Inefficient mechanics of breathing is one major risk factor for infants and young children because it increases work of breathing. In many ways pediatric anatomy and physiology predisposes a child to respiratory distress and respiratory failure. This article reviews the mechanics of breathing and discusses the differences in the pediatric airway that makes them more vulnerable to respiratory failure.
(Illustrations copyright Whitten, Pediatric Airway Management: A Step By Step Guide)
Mechanics of Normal Breathing
Normal quiet breathing is effortless. The rate is neither too fast nor too slow, however, rate varies greatly depending on age and metabolic rate. The chest rises and falls easily and symmetrically. Air flows into and out of the lungs through the open airway based on changes in air pressure.
Adult Chest Cavity Anatomy Makes Breathing Efficient
Let’s start by reviewing the adult mechanics of breathing. The angulation and rigidity of the ribs during the breathing cycle maximizes efficiency in the adult. The lungs are housed in a skeletal cage formed by the ribs. In order to initiate airflow into the lungs, pressure in the lungs must drop below atmospheric pressure. The body accomplishes this by expanding the airtight chest cavity, thereby decreasing the pressure inside. Two motions are involved:
- expansion of the rib cage by contraction of intercostal muscles
- contraction and descent of the diaphragm
The ribs form three functional groupings. The first rib attaches rigidly to the sternum to anchor the rib cage. It hardly moves during respiration.
The 8th through 12th ribs expand mostly laterally during inhalation. This effectively increases intra-abdominal space for organs pushed downward by the diaphragm. The motion is like a bucket handle, swinging up and down toward the side away from the centerline and expanding the width of the chest cavity.
The 2nd to 7th ribs flexibly expand mostly anterior-posterior with a little lateral motion. This motion is like a pump handle — mostly up and down in the front of the chest, expanding the depth of the chest cavity.
Diaphragmatic Contaction Is The Bellows
The diaphragms are two large dome-shaped sheets of muscle separating the thoracic cavities from the abdominal cavity. As the diaphragms contract with each inhalation, they act like a bellows. During inhalation the bellows descends and flattens, increasing intrathoracic volume and decreasing intrathoracic pressure. This pulls air into the lungs as they inflate.
During exhalation, the diaphragm and intercostals relax. As a result, the diaphragms rise and become dome shaped again, decreasing intrathoracic volume and raising intrathoracic pressure. Lungs deflate. The patient exhales. Unless there is obstruction, exhalation is passive, requiring little energy.
Full contraction of the intercostals and the diaphragm allows for much greater expansion of the chest cavity and produces a larger breath, assuming that air is free to flow into the lungs..
What Factors Affect Ease of Air Flow?
A variety of factors affect how easily that air flows:
- breathing rate
- too rapid or too slow a rate impairs air movement
- inspired tidal volume
- ventilating close to dead space volume causes CO2 levels to rise
- airway resistance
- smaller airways have higher resistance than larger airways
- increased resistance impairs airflow
- tissue resistance
- increased frictional resistance of lung tissues and chest wall increases work of breathing and limits tidal volume
- elastic recoil
- with weaker elastic recoil, airways tend to remain partially collapsed on exhalation rather than passively reinflate to baseline
- poor compliance makes it harder to distend the lungs, limiting air movement and increasing the work of breathing
Changes in any of these parameters can significantly affect adequacy of respiration and how hard it is to take a breath.
Anatomical Features That Increase Pediatric Work of Breathing
When the patient works hard to take a breath, for example against an obstruction, he generates a more negative pressure inside the chest cavity. The intercostal muscles more fully contract. Retractions, noisy breathing, and a rocking chest wall motion are common. As respiratory failure progresses, the pattern of respiration becomes more and more inefficient and ineffective. Work of breathing increases.
In the patient exhausted to the point of respiratory collapse, or in the patient with respiratory depression due to altered mental status, there may be little effort to breathe. Hypoventilation worsens hypoxia, hypercarbia, and respiratory acidosis. Level of sedation increases, further depressing respiratory drive.
Normal infants and small children have significant anatomic predispositions to serious disruption of their mechanics of breathing if they become sick or injured.
Evaluating the degree of respiratory compromise is a judgment call. Mild or potential obstruction may have no signs or symptoms at all. In certain patients such as facial burn victims or patients having a severe allergic reaction, mild airway obstruction can convert to total obstruction quickly as edema forms. Constant reassessment is important so that you may intervene early if necessary — before the airway is lost.
The Infant’s Chest Wall Increases The Work Of Breathing
In the infant or small child, the chest wall is more box-like in shape compared to the adult’s. The ribs are more at right angles to the vertebral column and won’t be angulated like an adult until age 10 years. This makes the pediatric chest wall mechanically less efficient and limits potential lung expansion.
Babies “belly breathe”. To take a deep breath, the infant’s chest therefore expands a little and the abdomen rises 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 with even partial airway obstruction.
Because chest wall structure and “belly breathing” limit the ability to increase tidal volume, the baby must rely on respiratory rate increases to compensate for stress. The harder a child tries to breathe, the less efficient and more labored breathing becomes.
You can see video of a toddler with croup and the signs of airway obstruction described above here.
Monitor Your Pediatric Patient Carefully
Watch for signs of airway obstruction.
Infants and toddlers tire easily when they have airway or respiratory compromise. Respiratory distress can easily progress to respiratory failure. Assess your patients carefully and monitor for change. Always ask yourself: “How well is my patient breathing?”
Follow this link to my review article in Airway Management on common problems in pediatric airway management and how to fix them:
Follow the link below for discussions and video of recognizing and treating airway obstruction.