Invasive mechanical ventilation induces, on the preterm neonate’s lungs, an interruption of both growth and alveolar maturation, as a direct consequence of the inflammation caused by the prolonged ventilatory support, which can increase the risk of development of bronchopulmonary dysplasia (1).
To reduce the incidence of this complication, the efficacy of noninvasive ventilation has been investigated over the past 40 years. The application of a continuous positive pressure to the airways (CPAP) is a noninvasive respiratory support technique that has been widely investigated more recently as a primary respiratory support and as a post extubation management strategy for those neonates at risk for respiratory distress syndrome (2,3)
Distinctive anatomical and physiological aspects of the neonate’s airway system
The rationale behind the efficacy of CPAP lies in some distinctive anatomical and physiological conditions of the neonate’s airway system. The neonate, as a matter of fact, is more inclined to airway obstruction and has more disadvantageous characteristics that make managing the work of breathing more difficult, such as:
- A higher position of the larynx that generates a disadvantageous angle to allow flow passage;
- A laxity of laryngeal, tracheal and bronchial cartilage;
- An accentuated vascularization of submucous corium, that can lead more easily to phlogistic and edematous phenomenon;
- A high chest wall compliance and a low lung compliance;
- A horizontal layout of ribs, which causes a weaker contractile activity during the ribcage’s expansion, followed by a muscular activity that is mostly diaphragmatic;
- A reduced insertion angle of the diaphragm compared to an adult, with a plain layout of the diaphragmatic dome that determines a paradoxical retraction movement of the ribcage during inspiration; that is associated with a lower lung filling and a reduced functional residual capacity (FRC);
- A reduction in FRC, in turn, may lead to a higher risk of developing of atelectatic phenomenon, alveolar decruitment and ventilation – perfusion mismatch (2).
In response to those difficulties, the neonate tries to increment his own FRC implementing some coping mechanisms that include an increased respiratory frequency, closed glottis expirations (which generate the most common sign of respiratory distress in the neonates, the expiratory grunt) and intensified tonic activity of the diaphragm. Nevertheless, this effort cannot be excessively prolonged, since respiratory muscles in the neonates are characterized by a paucity of type I fibers, also known as fatigue – resistant muscle fibers, therefore it could lead to a rapid muscular exhaustion.
Benefits of nasal CPAP
Nasal CPAP applies a constant positive pressure to the airways, superior to the atmospheric pressure, during the entire respiratory cycle, through nasal masks or nasal prongs. The pressure channeled through the nasopharyngeal cavity is kept between 4 and 9 cmH2O, pressure values considered to be effective to stabilize the upper airways, to maintain an adequate lung expansion and to prevent alveolar collapse (3-5).
Applying a constant positive pressure to the airways eases the neonate’s work of breathing, ensuring an adequate FRC (4). Moreover, the increased end expiratory lung volume contributes toward the stabilization of the highly compliant neonatal chest wall, improving lung mechanics and reducing thoraco-abdominal asynchrony (6).
The application of a positive pressure enables alveoli to be kept open, recruiting those alveoli that could easily collapse determining widespread atelectatic phenomenon.
Benefits are even more evident if we consider:
- The well-known low surfactant production that characterizes the lung of the preterm, which makes it even more difficult to reopen the alveoli after the expiration;
- The typical high chest wall – lung compliance ratio, which leads to a reduction in the resting volume, the residual volume at the end of a forced expiration, at a level that is extremely close to the lung closing volume;
- The respiratory effort exerted by the newborn to maintain an adequate FRC above the residual lung volume. The FRC is maintained above the residual lung volume during active breathing through an increase in the time constant and in respiratory rate. This is achieved through the persistence of accessory respiratory muscle activity during the expiratory phase and the adduction of the vocal cords during expiration, which allows an increase in the time constant of the respiratory system by reducing the inspiratory resistance. The high respiratory rate, in turn, contributes reducing expiratory time preventing the lung from emptying completely (2,7).
CPAP is therefore considered to be the first line intervention for both treatment and prevention of neonatal respiratory distress, with special efficacy in the prevention of bronchopulmonary dysplasia and the need for mechanical ventilation in very preterm neonates (8).
Ongoing nCPAP requires careful monitoring for the early identification of predictive failure signals and the prevention of related complications (9). Neonates on CPAP need some special precautions to be taken:
- Regular monitoring of vital signs is essential for the early identification of changes indicative of clinical deterioration: increased respiratory rate and heart rate are often indicative of excessive respiratory effort. One of the coping mechanisms put in place by the neonate to increase the FRC is the increase of respiratory rate, in order to reduce expiratory time and therefore increase the end expiratory lung volume (2,7);
- During nCPAP, an orogastric tube (preferable to the nasogastric tube to prevent excessive leakage from the interface) is required to prevent gastric distension. Gastric distension may lead to a worsening in respiratory mechanics, an increased work of breathing and a poor oxygenation. Partial compression of the diaphragm by the distended stomach prevents adequate diaphragmatic activity; that way, accessory respiratory muscles have to undertake all the work of breathing, inevitably leading to muscle exhaustion (10);
- Prone positioning plays a key role in promoting the effectiveness of nCPAP. The prone position improves PaO2 and reduces episodes of apnea (11); it determines an increase in lung volume, related to the dorsal expansion of the lung and the reduced cardiac and abdominal visceral weight transmission on the lung (12);
- Adequate humidification and adequate heating of gases reduce the work of breathing, improve comfort and tolerance of the treatment (13). The delivery of non – humidified and non – heated gases can instead generate irreversible damage to the respiratory mucosa, resulting in the loss of heating, thermal and depurative capacity that characterize it.
That’s all for today.
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