NEWBORN VENTILATORY STRATEGIES

newborn ventilatory strategies :
Continuous positive airway pressure: CPAP has been an important tool in the treatment of newborns with RDS. The mechanisms by which CPAP produces its beneficial effects include increased alveolar volumes, alveolar recruitment and stability, and redistribution of lung water. The results are usually an improvement in V/Q matching. However, high CPAP levels may lead to adverse effects. Multiple clinical trials have evaluated the use of CPAP in newborns with respiratory disorders. Meta-analyses generally conclude that CPAP is most beneficial early in established RDS. Prophylactic CPAP in preterm infants does not lead to a decreased incidence or severity of RDS and does not reduce the rate of complications or mortality.

Once the diagnosis of RDS is established, the administration of CPAP decreases oxygen requirements, decreases the need for mechanical ventilation, and may reduce mortality. However, the incidence of air leaks is increased among infants who receive CPAP. Optimal time to start CPAP depends on the severity of RDS. Early CPAP (ie, when the arterial-to-alveolar oxygen ratio is approximately >0.20) decreases the subsequent need for CMV and duration of respiratory assistance. Initiate CPAP in newborns with RDS when PaO2 is approximately less than 50 mm Hg on a FiO2 of 0.40 or more. Studies performed to determine whether CPAP facilitates successful extubation have not demonstrated consistent results.

Conventional mechanical ventilation: A complex interrelationship exists between the ventilator, the blood gas values, the mechanical characteristics of the respiratory system, and the infant's spontaneous respiratory efforts. Although attention often is focused on the effect of ventilator setting changes on blood gases, the ventilator changes may alter the pulmonary mechanics either acutely (eg, changes in PEEP affect compliance) or chronically (by predisposing to lung injury). Ventilator changes also may affect spontaneous breathing (eg, high PEEP decreases respiratory rate). An understanding of the basic pathophysiology of the underlying respiratory disorder is essential to optimize the ventilatory strategy. Aim for an adequate gas exchange without injuring the lungs; the ultimate goal is a healthy child without chronic lung disease.

A review of the major ventilatory parameters, which can be adjusted on pressure-limited time-cycled ventilators (ie, the most common type of ventilators used for CMV), is useful. These concepts also are applicable to volume ventilators.

Peak inspiratory pressure: Changes in PIP affect both PaO2 (by altering MAP) and PaCO2 (by its effects on tidal volume and thus, alveolar ventilation). Therefore, an increase in PIP improves oxygenation and decreases PaCO2. Use of a high PIP may increase the risk of volutrauma with resultant air leaks and BPD; thus, exercise caution when using high levels of PIP. The level of PIP required in an infant depends largely on the compliance of the respiratory system.

A useful clinical indicator of adequate PIP is gentle chest rise with every breath, which should not be much more than the chest expansion with spontaneous breathing. While absent breath sounds may indicate inadequate PIP (or a blocked and/or displaced ETT or even ventilator malfunction), the presence of breath sounds is not very helpful in determining optimal PIP. Adventitious sounds, such as crackles, often indicate disorders of lung parenchyma associated with poor compliance (requiring higher PIP), while wheezes often indicate increased resistance (affects the time constant).

Always use the minimum effective PIP. Making frequent changes in PIP in the presence of changing pulmonary mechanics, such as after the administration of surfactant in the management of RDS, may be necessary. Babies with chronic lung disease often have nonhomogeneous lung disease, leading to varying compliance of different regions of the lung and, therefore, differing requirements for PIP. This partially accounts for the coexistence of atelectasis and hyperinflation in the same lung.

Positive end-expiratory pressure: Adequate PEEP helps to prevent alveolar collapse, maintains lung volume at end-expiration, and improves V/Q matching. Increases in PEEP usually increase oxygenation associated with increases in MAP. However, in infants with RDS, a very elevated PEEP (>5-6 cm H20) may not improve oxygenation further and, in fact, may decrease venous return, cardiac output, and oxygen transport. High levels of PEEP also may decrease pulmonary perfusion by increasing pulmonary vascular resistance. By reducing delta (amplitude) pressure (PIP minus PEEP), an elevation of PEEP may decrease tidal volume and increase PaCO2.While both PIP and PEEP increase MAP and may improve oxygenation, they usually have opposite effects on PaCO2. Generally, older infants with chronic lung disease tolerate higher levels of PEEP without carbon dioxide retention and with improvements in oxygenation. PEEP also has a variable effect on lung compliance and may impact the PIP required. With RDS, an improvement in compliance occurs with low levels of PEEP, followed by a worsening of compliance at higher levels of PEEP (>5-6 cm H20). A minimum PEEP of 2-3 cm H20 is recommended, since endotracheal intubation eliminates the active maintenance of FRC accomplished by vocal cord adduction and closure of the glottis.

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. 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. 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. 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. 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.