RESPIRATORY PHYSIOLOGY IN THE DEVELOPING CHILD
Spontaneous ventilation requires adequate function of the central control of breathing, ventilator muscle function, and lung mechanics.
Respiratory failure occurs when the central respiratory drive and/or power are inadequate to overcome the respiratory load. When the cause of this imbalance is irreversible, the condition becomes chronic respiratory failure.
Developmental factors that affect breathing in the infant and young child may include soft thoracic cage, poorly developed intercostal muscles, lack of bucket-handle motion in the rib cage because of the horizontal alignment of the ribs, short diaphragm, fewer type I muscle fibers, smaller airways and increased resistance, and fewer air-exchange units. Less collateral ventilation and decreased stability of the air-exchange units increase likelihood of collapse.
Improvements in the treatment of acute respiratory failure, the development of subspecialties such as pediatric critical care and neonatology, and advancements in invasive (e.g., tracheostomy and positive pressure ventilation) and noninvasive (mask continuous positive airway pressure [CPAP] or bilevel positive airway pressure [BiPAP]) ventilation have led to an increase in the survival of pediatric patients with chronic respiratory failure.
Infants, children, and adolescents with disorders of central control of breathing, disease of the airways, residual lung disease after severe respiratory illness, persistent pulmonary hypertension, and neuromuscular disorders may experience hypercarbic and/or hypoxemic chronic respiratory failure. Although it is generally possible to identify a primary cause for the respiratory failure, many children have multiple causative factors.
Chronic respiratory failure is pulmonary insufficiency for a protracted period, usually 28 days or longer. The goals of long-term mechanical ventilation are to sustain and extend life, to enhance the quality of life, to reduce morbidity, to improve physical and psychological function, and to enhance growth and development. Patients with reversible neuropathies (e.g., Guillain-Barre syndrome, neuropathy of critical illness), bronchopulmonary dysplasia, pulmonary hypertension, airway abnormalities, and congenital heart disease before or after surgical intervention are on long-term ventilation as a bridge for full recovery. Patients are maintained on long-term ventilation until they recover from the initial insult. Patients with conditions such as central hypoventilation, progressive neuromuscular disease, and high quadriplegia may require ventilatory support indefinitely.
The preferred site for a patient's care after initial discharge with a ventilator is within the family home. When social circumstances do not allow this placement, patients may be placed in a highly skilled nursing facility.
Less than 1% of patients admitted to pediatric intensive care units require long-term noninvasive or invasive ventilatory assistance. The polio epidemic in the 1930s through 1950s led to widespread need of respiratory support for children. At that time a negative pressure ventilation device such as the iron lung was used.
The causes for long-term dependency on chronic ventilation or supplemental oxygen may be categorized as follows:
Respiratory pump: respiratory muscles, rib cage, ventral abdominal wall (neuromuscular disease, chest wall deformity, spinal cord injury, prune belly syndrome)
Respiratory drive: congenital central hypoventilation, brain or brainstem injury, central nervous system (CNS) tumors, metabolic disorders
Airway: obstructive sleep apnea, tracheomalacia, bronchomalacia
Pulmonary parenchymal or vascular issues: bronchopulmonary dysplasia (BPD), lung hypoplasia, recurrent aspiration, cystic fibrosis (CF), congenital heart disease prior to or following cardiac surgery, pulmonary hypertension
The goals of long-term ventilation are to sustain and extend life, enhance the quality of life, reduce morbidity, improve physical and psychological function, and enhance growth and development. This is achieved by maintaining normal oxygenation and ventilation and minimizing the work of breathing.
Chronically ventilated infants and children recovering from severe illness may benefit from normal family interactions. Whenever possible, home discharge on mechanical ventilation is preferred over long-term hospitalization.
Caring for a child on long-term ventilator support in the home is a complex, physically demanding, emotionally taxing, and expensive process. It changes the family routines, priorities, and lifestyle and may adversely affect family relationships.
The discharge process for a child likely to require long-term ventilator support should start early in the hospital, before stabilization and transition to a portable ventilator.
The decision to initiate long-term ventilation also depends on the disease process and prognosis. Children with degenerative neuromuscular disease such as spinal muscular atrophy (SMA) type I may present with respiratory failure very early in life, often triggered by the first episode of respiratory illness. Although some parents of infants with SMA may decide to limit treatment to palliative care, others choose noninvasive or invasive ventilator support.
Infants and young children with chronic lung disease or airway malacia may improve their respiratory function and wean off ventilator support with adequate ventilation, good nutrition, and measures that promote development and prevent further injury.
Successful home discharge depends also on the presence of adequate resources in the community to support the family. Some programs recommend professional nurses in the home to assist with around-the-clock home care. The level of care depends on adequate funding and the availability of nursing agencies with skilled nurses. Housing may also present a significant barrier to home discharge. Adequacy of access, space for the child, caretakers, equipment, and supplies are other variables. Environmental safety issues include building and electrical code violations and the need for ramps or lifts. Funding for home care for children is challenging and varies across the country.
RESPIRATORY EQUIPMENT FOR HOME CARE
Supplemental oxygen and positive pressure support can be administered by nasal cannula. The nasal cannula system can also deliver heated, supersaturated, high-flow gases. A number of devices are available for CPAP and BiPAP. These devices attach to nasal pillows, nasal mask, or full face mask. They are best suited for the management of obstructive sleep apnea. Long-term use of a facial mask in small children may result in midface dysplasia. This type of ventilation may also be used in children with mild respiratory insufficiency, recurrent atelectasis, or nocturnal hypoventilation.
Other noninvasive respiratory devices are the cuirass ventilator and the iron lung, both of which are negative pressure devices. The rocker bed may also be used for mild respiratory insufficiency due to muscular weakness.
Diaphragmatic pacing may be used for congenital central hypoventilation syndrome (CCHS) or in older children with high-level quadriplegia. Many patients on diaphragmatic pacing will require tracheostomy because of lack of coordination between diaphragmatic contraction and opening of the glottis.
POSITIVE PRESSURE VENTILATION
A ventilator used in the home should be small, lightweight, and quiet; be able to entrain ambient air; provide continuous flow; and be able to accommodate a wide range of settings: pressure, volume, pressure support, and rate in order to satisfy needs from infancy to adulthood. Battery power, both internal and external, can allow unrestricted portability in the home and the community. The equipment must also be impervious to electromagnetic interference and must be relatively easy to operate and troubleshoot.
Children who are chronically ventilated on positive pressure require a tracheostomy. The child's caregivers (parents, family members, and home nursing staff) are instructed in all aspects of tracheostomy care: stoma care, elective and emergency tracheostomy change, recognition of obstruction or decannulation, and appropriate suctioning techniques. The child's caregivers have to demonstrate competency prior to home discharge.
Thick, copious secretions may contribute to increased airway resistance and provide a substrate for bacterial and fungal growth. Respiratory infections in turn may also increase the amount of secretions and viscosity and thus contribute to airway clearance issues.
Patients with neuromuscular weakness often have discoordinated or even absent swallow and are at risk for aspiration of secretions and food. Reflux, resulting in aspiration, is also common. Many patients have poor or nonexistent cough and may have ciliary dysfunction.
Modalities that help with clearance of secretions include manual or mechanical percussion, postural drainage, vibration, and vest or wrap percussion therapy. Cough-assist devices and/or abdominal binders may enhance cough. Additionally, oropharyngeal or tracheal suction help clear secretions. Intermittent positive ventilation devices are useful adjuncts. Pharmacologic agents such as anticholinergics or botulinum toxin may help control secretions, or selective ligation of the salivary glands may be an option. In extreme cases laryngotracheal separation may be considered. In case of thick tenacious secretions, review of the patient's hydration and anticholinergic medications is indicated. Nebulized dornase alfa, N-acetylcysteine, hypertonic saline, or sodium bicarbonate may be helpful. In selected cases bronchoscopy may be indicated for removal of inspissated secretions and re-expansion of atelectatic lung.
PHYSICAL, OCCUPATIONAL, AND SPEECH THERAPY
Therapies are essential in the management of chronic respiratory failure. Some of the goals of physical therapy are strengthening of the muscles, with an emphasis on muscles essential to breathing such as truncal and abdominal. Occupational therapy goals are in support of child development. Child life therapy focuses on providing a developmentally appropriate environment and age-appropriate play. Speech therapy initially focuses on oromotor skills for feeding and communication. Evaluation of swallow is very important. Equally important is evaluation of hearing, as hearing loss is relatively common in ventilator-dependent patients. Sign language may be taught in order to ease communication.
Respiratory infections such as tracheitis, bronchitis, and pneumonia are common in patients with chronic respiratory failure. Community-acquired viral or bacterial pathogens are common. Bacterial infections must be distinguished from colonization in order to avoid overusage of antibiotics. Preventive measures such as immunizations and meticulous tracheostomy care are essential.
A patient who is ventilated in the home must be electronically and/or physically monitored at all times. Infants and young children, cognitively impaired patients, and patients who are tracheostomy dependent due to suprastomal obstruction must be under direct observation of the caregivers at all times. Caregivers should also closely monitor children whose pulmonary status is fragile or unstable. Continuous monitoring of heart rate and oxygen saturation is recommended during sleep and either continuous or intermittent monitoring during the daytime. Patients with pulmonary hypertension are prone to rapid drops in oxygen saturation. Patients with CCHS are particularly vulnerable to episodes of hypoxemia or hypercarbia.
Patients evaluated in the pulmonary clinic should be monitored at each visit for heart rate, oxygen saturation, and transcutaneous and end-tidal CO2 levels. Pulmonary function tests should be considered for patients who are old enough (over 5 years) and able to cooperate. Patients with pulmonary hypertension should have serial echocardiograms. An increased level of monitoring is recommended for patients who have improved enough and are in the process of weaning off ventilator support. In addition to physiologic parameters, patients must be monitored for signs of stress, agitation, or fatigue.
WEANING OFF VENTILATOR SUPPORT
Patients recovering from chronic respiratory failure who are on stable ventilator settings, on low PEEP, and on minimal or no oxygen support should be evaluated periodically for readiness to wean off mechanical ventilation. Barriers to weaning may include residual lung disease, pulmonary hypertension, impaired central control of breathing, and muscle weakness. Muscle weakness may be due to underlying neuromuscular disease; use of sedatives, analgesics, or steroids; past use of muscle relaxants; prolonged immobility; and overuse of ventilator support. It is important to titrate ventilator needs in order to exercise respiratory muscles; however, caution must be taken to avoid fatigue.
Transitioning from full ventilation to spontaneous breathing requires conditioning of the respiratory muscles. This can be achieved by gradually decreasing the pressure or the rate, sprints of pressure support, retraining the respiratory muscles by breathing against resistance, or spontaneous sprints off the ventilator. The weaning program may be initiated prior to discharge from the hospital or during clinic visits. Typically one may start with sprints of 15 minutes two to three times a day under direct observation of a caregiver. The sprints are lengthened gradually with continuous monitoring in the home and frequent clinic visits. The patient is also monitored for respiratory distress, weight gain, energy levels, behavior, and sleep patterns. When the patient is weaned down to six hours/day of ventilator support, a polysomnogram is ordered prior to complete liberation off the ventilator.
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