Vol. 14 •Issue 10 • Page 18
Selecting Ideal Pediatric Candidates for Noninvasive Positive Pressure Ventilation
The use of noninvasive positive pressure ventilation has steadily increased since its development as a way to prevent airway collapse during expiration in adults with obstructive sleep apnea.
It has evolved into a mainline therapy for a wide range of acute and chronic respiratory disorders, and it holds potential as an effective mode of therapy in the pediatric population.
NPPV is a method of respiratory assistance that incorporates a pressure-targeted, high-flow ventilator with an external mask interface. The primary purpose is to augment alveolar ventilation in the setting of respiratory insufficiency. NPPV is considered “preferred therapy” for acute episodes of hypercarbia in adults with chronic obstructive pulmonary disease, primarily to avoid endotracheal intubation and ventilator-associated pneumonia.
However, in contrast to the relatively large number of well done clinical trials in adult patients, no randomized trials have been conducted in children. The insights presented in this review are drawn from published case series and ongoing experience with NPPV in the intensive care unit and general care areas at Egleston Children’s Hospital in Atlanta.
Many questions remain, though. Before they can be answered, agencies such as the Food and Drug Administration, the manufacturers of NPPV devices, and government funding agencies must recognize the need for and ultimately fund interventional trials of NPPV for selected respiratory disorders in children.
Adolescents with restrictive pulmonary disorders caused by neuromuscular weakness (Duchenne’s muscular dystrophy, Type II spinal muscular atrophy) are ideal candidates for NPPV. These patients frequently have few respiratory symptoms for many years until early adolescence, and then they present with acute pneumonia.
They’re admitted to the ICU with respiratory failure, improve, but are often difficult to wean from mechanical ventilation. The situation can be frustrating and complicated by competing agendas – the need of the intensive care specialist to move patients out of the unit in a timely way, the patient’s resistance to an unexpected change in mode of care and lifestyle, and the family’s natural anxiety at the possibility of having to manage a tracheostomy.
The main attribute of NPPV in this situation is to facilitate weaning from endotracheal intubation (although this may fail), and to give the family and patient time to adapt to a mode of ventilatory assistance that’s effective but relatively noninvasive.
Tips for successful NPPV in this patient group include:
• an open mind as to the type of interface selected (the patient’s preferences are very important)
• NPPV combined with aggressive chest physiotherapy — including high-frequency chest wall oscillation and mechanical insufflator-exsufflator
• creative on/off schedules (especially around meals)
• documentation of adequate gas exchange in the sleep laboratory when available.
Although most adolescents with neuromuscular disorders and respiratory insufficiency do well with NPPV, some don’t like it, and therefore, it doesn’t work. Patients who decompensate quickly upon removal from NPPV should remain in the ICU while other approaches, including even more aggressive chest physiotherapy and tracheostomy, are considered.
Another tip is to not expect the PaCO2 to decrease quickly; this may take hours to occur or not at all. The important thing is to maintain a normal arterial pH, restore oxygenation to a safe level (optimally > 95 percent), and ease the child’s work of breathing. We use several factors to select an optimal treatment range for the SaO2, and we recommend keeping the SaO2 above 95 percent whenever there’s right ventricular dysfunction and an elevated pulmonary arterial pressure. Keep in mind that supplemental oxygen per se may blunt ventilatory drive in patients with long-standing hypercarbia, and the patient must be carefully monitored.
NPPV is also highly effective as a long-term treatment in adolescents with neuromuscular respiratory dysfunction. In this scenario, the goal of NPPV is to confer “probable clinical benefit” and not necessarily as a life-support device.
The natural history of respiratory dysfunction in patients with progressive neuromuscular weakness is well-characterized. In our experience, NPPV — even when started before symptoms of respiratory insufficiency — doesn’t improve lung function.
These adolescents often have silent but brief episodes of obstructive apnea with hypoxemia that occur during REM sleep, well in advance of obvious hypercarbia. The sleep laboratory is an ideal location to diagnose this problem and to institute NPPV. Clinical benefits of NPPV in this population include improved sleep quality and reduced daytime PaCO2.
ADVANCED CYSTIC FIBROSIS
Adolescents with advanced cystic fibrosis and severe obstructive changes in lung function are also reasonable candidates for nocturnal NPPV. The pattern of respiratory dysfunction in CF is completely different from neuromuscular disease.
Patients with advanced CF typically present in late childhood or early adolescence with airway obstruction, bronchiectasis and progressive hypoxemia. Hypercarbia, a poor prognostic indicator, isn’t seen until relatively late in advanced CF, and often occurs in association with severe malnutrition.
With the advent of successful lung transplantation for some children with advanced CF, the classic principle of “do not intubate” in these patients doesn’t necessarily hold. NPPV is an ideal “bridge” therapy to lung transplantation in CF patients who meet appropriate criteria. Treatment with supplemental oxygen alone in advanced CF exacerbates nocturnal hypoventilation, a complication that’s prevented by concurrent treatment with NPPV.
Overlap syndromes include a diverse group of challenging patients with more than one category of respiratory dysfunction. The most common example in pediatrics would be the child with spastic quadriplegia, often complicated by scoliosis and poor oral-motor function.
These children can have repeated episodes of respiratory distress characterized by stridor, hypoxemia and retractions, often in association with a respiratory infection. The pathophysiology includes a combination (overlap) of upper airway obstruction from spasticity of the muscle groups supporting the larynx and hyoid bone, restrictive lung dysfunction from chronic aspiration, and chest wall deformities. Many are malnourished and prone to respiratory muscle fatigue.
NPPV is extremely helpful in this group of patients and can be used to acutely relieve upper airway obstruction and decrease the work of breathing. In situations in which tracheostomy isn’t acceptable to the family or in the best interests of the child, long-term NPPV is a reasonable substitute.
In long-term use, meticulous attention must be paid to comfortable mask fit and avoiding dermal abrasion whenever possible using artificial membranes placed between the mask margins and skin. Effective NPPV with a nasal mask interface is sometimes difficult in children with cerebral palsy due to considerable oral leak. In this situation, chin straps, cervical collars, and switching to a full face mask are viable alternatives.
Spina bifida is another pediatric disorder with multiple “overlapping” pulmonary complications that’s amenable to treatment with NPPV.
Children with Prader-Willi syndrome and obesity hypoventilation syndrome present with obesity, severe obstructive apnea, hypoxemia, central alveolar hypoventilation and chest wall dysfunction. The primary differentiating feature from OSA is the presence of alveolar hypoventilation (hypercarbia) that’s present both awake and asleep.
Treatment with supplemental oxygen alone can aggravate the degree of hypoventilation. The natural history of both of these disorders is premature death from right ventricular dysfunction, if left untreated.
NPPV is an attractive mode of therapy in both syndromes, and it’s often used in an attempt to prevent insertion of an invasive airway and routine mechanical ventilation. The problem with NPPV in these disorders is that the “trigger” mode is useless when the patient makes no spontaneous ventilatory effort.
Furthermore, in our hands we haven’t consistently been able to lower PaCO2 with NPPV alone in children with Prader-Willi syndrome or obesity hypoventilation syndrome even at high distending pressures. In some children, overly high distending pressures with NPPV can induce even more central hypoventilation through an unknown mechanism. Thus, a trial of NPPV is reasonable in these patients, but it isn’t a proven substitute for weight loss, respiratory stimulants, and in severe cases, tracheostomy and positive pressure ventilation.
HYPOXEMIC RESPIRATORY FAILURE
Hypoxemic respiratory failure can occur in children with status asthmaticus, severe pneumonia, early phases of acute respiratory distress syndrome, acute chest syndrome, and bronchiolitis. NPPV is often used in children with severe hypoxemia in the belief that it’s preferable to standard care (bronchodilators, face mask oxygen, etc.) in preventing endotracheal intubation. Unfortunately, there are no published randomized trials conducted in pediatric age patients of NPPV vs. standard care in any of these disorders to prove or disprove this point.
In our experience, NPPV in children with severe asthma effectively improved oxygenation in most children. However, a substantial number worsened, and the intubation rate in our case series was high at 28 percent. Clearly we selected a high-risk group (by accident since it was a retrospective report) for NPPV, and the question remains whether early application of NPPV is effective in acute asthma complicated by marked hypoxemia.
Controlled clinical trials should be conducted with clear, clinically relevant outcome indicators, including intubation rate normalized to the number of ICU days, safety variables, and incidence of nosocomial pneumonia.
We have seen children with acute asthma treated with NPPV develop air leak syndromes. This isn’t to say that the air leak was definitely due to NPPV alone, but the risk of any form of positive airway pressure has to be carefully considered in children with obstructive airways disorders complicated by gas trapping (lung over-expansion) and respiratory distress.
Acute chest syndrome complicating vaso-occlusive crises in children with sickle cell anemia is highly amenable to NPPV. In this situation, chest pain, often in association with rib cage infarcts, promotes a rapid shallow breathing pattern and massive atelectasis. Lower respiratory infection is often present, and some children present with pulmonary infarcts and bone marrow emboli. The net effect of any one or a combination of these derangements is respiratory distress with a reduction in functional residual capacity (FRC) and hypoxemia.
We have effectively used continuous positive airway pressure and then NPPV in children with acute chest syndrome as a means to reduce hypoxemia and raise FRC. In our experience, application of NPPV early on — when the FiO2 reaches .30 or higher — is helpful to maintain lung expansion.
Further trials are indicated to test whether early application of NPPV or CPAP in children with acute chest syndrome truly prevents respiratory failure and the need for invasive modes of ventilation or merely delays more aggressive therapy.
CANDIDATES NOT LIKELY TO RESPOND
In small toddlers and infants with respiratory distress, NPPV with today’s commercial devices is probably no more effective and possibly even less effective than CPAP. The challenge posed by infants and toddlers with respiratory distress is the time constant of the respiratory system (determined by the amount of time for a step increase in external pressure to equilibrate with the air spaces), is too fast for the response features of commercial NPPV devices.
Thus, infants and toddlers don’t trigger the device in the spontaneous mode. As a result, the inspiratory pressure support feature, the primary advantage of NPPV or CPAP to raise tidal volume, doesn’t occur.
Modifications of the NPPV circuit tubing and response features so as to facilitate NPPV in small infants have been attempted, but these were done against manufacturers guidelines and aren’t recommended absent carefully designed, institutional review board-approved clinical trials.
We generally don’t attempt NPPV in children until they’re 3 to 5 years old. In this age range, CPAP is an acceptable alternative, but the effects of CPAP on tidal volume and hence alveolar ventilation can be unpredictable. We also wouldn’t attempt NPPV in any child with cardiovascular instability.
The most important principle in the application of NPPV in pediatric-age patients with respiratory insufficiency is that the clinician should never assume that it’s going to be effective. There’s no substitute for bedside adjustments based on the patient’s work of breathing, gas exchange and overall level of comfort.
Like every mode of respiratory care, the burden falls on the attending respiratory therapist to determine objectively that the mode of therapy is effective. Equally as important, the wise therapist should know in advance what action to take when things don’t work according to expectations.
W. Gerald Teague, MD, is in the division of pulmonary, allergy, cystic fibrosis and sleep medicine, Emory University School of Medicine, Atlanta. Pradip Kamat, MD, is in the department of critical care medicine, Children’s Health Care of Atlanta at Egleston.