Anemia of Prematurity - Portal Neonatal

Tidal volume is the volume of gas inhaled (or exhaled) with each breath. Frequency is the number of
breaths per minute. Dead space is the part of the tidal volume not involved in gas exchange, such as
the volume of the conducting airways. Dead space is relatively constant. Thus, increases in either
tidal volume or frequency increase alveolar ventilation and decrease partial pressure of carbon
dioxide, arterial (PaCO2). Because dead-space ventilation is constant, changes in tidal volume appear
more effective at altering carbon dioxide elimination than changes in frequency. For example, a 50%
increase in tidal volume (ie, 6-9 cc/kg) with a constant dead space (ie, 3 cc/kg) doubles alveolar
ventilation (3-6 cc/kg X frequency). In contrast, a 50% increase in frequency increases alveolar
ventilation by 50%, because an increase in dead space ventilation (dead space X frequency) occurs
when frequency is increased.
Even though increases in minute ventilation achieved with large tidal volume increase alveolar
ventilation more, the use of relatively small tidal volume and high frequencies usually are preferred as
volutrauma can be minimized.
Hypoxemia
Hypoxemia is usually the result of V/Q mismatch or right-to-left shunting, although diffusion
abnormalities and hypoventilation (eg, apnea) also may decrease oxygenation. V/Q mismatch is a
major cause of hypoxemia in infants with respiratory distress syndrome (RDS) and other causes of
respiratory failure. V/Q mismatch usually is caused by poor ventilation of alveoli relative to their
perfusion. Shunting can be intracardiac (eg, congenital cyanotic heart disease) and/or extracardiac
(eg, pulmonary). During conventional ventilation, oxygenation is determined largely by the fraction of
inspired oxygen (FiO2) and the mean airway pressure (MAP) (see Picture 2). MAP is the average
airway pressure during the respiratory cycle and can be calculated by dividing the area under the
airway pressure curve by the duration of the cycle as follows:
MAP = K (PIP – PEEP)
TI
--------
TI + TE
+ PEEP
This formula includes the constant determined by the flow rate and the rate of rise of the airway
pressure curve (K), peak inspiratory pressure (PIP), positive end-expiratory pressure (PEEP),
inspiratory time (TI), and expiratory time (TE). This equation indicates that MAP increases with
increasing PIP, PEEP, TI to TI + TE ratio, and flow (increases K by creating a more square waveform).
The mechanism by which increases in MAP generally improve oxygenation appears to be the
increased lung volume and improved V/Q matching. Although a direct relationship between MAP and
oxygenation exists, some exceptions are found. For the same change in MAP, increases in PIP and
PEEP enhance oxygenation more than changes in the ratio of TI to TE (I/E ratio). Increases in PEEP
are not as effective once an elevated level (>5-6 cm H2O) is reached and may not improve
oxygenation at all. In fact, a very high MAP may cause overdistention of alveoli, leading to right-to-left
shunting of blood in the lungs. If a very high MAP is transmitted to the intrathoracic structures, which
may occur with near normal lung compliance, cardiac output may decrease, and thus, even with
adequate oxygenation of blood, systemic oxygen transport (arterial oxygen content X cardiac output)
may decrease.
Blood oxygen content largely depends on oxygen saturation and hemoglobin level. Thus, transfusing
packed red blood cells to infants with anemia (hemoglobin >7-10 mg/dL) who are receiving assisted
ventilation is common practice. Oxygenation also depends on oxygen unloading at the tissue level,
which is determined strongly by the oxygen dissociation curve. Acidosis, increases in 2,3diphosphoglycerate,
and adult hemoglobin levels reduce oxygen affinity to hemoglobin and thus favor
oxygen delivery to the tissues.