Anemia of Prematurity - Portal Neonatal

Rate: Changes in frequency alter alveolar minute ventilation and, thus, PaCO2. Increases in rate and,
therefore, in alveolar minute ventilation decrease PaCO2 proportionally, and decreases in rate
increase PaCO2. Frequency changes alone (with a constant I/E ratio) usually do not alter MAP nor
substantially alter PaO2. Any changes in inspiratory time that accompany frequency adjustments may
change the airway pressure waveform and thus alter MAP and oxygenation.
Generally, a high-rate, low-tidal volume strategy is preferred (see Table 2). However, if a very short
expiratory time is employed, expiration may be incomplete. The gas trapped in the lungs can increase
FRC, thus decreasing lung compliance. Tidal volume decreases as inspiratory time is reduced
beyond a critical level depending on the time constant of the respiratory system. Thus, above a
certain ventilator rate during pressure-limited ventilation, minute ventilation is not a linear function of
frequency. Alveolar ventilation actually may fall with higher ventilatory rates as tidal volumes decrease
and approach the volume of the anatomic dead space.
Inspiratory and expiratory times: The effects of changes in inspiratory and expiratory times on gas
exchange are influenced strongly by the relationships of these times to the inspiratory and expiratory
time constant, respectively. An inspiratory time 3-5 times longer than the time constant of the
respiratory system allows relatively complete inspiration. A long inspiratory time increases the risk of
pneumothorax. Shortening inspiratory time is advantageous during weaning (see Table 4). In a
randomized trial, limitation of TI to 0.5 second, rather than 1 second, resulted in significantly shorter
duration of weaning. In contrast, patients with chronic lung disease may have a prolonged time
constant. In these patients, a longer inspiratory time (near 0.8 s) may result in improved tidal volume
and better carbon dioxide elimination.
Inspiratory-to-expiratory ratio: The major effect of an increase in the I/E ratio is to increase MAP
and thus improve oxygenation (see Table 3). However, when corrected for MAP, changes in the I/E
ratio are not as effective in increasing oxygenation as are changes in PIP or PEEP. A reversed
(inverse) I/E ratio (inspiratory time longer than expiratory time) as high as 4:1 has been demonstrated
to be effective in increasing PaO2; however, adverse effects may occur (see Table 3).
Although a decreased incidence of BPD with the use of reversed I/E ratios may be possible, a large,
well-controlled, randomized trial revealed only reductions in the duration of a high inspired oxygen
concentration and PEEP exposure with reversed I/E ratios, with no differences in morbidity or
mortality. Changes in the I/E ratio usually do not alter tidal volume, unless inspiratory and expiratory
times become relatively too short. Thus, carbon dioxide elimination usually is not altered by changes
in I/E ratio.
Fraction of inspired oxygen: Changes in FiO2 alter alveolar oxygen pressure and thus,
oxygenation. Because FiO2 and MAP both determine oxygenation, they can be balanced as follows:
• During increasing support, first increase FiO2 until approximately 0.6-0.7, when additional
increases in MAP are warranted.
• During weaning, first decrease FiO2 (to approximately 0.4-0.7) before reducing MAP, because
maintenance of an appropriate MAP may allow a substantial reduction in FiO2.
Reduce MAP before a very low FiO2 is reached, because a higher incidence of air leaks has been
observed if distending pressures are not weaned earlier.
Flow: Although not well studied in infants, changes in flow probably impact arterial blood gases
minimally as long as a sufficient flow is used. Flows of 5-12 L/min are sufficient in most newborns,
depending upon the mechanical ventilator and ETT being used. To maintain an adequate tidal
volume, high flows are needed when inspiratory time is shortened.