Introduction
The neonate has many differences in its pulmonary system from the adult. Infants are
not just small versions of adults. These differences must be addressed in the
stabilization of the infant after delivery to correctly assess its respiratory status. Even
if you do not routinely work in labor and delivery, these same issues arise if you are
treating a newborn infant in the Emergency Department, the acute care hospital,
outpatient clinic, or the home. The first set of differences is the anatomic differences,
followed by the physiologic differences.
I. Anatomic Differences
The infant's anatomy that directly affects the respiratory status of the newborn will be
discussed in seven major categories. They are:
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Head Size
Tongue
Nares
Upper Airway
Chest
Airways
Abdomen
So, let's discuss each one and with the discussion about how these areas are different,
discussion will also be presented as to the relevancy to a respiratory therapist.
A. Head Size
The head of the neonate is larger in proportion to the body
than an adult’s. The primary concern from a respiratory care
stand point is a mechanical issue. If the infant’s head falls
into a position that is not conducive to maintaining an open
airway, the infant may not be able to adjust his head position
to correct the situation. Often, this is detected in the nursery
by an oximeter alarm. The infant does not have an open
airway and he desaturates. The simple intervention of re-
positioning the infant’s head often completely corrects the situation. So, the take home
message is: if the infant’s respiratory status appears to be compromised - first reposition the head of the infant before trying more
aggressive types of therapy.
The size of the head is important for another reason.
Remember when you were a kid and your Mom told you
to put on a hat before going outside on a cold day? Well,
this principle is particularly true for a newborn infant.
The infant head is larger in proportion to its body than
the adult head. Therefore, even more heat loss occurs
through the head. Added to the issue is the fact that the
head is wet at delivery, the delivery room may be cool,
and the infant is carried through the cool room adding heat loss from convection.
Now, factor in the gestational age of the infant- the more premature, the poorer the
ability to regulate his own temperature. Drying of the head (and the rest of the body)
upon delivery is crucial to not cold stressing the infant.
B. The Tongue
The tongue is another body part on the infant that is proportionately larger. The
purpose of this big tongue is to create an adequate suck reflex so adequate nutrition is
consumed and the infant grows. As with the larger head, this proportionately larger
body part can compromise the pulmonary status of the infant. As all RCPs know, the
tongue can fall to the back of the pharynx and cause an airway obstruction. Infants
also have a large amount of lymphoid tissue in the area of
the pharynx. These two factors greatly increase the infant's
risk of upper airway occlusion.
As with adults, insertion of an oral airway is important in
maintaining a patent airway in the unconscious infant.
The big tongue also causes problems in stabilization of endotracheal tubes in
infants. If the infant has a strong suck reflex, the infant may "work" on the ET tube
and potentially extubate himself. The respiratory therapist must pay careful attention
to the security of the tape with infant airways even more so than adults (the added
issue of not having a cuffed ET tube enhances the infant’s ability to move the tube
with his tongue.
C. The Nares
The third factor in the infant's anatomical differences is the
nares. Many people refer to infants as obligate nose
breathers. Recent studies have shown that infants can
mouth breathe during both spontaneous breathing and with
nasal occlusion. So, a more up to date term would be
preferential nose breathers. Under normal circumstances,
the infant does breathe through his nose. Therefore, any
decrease in airway diameter due to secretions or
inflammation can significantly add to the infant's work of
breathing. Nasal flaring is one of the key signs of respiratory distress in the infant.
D. The Upper Airway
The upper airway is significantly different in the infant.
Let's look first at the epiglottis. It differs in the infant in three ways. It is:
-proportionately LARGER (than the adult's)
-less FLEXIBLE
-Omega shaped
These factors make it extremely susceptible to trauma. A gentle touch should be used
when intubating, suctioning, or examining the infant upper airway.
The infant larynx lies higher in the neck in relation to the cervical spine. The larynx
descends as the infant grows into a child and is similar to an adult airway by the age
of six.
The narrowest point in the infant airway is the cricoid ring (in
contrast to adults - which is the epiglottis). Due to the narrowing
of the airway to the cricoid ring, many people refer to the infant
airway as funnel shaped. This natural narrowing is the reason
we are able to use uncuffed endotracheal tubes in infants.
E. The Chest
At birth, infants have an increased A-P diameter similar to our adult COPD
patients . The infant’s A-P diameter of the thorax is very nearly equal to the diameter
measure laterally. This is very pronounced on a CXR which will display the
horizontal orientation of the ribs. As with our COPD’ers, this barrel chest does not
supply the "pump handle" effect to increase inspiration. The barrel chest increases the
infant’s work of breathing since their only choice if an increase in minute ventilation
is needed is to increase respiratory rate.
Another factor regarding the infant chest is that the ribs and sternum are primarily
cartilage- which creates poor chest wall stabilization. Therefore, when an infant
attempts to increase his tidal volume, the chest wall sinks in- this action is known as
retractions. Increasing tidal volumes therefore are attempted through the use of the
diaphragm alone. This is NOT an efficient method to increase tidal volume.
Therefore, the infant increases his respiratory rate to increase minute ventilation. You
will recall that this is a very inefficient method to increase alveolar minute ventilation
since gas must move through anatomic deadspace with each breath.
Poor development of accessory muscles- The accessory muscles are poorly
developed - and the younger in gestational age the infant is, the poorer the
development. Many of our COPD patients overcome or compensate for their
increased A-P diameter by conditioning their accessory muscles. The infant is not able
to do this. Therefore, once again, if the infant needs to increase his minute ventilationthe only choice he has is to increase his respiratory rates. Respiratory rates in a
premature infant may climb to over 100/minute.
F. The Airways
Okay, its probably obvious that a 3 pound infant has shorter
airways than a full size adult. But the actual numbers may impress
you. The length of the infant trachea is 57 mm - or approximately
2 inches! This is in contrast to the length of the adult trachea which is approximately 120 mm - or 5 inches (or 4.7 inches to be
proper - but 5 is probably easier to remember). Anyone who has
worked with an intubated infant knows how quickly mainstem
intubations and/or inadvertent extubations can occur.
Now let's talk about width. The diameter of the adult airway is
approximately 20 mm in diameter. The diameter of the neonate is
4-5 mm. So, if you calculate the area of each trachea you get:
Area = pi X r2
radius = 1/2 the diameter
pi = 3.14
So:
Adult
Infant
r = 1/2(20 ) =10 mm
r = 1/2(4) =2 mm
A = 3.14 X (10mm)2
A = 3.14 X (2 mm)2
A = 314 mm2
A = 12.56 mm2
Or, the adult airway has an area 25 times larger than the infant airway!
Now, let's look at the math when both airways have 1/2 mm (.5 mm) of edema. The
equation is the same:
Adult
Infant
r = 1/2(19 ) =9.5 mm
r = 1/2(3) =1.5 mm
A = 3.14 X (9.5mm)2
A = 3.14 X(1.5 mm)2
A = 283 mm2
A = 7 mm2
OR - to put it another way - if the adult airway has 1/2 mm of edema - the area will
decrease by 10%. If the infant airway has the same 1/2 mm of edema, the area of the
airway decreases by over 50%! The math clearly shows that infants will be more
severely affected by changes in airway diameter than adults.
G. The Abdomen
Picture a newborn infant in your head. Think of that cute, big round belly. The
infant's abdomen is proportionately larger than the adult's (or at least the ideal body
weight adult!). Just as many women at near term of their pregnancy complain they
cannot take a deep breath because the abdomen is pushing up the diaphragm- the same
can be a problem for the infant. Care should be taken in resuscitation efforts, CPAP
therapy via nasal prongs, or after feedings to prevent a further exacerbation of this
problem via gastric insufflation.
II. Physiologic Differences
Now that we have talked about the anatomic differences in the infant - let's turn to the
physiologic differences. The following four areas will be discussed:
A. Alveoli
A full term infant, at delivery has approximately 2 - 3 million
alveoli.The number will increase ten fold by the age of 8, but since
almost all of the gas diffusing area of the lung is contained in the
alveoli; infants do not have full benefit of the greatest defense
mechanism of the human lung - the massive reserves of gas diffusing
area. This is both good and bad. If the infant acquires a pulmonary
infection, the infant may have significantly more gas impairment than
an adult. However, if the infant's alveoli are damaged - perhaps from barotrauma from
mechanical ventilation - the infant can "grow" out of the dysfunction.
B. Surfactant
Something all RT's are taught in school- but may need a refresher is that surfactant is an important chemical in the lung to decrease
alveolar surface tension. The common analogy is the amount of
work required to inflate a completely deflated balloon in comparison
to the amount of work required to inflate a partially inflated balloon.
In the infant lung, surfactant production stabilizes at about 35 weeks
gestation. Luckily, since the early 1990's, artificial surfactant has become availableand its administration may be a unit for discussion in the future.
C. Ventilatory Reserve
The infant has a poor ventilatory reserve - for two reasons discussed earlier.
First, the inability of the infant to increase his tidal volume due to the cartilaginous
ribs and poorly developed accessory muscles. When distressed and needing increased
alveolar ventilation, the infant's only choice is to increase his rate. To review, this is
an inefficient method since anatomical deadspace must be ventilated with each breath.
Second, also discussed earlier, is the decreased number of alveoli in the infant. So if
some areas of the lung become ineffective, let's say do to infection, the infant's overall
status decreases more quickly than an adults. It should be noted that the full number of
alveoli do not develop until 8 years old - so toddlers and young children also can be
compromised significantly by inflammation or infection.
D. Metabolic Demands
When an adult has an increased body temperature - the metabolic demands
increase. This is also true for the infant:
Increased temperature = Increased metabolic demands
However, the reverse is not true in infants. If an infant is COLD stressed there is an
INCREASE in metabolic demand. If the stress is corrected quickly, the infant may
not suffer any permanent effects. However, if the cold stress is maintained, the
calories needed for growth will be used to try to regulate the temperature and continue
to stress the infant. Many different types of warming systems are used in neonatal
intensive care nurseries to keep the infant's caloric expenditure on temperature control
to a minimum. But for today's message - keep the infant WARM. Your care may take
place in the delivery room, in the nursery, or in the Emergency Department - wherever
you are - think WARM!
Summary
In this unit, we have discussed the anatomic and
physiologic differences in the infant from the
adult. It should be noted that if the infant is not
full term, these differences will be even more
pronounced. It also should be noted that changes
occur gradually - an example is that your
complete number of alveoli are not developed
until 8 years of age. So you may also want to
consider these differences when assessing infants and toddlers.
Suggested Reading
Burton, George (1997). Respiratory Care: A Guide to Clinical Practice. Fourth
Edition. Lippincott Publishers.
Scanlon,C., Spearman, C., Sheldon, R.(1995) Egan's Fundamentals of Respiratory
Care,Sixth Edition, Mosby Publishers.
Whitaker, Kent(1997). Comprehensive Perinatal and Pediatric Respiratory Care.
Second Edition. Delmar Publishers.
©Endless Education Ventures, 1998.