Work or breathing- adult vs. neonate:
Anatomically and physiologically the neonate’s pulmonary system differs from that of an
adult in 4 respects:
1.
High Oxygen consumption- The oxygen consumption of the infant is 7-9
mL/kg/min, whereas the adult is 3 mL/kg/min, therefore varying degrees of
airway obstruction have more impact on oxygen delivery and reserve in the
neonate.
2.
High closing volumes- the high closing volume of the neonate’s lungs are within
the range of the normal tidal volume. Closing volumes are the lung volumes at
which the alveoli close, resulting in shunting of blood by a closed alveolus.
3.
High ratio of minute ventilation to FRC- The neonate has increased alveolar
ventilation because of the need to increase oxygen delivery secondary to the high
oxygen consumption. Since the tidal ventilation for an infant is the same (mL/kg)
as an adult and the oxygen consumption is 3 times greater, the respiratory rate
must be 3 times greater. Therefore the alveolar ventilation is 3 times greater. The
ratio of minute ventilation to FRC is 5:1 in the neonate and 1.5:1 in the adult.
Clinically there is much more rapid induction of inhalational anesthesia, as well
as more rapid awakening.
4.
Pliable ribs- The neonate’s diaphragm is the major ventilatory muscle. This can
be a disadvantage in the neonate since the diaphragm results in greater negative
intrathoracic pressures; a pliable rib cage can result in retractions of ribs. This
results in less efficient ventilation with a high energy price for the effort. This is
why neonates are susceptible to fatigue with any condition that interferes with
pulmonary function
Which of the following respiratory indices is increased in neonates compared with
adults?
A Tidal volume (mL/kg)
B. pH
C. Alveolar ventilation (mL/kg/min)
D. FRC
E. PaCO2
Reference: Barash, P. Clinical Anesthesia. Fifth Edition. 2006. Pg 1186-1188.