632 SECTION V Pediatric Critical Care Pulmonary triggered, the ventilator attempts to reach the pressure support setting as quickly as possible As this pressure level is reached, the ventilator’s micr[.]
632 S E C T I O N V Pediatric Critical Care: Pulmonary triggered, the ventilator attempts to reach the pressure-support setting as quickly as possible As this pressure level is reached, the ventilator’s microprocessor determines the volume that has been delivered from the machine, compares this measurement with the desired Vt, and determines whether the minimum desired Vt will be reached If the ventilator determines that the ultimate delivered Vt will be equivalent to the set Vt, the breath is delivered as a pressure-support breath If the ventilator determines that the ultimate delivered Vt will be less than the set Vt, the breath changes from a pressure-limited to a volume-limited breath Automatic Tube Compensation Automatic tube compensation (ATC) is a technique for overcoming the imposed work of breathing caused by the increased resistance of artificial airways This is accomplished by using the known resistive characteristics of the artificial airways ATC is essentially pressure support in which the ventilator adjusts the delivered pressure to compensate for the imposed work of breathing by the artificial airways and the flow demand of the patient.16 To select the level of ATC required, the operator inputs the type of tube, endotracheal or tracheostomy, and percentage of compensation desired (10%–100%) Nontraditional Modes of Ventilation Airway Pressure Release Ventilation Airway pressure release ventilation (APRV) is a mode of mechanical ventilation used to improve oxygenation by maintaining an open lung with high mean airway pressures while allowing spontaneous breathing Additional ventilatory assistance is provided with intermittent release breaths, allowing exhalation below the target mean airway pressure for brief moments APRV is essentially pressure-control ventilation using an inverse I:E ratio, with spontaneous breaths allowed during the mandatory breaths (Fig 54.7) The mandatory breaths applied by APRV are time-triggered, pressure-targeted, time-cycled breaths Minute ventilation occurs with both spontaneous breaths and the periodic inflation and deflation that occur between the two levels of airway pressure Four major variables determine an APRV breath: Phigh, Plow, Thigh, and Tlow Phigh is defined as the pressure of the triggered mandatory breath, and Thigh is the duration that Phigh is maintained Plow is the release pressure, and Tlow is the time of the pressure release Phigh is the primary determinant of mean airway pressure and oxygenation; Thigh is inversely proportional to the number of release Thigh breaths and affects both carbon dioxide removal and, to a lesser extent, mean airway pressure Plow is typically set to cm H2O to allow lung recoil during a release breath, yet Tlow is set short enough that alveolar collapse does not occur before lung reinflation.17 Currently, some modern ventilators have incorporated features that allow spontaneous breathing through a demand-flow system during both the inspiratory and expiratory phases of a pressure-controlled, time-cycled breath These may be called by different names, such as biphasic positive airway pressure and intermittent mandatory pressure-release ventilation but are still classified under the acronym APRV Some ventilators also allow pressure support to be provided for the spontaneous breaths during these modes APRV has not been well studied in children Most of the articles published on this mode are case reports or small case series,18–20 mainly as a rescue therapy for refractory hypoxemia Most of the studies have shown improvement in oxygenation, likely related to the higher mean airway pressure with APRV.19,21 Proportional Assist Ventilation Proportional assist ventilation (PAV) is a mechanical ventilation mode that relies on the instantaneous elasticity and flow resistance of the respiratory system When PAV is initiated, the ventilator provides “test breaths” (controlled breaths with fixed flow and volume), which allows estimation of the respiratory system mechanics, minute ventilation, and work of breathing (resistive and elastic ventilator muscle loads) These are repeated at regular intervals to recompute the load estimations These estimations provide inputs for the PAV algorithm The clinician then sets the level of PAV (usually a percentage of the total work) to be provided breath by breath by the ventilator.22 Unlike conventional breaths—in which volume, flow, or pressure is predetermined—in PAV the ventilator applies the pressure in relation to the patient effort after the breath has started As the patient makes an inspiratory effort, the ventilator monitors the instantaneous rate and volume of gas flow, and then amplifies the patient’s effort during the breath by applying additional pressure PAV can be conceptualized as simply continuously adjusting pressure support ventilation (PSV), in which the level of pressure support is attuned as a multiple of the sum of the volume and flow signals The few clinical studies in infants and children demonstrate that PAV can maintain gas exchange with lower airway and transpulmonary pressure and improve thoracoabdominal synchrony but no differences in clinical outcomes.23 Tlow Spontaneous breaths Phigh Inspiratory time Pressure Limit Plow Time • Fig 54.7 Pressure scalar of airway pressure release ventilation CHAPTER 54 Mechanical Ventilation and Respiratory Care Neurally Adjusted Ventilatory Assist Neurally adjusted ventilatory assist (NAVA) is a mode of PSV in which changes in electromyographic activity of the diaphragm provide the trigger and determine the level of support for each breath.22 In NAVA, the ventilator assists each spontaneous breath with pressure that is proportional to the integral of the electrical activity of the diaphragm (EAdi) NAVA uses a specifically designed catheter to measure the transesophageal recording of EAdi For NAVA to be effective, it is critical that the esophageal catheter measuring EAdi be positioned correctly The EAdi signal is used to both trigger and cycle-off the breath; the magnitude of ventilator support is determined by a mathematic function that represents diaphragmatic electrical activity times a gain factor, as shown by the equation Paw NAVA level EAdi where Paw is the instantaneous airway pressure (cm H2O), EAdi is the instantaneous integral of the diaphragmatic electrical activity signal (µV), and the NAVA level (cm H2O/mV or per arbitrary unit) is a proportionality constant or gain factor Similar to PAV, this allows the patient to control the Vt and flow within the breath By coordinating the diaphragmatic activity and ventilator support, the delivered pressure should be synchronous with the patient’s spontaneous breath The level of assistance can be automatically adjusted depending on the changes in neural drive, respiratory mechanics, and inspiratory muscle function As NAVA uses EAdi to estimate respiratory center output, the phrenic nerve must be intact and functioning NAVA is designed to improve patient-ventilator synchrony Pressure, flow, volume, and inspiratory time are not predetermined The ventilator assists the spontaneous breath with pressure applied per millivolt of diaphragmatic electrical activity, and the work of breathing is shared between the patient and ventilator Similar to PAV, when the patient effort increases, the pressure applied by the ventilator during the breath also increases to unload the patient effort in NAVA Studies in infants and children demonstrate that NAVA is feasible and may present advantages in patient-ventilator synchrony and reduced sedation needs.11,24 Ventilation for Selected Underlying Pathophysiology Primary Respiratory Muscle Failure (“Respiratory Pump Failure”) The primary difficulty in these disorders is inadequate ventilation due to weakness of the respiratory muscles (pump failure) Vt and ventilatory rates are set to provide normal minute ventilation to maintain target Paco2 Complete control of ventilation may result in disuse muscle atrophy and complicate weaning from mechanical ventilation Therefore, spontaneous breathing should be encouraged as much as possible Fio2 is usually kept to a minimum (#0.30), as these disorders are not generally associated with inadequate oxygenation PEEP is usually set at a relatively low level (3–5 cm H2O) to allow weak respiratory muscles to generate an adequate driving pressure above PEEP In chronic hypoventilation, hypercarbia is often acceptable, provided that the arterial pH is within an acceptable range Obstructive Lung Diseases Respiratory failure due to lower airway obstruction poses a special problem in that the primary disease process is not relieved by the 633 addition of positive-pressure ventilation These diseases may require substantially longer exhalation times owing to the narrowing of small airways during exhalation If the expiratory time is inadequate to empty the lung, gas trapping often results in an endexpiratory alveolar pressure that is higher than the proximal airway pressure because of incomplete emptying of the alveoli, resulting in “auto-PEEP,” “intrinsic PEEP,” or “inadvertent PEEP.” Gas trapping worsens hyperinflation, with its attendant complications The level of PEEP selected for patients with lower airway obstruction is controversial There are two schools of thought: “low PEEP” and “high PEEP.” Low PEEP advocates usually apply a PEEP of to cm H2O because of the concern for pulmonary barotrauma from air trapping and alveolar hyperinflation Endexpiratory lung volume and therefore the level of alveolar inflation will not be affected by the level of proximal set PEEP as long as it is less than the amount of auto-PEEP Conversely, in adults with severe asthma, high levels of PEEP, which is closer to the level of auto-PEEP, have been shown to decrease the magnitude of air trapping and work of breathing without significant complications.25 The theory is that applied PEEP may be more homogeneously distributed throughout the airways than is intrinsic PEEP (auto-PEEP) In children with tracheomalacia or bronchomalacia, higher levels of PEEP may be necessary to distend the airways and prevent dynamic compression during exhalation Depression of cardiac output with subsequent hypotension may also occur during intubation because of the institution of positive airway pressure to already hyperinflated lungs This causes further impedance to venous return and increased pulmonary vascular resistance, which may be overcome with rapid intravenous fluid administration during this transition Restrictive Lung Diseases Restrictive parenchymal lung disorders, such as ARDS and pneumonia, primarily present with inadequate oxygenation due to diffuse subsegmental atelectasis, V/Q mismatching, and intrapulmonary shunting.26 The most effective method to improve oxygenation is increasing mean lung volume above closing volume by increasing the mean airway pressure through adequate PEEP The degree of intrapulmonary shunting, V/Q mismatching, alveolar edema, alveolar collapse, and decreased compliance is directly proportional to the severity of lung disease Therefore, arbitrary limits cannot be placed on the level of PEEP or mean airway pressure that will be necessary to maintain adequate gas exchange Optimal PEEP should be set above the critical closing pressure of the airways, which will prevent alveoli from closing during exhalation Analysis of static pressure-volume loops can be used to determine optimal PEEP, but this is a tedious process rarely done at the bedside However, dynamic pressure-volume loops may help determine whether alveolar collapse is occurring (Fig 54.8) As the lung is inflated, there may be an abrupt change in compliance, as denoted by the lower inflection point The flat portion of the pressure volume curve prior to this inflection point represents a state of low lung compliance when higher pressures are required to reopen collapsed alveoli Absence of the lower inflection point suggests the absence of alveolar collapse Similarly, as the lung reaches peak inflation, the pressure-volume slope may demonstrate a second period of low compliance, as noted with an upper inflection point It is generally thought the upper inflection point reflects overdistention of the alveoli The general recommendation is to minimize atelectrauma by titrating PEEP to a level above the lower inflection point while minimizing barotrauma by S E C T I O N V Pediatric Critical Care: Pulmonary Volume UIP Volume 634 LIP A PEEP PIP Pressure B PEEP PIP Pressure • Fig 54.8 Pressure-volume loop (A) demonstrating inadequate positive end-expiratory pressure (PEEP) and hyperinflation The presence of a flat, low compliance section at the beginning of the breath prior to the lower inflection point (LIP) represents pressure used to open closed alveoli without lung expansion A second change in compliance at the upper inflection point (UIP) represents “beaking” seen in hyperinflation, in which pressure continued to be delivered to a lung that has reached maximal stretch In loop B, PEEP has been increased to above the closing volume, resulting in disappearance of LIP, and the peak inspiratory pressure (PIP) has been reduced, eliminating the UIP and associated hyperinflation maintaining the end-inspiratory pressure below the upper inflection point It should be noted that the observation of a lower inflection point during dynamic loops taken by the ventilator (as opposed to static measurements) might be caused by factors unrelated to expiratory alveolar collapse (e.g., secretions, edema, tube malposition) Therefore, one should exercise caution when using these measurements to infer the adequacy of PEEP setting As PEEP is increased to maintain an open-lung ventilation strategy, Pplat may reach levels that contribute to ventilator-associated lung injury and pulmonary air leak As mentioned earlier, efforts should be made to maintain Pplat of 28 cm H2O or less to minimize barotrauma In settings of markedly reduced compliance, Vt may need to be targeted to to mL/kg ideal body weight Hypercapnia may be permitted under these circumstances, provided arterial pH is acceptable (permissive hypercapnia) With a high airway pressure maintained throughout inspiration, modes with a decelerating flow pattern provide higher mean airway pressure and maintain a higher mean lung volume as compared with constant flow ventilation.27,28 In the setting of the subsegmental atelectasis commonly seen in parenchymal lung diseases, several techniques of alveolar recruitment have been described in the literature These include manual inflation to high airway pressures, increase of PEEP in a stepwise manner, application of a sigh maneuver, and a combination of titrated levels of PEEP with increased inflation pressures Recruitment maneuvers often result in short-term improvements in oxygenation Unfortunately, not all patients have recruitable lungs; in these patients, sustained inflation or sigh maneuvers may result in hyperinflation and injury of healthier lung segments Prone positioning has been proposed as another strategy to improve gas exchange in patients with ARDS with severe hypoxemia by reducing gravity-dependent alveolar collapse in the larger, dorsal segments of the lung Oxygenation improves in most adult and pediatric patients with ARDS when they are placed prone.29–32 A recent multicenter randomized trial demonstrated that placing adults with severe ARDS in the prone position showed significant mortality benefit (16% vs 32%).33 However, the only randomized control trial of prone positioning in children with acute lung injury/ARDS did not show any difference in the number of ventilator-free days or other secondary end points.29 There are several potential reasons why the same mortality benefit seen in adults in the prone position has not been demonstrated in children Children present with heterogeneous etiologies of pediatric ARDS (PARDS) and have a lower baseline mortality, thus requiring a larger number of subjects to show a statistical difference Additionally, prior pediatric prone positioning studies have not focused solely on severe ARDS Children also have a more compliant chest wall than adults and may demonstrate less positional benefit in lung aeration The recent adult trial has renewed interest in the potential benefit of placing children in the prone position, and a recently initiated multicenter randomized control trial will study both prone positioning and high-frequency oscillatory ventilation in children with severe PARDS.34 Unilateral Lung Disease or Severely Differential Lung Disease Unilateral or asymmetric lung disease in infants and children poses special problems during mechanical ventilation Because of regional differences in compliance and resistance, the time constants for inflation and deflation may vary widely between lung segments, and delivered tidal volume tends to preferentially inflate the more compliant lung while underventilating the stiffer, more affected lung This may result in overinflation of the relatively “normal” lung and redistribute pulmonary blood flow away from the hyperinflated lung, exaggerating the V/Q mismatch and CHAPTER 54 Mechanical Ventilation and Respiratory Care potentially contributing to further barotrauma In such circumstances, simultaneous independent lung ventilation (SILV) may allow each lung to be ventilated according to its needs without affecting the other lung Potential indications for SILV include unilateral atelectasis or consolidation, emphysema, pneumonia, pneumothorax, and bronchopleural fistula In postoperative care, SILV can be used for lung reexpansion after thoracic surgery, correction of V/Q mismatch in the lung remaining dependent during surgery, and treatment of pulmonary complications arising during anesthesia and surgery (e.g., pneumothorax or aspiration syndrome) SILV requires a bilumen tube—one tube being the longer bronchial tube and the other being the shorter tracheal tube Usually, the bronchial tube is advanced into the right main stem bronchus so that the lungs can be ventilated separately SILV requires two ventilators, which permit the application of different ventilation settings for each lung.34 Heart Failure and Postoperative Management of Congenital Heart Disease The goals of respiratory management for congestive heart failure are to (1) prevent and relieve alveolar collapse from alveolar and interstitial edema (due to pulmonary vascular congestion), (2) decrease oxygen demand on the heart through a reduction in the work of breathing, and (3) decrease the afterload of the left ventricle As a general principle, the greater the inotropic support a heart needs, the greater should be the respiratory support provided Adequate PEEP will avoid atelectasis and its associated V/Q mismatch and increase in pulmonary vascular resistance Similarly, hyperinflation should be avoided, as it may also increase pulmonary vascular resistance and increase right ventricular afterload (see Chapter 32 for more detail on cardiopulmonary interactions) Following cardiac surgery, the requirement for mechanical ventilation depends on several factors: patient age, complexity of the cardiac lesion, complexity of the operative procedure, duration of bypass, duration of circulatory arrest, and postoperative cardiopulmonary status Prolonged intubation and mechanical ventilation are more likely in children younger than year of age, especially in those with more complex heart lesions, prolonged bypass and circulatory arrest times, postoperative respiratory failure, and hemodynamic instability.35 Adequate PEEP should be applied to prevent and relieve atelectasis, and support should be adequate to optimize work of breathing As the hemodynamic function improves, the level of support should be weaned as dictated by the clinical status.36 The choice of ventilatory parameters depends on the goals for each patient In patients with pulmonary hypertension or pulmonary vascular disease, mild hyperventilation to provide respiratory alkalosis will decrease pulmonary vascular resistance and right ventricular afterload Patients with marginal cardiac output may require higher levels of support, but a high mean airway pressure may impede systemic venous return In patients with passive pulmonary blood flow, such as those following the Glenn and Fontan operations, airway pressures should be maintained at a minimum, spontaneous ventilation should be encouraged, and extubation should occur as early as clinically possible.36 Diseases With Abdominal Distention Significant abdominal distention displaces the diaphragm superiorly and decreases alveolar lung volumes, especially at the lung 635 bases The resultant reduction in respiratory system compliance can increase the work of breathing during spontaneous breathing Positive pressure ventilation may cause differential hyperinflation of the apical regions, with normal or low volumes at the bases Increased PEEP will help mitigate basilar atelectasis, and therapy should be directed primarily toward reducing the intraabdominal pressure Neurologic and Neuromuscular Diseases Mild hyperventilation is an effective method of reducing intracranial pressure through cerebral vasoconstriction High intrathoracic pressure may impede venous return from the brain by increasing central venous pressure Thus it should be avoided, when possible, in the setting of increased intracranial pressure However, in the setting of multiorgan trauma, elevated mean airway pressure may be required to provide adequate oxygenation A careful balance of the physiologic principles is essential The goals of respiratory support in patients with acute selflimited neuromuscular diseases are to provide respiratory assistance to maintain adequate minute ventilation while minimizing disuse muscle atrophy from mechanical ventilation Spontaneous breathing should be encouraged as much as possible and neuromuscular blockade avoided Patient-Ventilator Asynchrony A primary goal of mechanical ventilation is the coordination of the patient’s respiratory muscles (i.e., spontaneous respiratory effort) and the ventilator (i.e., mechanical support) The patient should not be attempting to inspire when the ventilator is in the expiratory phase and should not be attempting to exhale when the ventilator is attempting to deliver a breath Patient-ventilator asynchrony is defined as a mismatch between the respiratory effort of the patient and ventilator Asynchrony can increase work of breathing, patient discomfort, and lung injury Patients exhibiting asynchrony during mechanical ventilation have shown improved oxygenation and ventilation after neuromuscular blockade, but it must be stressed that adjustments to the ventilator settings can often avoid the need for neuromuscular blockade Asynchrony Associated With Breath Triggering Asynchrony associated with breath triggering can be classified into (1) missed triggering, (2) delayed triggering, (3) autotriggering, (4) double triggering, and (5) reverse triggering.37 The first two forms of asynchrony cause air hunger by not providing a supported breath when the patient desires a breath, while the latter three forms provide mechanical breaths when the patient should not be receiving a breath Missed triggering: Missed triggering refers to a spontaneous breath that is not accompanied by a synchronous assisted/supported mechanical breath (Fig 54.9) Missed triggering results in a patient’s effort that fails to trigger a ventilator breath This is due to a mismatch between patient effort and trigger sensitivity Poor patient effort (often secondary to muscle weakness) results in a breath that is not sufficiently strong to reach the trigger threshold; while a trigger threshold set too high (e.g., insensitive trigger) prevents the patient from reaching the targeted threshold Ineffective triggering can be detected as either a pressure drop in the airway or a deflection in flow not followed by a ventilator breath Ineffective triggering can be 636 S E C T I O N V Pediatric Critical Care: Pulmonary Flow Missed triggers: patient attempting to inhale but does not receive a breath Delayed triggering: a pause between when the patient starts to inhale and ventilator provides a breath Pressure fluctuations may or may not be visible Pressure A Time Autotriggering Flow Double trigger Pressure B Time Patient quickly exhales and tries to take a new breath Flow Trigger Pressure “scooping” Pressure C Time • Fig 54.9 Scalars representing different types of patient-ventilator asynchrony (A) The trigger sensitivity is initially set such that the patient is unable to trigger a breath; a flow deflection signals attempted inhalation without an associated breath Subsequently, the trigger is set such that the patient attempts to inhale, and there is a delay before the ventilator responds with a breath (B) The patient initially experiences a double trigger event, with a second breath triggered prior to beginning exhalation, resulting in additional tidal volume delivered with an increase in pressure The patient then begins to experience autotriggering, with repeated breaths given immediately following exhalation The latter may be easily mistaken for tachypnea, but examination of the patient will reveal no respiratory effort (C) This patient is experiencing flow asynchrony in the form of inadequate flow demand The patient begins to inhale but is unable to draw enough flow to meet one’s needs, slightly exhales, then once again inhales The pressure waveform has a scooped appearance as the patient “sucks” flow from the circuit ... arbitrary unit) is a proportionality constant or gain factor Similar to PAV, this allows the patient to control the Vt and flow within the breath By coordinating the diaphragmatic activity and ventilator... hyperinflated lungs This causes further impedance to venous return and increased pulmonary vascular resistance, which may be overcome with rapid intravenous fluid administration during this transition... resultant reduction in respiratory system compliance can increase the work of breathing during spontaneous breathing Positive pressure ventilation may cause differential hyperinflation of the