyond their normal resting volume. This trapped air creates what’s called auto-PEEP, a buildup of residual pressure that makes each subsequent breath even harder to take. The lungs become stiffer as they overinflate, and the diaphragm flattens into a less efficient position.
The combination of high resistance and air trapping increases the metabolic cost of breathing significantly. Respiratory muscles that normally use a small fraction of the body’s energy output start consuming a much larger share. In infants and young children, whose respiratory muscles fatigue more quickly than adults’, this increased energy demand can deplete reserves fast. The child’s respiratory rate may initially climb as compensation, but eventually the muscles tire and breathing slows, a dangerous turning point.
Progression From Distress to Failure
PALS draws a clear line between respiratory distress and respiratory failure, and recognizing the shift is one of the most important skills the course teaches. In distress, the child is compensating: heart rate is elevated (tachycardia), the child appears anxious or agitated, and breathing effort is visibly increased but still producing adequate air movement.
Respiratory failure looks different. The heart rate drops (bradycardia replaces tachycardia), the child becomes lethargic or unresponsive, and breathing effort may actually appear to decrease, not because the obstruction has improved but because the child is exhausted. Cyanosis that persists despite supplemental oxygen is another hallmark. These are late and ominous signs. In pediatric patients, cardiac arrest is rarely a primary cardiac event. It is overwhelmingly the end result of uncorrected respiratory failure, which makes early recognition of increasing airway resistance so critical.
How PALS Addresses Increased Resistance
The overarching goal is to reduce resistance and support ventilation before the child deteriorates. The approach depends on whether the obstruction is in the upper or lower airway.
For upper airway obstruction caused by a foreign body, PALS recommends repeated cycles of 5 back blows alternating with 5 chest thrusts in infants. In children older than one year, the protocol shifts to 5 back blows alternating with 5 abdominal thrusts. Abdominal thrusts are not used in infants due to the risk of organ injury.
For obstruction caused by swelling or inflammation (croup, anaphylaxis, asthma), the strategy centers on reducing the swelling itself and opening the airway with bronchodilators or anti-inflammatory treatments appropriate to the cause. Keeping the child calm is not just a comfort measure; it reduces turbulent airflow, which as noted earlier can increase resistance from the fourth power to the fifth power of the radius reduction.
If breathing becomes absent or clearly inadequate, assisted ventilation is initiated. The AHA recommends a respiratory rate of 20 to 30 breaths per minute for infants and children who have a pulse and require assisted breathing or who have an advanced airway during CPR. The emphasis throughout PALS is that respiratory conditions are the leading cause of cardiac arrest in children, and appropriate interventions to support ventilation and oxygenation should begin quickly.