Basics of Mechanical Ventilation for the Outpatient Pediatrician

Advancements in medical technology have allowed a growing number of children with complex medical conditions and technology dependence to be discharged home from the hospital, including patients receiving mechanical ventilation. Familiarity with the principles of mechanical ventilator support, common modes used in the outpatient setting, and common complications is imperative in taking care of these children.After completing this article, readers should be able to: Describe the physiology of breathing.List the different types of respiratory support.Describe the basics of mechanical ventilation.List the indications for mechanical ventilation and home mechanical ventilation.Manage the initial settings of invasive mechanical ventilation.List the complications of hospital and home mechanical ventilation.The respiratory system consists of the airways, which conduct gases to and from the lungs, which function as the gas exchange system, and the respiratory muscles, which act as the ventilatory pump. The respiratory system’s primary function is gas exchange, which includes oxygenation and ventilation. Oxygenation refers to the transfer of oxygen from alveoli to arterial blood, and ventilation refers to removing carbon dioxide, a byproduct of cellular metabolism, from the pulmonary arterial blood. Respiratory failure ensues when the respiratory system fails to effectively supply oxygen or remove carbon dioxide, leading to hypoxemia, hypercapnia, or both. Infants and young children are at higher risk of increased work of breathing and respiratory failure due to anatomic and physiologic differences from adults (Table 1).Normal spontaneous breathing is negative-pressure breathing. It occurs via contraction of the diaphragm and, to a lesser extent, the external intercostal muscles. Negative-pressure breathing leads to expansion of the chest cavity, creating more negative intrapleural pressure, allowing air to fill the lungs. Normal exhalation, on the other hand, is a passive process. The inspiratory muscles relax, allowing the lung, chest wall, and abdominal structures to recoil, leading to air exhalation (Figure 1). Normal breathing constitutes a small proportion (<5%) of total oxygen consumption. The respiratory centers in the brainstem (medulla oblongata and pons) regulate rhythmic breathing. The dorsal respiratory group and the ventral respiratory group, which are present in the medulla oblongata, play a crucial role in generating rhythmic breathing patterns. The pneumotaxic and apneustic centers that are present in the pons modulate the medullary respiratory centers. Central and peripheral chemoreceptors regulate ventilation. Central chemoreceptors in the medulla oblongata and pons are very sensitive to changes in pH and Paco2; in cases of chronic hypercarbia, these receptors become less sensitive. Peripheral chemoreceptors in the carotid and aortic bodies respond to changes in Pao2, although to a lesser extent. Signals are sent from the respiratory centers to the respiratory muscles via the phrenic and intercostal nerves (Figure 2).Respiratory support may be needed either to assist with the work of breathing or to improve gas exchange. Respiratory support devices can be divided into oxygen delivery devices, noninvasive positive- or negative-pressure devices, and invasive mechanical ventilation (IMV).Oxygen delivery devices are categorized into low and high flow, depending on the liters of oxygen delivered per minute. All these devices are connected to an oxygen source, and the fraction of inspired oxygen (Fio2) delivered to the patient will depend on the percentage of room air mixed with the supplemental oxygen. The device that provides the most Fio2 to a spontaneously breathing child is the non-rebreather face mask. Compared with other high-flow devices, a high-flow nasal cannula (HFNC) also delivers heated and humidified air. Oxygen delivery devices, except for HFNC, may not provide ventilatory support. When patients develop hypercapnic respiratory failure or hypoxemic respiratory failure that is refractory to supplemental oxygen, positive-pressure ventilation might be needed. Various respiratory support devices are listed in Table 2.Mechanical ventilation is a life-support technology that assists patients who cannot breathe effectively due to respiratory, neuromuscular, or neurologic conditions. Mechanical ventilation can be provided in a variety of modes and with different interfaces. Some key concepts and types of ventilation are discussed in this section.1–3Positive-pressure ventilation refers to the delivery of air to the lungs using positive pressure via noninvasive ventilation (NIV) or IMV.NIV refers to delivering positive pressure to the airway via nasal or naso-oral interfaces without requiring endotracheal intubation or tracheostomy. Some of the benefits of NIV include improved patient comfort, avoidance of endotracheal intubation, decreased need for sedation, and decreased risk for hospital-acquired infections. Positive pressure is delivered via continuous positive airway pressure (CPAP) or as biphasic positive airway pressure (BiPAP). For NIV to be successful, the patient requires an intact upper airway, airway protective reflexes, and an adequate seal on the mask interface to avoid pressure loss.IMV requires endotracheal intubation or tracheostomy to provide positive pressure to the airways. IMV is indicated when the pulmonary function cannot be sustained with noninvasive modalities, as in cases of severe and rapidly progressing respiratory failure, significant neurologic or neuromuscular impairment, refractory shock with multiorgan dysfunction, or a combination of these conditions.Negative-pressure ventilation (NPV) mimics normal physiologic respiration. A chest shell (cuirass) is placed external to the chest, generating subatmospheric pressure to initiate inspiration. The chest wall moves outward and induces negative pressure in the intrapleural cavity, allowing alveolar recruitment and airflow. In contrast to normal spontaneous breathing, the exhalation process is active with NPV, as the cuirass increases the intra-alveolar pressure to allow air to flow out of the lungs. Similarly to NIV, pressure delivery during NPV may be continuous or biphasic. The most common modality used with NPV is continuous negative pressure. NPV might improve cardiac output by lowering intrathoracic pressure, thus increasing right ventricular (RV) preload and decreasing RV afterload. NPV cannot provide supplemental oxygen directly, so children with hypoxemia often need additional oxygen delivery sources. In their practice, pediatricians may encounter patients receiving NPV, as it is increasingly used in children with neuromuscular disorders such as spinal muscular atrophy.Conventional ventilators deliver a preset tidal volume or inspiratory pressure and a baseline positive end-expiratory pressure (PEEP) at a rate in the physiologic range for age. Inspiratory time is typically set shorter than the expiratory time to avoid hyperinflation while providing sufficient inspiratory time to ensure proper lung inflation.High-frequency oscillation ventilation (HFOV) delivers small tidal volumes (1–3 mL/kg) at a supraphysiologic frequency of 3 to 15 Hz (1 Hz = 60 oscillations per minute), which translates to rates of 180 to 900 breaths per minute. High mean airway pressures help with alveolar recruitment and adequate oxygenation. Ventilation is maintained by diffusion, turbulence, and movement of air between lung regions (pendelluft effect). HFOV is typically used in a hospital setting when conventional ventilation fails to maintain adequate oxygenation in patients with pediatric acute respiratory distress syndrome, or bronchopleural fistulas, or in neonates with meconium aspiration syndrome or persistent pulmonary hypertension.4A typical mechanical ventilator consists of a control panel (for user interface), a power source, a gas supply system, a central processing unit (the brain of the ventilator), a breathing circuit with a humidification system, and a patient interface (endotracheal or tracheostomy tube for invasive ventilation and various masks and cannulas for NIV) (Figure 3).Hospital ventilators are typically larger, more powerful, and more expensive than home ventilators. Hospital ventilators offer a wider range of adjustable parameters and monitoring capabilities to meet the patient’s requirements. They typically require a high-pressure gas source at 50 psi.Home ventilators are designed to be more compact, lightweight, portable, and easier to use. The ventilator is connected to an oxygen source (usually a concentrator) and a humidifier. A high-pressure gas source is not required for home ventilators. Pediatric home ventilators must offer a more sensitive trigger, generate lower tidal volume and flow rate, and have adequate audible alarms. Home ventilators should be able to operate with an internal battery when there is no access to electricity. Unlike hospital ventilators, home ventilators have limitations, including the inability to provide high peak pressures or Fio2 greater than 0.6. With improved technology, newer home ventilators have capabilities closer to hospital ventilators.The most common terminology used for mechanical ventilation is listed in Table 3.Various modes of mechanical breaths are described based on how inspiration is initiated, sustained (limited or controlled), and terminated. Individual brands of ventilators may deliver specific modes of mechanical breaths or use proprietary names for similar modes. In general, the following modes are available with most conventional mechanical ventilators (Figure 4). Controlled mandatory ventilation: The ventilator delivers breaths at a fixed rate and set volume or pressure, regardless of the patient’s respiratory efforts. The ventilator controls all phases of the breath. It is typically used in patients who are heavily sedated, chemically paralyzed, or unable to initiate their own breaths.Synchronized intermittent mandatory ventilation (SIMV): The ventilator provides breaths at a fixed set rate, synchronizing with the patient’s own inspiratory efforts. SIMV can be pressure-limited (preset pressure) or volume-limited (preset volume). The ventilator delivers controlled breaths if the patient’s spontaneous respiratory rate is lower than the set SIMV rate. It also allows the patient to take spontaneous breaths between the synchronized mandatory ones, which can be supported with volume or pressure.Assist-control (AC) ventilation: In AC ventilation, the patient can trigger their own breaths, and the ventilator will deliver a full ventilator breath, with the preset volume or pressure for each trigger, ensuring a minimum respiratory rate. Every triggered breath with AC is a full ventilator breath.CPAP (CPAP/pressure support PS): CPAP mode provides continuous positive pressure, allowing the patient to breathe spontaneously. The patient entirely controls the breathing cycle. A PS or volume support can be added to CPAP for spontaneous breaths.A control variable is the primary parameter the ventilator adjusts to achieve during inspiration. A limit (target) variable sets the maximum value the variable can attain during the inspiratory phase. The limit variable does not end the inspiration. A cycle variable ends the inspiration once the set variable value is achieved. A breath can be delivered by controlling the volume, pressure, time, or flow, with volume and pressure being the most used control variables. Volume control: A set tidal volume is delivered, while the airway pressure generated will vary depending on the airway resistance and respiratory compliance. This mode might be used when a tight Paco2 control is required and lung compliance is normal.Pressure control: The peak pressure is set to minimize the risk of barotrauma, while the delivered tidal volume varies depending on respiratory compliance and airway resistance.Ventilator graphics are useful for monitoring minute ventilation, respiratory mechanics, and improvement in respiratory function (Figures 5 and 6).Table 4 shows the starting settings for mechanical ventilation in the inpatient setting. In this section are some considerations for mechanical ventilation strategies in specific pathologies.Asthma and bronchiolitis are considered obstructive diseases due to increased airway resistance. Effective ventilation of these patients requires minimizing the risk of air trapping and hyperinflation. This can be accomplished by setting a low respiratory rate and a short inspiratory time, allowing sufficient time for complete exhalation.Patients with central nervous system dysfunction may have respiratory failure from loss of airway protection reflexes, impaired respiratory drive, diminished upper airway tone, or a combination thereof. In children with increased intracranial pressure, strict CO2 control is essential with guaranteed minute ventilation. Additionally, avoiding lung hyperinflation is important, as elevated intrathoracic pressures can compromise venous return from the brain.In patients with muscular disorders, muscle weakness leads to hypoventilation. When providing mechanical ventilation to these patients, encouraging spontaneous breathing is imperative to prevent muscle deconditioning and improve their chances of successful ventilator weaning. If oxygenation is not a concern, PEEP and Fio2 should be maintained at minimal levels.Over the past 20 to 30 years, advances in the medical field have contributed to a growing number of children using home mechanical ventilation (HMV). A Canadian epidemiological study reported a 37% increase in the number of patients discharged home with a ventilator over 13 years.5HMV aims to ensure adequate ventilation and oxygenation using stable ventilator settings without the need for continuous monitoring while maximizing growth and development. Ventilator mode and settings are initially established and adjusted in the inpatient setting, with close monitoring and titration to meet each patient’s needs.HMV should be considered when a patient develops progressive chronic respiratory failure or failure to wean from mechanical ventilation. Patients with chronic respiratory failure due to neuromuscular disorders, impaired neurologic control of breathing, or increased respiratory load from airway and lung pathology might benefit from HMV (Table 5).The choice of ventilation type and mode is determined by the cause and severity of the respiratory failure (Table 6).NIV can be delivered through nasal, oronasal, or full-face masks. The key to successful NIV is selecting a well-fitting mask to minimize leakage and maximize comfort. Most ventilators compensate for leaks by delivering a higher flow to reach the set pressure. Depending on the patient’s condition, CPAP or biphasic mode might be required.Some of the disadvantages of NIV are the risk of gastric content aspiration, especially in younger patients who cannot remove the mask; skin breakdown; and difficulty in clearing airway secretions. If a patient fails NIV, IMV via tracheostomy is the alternative for long-term home ventilatory support.Invasive HMV is delivered via tracheostomy. The stoma should be healed adequately before initiating HMV and considering discharge from the hospital. The gas delivered should be warmed and humidified to prevent the drying of the airway mucosa. Most ventilators have an active humidification device attached.Most home ventilators can provide volume- or pressure-controlled ventilation. Volume-control ventilation delivers a set tidal volume, while pressure-control ventilation delivers a set positive pressure. The benefit of pressure-controlled ventilation is that it can adapt to the patient’s irregular breathing patterns and compensate for some leaks. Some of the current home ventilators can deliver more advanced modes of ventilation.Single-limb circuits are often used in home mechanical ventilators because they are simpler and less bulky. These circuits cannot measure tidal volumes, so they are estimated using algorithmic calculations. Some single-limb circuits have a leak port to compensate for leaks and allow some CO2 removal. Other circuits have an exhalation valve to allow complete CO2 elimination. Double-limb circuits have an inhalation limb and an exhalation limb connected to inhalation and exhalation ports, respectively. Accurate measurement of tidal volumes is possible with double-limb circuits.NPV may be used in children with neuromuscular disease for long-term support. NPV is increasingly used for managing children with respiratory failure and spinal muscular atrophy.Children who require HMV, in general, have multiple underlying comorbidities; hence, they should be cared for by a multidisciplinary team that includes a primary care physician, pulmonologist, and respiratory therapist, along with other specialists depending on the patient’s comorbidities. Most children requiring home ventilation need continuous nursing and respiratory therapy care. Parents and caregivers should be trained to recognize and troubleshoot ventilator malfunction, tracheostomy dislodgement, and acute hypoxemia. The ventilator should have appropriately set alarms to alert caregivers of possible problems.Airway clearance techniques should be used routinely to prevent mucous plugging and subsequent atelectasis and to minimize the risk of respiratory infections. These techniques range from nebulization with mucolytics to the use of chest physiotherapy and cough assist devices.It is recommended that children be monitored with pulse oximeters, especially while asleep, alongside ventilator alarms.7 The equipment necessary at home should include batteries, suctioning equipment, supplemental oxygen, a nebulizer, a self-inflating bag and mask, and extra tracheostomy tubes.Weaning is the gradual process of shifting the responsibility for breathing from the ventilator back to the patient as the condition requiring mechanical ventilation improves. Weaning is typically guided by the patient’s respiratory effort, adequacy of ventilation and oxygenation, and decreased dependence on mechanical ventilator support. Children undergoing HMV require regular monitoring by pulmonologists to determine whether weaning off the ventilatory support is appropriate or feasible.The plugging of airways with thick mucus is most often seen in younger children and smaller-diameter tracheostomy cannulas. Infants have not yet developed alveolar collateral ventilation (pores of Kohn or channels of Lambert), so mucous plugging in smaller airways more frequently can lead to atelectasis. Humidification devices prevent the drying of secretions and decrease the risk of mucous plugging. Some patients might also need nebulized mucolytics.Accidental removal of the tracheostomy cannula happens more frequently in infants or when short tubing pulls on the cannula. Parents and caregivers should be trained before hospital discharge to manage the dislodgement of tracheostomy tubes and to perform cardiopulmonary resuscitation in case of an emergency. A spare cannula of the same and smaller size should always be available for emergent tube exchange.Mask leakage can occur when masks or headgear are not fitted properly. Even though a small leak is expected, larger leaks might compromise ventilation effectiveness. This may result in hypoxemia or hypercapnia and frequent nighttime awakenings, particularly in patients who require mechanical ventilation for obstructive sleep apnea.Midfacial deformity can occur, most commonly when masks used for NIV are improperly fitted. The high pressures exerted by the mask may interfere with normal facial bone growth. Younger children should be monitored regularly to ensure the mask fits properly, preventing skin breakdown and minimizing the risk of facial bone hypoplasia.Ventilator malfunctions can occur due to power failure, equipment defects, disconnection, or changes in the patient’s condition. Caregivers should be trained to recognize and troubleshoot the most common causes of ventilator malfunction, such as circuit disconnection, lack of power supply, or poor battery backup. Caregivers should also be appropriately trained to provide manual ventilation until help arrives. Families should have functioning supplies, such as suction machines, appropriately sized catheters, resuscitation bags, and nebulization machines if needed.Children with artificial airways are at higher risk for developing infections, such as tracheitis or ventilator-associated pneumonia. Children with artificial airways are often colonized with hospital-acquired organisms, like Pseudomonas, Klebsiella, and methicillin-resistant Staphylococcus aureus. If there are changes in tracheal or or increased oxygen or ventilatory should be and the patient should be (Table HMV should be as of a and of care that includes the primary and primary care low of undergoing HMV should be monitored with noninvasive such as pulse especially when on due to lack of trained caregivers who can troubleshoot common causes of ventilator should be for children with chronic on due to lack of to for the in this