Critical Care
Ashley L. Steed
Nikoleta S. Kolovos
RESPIRATORY FAILURE
Defined as the inability of the respiratory system to provide adequate oxygen to meet the body’s demands and/or excrete produced carbon dioxide. This failure is characterized by hypoxemia (decreased oxygen content in the blood), which may lead to hypoxia (tissue oxygen deprivation) as well as hypercapnia (increased carbon dioxide in the blood).
Causes of Hypoxemia and/or Hypercapnia
Alveolar Hypoventilation: Defined as inadequate minute ventilation.
Multiple etiologies including impaired respiratory drive from altered sensorium (i.e., sedation, coma, status epilepticus), upper airway obstruction, peripheral nervous system dysfunction (Guillain-Barré syndrome, botulism), or respiratory muscle weakness (muscular dystrophy, fatigue).
Physical examination: encephalopathy/apnea/hypopnea often seen in those with impaired respiratory drive; stridor/suprasternal retractions with upper airway obstruction; neuropathy/myopathy with underlying neuromuscular disorders.
Treatment: Supplemental oxygen may offset hypoxemia in mild cases, but noninvasive or invasive mechanical ventilation may be necessary in moderate to severe cases. Proper patient position or placement of an oral or nasal airway device may alleviate upper airway obstruction. Helium:oxygen (Heliox) gaseous mixtures help reduce turbulent flow and overcome increased resistance caused by upper airway obstruction.
Ventilation/Perfusion (V/Q) Mismatch: Ideally, ventilated lung units receive blood flow in order for gas exchange to occur. However, when ventilation and perfusion are not optimal, hyoxemia and hypercapnia can result. Ventilated alveoli that are not perfused are termed dead space (V/Q > 1). In contrast, when alveoli receive blood flow but are not ventilated, no gas exchange occurs and blood is thus “shunted” (V/Q < 1).
Various causes of V/Q mismatch exist. An extreme example of V/Q > 1 is pulmonary embolism. Pneumonia, atelectasis, and asthma can lead to V/Q < 1.
Physical examination: Typical of those patients in respiratory distress from multiple causes such as tachypnea, nasal flaring, and retractions. Specific etiologies may lead to crackles in patients with pneumonia or wheezing and prolonged expiration in patients with reactive airway disease. A massive pulmonary embolism may result in cardiovascular collapse.
Treatment: Aimed at treating the underlying cause (e.g., steroids for asthma or antibiotics for pneumonia). As with alveolar hypoventilation, supplemental oxygen may offset hypoxemia, but moderate to severe cases will require escalation of respiratory support.
Diffusion Impairment: Oxygen diffusion from the alveolar space into the blood depends on the area available for exchange, thickness of the alveolar wall, the partial pressure difference across the space, and the rate of blood flow. Less area available for gas exchange, increased thickness of the alveolar wall, decreased difference in the partial pressure of oxygen, or an increased rate of blood flow will limit oxygen diffusion. CO2 more readily diffuses across the alveolar surface, and thus its elimination is less affected by these changes.
Examples of diffusion impairment include pulmonary fibrosis and emphysematous changes.
Examination findings are nonspecific signs and symptoms of respiratory distress.
Treatment: Aimed at optimizing oxygen diffusion by increasing the surface area available for gas exchange (i.e., application of continuous positive pressure), limiting any underlying disease process resulting in a thickened alveolar wall, and/or increasing the difference in oxygen partial pressure with supplemental oxygen.
Additional Causes of Hypoxemia
Shunt: Venous blood bypasses ventilated alveoli and mixes with oxygenated blood.
Etiologies include anatomical shunts (i.e., intracardiac mixing, cerebral arteriovenous malformations) and in extreme cases when no ventilation reaches some airspaces and V/Q = 0. Hypoxic pulmonary vasoconstriction helps to limit the latter situation by limiting perfusion to areas of the lung with low ventilation via constriction of pulmonary arterioles thereby redirecting blood flow to alveoli with increased oxygen.
Examination findings vary from cyanosis without distress to shock due to extreme decrease in overall oxygen content in blood.
Treatment is aimed at the underlying process. For example, optimize the balance between pulmonary and systemic blood flow in patients with cardiac mixing defects until surgical correction is possible.
Low Inspired Partial Pressure of Oxygen: High altitudes have lower partial pressure of oxygen in the atmosphere, which directly diminishes the partial pressure difference driving oxygen delivery. Supplemental oxygen can be used if necessary.
RESPIRATORY SUPPORT
Noninvasive Support
Respiratory insufficiency can be supported with noninvasive strategies ranging from supplemental oxygen by nasal cannula to mechanical ventilation with a tight-fitting face or nasal mask depending on severity. This is distinguished from invasive support, which is supplied via an artificial airway (i.e., an endotracheal or tracheostomy tube).
Noninvasive mechanical support can be delivered with positive pressure during the inspiratory and expiratory phase. Biphasic positive airway pressure (BIPAP) delivers a higher pressure of gas during the inspiratory cycle (IPAP setting) and a lower pressure during the expiratory cycle (EPAP setting). The increased pressure
delivery is triggered via the patient’s inspiratory effort. Typical initial setting are IPAP 8-10 cm H2O and EPAP 4-5 cm H2O and are adjusted based on the patient’s work of breathing and oxygen delivery. Continuous positive airway pressure (CPAP) can be used without change in pressure through the respiratory cycle; settings are typically those used for EPAP. Additionally, a backup rate may be set and useful for patients with hypopnea or periodic apnea.
Common indications for noninvasive mechanical support include static or slowly progressing neuromuscular disease, central hypoventilation, chronic respiratory insufficiency and severe asthma exacerbations.
Advantages: avoidance of artificial airway and thus decreased sedation need as compared to that required for patient tolerance of endotracheal tube. Consequently, patient interactivity and mobility level is less diminished.
Disadvantages: requires some patient cooperation and therefore may require some sedation, particularly in younger children. These devices cannot deliver full ventilatory support, and assessment of pulmonary mechanics is more difficult. Gaseous gastric distention may occur, limiting ability to provide full enteral nutrition. Long-term use may cause skin breakdown or midface hypoplasia at sight of mask seal.
Invasive Support
Indications for invasive respiratory support include the following: respiratory failure, shock (with goal of decreasing systemic oxygen consumption via decreasing the oxygen demand necessary for work of breathing), need for controlled ventilation as therapy (i.e., treatment of intracranial hypertension) or to facilitate safety during procedures, upper airway disease, and inability to control or protect airway (i.e., altered sensorium).
Intubation
Preparation
Have ready access to the following: oxygen, suction equipment, appropriately sized mask, ventilation bag, lighted laryngoscope with appropriately sized blade, endotracheal tube of the expected size as well as one 0.5 mm larger and 0.5 mm smaller, stylet, CO2 detector, pulse oximetry, secure intravenous (IV) access, and ventilator. Consider having an appropriately sized laryngeal mask airway (LMA) and/or oral airway in case of difficulty with intubation.
Position the patient in such a manner that the oral, pharyngeal, and tracheal axes are aligned to achieve optimal view of the airway.
Pharmacotherapy (see Table 8-1). Sedatives and neuromuscular blocking agents are used for patient comfort and to facilitate visualization of the airway.
Review prior intubation history and records if available.
Signs of a Difficult Airway
Consider the presence of an anesthesiologist or otolaryngologist with advanced airway skills if the patient has signs of a difficult airway such as micrognathia, facial clefts, midface hypoplasia, maxillary protrusion, facial asymmetry, small mouth opening, short neck, limited cervical spine mobility, oral or upper airway bleeding, edema, or foreign bodies.
TABLE 8-1 Selection of Medications for Intubation | ||||||||||||||||||||||||||||||||||||||||
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AIRWAY MANAGEMENT
Rapid Sequence Induction and Emergent Intubation
Rapid sequence induction is rarely used in pediatric critical care prior to intubation given many patients’ inability to tolerate even brief lack of ventilation. However, patients at high risk for pulmonary aspiration (known recent oral intake, orofacial trauma, abdominal mass) should be considered for rapid sequence induction.
This is accomplished by preoxygenation and denitrogenation of the lungs utilizing 100% oxygen and a tight-fitting mask.
A defasciculating dose of neuromuscular blockade may be considered in patients with intracranial hypertension or ocular injury.
Isotonic fluids should be administered if the cardiovascular status is tenuous.
Cricoid pressure should be administered to prevent aspiration of stomach contents or blood.
Bag-Mask Ventilation
Most patients should undergo bag-mask ventilation to facilitate gas exchange during induction and after neuromuscular blockade.
Depending on the patients’ last enteral intake, minimization of time undergoing bag-mask ventilation and application of cricoid pressure is warranted to reduce aspiration risk.
A well-fitting mask that covers the nose and mouth is essential. An oral or nasal airway may also be necessary if upper airway obstruction occurs due to poor airway tone after administration of sedation and neuromuscular blockade.
Two people may be needed, one to ensure an adequate seal via positioning the patient and mask optimally and one to operate the bag.
If a patient cannot be ventilated via a bag and mask, do not give neuromuscular blockade. Emergently consult anesthesiology or otolaryngology personnel while placing rescue airway devices (i.e., oral airway or LMA).
Ventilating Bags
Self-inflating bags: do not require adequate seal or gas source to fill (can pull gas from environment)
Flow-inflating bags (i.e., anesthesia bags): fill only when connected to a gas source and have an adequate seal, require regulation of pressure via a flow-control valve, and allow operator to gauge lung compliance (change in lung expansion for a delivered pressure)
Selection of Laryngoscopy Blade and Endotracheal Tubes
Blade types
Miller: straight blade with a slightly curved tip, positioned posterior to the epiglottis allowing visualization of glottis by lifting the epiglottis upward. Particularly helpful for those with relatively large and floppy epiglottises (i.e., infants).
Macintosh: curved blade, positioned in the vallecula, anterior to the epiglottis, such that the epiglottis is lifted upward indirectly to expose the glottis.
Endotracheal tubes
Cole formula estimates endotracheal tube size based on age:
Tube size (mm internal diameter) = age (years)/4 + 4
Infants should typically be intubated with a 3.0-3.5-mm endotracheal tube
Less reliable for infants and patients with Trisomy 21; these patients often require a 0.5 mm smaller endotracheal tube than that estimated by the Cole formula.
During intubation, the endotracheal tube should pass easily through the glottis, allowing a leak at 15 cm H2O of air pressure. However, the leak must not be large enough that effective ventilation is compromised. If so, the endotracheal tube should be replaced with a larger tube or one with a cuff such that effective ventilation is assured.
Cuffed versus uncuffed endotracheal tubes: The narrowest portion of the airway in the young child is the subglottic region, thereby allowing for a reasonable fit of the endotracheal tube after easy passage through the glottis and into the subglottic region. However, in adults, the narrowest portion of the airway is at the level of the vocal cords, and therefore cuffed endotracheal tube are utilized in order to facilitate effective gas exchange. Traditionally, uncuffed endotracheal tubes were used in children <8 years of age, but more recently cuffed endotracheal tubes have been utilized in younger children as well. Patients typically require a tube 0.5 mm internal diameter smaller than estimated by the Cole formula when utilizing a cuffed tube (see Duracher and Newth, suggested readings).
During and After Intubation
When the airway is visualized, the endotracheal tube should be observed to pass through the vocal cords into the glottis. Stop endotracheal tube advancement after the cuff has passed through the glottis or at a predetermined placement, noted by marking on the endotracheal tube or by the following approximation:
3 × endotracheal tube size (in mm) = appropriate endotracheal tube depth (cm)
Tube position should be confirmed with CO2 detection, symmetrical chest wall rise, equal auscultation over the chest wall, and favorable gas exchange.
Chest radiography is useful to evaluate depth of endotracheal tube placement.
Mechanical Ventilation
Use of positive pressure to move gas into the lungs in order to achieve oxygenation and ventilation. More specifically, a ventilator delivers a regulated gas flow, which generates a pressure that is transmitted to the lungs (airway pressure) that moves a volume (tidal volume) of gas.
Major determinants of oxygenation are alveolar lung volume and fraction of inspired oxygen (FiO2). Alveolar lung volume is affected primarily by measures that determine mean airway pressure, such as positive end expiratory pressure (PEEP), inspiratory time, and peak airway pressure. The major determinants of CO2 clearance (i.e., ventilation) is minute ventilation, defined as the amount of gas moved into and out of the lungs per minute. Minute ventilation is determined by tidal volume achieved and the respiratory rate.
Although modern ventilators offer different modes, the overriding goal is to select a strategy that maintains oxygenation and ventilation, is comfortable to the patient, and minimizes ventilator-induced lung injury and complications such as pneumothorax, cardiovascular compromise, and respiratory muscle atrophy.
Modes of Conventional Mechanical Ventilation
Modern ventilators can provide different strategies of gas delivery as determined by the mode selected. Modes differ by the parameters set by the clinician, such as the timing and pattern of breathing (mandatory, assisted, supported or spontaneous) as well as how that support is delivered (regulated by flow or pressure). The most common initial assisted modes of ventilation are those in which the clinician sets a respiratory rate and either a tidal volume (volume control) or peak airway pressure (pressure control).
Mandatory Ventilation
In controlled mandatory ventilation, the ventilator delivers a set number of breaths per minute with a set tidal volume (volume control) or pressure (pressure control) with a fixed inspiratory time regardless of patient effort and provided no gas flow between delivered breaths. The addition of a continuous gas flow allowed patient spontaneous breathing, and this mode of ventilation is called intermittent mandatory ventilation.
Assisted or Supported Ventilation
Further advances in technology have allowed ventilator modes that synchronize delivered breaths with the patient’s respiratory effort, which allows greater comfort and synchrony.
With assisted ventilation (regardless of whether gas delivery is regulated by volume or pressure) in a spontaneously breathing patient, the ventilator provides a breath in response to the patient’s inspiratory effort. The ventilator senses this effort via a change in continuous gas flow (flow triggered) or a change in pressure (pressure triggered). Flow triggering is more sensitive to patient’s inspiratory initiation. Therefore, the breath is temporally synchronized with the patient and is delivered to a preset tidal volume (volume control) or pressure (pressure control). The breath is delivered with a preset inspiratory time, which may be uncomfortable in a spontaneously breathing patient. A mandatory number of breaths (preset rate) are given; if no patient respiratory effort is detected, the ventilator delivers the breaths at fixed intervals similar to mandatory ventilation. Spontaneous breathing above the preset respiratory rate can be supported. The patient determines the inspiratory time for these supported breaths.
Assisted ventilation is able to provide complete support for those patients with weak respiratory effort. A common strategy for weaning from assisted mechanical ventilation is to decrease the number of preset mandatory breaths and rely more on supported spontaneous breathing as the patient improves.
With supported ventilation, frequency and inspiratory time of gas delivery is entirely regulated by patient effort. Therefore, this strategy of ventilation can only be used in patients with adequate respiratory drive. Support is delivered by additional positive pressure in synchrony to the patient’s effort in a decelerating flow pattern. The level of support is determined by a preset pressure (i.e., pressure support as described above) or a preset tidal volume (termed volume support) in which mode the ventilator modifies the pressure support needed to achieve the preset tidal volume. In pressure support, the tidal volume achieved is dependent on the patient’s effort and the respiratory system compliance and therefore must be carefully monitored. In volume support, the tidal volume achieved is the set parameter; therefore, careful attention must be paid to the pressure support needed to obtain that tidal volume.
Supported ventilation is a frequently used mode as patients are weaning from mechanical respiratory support. This form of ventilation also decreases the inspiratory work needed to overcome endotracheal tube impedance.
Spontaneous Ventilation
A constant level of pressure is maintained through the respiratory circuit while the patient breaths spontaneously (CPAP). The application of CPAP improves gas exchange and decreases respiratory work by maintaining end expiratory lung volume (i.e., preventing derecruitment and thus airway collapse).
Therefore, patients must have an adequate respiratory drive and strength. This mode of ventilation is also useful in weaning patients from mechanical respiratory support.
Strategies to Deliver Respiratory Support
As outlined above, preset ventilator parameters are used to ensure appropriate oxygenation, ventilation, and patient comfort.
Common mandatory and assist modalities used are those in which the clinician sets the respiratory rate and either the tidal volume (volume control, which is delivered via a constant gas flow pattern) or the peak airway pressure (pressure control, which is delivered via a constant pressure). Thus, volume control will guarantee a minute ventilation but may require high airway pressures depending on respiratory compliance; additionally, for a given tidal volume, volume control leads to higher peak airway pressures than pressure control due to the constant flow pattern. Alternatively, pressure control delivers gas with a constant pressure in a decelerating flow pattern, which may improve gas distribution among lung units with individual differences in compliance. Although this strategy does not guarantee minute ventilation, it does provide the ability to control airway pressure.
For patients with poor respiratory compliance, such as severe pneumonia or acute respiratory distress syndrome, pressure control modes of ventilation may be safer by limiting the high airway pressures that risk complications such as pneumothorax.
For patients with moderate leaks around the endotracheal tube, pressure control may achieve effective ventilation by overcoming the leak with increased pressure settings. Volume control is not useful in this setting as ventilator assessment of volume is inaccurate given the leak.
Modern ventilators offer pressure-regulated volume control, which combines the advantage of guaranteed minute ventilation with a preset high pressure limit. Gas delivery is provided by a constant pressure with a decelerating inspiratory flow pattern aimed at delivering a preset tidal volume. Delivered tidal volume is assessed by the ventilator, and pressure needed to achieve preset tidal volume is adjusted as respiratory compliance varies. This strategy may minimize the risk of ventilatorinduced lung injury and maximize patient comfort. (See Table 8-2 for a comparison of common ventilator strategies.)
Setting Conventional Ventilator Parameters
Tidal volume: The average resting tidal volume for a spontaneously breathing, nonintubated child is 5-7 mL/kg with larger “sigh” breaths periodically interspersed; average adult tidal volumes are 350-600 mL depending on lung size. Modern ventilators no longer require larger preset tidal volumes to compensate for inadequate gas delivery secondary to ventilator circuit compliance; therefore, physiologic tidal volumes are chosen (typically 6-8 mL/kg given lack of “sigh” breaths). An adequate tidal volume should generate adequate chest rise.
Rate: A physiologic norm for age is selected and then adjusted with particular attention paid to the patient’s ability to fully exhale and gas exchange.
Inspiratory time: A physiologic, age-specific time is selected, resulting in an average inspiratory:expiratory ratio of 1:2. Reasonable starting points are 0.4-0.5 seconds for infants, 0.6-0.8 seconds for younger children, and 0.8-1.2 seconds for adolescents and adults. Particular attention must be paid to the patient’s respiratory rate and inspiratory time such that full exhalation is achieved between breaths. In patients with obstructive pulmonary disease, this physiology is particularly important and may necessitate lower than normative set respiratory rates on a ventilator.
Positive end expiratory pressure (PEEP): Depending on the patient’s lung compliance and the need for intubation, the PEEP should be adjusted to maintain lung recruitment at functional residual capacity, which is the starting lung volume at which lung compliance is optimal. A starting value of 5 cm H2O is often sufficient
for most patients with reasonable lung compliance; increases are typically made in 1-2 cm H2O increments. Careful attention must be paid to the hemodynamic effects as excessive PEEP will decrease systemic venous return and therefore right heart preload and consequently cardiac output. In addition, under or overdistention of the lung by suboptimal PEEP will impair gas exchange. Assessment of lung distention is aided by chest radiography.
FiO2: The need for supplemental oxygen is based on the pathophysiology necessitating intubation, and its use will be determined by clinical circumstances and titrated to maintain appropriate oxygen delivery to the body. Attempts should be made to limit its use to nontoxic levels, typically <60%, by also targeting optimal ventilator strategies, airway clearance, and evacuation of pneumothoraces.
TABLE 8-2 Comparison of Common Conventional Ventilator Strategies | ||||||||||||||||||||||||||||||||
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High-Frequency Oscillatory Ventilation
High-frequency oscillatory ventilation (HFOV) is most often used in the pediatric setting as a rescue ventilation strategy for those patients with severe hypoxia or hypercapnia despite optimal conventional ventilator management. The mode of ventilation uses high mean airway pressures to facilitate alveolar recruitment and maintenance with superimposed sinusoidal oscillations achieving small changes in lung volumes at supraphysiologic frequencies (3-15 Hz corresponding to 180-900 cycles per minute) to promote favorable gas exchange. This form of ventilation may also induce less ventilatory associated lung injury by minimizing lung stretch.
Setting HFOV Parameters
Mean Airway Pressure: Main determinant of oxygenation. This pressure is typically set 5 cm H2O above that used during conventional mechanical ventilation and increased until adequate oxygenation is achieved. When weaning pressure in HFOV, the mean airway pressure is typically decreased in increments of 1 cm H2O transition to conventional ventilation is considered when mean airway pressure needed to achieve optimal gas exchange is feasible on a conventional ventilator (typically <20 cm H2O).
ΔP: The amplitude (i.e., size of the oscillations) is a key determinant of ventilation and is adjusted to achieve adequate gas exchange and vibration (“jiggle”) of the patient, typically targeted to the level of the groin. Incremental adjustments to the amplitude are typically made by 2-3 cm H2O.
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