Pediatric Respiratory Failure and Mechanical Ventilation
Respiratory distress is a common initial symptom for which children are brought to medical attention and ultimately need admission to the PICU. Compared with adults, children typically have smaller airways, decreased reserves, and inadequate compensatory mechanisms, which can lead to quick decompensation and ultimate respiratory arrest if not promptly and adequately managed. This section reviews common reasons for respiratory failure in children as well as noninvasive and invasive means of support.
Respiratory failure and the need for invasive respiratory support remain common indications for PICU admission, in large part due to the childhood diseases of bronchiolitis and asthma and an increasing number of children who require chronic respiratory support at home. The two types of respiratory failure are hypoxemic and hypercarbic/hypercapnic:
Hypoxemic respiratory failure: The patient is unable to maintain adequate arterial oxygen saturation. Common etiologies include pneumonia, bronchiolitis, and asthma.
Hypercarbic/hypercapnic respiratory failure: The patient is unable to maintain adequate ventilation. Common etiologies include bronchiolitis, asthma, and neuromuscular disease, and hypoventilation (e.g., related to opioid overdose).
Some patients may experience both hypoxemia and hypercarbic respiratory failure, depending on the underlying disease process. Respiratory failure requiring mechanical ventilation can also be seen in patients who have healthy lungs, including those with altered mental status (e.g., status epilepticus) who need airway protection and certain postoperative patients (e.g., patients with massive fluid shifts).
In recent years, use of noninvasive ventilation for patients with respiratory failure has increased. Noninvasive ventilation has been proven effective for both hypoxemic and hypercarbic respiratory failure and is associated with decreased length of PICU stay and hospital costs. In practice, noninvasive ventilation is often used in a stepwise approach to increasing support, as follows:
High-flow nasal cannula (HFNC) delivers humidified and warmed air by a nasal interface. Flow rates and fraction of inspired oxygen (FiO2) may be titrated independently and provide some amount of distending pressure to help reduce work of breathing and improve oxygenation.
Continuous positive airway pressure (CPAP) provides a constant airway pressure similar to positive end-expiratory pressure (PEEP). CPAP is delivered by various interfaces, including naso-oral mask, nasal mask, and nasal prongs.
Bilevel positive airway pressure (BPAP) expands on CPAP by adding a synchronized inspiratory pressure utilizing the same delivery interfaces available for CPAP.
Mechanical ventilation is used in the PICU to care for patients in respiratory failure. Assist control and pressure support are the primary modes of mechanical ventilation used in pediatric practice.
Assist control: When the ventilator is set to assist control, it will provide fully supported breaths that can be patient triggered or time triggered (i.e., with a set respiratory rate). Assist-control mode can be controlled by pressure or volume or a combination of both:
- Pressure control: Breaths are delivered by providing a set peak inspiratory pressure and inspiratory time. Tidal volume and inspiratory flow are variable and determined by lung compliance and airway resistance.
- Volume control: Breaths are delivered by providing a set tidal volume and inspiratory flow rate. Peak inspiratory pressures are variable.
Pressure support: When the ventilator is set to pressure support, it will provide airway pressure only when the patient initiates a breath. The patient controls inspiratory time and tidal volume. Pressure support is most often used as a weaning mode or in spontaneously breathing children, to offload airway resistance and work of breathing.
Mechanical Ventilation Waveforms
Pressure, volume, and flow tracings are depicted for each of the most common breaths provided by mechanical ventilation. Pressure, breath cycle time, flow, and volume may be set depending on the breath type utilized. Solid lines depict the set variable and dotted lines depict the dependent, or dynamic, variable of each breath within a cycle. Within pressure or volume breaths, assist and control differ only by whether they are machine or patient triggered. Pressure support is unique from pressure control/assist in that breath duration is controlled by the patient. Adapted from Murray and Nadel’s Textbook of Respiratory Medicine, 6th Edition, 2016.
Complications of Mechanical Ventilation
Although mechanical ventilation can be lifesaving for children, it is associated with the following important risks and complications that should be considered:
Ventilator-Induced Lung Injury (VILI)
- Barotrauma: caused by overdistention of the alveoli due to excessive airway pressure (can lead to air leak syndromes requiring additional interventions, including chest-tube insertion)
- Volutrauma: caused by excessive tidal volume, leading to stretch injury of the alveoli (can lead to air leak syndromes requiring additional interventions, including chest-tube insertion)
- Atelectrauma: caused by inadequate PEEP, leading to areas of repetitive atelectasis with each breath
- Oxytrauma: caused by the creation of reactive oxygen species due to excessive FiO2 delivery
Asynchrony: Patient–ventilator dyssynchrony has been associated with worse outcomes, including higher mortality (see Table 2 in Mechanical Ventilation: State of the Art for a summary table of different types of dyssynchrony during ventilation).
The following figure shows lung injury caused by forces generated by ventilation at low and high lung volumes.
Lung Injury Caused by Forces Generated by Ventilation at Low and High Lung Volumes