Handbook of Clinical Neurology, Vol. 119 (3rd series) Neurologic Aspects of Systemic Disease Part I Jose Biller and Jose M. Ferro, Editors © 2014 Elsevier B.V. All rights reserved
Chapter 19
Acute and chronic respiratory failure SABIN OANA* AND JAYANTA MUKHERJI Department of Anesthesiology, Loyola University Medical Center, Maywood, IL, USA
INTRODUCTION Respiratory failure (RF) is defined as failure of oxygenation and/or carbon dioxide (CO2) elimination. Hypoxemia exists if arterial oxygen tension (PaO2) is below 60 millimeters of mercury (mmHg). Hypercapnia is present if arterial CO2 tension (PaCO2) is above 50 mmHg. There are three common mechanisms that could lead to RF: right to left shunt (cardiac or intrapulmonary), ventilation/perfusion mismatch, and hypoventilation. In addition, low partial pressure of inspired oxygen, diffusion impairment, and high partial pressure of inspired CO2 have also been described as rare causes of RF (Bartter et al., 2011). Another way of looking at the pathophysiology is to imagine the respiratory system as consisting of two parts: the lung and the pump that moves the lung (respiratory centers, spinal cord and respiratory peripheral nerves, respiratory muscles, chest wall). Failure of the lung is primarily manifested as hypoxemia (hypoxemic or type I failure; shunt physiology) and failure of the pump results mainly in hypercapnia (hypercarbic or type II failure; hypoventilation is the principal mechanism). Recent literature mentions a type III or perioperative failure, associated mainly with lung atelectasis, and a type IV, related to hypoperfusion of respiratory muscles in shock (Kress and Hall, 2012). The majority of neurologic diseases that progress to RF are therefore type II failures and are characterized by either a reduced drive to breathe or weak respiratory muscles. Depending on the speed of onset, RF can be classified as acute, acute on chronic, or chronic. RF usually supervenes insidiously in chronic progressive neuromuscular diseases (NMDs) such as amyotrophic lateral sclerosis (ALS). There can also be an acute superimposed factor (e.g., respiratory infection) that precipitates their
deterioration. On rare occasions, however, RF may herald disease onset. More commonly, there are acute conditions that are associated with RF (e.g., Guillain–Barre´ syndrome (GBS)). Other entities such as critical illness neuropathy and myopathy are often first diagnosed in the critical care unit. The initial manifestations of RF usually appear during sleep and, as the diseases progress, they are also present during the daytime. In normal individuals, during sleep, especially in the rapid eye movement state, there is hypotonia of respiratory muscles and of the airway as well as decreased chemosensitivity to CO2. These otherwise normal changes produce an abnormal degree of hypoventilation and hypoxemia in individuals affected by NMD (Chokroverty, 2001). Disruption of any segment of the neural pathway of respiration can produce RF. RF may be classified according to the anatomic site of origin into: central nervous system disorders, spinal diseases, peripheral nervous system dysfunction, and muscle diseases. Previous work reviewed the need for primary mechanical ventilation (MV) in patients affected by peripheral nervous system diseases (Cabrera Serrano and Rabinstein, 2010). The most frequent conditions were myasthenia gravis (MG), GBS, myopathies, and ALS, together accounting for 75% of cases. However, 55% of patients did not have a diagnosis on admission and 10% never had a final diagnosis.
GENERAL APPROACH TO DIAGNOSIS Respiratory symptoms in NMDs include dyspnea or shortness of breath with its variants orthopnea (dyspnea when supine) and platypnea (dyspnea when upright). Very commonly, and generally early in the course of diseases, there are various sleep disturbances (restless
*Correspondence to: Sabin Oana, M.D., Assistant Professor of Anesthesiology, Loyola University Medical Center, Department of Anesthesiology, Bldg 103, Rm 3102, 2160 S 1st Ave, Maywood, IL 60153-3328, USA. Tel: þ1-708-216-8866, Fax: þ1-708-216-1249, E-mail: [email protected]
274
S. OANA AND J. MUKHERJI
sleep, frequent awakenings, nightmares, and migraines) and related symptoms of daytime fatigue. Difficulty in speech and swallowing are markers of bulbar symptoms (i.e., of the cranial nerves that originate in the medulla) that predispose the patient to ineffective cough and aspiration pneumonia. Physical examination can reveal tachypnea, shallow breathing, accessory inspiratory and expiratory muscle contraction (parasternal, sternocleidomastoid, scalene, intercostal, and abdominal) and ribcage–abdominal asynchrony. Of note, the reliability and reproducibility of the clinical examination with neurologic focus of a RF patient are greatly reduced, especially in the intensive care setting. In cases with mild hypoxia, one can encounter headaches or inattentiveness, lethargy or delirium, somnolence or anxiety, tremors or seizures that, untreated, progress toward loss of consciousness and unresponsiveness. Hypoxic-ischemic encephalopathy is a complication of prolonged hypoxia during cardiac and respiratory arrest. A special type is that encountered during birth, which can cause long-term damage including intellectual developmental disorder and cerebral palsy. A low percentage of adults can regain consciousness after cardiopulmonary arrest but still suffer long-term neurologic sequelae, including motor, memory, or personality changes (Dreibelbis and Jozefowicz, 2010). A special type of RF occurs when the inspired oxygen is low, as, for example, during climbing at high altitudes. Acute mountain sickness can progress to high altitude cerebral edema and is manifested as dizziness, somnolence, confusion, and decreased consciousness. Arterial blood gas analysis is one the first steps in the characterization and differential diagnosis of RF. It entails measuring pH, PaO2, PaCO2, and bicarbonate.
These should provide enough data to diagnose an acute versus chronic, respiratory versus metabolic acid–base disorder. Once the respiratory component is assessed, there are multiple indices of hypoxemia that can be calculated to aid in the more detailed diagnosis of RF (i.e., differentiating pulmonary versus extrapulmonary causes). Each has its own merits and limitations. Tension-based indices are based on alveolar air equation and content-based indices are based on oxygen content equation (Siggaard-Andersen and Gothgen, 1995; Wandrup, 1995) (see Table 19.1). Electrophysiologic studies consist of nerve conduction studies (motor and sensory) as well as needle electromyography (EMG). More specialized testing includes: neuromuscular junction testing with repetitive nerve stimulation, single-fiber EMG and train of four stimulation, respiratory EMG with percutaneous or needle stimulation of phrenic nerve and diaphragm, and direct muscle stimulation (Dhand, 2006). In axonal disease there is normal conduction but reduced amplitude, whereas in demyelinating disease there is slow conduction with normal potential amplitude. For needle EMG the neurogenic pattern shows spontaneous activity, large potentials with mild contraction and less than complete recruitment and interference with full contraction. The myopathic pattern might demonstrate some spontaneous activity with small potentials during mild contraction but small amplitude and full interference patterns with full stimulation. Pulmonary function testing in NMDs encompasses spirometry, lung volume and capacity, and measurement of respiratory muscle strength (Aboussouan, 2005). The most common clinical picture is that of a restrictive respiratory disorder, albeit with some
Table 19.1 Oxygen parameters* Equation
Formula
Alveolar pressure of oxygen
PaO2 ¼ FiO2 (Patm PH2O) PaCO2/RQ or PaO2 700 FiO2 1.2 PaCO2 CaO2 ¼ 1.34 Hgb SaO2 þ 0.03 PaO2 Qs/Qt ¼ (CcO2 CaO2) (CcO2 CvO2) or Qs/Qt ¼ (1 SvO2) (1 SaO2) Vd/Vt ¼ (PaCO2 PeCO2) PaCO2
Arterial oxygen content equation Shunt equation
Dead space fraction
Normal values
Less than 5% (0.05)
Below 0.33(33%)
*Where: PaO2, alveolar partial pressure of oxygen in mmHg; FiO2, inspired fraction of dry oxygen; Patm, atmospheric pressure (760 mmHg at sea level); PH2O, partial pressure of water vapor (47 mmHg); PaCO2, alveolar partial pressure of CO2 in mmHg; RQ, respiratory quotient or CO2/O2 exchange ratio (approx 0.8); Hgb, hemoglobin concentration; CaO2, CcO2, CvO2 are oxygen content of arterial, capillary, and mixed venous blood; SvO2 and SaO2 are the saturation of hemoglobin in mixed venous blood and, respectively, arterial blood; PeCO2, partial pressure of expired CO2.
ACUTE AND CHRONIC RESPIRATORY FAILURE particularities. Weakness in expiratory muscles generates a decreased expiratory reserve volume but with preserved functional residual capacity. In consequence there is elevated residual volume. Associated weakness in inspiratory muscles decreases the inspiratory reserve volume. The overall result is a marked decrease in vital capacity (VC) with a preserved total lung capacity. Diaphragmatic dysfunction has a characteristic 20–50% decrease in VC when supine. Maximal voluntary ventilation could be reduced. Strength of the respiratory muscles is assessed by measuring the maximum inspiratory and expiratory pressures (MIP and MEP). More recently, sniff inspiratory pressures (nasal (SNIP), esophageal, and gastric) have also been employed to provide data regarding the respiratory muscles. Transdiaphragmatic pressure is the difference between the esophageal and gastric pressures and is sensitive for diaphragmatic dysfunction (Steier et al., 2007). Genetic testing is usually recommended in inherited diseases with specific presentations like familial ALS, spinal and bulbar muscular atrophy, Charcot–Marie–Tooth (CMT) 1A, myotonic dystrophy, Duchenne muscular dystrophy (DMD) (Burgunder et al., 2011), and congenital central hypoventilation syndrome (CCHS).
SPECIFIC DISEASES Stroke RF in stroke occurs in about 2–4% of patients and is an ominous sign, traditionally being associated with high mortality. The treatment of such patients should be performed in acute stroke units, resulting in a decrease in mortality (Gattellari et al., 2009). Younger patients with posterior circulation involvement have better prognosis (Rabinstein and Wijdicks, 2004). Common indications for MV in stroke patients are neurologic reasons (coma, loss of brainstem reflexes, brain edema with impending herniation), general deterioration in clinical condition, cardiopulmonary causes (pneumonia, aspiration, pulmonary embolism), or elective
275
for neuroradiologic or neurosurgical procedures (Steiner et al., 1997). There are several patterns of impaired breathing described after stroke that could help in the localization of stroke (Brazis et al., 2011) (see Table 19.2). Upper airway obstruction is common in stroke patients, obstructive sleep apnea (OSA) being considered both a risk and a consequence for stroke. In OSA patients with stroke, continuous positive airway pressure (CPAP) should be offered, as it reduces mortality (Martı´nez-Garcı´a et al., 2009). Dysphagia is very frequent and can be diagnosed early and safe with the help of flexible fiberoptic endoscopy (Warnecke et al., 2009). More recent studies found that aggressive treatments with thrombolysis, decompressive hemicraniectomy, MV, and feeding tubes are achieving functional independence for an unexpectedly high number of patients (Seder and Mayer, 2009).
Amyotrophic lateral sclerosis ALS is being increasingly recognized as a disease of diverse genotype, phenotype, and rate of progression (El Escorial Criteria Revisited) (Brooks et al., 2000). RF is rare at the onset, but is the most frequent cause of death. It signifies degeneration of the center and/or of the neurons of the phrenic nerve (Kiernan et al., 2011). Respiratory symptoms in ALS are dyspnea on exertion, orthopnea, disturbed sleep, daytime somnolence, and vivid nightmares (Wijesekera and Leigh, 2009). Respiratory signs in ALS include tachypnea, use of accessory muscles, and paradoxical movement of the abdomen. As far as respiratory investigations are concerned, forced vital capacity (FVC) is increasingly recognized as being an insensitive marker for early RF. In order to predict the need for noninvasive ventilation (NIV), recent focus has been on measurements such as nocturnal oximetry, nocturnal desaturations less than 90% for more than 1 full minute, MIP, supine FVC, transdiaphragmatic pressure, SNIP. Common indications for NIV are: orthopnea, SNIP 35 cmH2O) leading to volutrauma. Lower TVs of 6–8 mL/kg limit plateau pressures, prevent release of inflammatory mediators, and reduce ventilatorinduced lung injury. Lung protective strategies in ARDS include using lower TV and higher PEEP to avoid alveolar overdistention and derecruitment (Petrucci and Iacovelli, 2007). PEEP prevents alveolar derecruitment by restoring the functional residual volume to the physiologic range. Permissive hypercapnia is a strategy whereby lungs are protected during MV by adoption of deliberate alveolar hypoventilation. Hypercapnic acidosis is a consequence of this strategy and not a goal. Lung protective ventilation impairs CO2 clearance and may lead to right heart dysfunction or increased intracranial pressure. Extracorporeal CO2 removal has the potential to optimize lung protective ventilation by uncoupling oxygenation and CO2 clearance (Cove et al., 2012).
Complications in mechanically ventilated patients Respiratory complications may result from MV itself. A bedside evaluation of patient, airway, and ventilator settings to monitor for ventilator–patient dysynchrony, oxygenation, gas exchange and airway pressures (peak and plateau), and compliance should be undertaken (Tables 19.4 and 19.5). Cardiovascular complications may be secondary to sepsis, or from the effects of positive pressure ventilation. Cardiac output is diminished with decreased venous return from high airway pressures, decreased intravascular volume, fluid sequestration from sepsis, and trauma.
Table 19.4 Differential diagnosis of hypoxemia and/or hypoventilation during mechanical ventilation Patient-related
Airway-related
Ventilator-related
Tension pneumothorax Atelectasis Pneumonia Bronchospasm Pleural effusion Pulmonary edema Laryngospasm Airway trauma
Right mainstem intubation Kinked endotracheal tube, occlusion due to biting Endotracheal tube cuff leak, herniation Inadvertent extubation with neck extension, migration of tube Unrecognized esophageal intubation
Ventilator circuit leak Low FiO2 Inappropriate ventilatory settings Patient–ventilator dysynchrony
ACUTE AND CHRONIC RESPIRATORY FAILURE Table 19.5 Diagnostic approach of hypoxemia and/or hypoventilation during mechanical ventilation Bag ventilate with FiO2 of 1.0 Auscultate and check capnogram to confirm endotracheal tube placement Chest X-ray, CT scan to diagnose lung parenchymal conditions Arterial blood gas to determine hypoxemia, hypoventilation, acidosis Fiberoptic bronchoscope to ascertain endotracheal tube position, bronchoscopy for removal of mucus plugs, bronchoalveolar lavage for pneumonia Echocardiogram to rule out intracardiac shunts Ventilator: check for disconnection, leaks or blockages Ventilator: check difference between exhaled and set TV, airway pressure profile, peak and plateau pressures, check response to lung recruitment and PEEP FiO2, fraction of inspired oxygen; CT, computed tomography; PEEP, positive end-expiratory pressure; TV, tidal volume.
Gastrointestinal complications such as upper GI bleed, gastric stasis, and ileus may be caused by shock, hypoxemia, drugs that inhibit gastrointestinal motility such as narcotics, antipsychotics, or hypokalemia. Sepsis syndrome. The inflammatory response is triggered by infection and is a series of humoral and cellular cascades. The mediators of sepsis, neutrophils, cytokines, coagulation factor, prostaglandins, and nitric oxide, target the endothelium and increase capillary permeability. This is expressed in the lungs as noncardiogenic pulmonary edema.
Noninvasive ventilation NIV is largely utilized in patients with acute hypercapnic RF, acute cardiogenic pulmonary edema, and acute exacerbation of COPD. The level of success is variable in hypoxemic forms of RF and is useful in select populations with no contraindications such as multiple organ failure, loss of consciousness, or hemodynamic instability. NIV can be used either as a primary ventilation mode for acute RF or for weaning patients from MV (Peter et al., 2002). It preserves airway defense mechanisms, lowers sedation requirements, and allows patients to speak, eat, and clear secretions. NIV unloads inspiratory muscles and reduces the work of breathing. The mask interface has to fit well and the patient should be comfortable. Nasal masks may be used but are ineffective in patients likely to mouth breathe. The applied pressure starts at 8–12 cmH2O and is adjusted according to the patient’s tolerance and desired TV. NIV has been associated with reduced infections (pneumonia, sinusitis) and hospital stay, when compared with MV delivered via an endotracheal tube
281
(Brochard, 2003). Immunocompromised patients with RF may benefit from a reduced need for intubation and reduction in mortality (Hilbert et al., 2001). In patients with COPD exacerbation, NIV counters the effects of auto-PEEP and diminishes dyspnea and the work of breathing (Keenan et al., 2003). In cardiogenic pulmonary edema, positive pressure breathing restores functional residual capacity (FRC), improves ventilation/perfusion ratios, and reduces afterload (Vital et al., 2008). NIV is contraindicated in patients who are unable to protect airway due to altered mental status, agitation and excessive secretions, or develop hemodynamic instability, respiratory arrest, and myocardial ischemia. In general, younger, cooperative patients with intact dentition, having less severe illness, hypercarbia (PaCO2 > 45 mmHg, 7.10), have a higher rate of success with NIV (Hill et al., 2007). Some centers have reported the use of a pumpless extracorporeal lung-assist device for extracorporeal CO2 removal allowing them to avoid invasive MV in patients with acute hypercapnic RF not responding to NIV (Kluge et al., 2012).
Weaning from mechanical ventilation Weaning usually implies two closely related aspects of care, discontinuation of MV and removal of any artificial airway. The clinician determines when a patient is ready to resume spontaneous ventilation. Once a patient is able to sustain spontaneous breathing, a second decision is made whether the artificial airway can be removed by assessing the patient’s mental status, airway protective mechanisms, and ability to cough and clear secretions (Alia and Esteban, 2000) (see Tables 19.6 and 19.7). Table 19.6 Checklist for weaning and extubation Patient awake and responds appropriately, aspiration risks assessed No agitation, cooperative, adequate pain control Hemodynamically stable, normal acid–base status, no electrolyte disturbance Pulmonary gas exchange with acceptable parameters for oxygenation and ventilation Cardiogenic pulmonary edema (fluid overload, left ventricular dysfunction) resolving Noncardiogenic pulmonary edema (pneumonia, acute lung injury) resolving Respiratory mechanics optimized, abdominal distention Underlying cause for ventilator dependency resolving
282
S. OANA AND J. MUKHERJI
Table 19.7 Causes of failed extubation Upper airway obstruction Obstructive sleep apnea Tracheal stenosis Laryngeal edema Airway compression from neck hematoma Airway trauma Inability to clear secretions Altered mental status Poor VC Muscle fatigue Aspiration Emesis from tube feeds Gastroesophageal reflux
The most effective means of weaning follows a systematic approach that includes a daily assessment of weaning readiness, along with interruption of sedation infusions and spontaneous breathing trials. Evidencebased practice dictates using protocols and checklists as decision support tools. Most studies of weaning protocols applied by nonphysician healthcare providers suggest faster weaning and shorter duration of ventilation and ICU stay (Haas and Loik, 2012). Tracheostomy should be considered in head-injured or critically ill patients as soon as the need for prolonged intubation (longer than 14 days) is identified. Mortality is not worse with tracheotomy and may be improved with earlier provision (Durbin, 2010).
WEANING PARAMETERS Sleep-deprived patients in the ICU manifest signs of agitation and lethargy. Establishing sleep patterns and daily orientation to the day, time, and surroundings are helpful. Daily interruption of sedative infusions is advocated to avoid prolonged sedation and increased duration of MV (Kress et al., 2000). The patient is assessed for respiratory drive, muscle strength, and the imposed workload before attempting weaning. Muscle fatigue results from muscle atrophy or electrolyte imbalance, NMD, increased work of breathing from acute bronchospasm, pulmonary edema, or depleted respiratory drive from excessive sedatives and narcotics. Weaning is considered successful if patients do not require resumption of ventilatory support within 48–72 hours after extubation (MacIntyre, 2004). About 20% of patients may fail their first attempt at weaning and may require prolonged support (Rothaar and Epstein, 2003). A somewhat particular cause of failure is due to postextubation pulmonary edema, either from loss of positive pressure ventilation with increased afterload in left ventricular dysfunction or from negative pressure pulmonary edema. Unplanned extubation requiring reintubation is detrimental as it increases mortality and the chances of prolonged MV, and ICU stay (Seymour et al., 2004). Evidence-based practice supports early attempts at weaning in a protocol-driven fashion (Robertson et al., 2008). Prolonged MV has been associated with pneumonia, gastrointestinal bleeding, and deep venous thrombosis. It is difficult to determine the optimal time for extubating patients with neurologic deficit from brain injury. Impaired airway reflexes and inability to clear secretions creates conditions for aspiration risks. However, delaying extubation of patients breathing spontaneously can carry an increased risk for pneumonia.
No index has proven to be ideal. The work of breathing is more predictive of successful weaning outcome in long-term MV patients. Weaning criteria consist of TV, VC, RR, MIP and rapid shallow breathing index (RSBI). The sensitivity and specificity of the weaning parameters are relatively poor (El-Khatib and BouKhalil, 2008). VC is measured upright rather that supine, because diaphragmatic paralysis may cause a 30% positional reduction. Patients may be too weak to sustain prolonged inspiratory effort or fail to cooperate. It is an unreliable measure of strength and fails to predict weaning outcome (threshold is 10–15 mL/kg for VC) (Yang, 1992). MIP provides a better negative than positive predictive value and may be performed in uncooperative intubated patients. A MIP value exceeding 30 cm of H2O is associated with successful extubation. RSBI is the ratio of RR/TV and is an accurate predictor of weaning outcome when lower than 105. It is not dependent on patient cooperation and effort (Chao and Scheinhorn, 2007).
WEANING MODES AND PROTOCOL-DRIVEN WEANING The optimal mode of weaning from MV remains controversial. Most patients do not require progressive withdrawal of support during weaning. The commonly used techniques of weaning are T-piece, SIMV, or PS. Spontaneous breathing trials using PS or T-piece is commenced if RSBI is less than 107. Daily T-piece trials are equivalent to a PS mode of weaning, and are superior to the SIMV mode (Esteban et al., 1995). PS provides a progressive unloading of inspiratory muscles compared with SIMV. NIV has been used as a method to support ventilation following early extubation (Burns et al., 2010). A successful trial for duration of 30 minutes up to 2 hours assures successful extubation (Epstein, 2009).
ACUTE AND CHRONIC RESPIRATORY FAILURE A protocol-driven weaning is based on best evidence, and is less influenced by personal decisions, allowing a systemic approach to learning and quality monitoring. It leads to a significant reduction in the duration of MV and complications (Blackwood et al., 2010). Protocolized weaning may be computer driven and has reduced MV duration when compared with physiciancontrolled weaning (Lellouche et al., 2006).
Management of patients on prolonged mechanical ventilation Prolonged mechanical ventilation (PMV), according to the Centers for Medicare and Medicaid Services, includes patients who have required more than 6 hours of MV for greater than 21 consecutive days and who are eligible for transfer to long-term acute care (White et al., 2008). In single center studies, about 3–7% of ICU patients meet PMV criteria (MacIntyre et al., 2005). Unlike the 48–72 hour criteria used in acute RF, patients on PMV are considered weaned if liberated from MV for 7 consecutive days. Comorbidities commonly associated with PMV include include malignancy, COPD, immunosuppression, poor nutritional status, polyneuropathy, myopathy, sepsis, recurrent aspiration, sedation, sleep deprivation, and delirium. Diaphragmatic contractile dysfunction is present in PMV with as few as 18 hours of MV, resulting in diaphragmatic atrophy (Powers et al., 2009). Managing patients on PMV involves more than ventilator weaning and has a rehabilitative focus, involving nutritional support, respiratory muscle training, physical and occupational therapy including speech therapy, psychological and social services. Long-term facilities caring for PMV patients have some environmental advantages in that they are relatively quiet, with distinct day–night cycles, and, unlike an acute ICU, they are geared towards receiving family members, encouraging mobility, providing more staff–patient interactions and counseling.
MANAGEMENT OF OROPHARYNGEAL AND TRACHEOBRONCHIAL SECRETIONS NMDs may compromise the effectiveness of the cough by producing weakness of the involved skeletal muscles. A weakened cough leads to inability to clear secretions and places patients at risk for atelectasis, mucus plugging, and pneumonia. A PCEF less than 2.7 L/sec produces an ineffective cough, with retention of airway secretions predisposing to pneumonia. Patients with low VC (less than 1.5 L) and vocal cord dysfunction
283
are susceptible to retained secretions (Kang and Bach, 2000). Anticholinergic medications inhibit the saliva production but their use is limited by side-effects related to blurred vision and urinary retention. Injection of botulinum into salivary glands produces local functional denervation and has been successfully used to treat sialorrhea in stroke, parkinsonism, and ALS (Ondo et al., 2004). Patients experience relief within days and the therapeutic effect usually lasts for approximately 3–4 months, when reinjection may be required. The use of anticholinergics and botulinum toxin can produce thick, tenacious secretions, which are difficult to mobilize in the setting of compromised musculature. This can be minimized by maintenance of adequate hydration and by coadministration of medications that thin secretions, such as guaifenesin or N-acetylcysteine (Elman et al., 2005). Physical modalities usually employed in handling secretions include manual suctioning, respiratory physical therapy, postural drainage, and glossopharyngeal breathing (GPB). Postural drainage allows the mobilization of tracheobronchial secretions by gravity-assist positioning, deep breathing with or without chest percussions. When secretions reach the upper airway they are expelled via coughing. Incentive spirometry is commonly used postoperatively to reduce pulmonary complications. While deep breaths taken with an incentive spirometer may help reduce atelectasis, there is no evidence that it helps in reducing pulmonary complications after upper abdominal surgery (Guimara˜es et al., 2009). GPB is useful for airway clearance in patients with low VC, as in NMD. It is accomplished by a series of air gulping actions generated by the lips, pharynx, and palate while the larynx serves as a valve maintaining air in the lungs between gulps. Both GPB and air stacking can increase lung volumes and, thereby, cough flows. GPB helps supplement mechanical insufflations and may be used to decrease daytime ventilator use (Bach et al., 2007). Oscillating devices assist in clearance of airway secretions. They generate intra- or extrathoracic oscillations orally or external to the chest wall. Mechanical vibration of the chest using vests that provide high frequency chest wall oscillations is believed to aid in the mobilization of tracheobronchial secretions by producing shearing forces that thin mucus. Forced expiratory maneuvers with Flutter VRP1® (Scandipharm Inc., Birmingham, AL) creates an oscillatory positive pressure and vibration within the airways which facilitates mucus mobilization and expulsion. There was no clear evidence that oscillation is a more or less effective intervention overall than other forms of physiotherapy (Morrison and Agnew, 2009).
284
S. OANA AND J. MUKHERJI
MANAGEMENT OF RESPIRATORY FAILURE IN PATIENTS WITH NEUROMUSCULAR DISEASE Improving pulmonary gas exchange requires an assortment of maneuvers ranging from providing supplemental oxygen and bronchodilators, to use of pulmonary support to sustain the weak respiratory muscles. The use of CPAP or low span BiPAP (inspiratory–expiratory pressure difference
Chapter 19
Acute and chronic respiratory failure SABIN OANA* AND JAYANTA MUKHERJI Department of Anesthesiology, Loyola University Medical Center, Maywood, IL, USA
INTRODUCTION Respiratory failure (RF) is defined as failure of oxygenation and/or carbon dioxide (CO2) elimination. Hypoxemia exists if arterial oxygen tension (PaO2) is below 60 millimeters of mercury (mmHg). Hypercapnia is present if arterial CO2 tension (PaCO2) is above 50 mmHg. There are three common mechanisms that could lead to RF: right to left shunt (cardiac or intrapulmonary), ventilation/perfusion mismatch, and hypoventilation. In addition, low partial pressure of inspired oxygen, diffusion impairment, and high partial pressure of inspired CO2 have also been described as rare causes of RF (Bartter et al., 2011). Another way of looking at the pathophysiology is to imagine the respiratory system as consisting of two parts: the lung and the pump that moves the lung (respiratory centers, spinal cord and respiratory peripheral nerves, respiratory muscles, chest wall). Failure of the lung is primarily manifested as hypoxemia (hypoxemic or type I failure; shunt physiology) and failure of the pump results mainly in hypercapnia (hypercarbic or type II failure; hypoventilation is the principal mechanism). Recent literature mentions a type III or perioperative failure, associated mainly with lung atelectasis, and a type IV, related to hypoperfusion of respiratory muscles in shock (Kress and Hall, 2012). The majority of neurologic diseases that progress to RF are therefore type II failures and are characterized by either a reduced drive to breathe or weak respiratory muscles. Depending on the speed of onset, RF can be classified as acute, acute on chronic, or chronic. RF usually supervenes insidiously in chronic progressive neuromuscular diseases (NMDs) such as amyotrophic lateral sclerosis (ALS). There can also be an acute superimposed factor (e.g., respiratory infection) that precipitates their
deterioration. On rare occasions, however, RF may herald disease onset. More commonly, there are acute conditions that are associated with RF (e.g., Guillain–Barre´ syndrome (GBS)). Other entities such as critical illness neuropathy and myopathy are often first diagnosed in the critical care unit. The initial manifestations of RF usually appear during sleep and, as the diseases progress, they are also present during the daytime. In normal individuals, during sleep, especially in the rapid eye movement state, there is hypotonia of respiratory muscles and of the airway as well as decreased chemosensitivity to CO2. These otherwise normal changes produce an abnormal degree of hypoventilation and hypoxemia in individuals affected by NMD (Chokroverty, 2001). Disruption of any segment of the neural pathway of respiration can produce RF. RF may be classified according to the anatomic site of origin into: central nervous system disorders, spinal diseases, peripheral nervous system dysfunction, and muscle diseases. Previous work reviewed the need for primary mechanical ventilation (MV) in patients affected by peripheral nervous system diseases (Cabrera Serrano and Rabinstein, 2010). The most frequent conditions were myasthenia gravis (MG), GBS, myopathies, and ALS, together accounting for 75% of cases. However, 55% of patients did not have a diagnosis on admission and 10% never had a final diagnosis.
GENERAL APPROACH TO DIAGNOSIS Respiratory symptoms in NMDs include dyspnea or shortness of breath with its variants orthopnea (dyspnea when supine) and platypnea (dyspnea when upright). Very commonly, and generally early in the course of diseases, there are various sleep disturbances (restless
*Correspondence to: Sabin Oana, M.D., Assistant Professor of Anesthesiology, Loyola University Medical Center, Department of Anesthesiology, Bldg 103, Rm 3102, 2160 S 1st Ave, Maywood, IL 60153-3328, USA. Tel: þ1-708-216-8866, Fax: þ1-708-216-1249, E-mail: [email protected]
274
S. OANA AND J. MUKHERJI
sleep, frequent awakenings, nightmares, and migraines) and related symptoms of daytime fatigue. Difficulty in speech and swallowing are markers of bulbar symptoms (i.e., of the cranial nerves that originate in the medulla) that predispose the patient to ineffective cough and aspiration pneumonia. Physical examination can reveal tachypnea, shallow breathing, accessory inspiratory and expiratory muscle contraction (parasternal, sternocleidomastoid, scalene, intercostal, and abdominal) and ribcage–abdominal asynchrony. Of note, the reliability and reproducibility of the clinical examination with neurologic focus of a RF patient are greatly reduced, especially in the intensive care setting. In cases with mild hypoxia, one can encounter headaches or inattentiveness, lethargy or delirium, somnolence or anxiety, tremors or seizures that, untreated, progress toward loss of consciousness and unresponsiveness. Hypoxic-ischemic encephalopathy is a complication of prolonged hypoxia during cardiac and respiratory arrest. A special type is that encountered during birth, which can cause long-term damage including intellectual developmental disorder and cerebral palsy. A low percentage of adults can regain consciousness after cardiopulmonary arrest but still suffer long-term neurologic sequelae, including motor, memory, or personality changes (Dreibelbis and Jozefowicz, 2010). A special type of RF occurs when the inspired oxygen is low, as, for example, during climbing at high altitudes. Acute mountain sickness can progress to high altitude cerebral edema and is manifested as dizziness, somnolence, confusion, and decreased consciousness. Arterial blood gas analysis is one the first steps in the characterization and differential diagnosis of RF. It entails measuring pH, PaO2, PaCO2, and bicarbonate.
These should provide enough data to diagnose an acute versus chronic, respiratory versus metabolic acid–base disorder. Once the respiratory component is assessed, there are multiple indices of hypoxemia that can be calculated to aid in the more detailed diagnosis of RF (i.e., differentiating pulmonary versus extrapulmonary causes). Each has its own merits and limitations. Tension-based indices are based on alveolar air equation and content-based indices are based on oxygen content equation (Siggaard-Andersen and Gothgen, 1995; Wandrup, 1995) (see Table 19.1). Electrophysiologic studies consist of nerve conduction studies (motor and sensory) as well as needle electromyography (EMG). More specialized testing includes: neuromuscular junction testing with repetitive nerve stimulation, single-fiber EMG and train of four stimulation, respiratory EMG with percutaneous or needle stimulation of phrenic nerve and diaphragm, and direct muscle stimulation (Dhand, 2006). In axonal disease there is normal conduction but reduced amplitude, whereas in demyelinating disease there is slow conduction with normal potential amplitude. For needle EMG the neurogenic pattern shows spontaneous activity, large potentials with mild contraction and less than complete recruitment and interference with full contraction. The myopathic pattern might demonstrate some spontaneous activity with small potentials during mild contraction but small amplitude and full interference patterns with full stimulation. Pulmonary function testing in NMDs encompasses spirometry, lung volume and capacity, and measurement of respiratory muscle strength (Aboussouan, 2005). The most common clinical picture is that of a restrictive respiratory disorder, albeit with some
Table 19.1 Oxygen parameters* Equation
Formula
Alveolar pressure of oxygen
PaO2 ¼ FiO2 (Patm PH2O) PaCO2/RQ or PaO2 700 FiO2 1.2 PaCO2 CaO2 ¼ 1.34 Hgb SaO2 þ 0.03 PaO2 Qs/Qt ¼ (CcO2 CaO2) (CcO2 CvO2) or Qs/Qt ¼ (1 SvO2) (1 SaO2) Vd/Vt ¼ (PaCO2 PeCO2) PaCO2
Arterial oxygen content equation Shunt equation
Dead space fraction
Normal values
Less than 5% (0.05)
Below 0.33(33%)
*Where: PaO2, alveolar partial pressure of oxygen in mmHg; FiO2, inspired fraction of dry oxygen; Patm, atmospheric pressure (760 mmHg at sea level); PH2O, partial pressure of water vapor (47 mmHg); PaCO2, alveolar partial pressure of CO2 in mmHg; RQ, respiratory quotient or CO2/O2 exchange ratio (approx 0.8); Hgb, hemoglobin concentration; CaO2, CcO2, CvO2 are oxygen content of arterial, capillary, and mixed venous blood; SvO2 and SaO2 are the saturation of hemoglobin in mixed venous blood and, respectively, arterial blood; PeCO2, partial pressure of expired CO2.
ACUTE AND CHRONIC RESPIRATORY FAILURE particularities. Weakness in expiratory muscles generates a decreased expiratory reserve volume but with preserved functional residual capacity. In consequence there is elevated residual volume. Associated weakness in inspiratory muscles decreases the inspiratory reserve volume. The overall result is a marked decrease in vital capacity (VC) with a preserved total lung capacity. Diaphragmatic dysfunction has a characteristic 20–50% decrease in VC when supine. Maximal voluntary ventilation could be reduced. Strength of the respiratory muscles is assessed by measuring the maximum inspiratory and expiratory pressures (MIP and MEP). More recently, sniff inspiratory pressures (nasal (SNIP), esophageal, and gastric) have also been employed to provide data regarding the respiratory muscles. Transdiaphragmatic pressure is the difference between the esophageal and gastric pressures and is sensitive for diaphragmatic dysfunction (Steier et al., 2007). Genetic testing is usually recommended in inherited diseases with specific presentations like familial ALS, spinal and bulbar muscular atrophy, Charcot–Marie–Tooth (CMT) 1A, myotonic dystrophy, Duchenne muscular dystrophy (DMD) (Burgunder et al., 2011), and congenital central hypoventilation syndrome (CCHS).
SPECIFIC DISEASES Stroke RF in stroke occurs in about 2–4% of patients and is an ominous sign, traditionally being associated with high mortality. The treatment of such patients should be performed in acute stroke units, resulting in a decrease in mortality (Gattellari et al., 2009). Younger patients with posterior circulation involvement have better prognosis (Rabinstein and Wijdicks, 2004). Common indications for MV in stroke patients are neurologic reasons (coma, loss of brainstem reflexes, brain edema with impending herniation), general deterioration in clinical condition, cardiopulmonary causes (pneumonia, aspiration, pulmonary embolism), or elective
275
for neuroradiologic or neurosurgical procedures (Steiner et al., 1997). There are several patterns of impaired breathing described after stroke that could help in the localization of stroke (Brazis et al., 2011) (see Table 19.2). Upper airway obstruction is common in stroke patients, obstructive sleep apnea (OSA) being considered both a risk and a consequence for stroke. In OSA patients with stroke, continuous positive airway pressure (CPAP) should be offered, as it reduces mortality (Martı´nez-Garcı´a et al., 2009). Dysphagia is very frequent and can be diagnosed early and safe with the help of flexible fiberoptic endoscopy (Warnecke et al., 2009). More recent studies found that aggressive treatments with thrombolysis, decompressive hemicraniectomy, MV, and feeding tubes are achieving functional independence for an unexpectedly high number of patients (Seder and Mayer, 2009).
Amyotrophic lateral sclerosis ALS is being increasingly recognized as a disease of diverse genotype, phenotype, and rate of progression (El Escorial Criteria Revisited) (Brooks et al., 2000). RF is rare at the onset, but is the most frequent cause of death. It signifies degeneration of the center and/or of the neurons of the phrenic nerve (Kiernan et al., 2011). Respiratory symptoms in ALS are dyspnea on exertion, orthopnea, disturbed sleep, daytime somnolence, and vivid nightmares (Wijesekera and Leigh, 2009). Respiratory signs in ALS include tachypnea, use of accessory muscles, and paradoxical movement of the abdomen. As far as respiratory investigations are concerned, forced vital capacity (FVC) is increasingly recognized as being an insensitive marker for early RF. In order to predict the need for noninvasive ventilation (NIV), recent focus has been on measurements such as nocturnal oximetry, nocturnal desaturations less than 90% for more than 1 full minute, MIP, supine FVC, transdiaphragmatic pressure, SNIP. Common indications for NIV are: orthopnea, SNIP 35 cmH2O) leading to volutrauma. Lower TVs of 6–8 mL/kg limit plateau pressures, prevent release of inflammatory mediators, and reduce ventilatorinduced lung injury. Lung protective strategies in ARDS include using lower TV and higher PEEP to avoid alveolar overdistention and derecruitment (Petrucci and Iacovelli, 2007). PEEP prevents alveolar derecruitment by restoring the functional residual volume to the physiologic range. Permissive hypercapnia is a strategy whereby lungs are protected during MV by adoption of deliberate alveolar hypoventilation. Hypercapnic acidosis is a consequence of this strategy and not a goal. Lung protective ventilation impairs CO2 clearance and may lead to right heart dysfunction or increased intracranial pressure. Extracorporeal CO2 removal has the potential to optimize lung protective ventilation by uncoupling oxygenation and CO2 clearance (Cove et al., 2012).
Complications in mechanically ventilated patients Respiratory complications may result from MV itself. A bedside evaluation of patient, airway, and ventilator settings to monitor for ventilator–patient dysynchrony, oxygenation, gas exchange and airway pressures (peak and plateau), and compliance should be undertaken (Tables 19.4 and 19.5). Cardiovascular complications may be secondary to sepsis, or from the effects of positive pressure ventilation. Cardiac output is diminished with decreased venous return from high airway pressures, decreased intravascular volume, fluid sequestration from sepsis, and trauma.
Table 19.4 Differential diagnosis of hypoxemia and/or hypoventilation during mechanical ventilation Patient-related
Airway-related
Ventilator-related
Tension pneumothorax Atelectasis Pneumonia Bronchospasm Pleural effusion Pulmonary edema Laryngospasm Airway trauma
Right mainstem intubation Kinked endotracheal tube, occlusion due to biting Endotracheal tube cuff leak, herniation Inadvertent extubation with neck extension, migration of tube Unrecognized esophageal intubation
Ventilator circuit leak Low FiO2 Inappropriate ventilatory settings Patient–ventilator dysynchrony
ACUTE AND CHRONIC RESPIRATORY FAILURE Table 19.5 Diagnostic approach of hypoxemia and/or hypoventilation during mechanical ventilation Bag ventilate with FiO2 of 1.0 Auscultate and check capnogram to confirm endotracheal tube placement Chest X-ray, CT scan to diagnose lung parenchymal conditions Arterial blood gas to determine hypoxemia, hypoventilation, acidosis Fiberoptic bronchoscope to ascertain endotracheal tube position, bronchoscopy for removal of mucus plugs, bronchoalveolar lavage for pneumonia Echocardiogram to rule out intracardiac shunts Ventilator: check for disconnection, leaks or blockages Ventilator: check difference between exhaled and set TV, airway pressure profile, peak and plateau pressures, check response to lung recruitment and PEEP FiO2, fraction of inspired oxygen; CT, computed tomography; PEEP, positive end-expiratory pressure; TV, tidal volume.
Gastrointestinal complications such as upper GI bleed, gastric stasis, and ileus may be caused by shock, hypoxemia, drugs that inhibit gastrointestinal motility such as narcotics, antipsychotics, or hypokalemia. Sepsis syndrome. The inflammatory response is triggered by infection and is a series of humoral and cellular cascades. The mediators of sepsis, neutrophils, cytokines, coagulation factor, prostaglandins, and nitric oxide, target the endothelium and increase capillary permeability. This is expressed in the lungs as noncardiogenic pulmonary edema.
Noninvasive ventilation NIV is largely utilized in patients with acute hypercapnic RF, acute cardiogenic pulmonary edema, and acute exacerbation of COPD. The level of success is variable in hypoxemic forms of RF and is useful in select populations with no contraindications such as multiple organ failure, loss of consciousness, or hemodynamic instability. NIV can be used either as a primary ventilation mode for acute RF or for weaning patients from MV (Peter et al., 2002). It preserves airway defense mechanisms, lowers sedation requirements, and allows patients to speak, eat, and clear secretions. NIV unloads inspiratory muscles and reduces the work of breathing. The mask interface has to fit well and the patient should be comfortable. Nasal masks may be used but are ineffective in patients likely to mouth breathe. The applied pressure starts at 8–12 cmH2O and is adjusted according to the patient’s tolerance and desired TV. NIV has been associated with reduced infections (pneumonia, sinusitis) and hospital stay, when compared with MV delivered via an endotracheal tube
281
(Brochard, 2003). Immunocompromised patients with RF may benefit from a reduced need for intubation and reduction in mortality (Hilbert et al., 2001). In patients with COPD exacerbation, NIV counters the effects of auto-PEEP and diminishes dyspnea and the work of breathing (Keenan et al., 2003). In cardiogenic pulmonary edema, positive pressure breathing restores functional residual capacity (FRC), improves ventilation/perfusion ratios, and reduces afterload (Vital et al., 2008). NIV is contraindicated in patients who are unable to protect airway due to altered mental status, agitation and excessive secretions, or develop hemodynamic instability, respiratory arrest, and myocardial ischemia. In general, younger, cooperative patients with intact dentition, having less severe illness, hypercarbia (PaCO2 > 45 mmHg, 7.10), have a higher rate of success with NIV (Hill et al., 2007). Some centers have reported the use of a pumpless extracorporeal lung-assist device for extracorporeal CO2 removal allowing them to avoid invasive MV in patients with acute hypercapnic RF not responding to NIV (Kluge et al., 2012).
Weaning from mechanical ventilation Weaning usually implies two closely related aspects of care, discontinuation of MV and removal of any artificial airway. The clinician determines when a patient is ready to resume spontaneous ventilation. Once a patient is able to sustain spontaneous breathing, a second decision is made whether the artificial airway can be removed by assessing the patient’s mental status, airway protective mechanisms, and ability to cough and clear secretions (Alia and Esteban, 2000) (see Tables 19.6 and 19.7). Table 19.6 Checklist for weaning and extubation Patient awake and responds appropriately, aspiration risks assessed No agitation, cooperative, adequate pain control Hemodynamically stable, normal acid–base status, no electrolyte disturbance Pulmonary gas exchange with acceptable parameters for oxygenation and ventilation Cardiogenic pulmonary edema (fluid overload, left ventricular dysfunction) resolving Noncardiogenic pulmonary edema (pneumonia, acute lung injury) resolving Respiratory mechanics optimized, abdominal distention Underlying cause for ventilator dependency resolving
282
S. OANA AND J. MUKHERJI
Table 19.7 Causes of failed extubation Upper airway obstruction Obstructive sleep apnea Tracheal stenosis Laryngeal edema Airway compression from neck hematoma Airway trauma Inability to clear secretions Altered mental status Poor VC Muscle fatigue Aspiration Emesis from tube feeds Gastroesophageal reflux
The most effective means of weaning follows a systematic approach that includes a daily assessment of weaning readiness, along with interruption of sedation infusions and spontaneous breathing trials. Evidencebased practice dictates using protocols and checklists as decision support tools. Most studies of weaning protocols applied by nonphysician healthcare providers suggest faster weaning and shorter duration of ventilation and ICU stay (Haas and Loik, 2012). Tracheostomy should be considered in head-injured or critically ill patients as soon as the need for prolonged intubation (longer than 14 days) is identified. Mortality is not worse with tracheotomy and may be improved with earlier provision (Durbin, 2010).
WEANING PARAMETERS Sleep-deprived patients in the ICU manifest signs of agitation and lethargy. Establishing sleep patterns and daily orientation to the day, time, and surroundings are helpful. Daily interruption of sedative infusions is advocated to avoid prolonged sedation and increased duration of MV (Kress et al., 2000). The patient is assessed for respiratory drive, muscle strength, and the imposed workload before attempting weaning. Muscle fatigue results from muscle atrophy or electrolyte imbalance, NMD, increased work of breathing from acute bronchospasm, pulmonary edema, or depleted respiratory drive from excessive sedatives and narcotics. Weaning is considered successful if patients do not require resumption of ventilatory support within 48–72 hours after extubation (MacIntyre, 2004). About 20% of patients may fail their first attempt at weaning and may require prolonged support (Rothaar and Epstein, 2003). A somewhat particular cause of failure is due to postextubation pulmonary edema, either from loss of positive pressure ventilation with increased afterload in left ventricular dysfunction or from negative pressure pulmonary edema. Unplanned extubation requiring reintubation is detrimental as it increases mortality and the chances of prolonged MV, and ICU stay (Seymour et al., 2004). Evidence-based practice supports early attempts at weaning in a protocol-driven fashion (Robertson et al., 2008). Prolonged MV has been associated with pneumonia, gastrointestinal bleeding, and deep venous thrombosis. It is difficult to determine the optimal time for extubating patients with neurologic deficit from brain injury. Impaired airway reflexes and inability to clear secretions creates conditions for aspiration risks. However, delaying extubation of patients breathing spontaneously can carry an increased risk for pneumonia.
No index has proven to be ideal. The work of breathing is more predictive of successful weaning outcome in long-term MV patients. Weaning criteria consist of TV, VC, RR, MIP and rapid shallow breathing index (RSBI). The sensitivity and specificity of the weaning parameters are relatively poor (El-Khatib and BouKhalil, 2008). VC is measured upright rather that supine, because diaphragmatic paralysis may cause a 30% positional reduction. Patients may be too weak to sustain prolonged inspiratory effort or fail to cooperate. It is an unreliable measure of strength and fails to predict weaning outcome (threshold is 10–15 mL/kg for VC) (Yang, 1992). MIP provides a better negative than positive predictive value and may be performed in uncooperative intubated patients. A MIP value exceeding 30 cm of H2O is associated with successful extubation. RSBI is the ratio of RR/TV and is an accurate predictor of weaning outcome when lower than 105. It is not dependent on patient cooperation and effort (Chao and Scheinhorn, 2007).
WEANING MODES AND PROTOCOL-DRIVEN WEANING The optimal mode of weaning from MV remains controversial. Most patients do not require progressive withdrawal of support during weaning. The commonly used techniques of weaning are T-piece, SIMV, or PS. Spontaneous breathing trials using PS or T-piece is commenced if RSBI is less than 107. Daily T-piece trials are equivalent to a PS mode of weaning, and are superior to the SIMV mode (Esteban et al., 1995). PS provides a progressive unloading of inspiratory muscles compared with SIMV. NIV has been used as a method to support ventilation following early extubation (Burns et al., 2010). A successful trial for duration of 30 minutes up to 2 hours assures successful extubation (Epstein, 2009).
ACUTE AND CHRONIC RESPIRATORY FAILURE A protocol-driven weaning is based on best evidence, and is less influenced by personal decisions, allowing a systemic approach to learning and quality monitoring. It leads to a significant reduction in the duration of MV and complications (Blackwood et al., 2010). Protocolized weaning may be computer driven and has reduced MV duration when compared with physiciancontrolled weaning (Lellouche et al., 2006).
Management of patients on prolonged mechanical ventilation Prolonged mechanical ventilation (PMV), according to the Centers for Medicare and Medicaid Services, includes patients who have required more than 6 hours of MV for greater than 21 consecutive days and who are eligible for transfer to long-term acute care (White et al., 2008). In single center studies, about 3–7% of ICU patients meet PMV criteria (MacIntyre et al., 2005). Unlike the 48–72 hour criteria used in acute RF, patients on PMV are considered weaned if liberated from MV for 7 consecutive days. Comorbidities commonly associated with PMV include include malignancy, COPD, immunosuppression, poor nutritional status, polyneuropathy, myopathy, sepsis, recurrent aspiration, sedation, sleep deprivation, and delirium. Diaphragmatic contractile dysfunction is present in PMV with as few as 18 hours of MV, resulting in diaphragmatic atrophy (Powers et al., 2009). Managing patients on PMV involves more than ventilator weaning and has a rehabilitative focus, involving nutritional support, respiratory muscle training, physical and occupational therapy including speech therapy, psychological and social services. Long-term facilities caring for PMV patients have some environmental advantages in that they are relatively quiet, with distinct day–night cycles, and, unlike an acute ICU, they are geared towards receiving family members, encouraging mobility, providing more staff–patient interactions and counseling.
MANAGEMENT OF OROPHARYNGEAL AND TRACHEOBRONCHIAL SECRETIONS NMDs may compromise the effectiveness of the cough by producing weakness of the involved skeletal muscles. A weakened cough leads to inability to clear secretions and places patients at risk for atelectasis, mucus plugging, and pneumonia. A PCEF less than 2.7 L/sec produces an ineffective cough, with retention of airway secretions predisposing to pneumonia. Patients with low VC (less than 1.5 L) and vocal cord dysfunction
283
are susceptible to retained secretions (Kang and Bach, 2000). Anticholinergic medications inhibit the saliva production but their use is limited by side-effects related to blurred vision and urinary retention. Injection of botulinum into salivary glands produces local functional denervation and has been successfully used to treat sialorrhea in stroke, parkinsonism, and ALS (Ondo et al., 2004). Patients experience relief within days and the therapeutic effect usually lasts for approximately 3–4 months, when reinjection may be required. The use of anticholinergics and botulinum toxin can produce thick, tenacious secretions, which are difficult to mobilize in the setting of compromised musculature. This can be minimized by maintenance of adequate hydration and by coadministration of medications that thin secretions, such as guaifenesin or N-acetylcysteine (Elman et al., 2005). Physical modalities usually employed in handling secretions include manual suctioning, respiratory physical therapy, postural drainage, and glossopharyngeal breathing (GPB). Postural drainage allows the mobilization of tracheobronchial secretions by gravity-assist positioning, deep breathing with or without chest percussions. When secretions reach the upper airway they are expelled via coughing. Incentive spirometry is commonly used postoperatively to reduce pulmonary complications. While deep breaths taken with an incentive spirometer may help reduce atelectasis, there is no evidence that it helps in reducing pulmonary complications after upper abdominal surgery (Guimara˜es et al., 2009). GPB is useful for airway clearance in patients with low VC, as in NMD. It is accomplished by a series of air gulping actions generated by the lips, pharynx, and palate while the larynx serves as a valve maintaining air in the lungs between gulps. Both GPB and air stacking can increase lung volumes and, thereby, cough flows. GPB helps supplement mechanical insufflations and may be used to decrease daytime ventilator use (Bach et al., 2007). Oscillating devices assist in clearance of airway secretions. They generate intra- or extrathoracic oscillations orally or external to the chest wall. Mechanical vibration of the chest using vests that provide high frequency chest wall oscillations is believed to aid in the mobilization of tracheobronchial secretions by producing shearing forces that thin mucus. Forced expiratory maneuvers with Flutter VRP1® (Scandipharm Inc., Birmingham, AL) creates an oscillatory positive pressure and vibration within the airways which facilitates mucus mobilization and expulsion. There was no clear evidence that oscillation is a more or less effective intervention overall than other forms of physiotherapy (Morrison and Agnew, 2009).
284
S. OANA AND J. MUKHERJI
MANAGEMENT OF RESPIRATORY FAILURE IN PATIENTS WITH NEUROMUSCULAR DISEASE Improving pulmonary gas exchange requires an assortment of maneuvers ranging from providing supplemental oxygen and bronchodilators, to use of pulmonary support to sustain the weak respiratory muscles. The use of CPAP or low span BiPAP (inspiratory–expiratory pressure difference
