Continuous positive airway pressure (CPAP) is the application of a continuous positive distending pressure to the airways. It is a technique that is used to assist spontaneously breathing infants and children suffering from respiratory distress syndrome in an attempt to prevent the need for mechanical ventilation. It accomplishes this by increasing the functional residual capacity (FRC), increasing compliance, decreasing total airway resistance, and decreasing respiratory rate, which are the desired outcomes of nasal CPAP.
In RDS, alveolar surfactant quantity and function are insufficient to maintain alveolar geometry, thus causing surface tension to increase. The increased surface tension in the alveoli causes an ever-decreasing FRC. With each breath, the patient must overcome the higher surface tension, and work of breathing increases. The administration of a continuous positive pressure to the airway physically holds the alveoli and airways open during exhalation and increases FRC. With an increase in FRC, lung compliance improves, easing the work of breathing, increases a patients PaO2 while allowing a decrease in the FiO2 and its accompanying toxic side-effects. CPAP may be administered to the neonate or infant through an endotracheal tube in the trachea. Alternative airways used on neonates include non-invasive nasal mask or nasal prongs.
CPAP is useful for the treatment of conditions resulting in airway or alveolar instability. Five general indications for CPAP exist. The first indication is any disease or condition that causes a decrease in the FRC. Causes of a decreased FRC include: infectious processes such as pneumonia; a loss of lung volume as seen with atelectasis, pulmonary edema, or thoracotomy; an inability for gas to reach the alveoli as occurs in meconium aspiration, or severe airway blockage with mucus; conditions that lower lung surfactant such as RDS; and other conditions, including transient tachypnea of the newborn and left-to-right shunting present with certain cardiac defects.
The second indication includes those processes that cause airway collapse. One of the primary causes of airway collapse is tracheobronchial malacia, in which the cartilage of the trachea is abnormal and does not offer the necessary rigidity to prevent collapse during inspiration and expiration. Airway collapse can lead to apnea, making CPAP helpful in treating apnea.
A third indication is to assist in weaning the patient from mechanical ventilation. A study by Tapia and associates, however, failed to demonstrate any difference in extubation outcome whether CPAP was used or not.
The fourth and fifth indications may be seen in the above conditions, but are not exclusive to them and are listed separately. An abnormal physical examination that shows a 30-40% increase in respiratory rate, retractions, grunting, flaring or cyanosis is the fourth indication. The fifth involves blood gas abnormality. Assuming ventilation is adequate, the inability to maintain the PaO2 greater than 50 mm Hg at an FiO2 of 60% is in indication for CPAP.
CPAP is most effective when it is instituted early in the progression of the disease. Initial pressures should start at between 4 and 5 cmH2O. The pressure is increased in increments of 2, as needed, to achieve the desired PaO2 level. A decrease in work of breathing, decreased retractions, nasal flaring and grunting, improved aeration on the chest radiograph, and subjectively improved patient comfort are all indications that CPAP has been successful.
Classification of Breath and Waveforms
CPAP breaths are classified as pressure controlled, pressure triggered, pressure limited and pressure cycled. The baseline variable is pressure. All breaths are spontaneous. That is, all breaths are initiated by patient effort and all breaths end owing to the patient’s compliance and resistance characteristics.
The principle hazard of CPAP therapy is that associated with high pressures. In the presence of excessive pressures, pulmonary blood flow is diminished secondary to the compression of pulmonary vessels. Cardiac output may also be reduced owing to the decrease in venous return to the heart. For these reasons, CPAP is not useful in the patient with persistent pulmonary hypertension of the newborn (PPHN) and other diseases where the problem is not one of alveolar instability.
Additional hazards include renal effects such as a decrease in the glomerular filtration rate, sodium excretion, and urine output. CPAP also elevates intracranial pressure, increasing the incidence of hemorrhage. Further hazards include pneumothorax, nasal obstruction, gastric distention, and necrosis or erosion of the nasal septum. Nasal deformities from the use of nasal prongs have also been recognized.
CPAP should not be used in the presence of upper airway abnormalities such as choanal atresia, cleft palate, or tracheoesophageal fistula, because it could be ineffective or dangerous. CPAP increases intrapulmonary pressure; therefore it should not be used in cases of untreated air leaks such as pneumothorax, pneumomediastinum, pneumopericardium, and pulmonary interstitial emphysema. The increase in intrathoracic pressure may also further worsen cardiovascular instability and should not be used in those patients Secondly, because the patient must maintain spontaneous ventilation, CPAP should not be used on the severely apneic patient who experiences episodes of desaturation or bradycardia. Any patient who cannot maintain an adequate spontaneous tidal volume and therefore have hypercapnic respiratory failure should not be treated with CPAP. Neonates with untreated congenital diaphragmatic hernia should not be treated with CPAP.
Weaning from CPAP
As soon as the patient begins to show signs of clinical improvement, the FiO2 is decreased. The CPAP is then lowered in increments of 2 cm H20 as tolerated and indicated by the blood gas status. Continuous monitoring of blood gases with a transcutaneous monitor and/or a pulse oximeter is recommended during the weaning phase. CPAP is then removed and the patient is placed on a nasal cannula at the preexisting FiO2, which is then weaned as tolerated.
Hall, B. (1998). B1. Retrieved from https://commons.wikimedia.org/wiki/File:Premature_infant_CPAP.jpg (Links to an external site.)
For this assignment, write a paper of at least 500 words that discusses CPAP in relation to respiratory distress syndrome in the newborn. Discuss common settings, application techniques and hazards.