637CHAPTER 54 Mechanical Ventilation and Respiratory Care resolved by setting the trigger sensitivity low enough to capture the patient’s efforts but not result in autotriggering (see follow ing discu[.]
CHAPTER 54 Mechanical Ventilation and Respiratory Care resolved by setting the trigger sensitivity low enough to capture the patient’s efforts but not result in autotriggering (see following discussion) Delayed triggering: Delayed triggering is defined as a lag from sensing the trigger to delivering the mechanical breath (see Fig 54.9) This is usually intrinsic to the trigger sensitivity and electronic response of the ventilator Autotriggering: When a breath is delivered neither in response to a scheduled event nor triggered by a patient effort, it is referred to as autotriggering (see Fig 54.9) It is best detected with esophageal monitoring, in which a breath is delivered in the absence of a diaphragmatic contraction Ventilator factors associated with autotriggering are a low triggering threshold, circuit leak, leak around endotracheal tube, and water in the circuit Patient-related factors include cardiac oscillation and low respiratory drive Double triggering: When two consecutive inspirations occur within an interval of less than half of the mean inspiratory time, it is referred to as double triggering (see Fig 54.9) The usual reason for double triggering is a mismatch, with the neural inspiratory signal being longer than the machine’s inspiratory time This results in the second breath being triggered after the first breath has been delivered Double triggering can also occur when a pressure-supported breath has a high termination criterion When these breaths are stacked, the peak inspiratory pressure may be increased by the second breath While missed trigger events are due to the low patient effort relative to the trigger threshold, double triggering is usually due to excessive patient demand or effort Double triggering may also occur with sighs or coughing Reverse triggering: Reverse triggering occurs when the ventilator delivers a breath not triggered by the patient (usually a timetriggered breath) and the distension of the lungs causes the diaphragm to contract, triggering a spontaneous breath In this setting, the ventilator may sense the pressure reduction in the circuit and prolong the flow into the patient, delivering larger than intended tidal volumes If the ventilator does not recognize the pull of flow into the patient, it may prematurely terminate the breath while the patient is still trying to inhale Asynchrony During Inspiration (Flow Asynchrony) Once a breath is initiated, the flow of gas into the patient must match the needs of the patient, providing a “physiologic” breath The most common form of flow asynchrony occurs when the patient’s demand exceeds the delivered flow.37 The pressure-time curve in ventilator graphics display can be useful in assessing flow asynchrony A commonly witnessed pattern for inadequate flow is an M-shaped flow pattern as the patient maximizes the available flow, slightly exhales, and then inhales again to the maximal available flow (see Fig 54.9) Less common is a delivered flow that is in excess of the patient’s needs, resulting in larger Vt Asynchrony During Cycling Cycling asynchrony occurs when the patient’s desire to exhale and ventilator cycling criteria are mismatched.37 Premature cycling refers to the ventilator terminating inspiration while the patient is still maintaining an inspiratory effort This most commonly occurs in the presence of a leak in the ventilator circuit or around the endotracheal tube, which will often lead to air hunger Delayed cycling refers to the prolongation of inspiration beyond the 637 start of the patient’s expiration This may be associated with adverse effects such as increasing hyperinflation and intrinsic PEEP due to a shortened expiratory time as well as patient discomfort.38 Use of Neuromuscular Blockade Neuromuscular blockers are often used as adjuncts to mechanical ventilation Although spontaneous breathing should be encouraged whenever possible, respiratory muscle paralysis may become necessary at times The indications for the use of neuromuscular blocking agents during mechanical ventilation include (1) unmanageable asynchrony between the patient and ventilator; (2) entities in which there is a need to decrease oxygen demand of skeletal muscles, especially in patients with hemodynamic instability or severe heart failure; or (3) prevention of coughing, especially in patients with intracranial hypertension or after complex abdominal or airway surgery The benefit of neuromuscular blockade early in the disease course of adults with severe ARDS remains a matter of debate.39,40 Neuromuscular blockade may also be considered in patients when ventilation is to be controlled such that a specific targeted minute ventilation is delivered Prolonged neuromuscular blockade should be avoided, as it tends to promote muscle atrophy and debility This, in turn, will generally prolong weaning from mechanical ventilation High-Frequency Ventilation High-frequency ventilation (HFV) refers to diverse modes of ventilation generally characterized by a supraphysiologic ventilatory frequency and Vt less than or equal to physiologic dead space Five distinct methods of HFV are recognized: highfrequency positive pressure ventilation (HFPPV), high-frequency jet ventilation (HFJV), high-frequency oscillatory ventilation (HFOV), high-frequency percussive ventilation (HFPV), and high-frequency chest wall oscillation (HFCWO) Only HFJV, HFOV, and HFPV have been extensively used clinically A version of HFCWO (vest therapy) is used for airway clearance (see pulmonary hygiene section that follows) but not primarily for ventilation The principal theoretical advantage for HFV lies in the ability to ventilate effectively with minimal alveolar stretch This may be coupled with a high mean airway pressure to improve lung expansion and oxygenation or a low mean airway pressure to reduce intrathoracic pressure for patients with air leak syndromes or passive pulmonary blood flow HFV (particularly HFJV) has been shown to support adequate gas exchange with severe pulmonary interstitial emphysema (PIE) in neonates and in the setting of bronchopleural fistula.41 HFOV use in adults has recently declined owing to data demonstrating questionable utility and potential harm.42 Nevertheless, HFOV remains in use in many neonatal and pediatric ICUs and is the subject of an ongoing clinical trial in severe PARDS.34 Mechanism of Gas Flow in High-Frequency Ventilation in the Normal Lung The exact primary mechanism of gas transport in HFV remains unclear, but it is likely that HFV combines multiple forms of gas transport.43,44 Mechanisms involved in gas transport during HFV include accelerated axial dispersion, increased collateral flow through pores of Kohn, intersegmental gas mixing or pendelluft phenomenon, Taylor dispersion, asymmetric gas flow profiles, 638 S E C T I O N V Pediatric Critical Care: Pulmonary and gas mixing within the airway due to the nonlinear pressurediameter relationship of the bronchi High-Frequency Jet Ventilation HFJV refers to delivery of inspiratory gases through a jet injector at a high velocity into the trachea with interruption of the flow stream at a rate of 100 to 400 cycles/min (Fig 54.10) The primary indication for HFJV is air leak syndrome, but HFJV may be used successfully in patients with bronchiolitis, congenital diaphragmatic hernia, congenital heart disease, and other disease states The main controls in HFJV are the driving pressure, inspiratory time, and rate The driving pressure is usually coupled with CO2 CO2 CO2 • Fig 54.10 High-frequency jet ventilation CO2, Carbon dioxide PEEP (provided through a separate bias flow circuit) to achieve a desired mean airway pressure and chest wall oscillation and is then adjusted according to the level of lung expansion and gas exchange The typical rate is 320 to 480 breaths/min, although rates as low as 240 breaths/min can be used for those patients needing a longer expiratory time The optimal rate is dependent on the child’s size and disease process Tidal volume increases with higher driving pressure and increased inspiratory time In certain circumstances, conventional ventilation can be combined with HFJV to provide “sigh breaths” of approximately to 10 mL/kg to prevent atelectasis and maintain lung volumes during HFJV It should be noted that the need for these sigh breaths can often be eliminated with the use of an adequately high mean airway pressure Gas trapping can be minimized with an inspiratory time close to 20%, longer expiratory time (i.e., lower rate) and lower driving pressures High-Frequency Oscillatory Ventilation HFOV refers to ventilation by oscillatory flow with alternating positive and negative pressures in the airway with Vt to mL/ kg and frequency to 15 Hz HFOV is most commonly used as a rescue therapy for restrictive lung diseases, such as ARDS, that are refractory to conventional ventilation.43 Oscillations are produced by a diaphragm connected to a piston (Fig 54.11) The main controls in HFOV are mean airway pressure, oscillatory pressure amplitude (power), frequency, and inspiratory time Mean airway pressure determines lung volume and is the primary determinant of oxygenation along with Fio2 Oscillatory pressure amplitude is the change in pressure around the mean airway pressure produced by forward and backward displacement of the piston These pressures are attenuated in the distal airways owing to the impedance of the endotracheal tube and proximal airways Vt is determined by the amplitude and duration of each stroke Oscillatory amplitude is a primary determinant of Vt and therefore minute ventilation as well Frequency is usually set in the range of to 12 Hz For a given amplitude, a lower frequency will increase Vt and improve minute ventilation but will also result in less attenuation of pressures along the airways; the larger effective alveolar Vt is therefore less lung protective Inspiratory time is generally set at 33% of the total cycle time A higher inspiratory time will also result in a larger effective tidal volume but can greatly increase the risk of gas trapping Bias flow To patient Magnetically driven piston Expiratory valve • Fig 54.11 High-frequency oscillatory ventilation CHAPTER 54 Mechanical Ventilation and Respiratory Care 639 Convective pressure rise VDR Pressure Pulsatile flow rate PEEP Oscillatory PEEP Time • Fig 54.12 High-frequency percussive ventilation High-Frequency Percussive Ventilation HFPV is most commonly used by burn centers for inhalation injury, as it combines aspects of HFJV and conventional ventilation and adds the secretion mobilization benefits of percussive ventilation It may also be used for ARDS, plastic bronchitis, and other disease states.45 In HFPV, gas flow is pneumatically driven and delivers subphysiologic Vt at high rates (up to 500 breaths/min) using the volume diffusion respirator High- and low-pressure circuits attach to a system called the Phasitron, which is a sliding Venturi that acts as both an inspiratory and expiratory valve There are seven control variables: (1) peak inspiratory pressure, (2) PEEP, (3) CPAP, (4) inspiratory time, (5) expiratory time, (6) percussive frequency, and (7) rate The Vt delivery is a product of the peak inspiratory pressure setting with subtidal volumes produced by the oscillatory function During inspiration, lung volumes are progressively increased in a controlled, stepwise fashion by repetitively diminishing subtidal volume deliveries until an oscillatory plateau is entered and maintained (Fig 54.12) At the end of inspiration, the lung is allowed to empty passively (with continued oscillations) until the preset expiratory baseline is reached Adverse Effects of Mechanical Ventilation The success of mechanical ventilation hinges on the balance of beneficial and deleterious effects The beneficial effects in the lung are related to improvements in pulmonary mechanics and gas exchange, which are often seen immediately Ventilator-induced lung injury may not be immediately appreciated but is closely tied to meaningful clinical outcomes; short-term improvement in gas exchange will typically not lead to improved outcomes if achieved with toxic ventilator settings Airway Injury Oropharyngeal and nasopharyngeal injuries secondary to the endotracheal tube are uncommon but may include ulceration of the ala nasi from pressure necrosis following prolonged nasotracheal intubation, or ulceration may occur at the angles of the mouth from tight taping of orotracheal tubes Palatal grooves and traumatic cleft palate can occur in infants Laryngeal injury may extend from minor swelling to ulceration of the mucosa of the vocal cords and aryepiglottic folds Similarly, injuries in the subglottic region may extend from minor swelling to major ulceration, and healing of severe injuries may lead to scarring or granuloma formation with airway obstruction The majority of the subglottic tracheal lesions are due to compression of the tracheal mucosa by the endotracheal tube High-pressure cuffs, low cardiac output state, upper respiratory tract infection, duration of intubation, and head-neck movement all increase the risk of tracheal injury However, these injuries have become less common with modern endotracheal tubes.46 Airway injury can also result from suction catheters Necrotizing tracheobronchitis is a severe form of airway injury seen in patients on mechanical ventilation, which is characterized by extensive ulceration and mucosal damage The sequelae of tracheal injuries include tracheal stenosis, tracheomalacia, tracheoesophageal fistula, and tracheoinnominate artery fistula Injury to the airway can be prevented by attention to several details The endotracheal tube should be of the proper size and should be inflated with less than 20 cm H2O to avoid pressure necrosis in the adjacent tracheal mucosa Excessive pressure on the skin should be avoided while taping the endotracheal tube Excessive patient movement should be prevented by targeted sedation Suctioning should be gentle, preferably with a catheter with multiple side holes, and suction catheters should not be routinely advanced beyond the tip of the endotracheal tube Effects on the Lung Adverse effects of mechanical ventilation on the lung may be due to the following factors: (1) high airway pressures, (2) overdistention of the alveoli, (3) cyclic closing and reopening of alveoli, (4) inflammation and cytokine exposure, (5) altered mucociliary clearance, (6) impaired lung water clearance, and (7) oxygen toxicity In many cases, it may be difficult to discern the contribution of lung injury from the ventilator from the pathologic effects of the underlying disease process(es) Pulmonary barotrauma is a general term that encompasses many entities of parenchymal injury Increased airway pressure may cause hyperinflation of the alveoli, increased alveolar dead space, impaired venous return, compressed alveolar vessels, and risk for air leak syndromes Alveolar rupture from overdistended alveoli is the most clinically apparent manifestation of pulmonary barotrauma Air leak may occur from the lung into the pleura (pneumothorax), interstitium (PIE), mediastinum (pneumomediastinum), pericardium (pneumopericardium), 640 S E C T I O N V Pediatric Critical Care: Pulmonary peritoneal cavity (pneumoperitoneum), and subcutaneous tissue (subcutaneous emphysema) Even though the term implies high airway pressures as the main mechanism of parenchymal injury, pulmonary barotrauma is often multifactorial The physiologic consequences of extra-alveolar air may range from no adverse effect to life-threatening cardiorespiratory compromise A pneumothorax may be small and inconsequential or may be large and under tension, necessitating immediate evacuation of the pleural air PIE may decrease lung compliance and increase pulmonary vascular resistance Pneumomediastinum will typically track along fascial planes either cephalad to produce subcutaneous emphysema or caudad to produce pneumoperitoneum or pneumoretroperitoneum Pneumomediastinum rarely requires evacuation and will self-resolve once the impetus for air leak has been halted Pneumopericardium can range from a minimally inconsequential amount of air to life-threatening cardiac tamponade Cardiovascular compromise is an indication for immediate evacuation of pericardial air An uncommon but important form of pulmonary air leak is a bronchopleural fistula, in which a fistulous track develops between the bronchus and pleural space This results in an almost continuous flow of air from the airway into the pleural space The fistula flow is wasted ventilation and may result in hypercarbia Attempts to increase minute ventilation by increasing Vt will only serve to increase the fistula flow by increasing the pressure gradient across the fistula If attempts to decrease airway pressures with conventional ventilation are not possible without compromising gas exchange, a trial of HFV may be considered Barotrauma can be minimized by avoiding factors that predispose to pulmonary air leakage The principal factors that can be controlled are airway pressures and lung volumes As long as acceptable gas exchange is maintained, every effort should be made to reduce airway pressures to a minimum Hyperinflation must be avoided When the lung disease is severe, deliberate hypercarbia may be tolerated provided that the arterial pH is acceptable.13 Inspired oxygen concentration should be maintained at nontoxic levels (usually ,0.50) Ventilator-induced lung injury is described in more detail in Chapter 48 Effects on the Circulatory System The cardiovascular effects of positive intrathoracic pressure are complex and depend on many factors, including the underlying lung disease etiology, uniformity of lung disease, transmission of airway pressure to the pleural space, and lung volume Positive intrathoracic pressure impedes right ventricular filling by decreasing the pressure gradient for systemic venous return Positive airway pressure can also increase pulmonary vascular resistance when the airway pressure significantly exceeds FRC Positive intrathoracic pressure has also been shown to decrease left ventricular afterload The net effect is a combination of all effects mentioned earlier and the reflex cardiovascular adjustments that accompany these changes See Chapter 32 for more details on cardiopulmonary interactions Specialty Gases Inhaled Nitric Oxide Nitric oxide is a potent vasodilator; inhaled nitric oxide (iNO) produces selective pulmonary vasodilation in any segment of ventilated lung Indications for iNO include primary pulmonary hypertension, pulmonary hypertension after repair of congenital heart disease, congenital diaphragmatic hernia, and isolated right heart failure Randomized controlled studies have shown that iNO safely improves arterial oxygen levels in babies with pulmonary hypertension and decreases the need for extracorporeal membrane oxygenation (ECMO) therapy.47 In multicenter studies in children and adults with ARDS,48,49 there were no differences in ventilator-free days and no effect on mortality between treatment groups Physiologically, iNO preferentially vasodilates capillaries in well-ventilated alveoli, improving V/Q mismatch and oxygenation, but not affecting the underlying disease pathology As such, iNO may be used as a bridge to a different therapy for ARDS (e.g., HFV or ECMO) but should not be employed routinely Not all patients respond to iNO; after a trial period, iNO should be continued only in those patients who show a clinical response Nitric oxide also serves as an oxidizing agent to convert hemoglobin to methemoglobin; therefore, methemoglobin levels should be monitored during its administration Helium-Oxygen Mixture Helium-oxygen (heliox) mixture has a much lower density as compared with oxygen-nitrogen mixture This results in reduced resistance to breathing during turbulent flow states and makes low-resistance laminar flow more likely.50 On the basis of the physics of airflow and the properties of heliox, the following behaviors can be predicted with its use: (1) heliox will result in a higher flow when transairway pressures are held constant, (2) heliox will result in a lower airway pressure when airflow is constant, (3) density-dependent flow meters will underestimate flow, (4) heliox can decrease the degree of air trapping and hyperinflation associated with lower airway obstruction, (5) heliox can decrease the work of breathing, and (6) heliox can result in better deposition of aerosols.51 Helium is usually administered through a tight-fitting face mask, high-flow nasal cannula, or invasively through a heliox-compatible ventilator Several studies have shown that heliox administration improves symptoms and relieves the respiratory distress associated with upper airway obstruction due to viral croup, subglottic narrowing, and postextubation stridor.52,53 Studies of heliox in children with asthma demonstrate improved delivery of aerosolized bronchodilators with the addition of heliox.51,54 Most clinical studies also show improvement in clinical scores of respiratory distress, relief of wheezing, and faster resolution of symptoms, but fail to show consistent improvement in other clinical outcomes, such as admission rates or length of stay.51,54 Heliox has also been used in acute bronchiolitis in infants, with improvement in the clinical score and work of breathing.55 Altering Pulmonary Vascular Resistance With Adjusted Inspired Oxygen and Carbon Dioxide Concentrations Low alveolar oxygen tension increases pulmonary vascular resistance (hypoxic pulmonary vasoconstriction), while high alveolar oxygen tension decreases pulmonary vascular resistance In states of increased pulmonary vascular resistance, such as pulmonary hypertension, increasing the Fio2 can be a powerful vasodilator as long as the vascular bed has normal reactivity Similarly, with certain types of “mixing” congenital heart lesions, such as hypoplastic left heart syndrome, it is critical to control pulmonary CHAPTER 54 Mechanical Ventilation and Respiratory Care blood flow and prevent pulmonary overcirculation One approach is to decrease the Fio2 to less than 0.21 with a blending of room air with nitrogen Several studies have shown that hypoxic gas mixtures can be used both preoperatively and postoperatively to balance the pulmonary and systemic circulations, but this approach has fallen out of favor due to concerns for combining hypoxia with a baseline state of reduced oxygen delivery Another approach, especially in patients undergoing mechanical ventilation either preoperatively or postoperatively, is to increase the fraction of inspired CO2 concentration (Fico2).56 Increased Fico2 and the concomitant increase in Paco2 also increases pulmonary vascular resistance and limits pulmonary blood flow One of the difficulties with a boost in Fico2 is increased spontaneous ventilatory drive due to an increased Paco2 This increases the work of breathing and, with marginal cardiac reserve, may impose undue strain on the heart Therefore, neuromuscular blockade and total ventilatory support may be necessary with increased Fico2 to avoid an increased workload on the heart Respiratory Care During Mechanical Ventilation Pulmonary Hygiene The primary objectives of airway clearance therapy are to prevent and treat atelectasis from mechanical obstruction of airways and remove toxic substances, including infective materials, proteolytic enzymes, and other mediators of inflammation The most effective method of clearing secretions is a combination of changing body position and vigorous coughing by the patient If the patient is unable to cough effectively, chest physiotherapy with or without active suctioning of the trachea may be beneficial Handheld mechanical devices may be used in lieu of a caretaker’s hands for chest percussion or vibration Hyperoxygenation prior to endotracheal suctioning can help mitigate associated desaturations and potential hemodynamic changes Chest physiotherapy refers to a variety of respiratory maneuvers performed to aid in the clearance of airway secretions and promotion of lung expansion, including (1) postural drainage, (2) chest percussion and chest vibration, and (3) deep breathing exercises The efficacy of chest physiotherapy in intubated patients is unclear.57 Several devices have also been used as an adjunct to standard chest physiotherapy: the intrapulmonary percussive ventilator (IPV); mechanical insufflator-exsufflator (CoughAssist, Philips Respironics); FLUTTER (Axcan Pharma) mucus clearance device and Acapella devices; intermittent positive pressure breathing (IPPB); mechanical percussors; and percussive vest devices The IPV device delivers high-flow jets of air to the airways by a pneumatic flow interrupter at a rate of 100 to 300 cycles/min through a mouthpiece The patient controls variables such as inspiratory time, peak pressure, and delivery rates IPV has been shown to be beneficial for secretion clearance (particularly for cystic fibrosis patients) and improvement in atelectasis in intubated patients.58,59 CoughAssist is a portable, electric mechanical insufflation-exsufflation device that attempts to simulate a cough by using a blower and valve to alternately apply a positive and then a negative pressure to a patient’s airway to assist the patient in clearing retained bronchopulmonary secretions This approach has been shown to be of particular benefit in patients with neuromuscular weakness.60 The FLUTTER and Acapella devices are small, handheld devices that provide positive expiratory 641 pressure (PEP) Exhaling through the device creates oscillations in the airway, resulting in loosening of mucus Other PEP devices are used with a small volume nebulizer and function in conjunction with medication delivery There is no clear evidence that PEP is more or less effective than other forms of physiotherapy IPPB devices use pressure to passively fill the lungs in conjunction with a patient’s breath and may also nebulize inhaled medication A high-frequency chest wall vibrating/oscillating vest device has been shown to mobilize secretions in patients with cystic fibrosis and is commonly used as an adjunct airway clearance device in children with a reduced ability to clear secretions due to neuromuscular abnormalities.60,61 Humidification Systems During spontaneous breathing, inspired air is warmed and almost completely humidified as it passes through the upper airways The use of an endotracheal or tracheostomy tube bypasses the natural warming and humidifying functions of the upper airway, leaving the mucosal surface below the artificial airway to provide both humidification and heat to the inspired air This may adversely affect mucociliary clearance, and delivered dry air may result in irritated, friable mucosa Also, if the inspired gases are not warmed to the body temperature, insensible water loss in the lung is increased Humidifiers can be classified into those that provide only humidity and those that provide both heat and humidity The nonheated designs are pass-over or blow-by, bubble, and jet humidifiers In the clinical setting, the amount of humidity provided by these simple humidifiers is about the same and is determined by time of contact with the gas and water, temperature of both the gas and water, and surface area of contact of the gas-water interface The efficiency of humidification increases as the time of contact and/or surface area of contact increases Heating the gas prior to humidification allows for higher relative humidity Aerosol Therapy Aerosolized drug administration is used for the delivery of medications, including b2-agonists, atropine, ipratropium bromide, cromolyn sodium, antiviral agents, corticosteroids, antibiotics, surfactant, pentamidine, and mucolytics Aerosolization increases the therapeutic index of the drug by delivering it directly to the site of action while minimizing systemic side effects The factors that affect deposition of aerosol particles are gravity, viscosity of the gas, kinetic activity of the particles, particle inertia, physical nature of the particle, temperature and humidity of the aerosol, and the ventilatory pattern Compared with adults, deposition of aerosolized particles in infants and children is poor because of the small airway caliber, relatively greater airway resistance, high respiratory rate with a short inspiratory time, increased chest wall compliance, ineffective coordination effort, and inconsistent breath-holding maneuvers Four types of aerosol delivery systems are available for clinical use: jet or pneumatic nebulizers, ultrasonic nebulizers, metereddose inhalers, and dry-powder inhalers.62 A jet nebulizer uses the Bernoulli principle to create an aerosol The size of the particle depends on the jet flow rate and size of the capillary tube Baffles placed in the path of the aerosols tend to remove larger particles, allowing delivery of smaller particles to the patient A pneumatic nebulizer creates an aerosol using the same principle as the jet nebulizer but may use the main gas flow or a side stream nebulizer ... mixture has a much lower density as compared with oxygen-nitrogen mixture This results in reduced resistance to breathing during turbulent flow states and makes low-resistance laminar flow more... boost in Fico2 is increased spontaneous ventilatory drive due to an increased Paco2 This increases the work of breathing and, with marginal cardiac reserve, may impose undue strain on the heart Therefore,...638 S E C T I O N V Pediatric Critical Care: Pulmonary and gas mixing within the airway due to the nonlinear pressurediameter relationship of the bronchi High-Frequency