Chapter 42. Restrictive Disease of the Respiratory System and the Abdominal Compartment Syndrome

• Scoliotic curves greater than 100° may cause dyspnea; curves greater than 120° are associated with alveolar hypoventilation and cor pulmonale.
• Most patients with chest wall deformity survive their first episode of acute respiratory failure. Common precipitants include upper and lower respiratory tract infections and congestive heart failure.
• Biphasic positive airway pressure may be effective in patients with acute hypercapnic respiratory failure.
• Low tidal volumes and high respiratory rates likely minimize the risk of barotrauma during mechanical ventilation; however, gradual institution of anti-atelectasis measures may improve gas exchange and static compliance.
• Nocturnal hypoxemia is common and may contribute to cardiovascular deterioration; routine polysomnography is recommended.
• Strategies for management of patients with chronic ventilatory failure include daytime intermittent positive pressure ventilation, nocturnal noninvasive ventilation, and ventilation through tracheostomy.
• Abdominal compartment syndrome (ACS) is caused by an acute increase in intra-abdominal pressure resulting from a number of surgical and medical conditions.
• By elevating the diaphragm and decreasing respiratory system compliance, ACS causes a restrictive defect. However, ACS affects a number of other organs and may cause multiorgan system failure.
• Diagnosis relies on measurement of intra-abdominal bladder pressure and identification of organ dysfunction.
• The abdomen should be decompressed before critical organ dysfunction develops.
• Failure to recognize and treat ACS portends a poor prognosis.
• Acute deterioration in respiratory status can occur from disease progression or a number of other infectious and noninfectious processes.
• Patients with idiopathic pulmonary fibrosis admitted to the ICU with acute respiratory failure have an extremely poor prognosis.
• If mechanical ventilation is deemed appropriate, the use of low tidal volumes and high respiratory rates during mechanical ventilation likely minimizes ventilator-induced lung injury.
• Idiopathic pulmonary fibrosis is typically refractory to drug treatment.
• Lung transplantation is a viable option in selected patients with end-stage fibrosis.

Thoracic cage deformity and pulmonary fibrosis both result in a restrictive limitation to breathing. Although relatively rare in the context of pulmonary intensive care, these disorders present unique challenges that complicate ICU management. More commonly a tense and distended abdomen decreases thoracic volume and respiratory system compliance. In this chapter, we describe the pathophysiologic derangements in cardiopulmonary function associated with these disorders and how they affect management during acute illness. A primary goal of this chapter is to offer a strategy for cardiovascular management and mechanical ventilation that minimizes the risk of ventilator-induced complications and maximizes the chance for early, successful extubation.

Although a number of disorders can deform and restrict the movement of the respiratory system (Table 42-1), kyphoscoliosis (KS) is the prototypical cause of severe thoracic deformity. Kyphoscoliosis is the combination of kyphosis (posterior deformity of the spine) and scoliosis (lateral deformity of the spine). It is far more common than isolated cases of kyphosis or scoliosis, placing as many as 200,000 people in the United States at risk of developing respiratory failure.1 Most cases are idiopathic and begin in childhood. Other cases result from congenital defects, poliomyelitis, thoracoplasty, syringomyelia, vertebral and spinal cord tumors, and tuberculosis.

The pathophysiologic consequences of KS correlate with the degree of spinal curvature, but there is considerable variability.1,2 Patients with severe deformity can lead long and relatively symptom-free lives,3 while patients with lesser degrees of curvature may develop ventilatory failure and cor pulmonale at a relatively young age. The reason for this variability is not clear, although sleep-disordered breathing appears to contribute to clinical deterioration in some cases.4,5

The combination of a moderate kyphotic deformity and a moderate scoliotic deformity is functionally equivalent to a severe deformity of either alone.2 Of the two, however, scoliosis produces greater physiologic derangements. In KS, scoliotic curves less than 70° (Fig. 42-1) rarely cause problems, while angles greater than 70° increase the risk of respiratory failure.1 The earlier in life this angle is achieved, the greater the risk of eventually developing respiratory failure because curvature increases by an average of 15° over 20 years from an initial angle of 70°.6,7 Angles greater than 100° can cause dyspnea. Angles ≥120° can result in alveolar hypoventilation and cor pulmonale.1

In order to decrease respiratory effort, patients with severe deformity take rapid and shallow breaths. Examination of the chest reveals decreased excursion and crackles or coarse wheezes from atelectasis or failure to clear secretions. Cardiac examination may demonstrate a loud P2, right ventricular heave, or jugular venous distention, indicating the presence of pulmonary hypertension.2

Respiratory Mechanics

Kyphoscoliosis reduces total lung capacity (TLC) and functional residual capacity (FRC) (Fig. 42-2). Residual volume (RV) may be normal or decreased to a lesser extent than FRC. Vital capacity (VC), inspiratory capacity (IC = TLC − FRC), and expiratory reserve volume (ERV = FRC − RV) are all decreased. Interestingly, in adolescents pulmonary function is only weakly related to the angle of scoliosis. In these patients, VC is also influenced by the degree of thoracic kyphosis, location of the curve, and number of vertebral bodies involved.8 Furthermore, spinal column rotation, respiratory muscle strength, and duration of the curve are not clearly related to pulmonary function in these patients.8 Decreasing chest wall compliance with age increases the risk of developing ventilatory failure.9

Patients with fibrothorax or thoracoplasty have similar abnormalities.1 By contrast, obesity mainly reduces FRC and ERV without much change in RV, VC, or TLC. In patients with ankylosing spondylitis, ERV and IC excursions are restricted around a normal FRC, such that RV increases and TLC decreases to reduce VC, a pattern similar to that seen in neuromuscular diseases of the chest wall.

In each of these disorders, it is the chest wall that limits the excursion of the respiratory system; the lungs and respiratory muscles are affected secondarily and to a lesser degree. In health, TLC is largely determined by the pressure-volume (P-V) curve of the lung, but in KS the P-V curve of the noncompliant chest wall dominates, lowering TLC and FRC while RV is relatively spared (Fig. 42-3). Note that the P-V curve of the respiratory system is shifted downward and to the right, requiring patients to generate large efforts to take in little air. Normal lung compliance and respiratory muscle strength are assumed in Fig. 42-3, although reductions in both contribute to low lung volumes in selected patients. Indeed in four patients with severe KS requiring mechanical ventilation for acute respiratory failure, both lung and chest wall compliance was decreased.10 Decreased lung compliance may occur as a result of infection, edema, atelectasis, or abnormalities in alveolar surface tension and may respond to intermittent positive-pressure ventilation (see below).11

Inspiratory muscle dysfunction occurs when the deformed thorax places inspiratory muscles at a mechanical disadvantage or there is respiratory muscle fatigue.12 When KS is a manifestation of neuromuscular disease (e.g., postpolio syndrome), inspiratory muscles may be affected directly by the neuromuscular disease.

Gas Exchange

Significant daytime hypoxemia rarely occurs until the development of daytime hypercapnia.1 However, nocturnal hypercapnia with hypoxemia occurs early, particularly during rapid eye movement (REM) sleep, and may underlie cardiovascular deterioration in some patients.4,5

The alveolar-arterial gradient [(a-a)O2] is usually ≤25 mm Hg, even in late stages of KS.1 This modest increase in (a-a)O2 results primarily from ventilation-perfusion (V̇/Q̇) inequality caused by atelectasis or underventilation of one hemithorax. V̇/Q̇ inequality further contributes to a low diffusing capacity, as does the failure of the vascular bed to grow normally in a distorted chest.

Alveolar hypoventilation results in part from an increase in the dead space to tidal volume ratio (Vds/Vt). This ratio is increased because Vt is reduced in hypercapnic patients, whereas anatomic and alveolar dead space are usually normal.1 Minute ventilation is often normal but maintained by higher respiratory rates. The use of small Vt minimizes the work of breathing and is a sign of inspiratory muscle dysfunction.12,13 As inspiratory muscle strength falls, the partial pressure of arterial carbon dioxide (PaCO2) rises and further affects diaphragm function.11,14

Ventilatory response to high concentrations of inspired CO2 is normal in normocapnic patients with KS. However, in hypercapnic patients, the response is blunted by buffering of the signal by elevated cerebrospinal fluid bicarbonate, or by a derangement in the central drive to breathe.13

Effects on the Pulmonary Circulation

A further consequence of severe KS is pulmonary hypertension and cor pulmonale. Left untreated, patients with cor pulmonale typically die within 1 year.2 Initially, pulmonary hypertension occurs only with exercise, but over time it occurs at rest as well. Pulmonary hypertension is usually caused by increased pulmonary vascular resistance (PVR) and not elevated left atrial pressure.1 Thus there is an increased gradient between the pulmonary artery diastolic pressure and the pulmonary capillary wedge pressure. Identifying and treating reversible conditions such as pulmonary embolism, hypoxemia, and sleep-disordered breathing lowers pulmonary artery pressure and delays the onset of right ventricular failure.15–17 However, there also may be irreversible changes associated with proliferation of the media in smaller, precapillary pulmonary vessels.2 The mechanism by which this occurs is not known, but blood flow through vessels narrowed by low lung volumes, blood flow through fewer vessels, and the vascular effects of chronic hypoxia and hypercapnia are likely important. Kinking of larger vessels as they travel through deformed lung may further increase pulmonary artery pressures in some cases.1

Outcome

In 20 patients with mean deformity of 113° in acute respiratory failure (ARF) for the first time, admission blood gases showed severe arterial hypoxemia (partial pressure of arterial oxygen [PaO2] of 35 ± 7 mm Hg), acute-on-chronic hypercapnia (PaCO2 of 63 ± 9 mm Hg), and mild arterial acidemia (pH of 7.34 ± 0.08).18 Cor pulmonale was present in 60% of patients. Seven (35%) patients required intubation and mechanical ventilation, while the remaining patients were managed successfully without mechanical ventilation in an age when noninvasive positive-pressure ventilation (NPPV) was not routinely available. There were no statistical differences in admission blood gases, cause of respiratory failure, age, or degree of spinal curvature between patients who required mechanical ventilation and those who did not. Outcome was surprisingly good. All patients survived their initial episode of ARF and subsequently experienced 2.4 episodes of ARF each during the follow-up period (median of 6 years). Median survival after the first episode of ARF was 9 years. On discharge, mean PaO2 was 63 mm Hg and mean PaCO2 was 55 mm Hg. This study demonstrates the utility of aggressive treatment of ARF, even in cases of severe deformity.

Etiology

ARF is usually precipitated by pneumonia, upper respiratory tract infection, or congestive heart failure.18 Triggers are often minor and may remain obscure, but even trivial insults can precipitate ARF when respiratory muscle reserve is decreased and work of breathing is increased. Because chest wall deformity theoretically affects the swallowing mechanism, aspiration should be considered in the differential diagnosis of ARF. Risk factors for clotting (pulmonary hypertension and decreased mobility) mandate consideration of pulmonary embolism, particularly when the cause of deterioration is obscure. Finally, identifying and treating airflow obstruction can help restore the delicate balance between strength and respiratory system load. Limited data suggest that airway resistance is increased in mechanically ventilated patients with KS (∼20 cm H2O/L per second) and refractory to bronchodilators.10 Bronchoscopic examination of patients with bronchodilator-unresponsive increases in airway resistance may reveal torsion and narrowing of central airways.19

Oxygen Therapy

A primary goal is to correct hypoxemia. This is best accomplished by increasing the fraction of inspired oxygen until an oxygen saturation of 90% to 92% is achieved. Adequate saturation by pulse oximetry should be confirmed by subsequent blood gas analysis, which also helps establish the acid-base status. If adequate oxygenation cannot be achieved with face-mask oxygen, biphasic positive airway pressure (BIPAP) should be initiated unless there are indications for intubation (see below).

Hypoxemia, in addition to its many adverse effects, causes pulmonary vasoconstriction and may precipitate right ventricular failure in patients with pre-existing right heart disease (see Chap. 26). Its causes include alveolar hypoventilation, V̇/Q̇ inequality, and intrapulmonary shunt. Right-to-left intracardiac shunts have also been reported in the setting of thoracic deformity.20 Low mixed venous PO2 (Pv̅O2), a frequent finding in patients with pulmonary hypertension and low cardiac output, further lowers arterial oxygenation when there is V̇/Q̇ inequality.

Hemodynamic Management

Evaluation of shock in patients with KS is similar to that described elsewhere in this text. When sepsis causes shock, patients with KS and pulmonary hypertension may not mount the usual hyperdynamic response. Hypotensive patients not responding to an initial aggressive volume challenge should be considered for right heart catheterization and/or bedside echocardiography to further direct therapy. Mechanical ventilation is indicated for nearly all patients with persistent shock, in part to redirect blood flow from the diaphragm, which can be as much as 25% of the cardiac output. Mechanical ventilation and sedation decrease oxygen consumption (and thus supplemental oxygen requirement) and lactic acid generation.

When right ventricular failure causes shock, a vicious cycle ensues. As the right ventricle fails, cardiac output and systemic blood pressure fall, limiting perfusion to the right ventricle from the aortic root. Right ventricular end-diastolic volume increases and shifts the interventricular septum to the left, decreasing left ventricular compliance, and further reducing cardiac output and blood pressure. Ensuring an adequate circulating volume and correcting hypoxemia to reduce pulmonary vasoconstriction are the first goals of therapy. Increasing systemic blood pressure and thus perfusion pressure to the right ventricle with norepinephrine may be helpful.21

To exclude venous thromboembolism we routinely perform lower extremity Doppler exams; however, roughly 50% of patients with acute pulmonary embolism have negative lower extremity Doppler exams. Serially negative Doppler exams provide an added sense of security, as does a negative D-dimer. Chest computed tomography (CT) with pulmonary embolism protocol is preferable to ventilation-perfusion imaging in chest wall deformity, and may provide additional clues regarding the etiology of respiratory failure. In select cases, pulmonary angiography is required to establish a firm diagnosis. In the absence of venous thromboembolism, preventive therapy with unfractionated heparin or low molecular weight heparin is indicated.

Noninvasive Ventilation

Decreased pulmonary compliance lowers lung volume, which in turn limits cough efficiency and mucus clearance.22 To improve compliance and treat atelectasis, short periods (15 to 20 minutes) of intermittent positive-pressure ventilation (IPPV) delivered by mouthpiece 4 to 6 times daily using inflation pressures between 20 and 30 cm H2O have been recommended.11 IPPV increases lung compliance by 70% for up to 3 hours in acutely ill patients, suggesting that IPPV lowers surface tension by altering the surfactant lining layer.11 Alternatively, a volume-preset, time-cycled device may be used.23

In patients with acute ventilatory failure, NPPV by full face mask or nasal mask should be considered first-line therapy (see Chap. 33). Advantages of NPPV over invasive ventilation include decreased need for sedation and paralysis, decreased incidence of nosocomial pneumonia, decreased incidence of otitis and sinusitis, and improved patient comfort. Disadvantages include increased risk of aspiration and skin necrosis, and less control of the patient's ventilatory status compared with invasive ventilation.24

Although nocturnal NPPV is firmly established in the management of KS patients with chronic respiratory failure,25,26 there are limited data regarding its efficacy in acutely ill patients.27–29 In one report of the use of noninvasive ventilation in 164 patients with heterogeneous forms of ARF, only five patients had restrictive lung disease.30 All five patients improved on noninvasive ventilation, although one subsequently required intubation. Noninvasive ventilation was also helpful in four patients with KS and ARF failing conventional medical therapy.31

Following the guidelines of Meduri and colleagues,30 we initiate NPPV using a loose-fitting full face mask. We start with 0 cm H2O continuous positive airway pressure (CPAP) and 10 cm H2O pressure support, and increase CPAP to 3 to 5 cm H2O and pressure support to the level required to achieve an exhaled tidal volume ≥7 mL/kg and a respiratory rate ≤25/min.

Noninvasive negative pressure ventilators are not feasible in most acute situations because they generally require patients to lie flat and coordinate their breaths with the ventilator. Difficulties with fit and applying the device adequately to the distorted chest wall further complicate their use. Still, negative pressure ventilators have averted intubation in rare cases,18 and have been used successfully in the long-term management of patients with KS.32

Intubation and Mechanical Ventilation

Intubation is indicated for cardiopulmonary arrest, impending arrest, refractory hypoxemia, mental status changes, and shock. Intubation can be difficult because of spinal curvature and tracheal distortion, and because patients with small lung volumes desaturate quickly. Assessment of the upper airway with fiberoptic bronchoscopy may be useful in some cases. During the peri-intubation period, a fraction of inspired oxygen (FiO2) of 1.0 is desirable, although it should be decreased to nontoxic levels if possible once the patient has been stabilized on the ventilator. Decreasing O2 consumption with sedatives, use of positive end-expiratory pressure (PEEP), and increasing Pv̅O2 are strategies that allow for nontoxic FiO2 in most patients. Positional maneuvers, such as placing the patient in the lateral decubitus position, may improve oxygenation in patients with asymmetric chest walls, but care must be taken to secure the airway.

Ventilatory failure results from an imbalance between respiratory muscle strength and respiratory system load. Thus identifying and correcting reversible elements of this imbalance is fundamental to restoring the ability to breathe. While this occurs, patients should be ventilated to baseline values of PaCO2 to avoid alkalemia and bicarbonate wasting.

Respiratory muscle fatigue is treated with 48 to 72 hours of complete rest on the ventilator, with early nutritional supplementation and correction of metabolic irregularities. To rest, patients must be comfortable, quiet, and synchronized with the ventilator. If the patient is not synchronized, work of breathing remains high despite ventilator settings that appear to supply most of the minute ventilation.

In patients with bronchospasm and an increase in the peak-to-plateau gradient, we add inhaled bronchodilators and consider systemic steroids. With attention to delivery technique, bronchodilator responsiveness is assessed by measuring airway resistance 15 to 30 minutes after a treatment. Bronchoscopy may be indicated in nonresponders to exclude bronchial torsion that may benefit from placement of an endobronchial stent.19Theophylline titrated to a serum level of 10 μg/mL may increase respiratory muscle strength, help clear secretions, and decrease airway resistance without significant toxicity.

Although there are no controlled trials to help guide ventilator management in patients with thoracic deformity, we suggest small tidal volumes (6 to 7 mL/kg) and high respiratory rates (20 to 30/min) to minimize the hemodynamic effects of positive-pressure ventilation and the risk of barotrauma. We maintain plateau pressures <30 cm H2O to avoid overdistention beyond physiologic TLC and hyperinflation-induced lung injury (pneumothorax, pneumomediastinum, interstitial emphysema, and volutrauma). One consequence of small tidal volume ventilation is reduced alveolar ventilation and hypercapnia. Fortunately, hypercapnia is generally well tolerated as long as PaCO2 does not exceed 90 mm Hg and acute increases in PaCO2 are avoided. Associated low values of arterial pH are generally well tolerated. Hypercapnia does cause cerebral vasodilation, cerebral edema, decreased myocardial contractility, vasodilation with a hyperdynamic circulation, and pulmonary vasoconstriction. Accordingly, it should be avoided in patients with raised intracranial pressure (as might occur in the setting of anoxic brain injury after arrest) and severely depressed myocardial function. The use of small tidal volumes demands added attention to lung volume recruitment and prevention of atelectasis. To this end, we initially apply 5 cm H2O of PEEP to prevent alveolar closure at end-expiration, and gradually increase tidal volume when atelectasis is suspected, keeping plateau pressure <30 cm H2O.

In refractory hypoxemia a trial of increasing PEEP (in an attempt to achieve 90% saturation of the arterial blood with a FiO2 ≤0.6) helps clarify the pathophysiology. To avoid overdistention at end-inspiration, tidal volume should be decreased during PEEP titration. Since the chest wall is stiff in KS, high alveolar pressures increase pleural pressure more than in diseases characterized by stiff lungs, thereby further decreasing venous return to the right atrium and reducing cardiac output. PEEP may also increase pulmonary vascular resistance and worsen right-to-left intracardiac shunt.

The approach to liberation from mechanical ventilation is similar to that described elsewhere in this text. We favor early determination of respiratory muscle strength as assessed by the maximum negative inspiratory force (NIF), and of respiratory system load as determined by the resistive and static pressures generated during positive pressure ventilation. Inadequate strength for a given load manifests as a rapid shallow breathing pattern, which generates a high frequency:tidal volume ratio. A slower and deeper pattern is achieved when strength increases and/or load decreases. Respiratory muscle strength is improved by correction of shock, anemia, acidosis, electrolyte abnormalities, and with the institution of nutrition, and a nonfatiguing graded program of respiratory muscle exercise. Treating pulmonary edema, atelectasis, pneumonia, and airflow obstruction decreases load. When time of extubation is near, ventilation with tidal volumes that mimic the patient's spontaneous tidal volume allows for a smoother transition to spontaneous breathing. In borderline cases, NPPV can be used to facilitate return to spontaneous breathing and reduce ICU length of stay.33 Tracheostomy may be required in more difficult cases and may be technically difficult in patients with cervical spine curvature and a distorted airway.

Long-Term Management

Primary considerations for long-term management include the use of home oxygen therapy and nocturnal NPPV. Patients with moderate to severe KS may demonstrate significant oxygen desaturation on exercise that is prevented by ambulatory oxygen therapy.34 Derangements in breathing pattern and oxygen desaturation during sleep should be excluded with polysomnography once the patient is stable. A broad spectrum of abnormalities, including central and obstructive sleep apnea, has been identified that may contribute to chronic hypoxemia, cor pulmonale, and early death. Chronic ventilatory failure is an indication for noninvasive nocturnal ventilatory support, which can improve daytime blood gases, sleep pattern, and respiratory muscle strength.35 Other useful strategies include daytime IPPV (25 cm H2O for several minutes, 4 to 6 times daily), negative-pressure ventilation, and nighttime mechanical ventilation through a tracheostomy. The role of orthopedic surgery in adolescents is debated and beyond the scope of this chapter.

We recommend serial assessments of right ventricular function and pulmonary artery pressure by echocardiography. The presence of pulmonary hypertension gives added importance to oxygen therapy and mandates exclusion of other treatable causes such as pulmonary embolism.

Definition and Etiology

Abdominal compartment syndrome (ACS) is characterized by critical organ dysfunction within and beyond the abdomen resulting from increased intra-abdominal pressure.36–38 Importantly, the critical value for intra-abdominal hypertension that defines ACS varies from patient to patient, and is dependent on abdominal wall compliance and tissue perfusion pressure.39 Studies have suggested that an intra-abdominal pressure >25 mm Hg is associated with the development of ACS; other reports have demonstrated that pressures as low as 10 mm Hg can lead to organ dysfunction in select cases.40,41 Conditions that increase abdominal wall compliance (e.g., prior pregnancy, obesity, and cirrhosis) appear to be protective, while inflexible surgical closures or scars increase risk.42,43 Given these considerations, the use of a strict cutoff value for intra-abdominal hypertension to define ACS is not recommended. Rather a combination of elevated intra-abdominal pressure and the presence of organ dysfunction is required for proper diagnosis (see below).

A number of surgical and nonsurgical conditions have been associated with intra-abdominal hypertension. Particularly at risk is the trauma patient requiring large-volume resuscitation and emergent abdominal surgery. Use of temporary closure techniques as an alternative to tight surgical closure increases abdominal wall compliance and minimizes intra-abdominal pressure in patients at risk.44 Other conditions associated with ACS are included in Table 42-2.

Cardiopulmonary Effects

Cephalad displacement of the diaphragm results in mechanical compression, atelectasis, V̇/Q̇ inequality, decreased respiratory system compliance, and increased work of breathing.45 These effects are associated with a fast and shallow respiratory pattern, hypoxemia, and hypercapnia. In mechanically ventilated patients peak and plateau pressures are elevated, leading to a common recommendation to lower tidal volume until plateau pressure is <30 cm H2O.

Elevation of the diaphragm also reduces ventricular compliance and contractility; by reducing inferior vena caval blood flow, ACS decreases cardiac preload to further drop cardiac output.46 Hypovolemia further aggravates this physiology, whereas hypervolemia is protective as intra-abdominal pressure increases.47,48 Despite decreased venous return, pulmonary capillary wedge pressure does not always fall; however, intravascular pressure in this setting does not adequately reflect transmural pressure or intravascular volume.

Gastrointestinal, Renal, and Central Nervous System Effects

Intra-abdominal hypertension decreases mesenteric arterial flow, an effect that is amplified by decreased cardiac output, hemorrhage, and hypovolemia.49 Direct compression of mesenteric veins increases venous pressure, promoting visceral edema and further increases in intra-abdominal pressure that decrease gut perfusion.50 The final event is bowel wall ischemia and translocation of bacteria through a disrupted mucosal barrier that may be central to multiorgan system failure.51

Increased venous resistance is also central to the rise in renal venous pressure and renal failure in ACS.52 As intra-abdominal pressure increases, urine output falls with oliguria potentially developing at an intra-abdominal pressure of 15 mm Hg and anuria at 30 mm Hg.53

There is an association between abdominal compartment syndrome and raised intracranial pressure, likely mediated by the effects of intra-abdominal pressure on central venous pressure.54 A critical situation occurs when systemic hypotension and increased intracranial pressure combine to decrease cerebral perfusion pressure.

Diagnosis

The diagnosis of ACS should be considered in any patient with a tense or distended abdomen who also has hemodynamic instability, a falling urine output, mental status changes, or progressive lactic acidosis. Failure to recognize that intra-abdominal hypertension can occur in the absence of abdominal distension or that multiorgan failure is a manifestation of ACS can lead to a missed diagnosis. The importance of early detection is emphasized because mortality rates in established ACS are generally >50%.55

Although there are clues to ACS on abdominal CT imaging, definitive diagnosis requires measurement of intra-abdominal pressure.56 The usual approach is to use intra-abdominal bladder pressure (IABP) as a surrogate measure of intra-abdominal pressure, based on extensive data that indicate that the two are strongly correlated.57 The technique relies on measurements taken directly from a Foley catheter with a pressure-transducer zeroed to the level of the pubic symphysis.57 Accurate measurements rely on ready transmission of intra-abdominal pressure to the bladder, and the bladder not adding its own effects to pressure measurements. Thus there is potential for inaccuracy when the bladder is diseased or when there is critical pathology in the vicinity of the bladder. In general, ACS is not likely when IABP is <10 mm Hg, but should be considered when the pressure is >25 mm Hg. As mentioned, however, strict cutoff values for IABP are neither sensitive nor specific enough to establish ACS, requiring concurrent assessment of organ dysfunction at any level of intra-abdominal pressure.55

Treatment

Abdominal compartment syndrome resulting from tense ascites is an indication for immediate paracentesis. Pre- and postparacentesis measurements of IABP, urine output, hemodynamics, and airway pressures help confirm ACS and demonstrate the benefit of therapeutic intervention. In most other cases of ACS, prompt surgical decompression is indicated. Determining the appropriate timing for surgical intervention is challenging and may require serial measurement of IABP and assessment of organ function. It is vital to intervene before the development of critical organ dysfunction. To aid in the determination of timing of surgery, some surgeons assess abdominal perfusion pressure (APP = MAP − IABP). An APP <50 mm Hg is associated with a poor outcome and the need for quick intervention.58 While the operating room is prepared, volume resuscitation and low tidal volume ventilation are useful temporizing measures.

When normal air spaces and blood vessels are replaced by fibrotic tissue, the lungs become small and stiff. In some disease processes there may be adjacent areas of inflammation amenable to immunosuppressive therapy. However, it is generally well accepted that fibrosis without inflammation is unresponsive to current pharmacologic treatment.

A number of acute and chronic disorders of known and unknown etiology are associated with pulmonary fibrosis. These include idiopathic pulmonary fibrosis (IPF), which is highlighted in this chapter.

As the name implies, IPF is a disease of unknown etiology, characterized by the histologic appearance of usual interstitial pneumonia.59,60 It most often presents insidiously in patients >50 years old and is progressive regardless of therapy. The hallmarks of IPF are dyspnea and cough. Initially dyspnea occurs only with exercise, but even limited dyspnea can bring about a sedentary lifestyle that further adds to functional disability. As dyspnea progresses, patients develop a rapid and shallow breathing pattern, often maintaining higher-than-normal levels of minute ventilation.61 Constitutional symptoms include fatigue, malaise, and weight loss; symptoms of sleep-disordered breathing may occur.

Auscultation of the chest frequently reveals dry, “velcro-like” crackles heard loudest at the bases of the lungs; in the late stages of the disease, cardiac examination may reveal signs of pulmonary hypertension. Clubbing of the fingers and toes commonly occurs in IPF (and asbestosis) and rarely in other interstitial lung diseases.

Laboratory Abnormalities

Hypoxemia that worsens with exercise or sleep and respiratory alkalosis are common in IPF. The development of hypercapnia is an ominous sign of imminent death. Despite hypoxemia, polycythemia is rare.62 Elevations of sedimentation rate, serum immunoglobulins, antinuclear antibodies, and rheumatoid factor can occur; an increase in angiotensin-converting enzyme or antineutrophil cytoplasmic antibodies suggests a different diagnosis.59

The radiographic distribution of infiltrates often suggests the underlying disorder. Peripheral reticular opacities most prominent at the bases and low lung volumes are classic in IPF. Traction bronchiectasis occurs when fibrotic lung tissue tethers open adjacent airways. Ground-glass opacifications are either absent or minimal.59 Subpleural honeycombing at bases of both lungs is characteristic of IPF.59

Other causes of lower lobe–predominant infiltrates include fibrosis associated with connective tissue disorders, asbestosis, and chronic aspiration. Upper lobe–predominant lesions include sarcoidosis, tuberculosis, fungal infections, silicosis, allergic bronchopulmonary aspergillosis, eosinophilic granuloma, ankylosing spondylitis, berylliosis, cystic fibrosis, and hypersensitivity pneumonitis. If hilar adenopathy is present, sarcoidosis, tuberculosis, histoplasmosis, malignancy, and berylliosis should be considered. Pleural effusion suggests lymphangioleiomyomatosis, connective tissue disorder, asbestosis, or drug-induced lung disease. Extensive parenchymal cysts occur in eosinophilic granuloma or lymphangioleiomyomatosis. These conditions can result in diffuse infiltrates with normal or increased lung volumes, as can a mixed process of emphysema and IPF.

Respiratory Mechanics

In end-stage pulmonary fibrosis, pulmonary function tests typically show reduced TLC, VC, and IC (see Fig. 42-2). FRC and RV are also reduced, though usually to a lesser extent than TLC or VC. Rarely, RV is normal when there is early airway closure or decreased elastic recoil pressure at low lung volumes.63 Both the forced vital capacity (FVC) and the forced expiratory volume in 1 second (FEV1) are decreased, but FEV1/FVC is increased; in this instance, high expiratory flow rates relative to volume reflect increased elastic recoil pressure. Airway resistance is usually normal or low, although reversible and irreversible obstructive defects do occur.

Since chest wall compliance and respiratory muscles are normal in most patients with pulmonary fibrosis,64 lung volumes are affected by changes in the pressure-volume relationship of the noncompliant lung (Fig. 42-4).64 The P-V curve is shifted downward and to the right, such that increased elastic recoil of the lung limits TLC despite a very large transpulmonary pressure. This requires patients to generate large negative pleural pressures during inspiration, and is the reason why patients prefer a fast and shallow respiratory pattern. Smaller tidal volumes are an adaptive response to minimize work of breathing, which can be five to six times normal.65

Gas Exchange

Exercise-induced hypoxemia and a low single-breath diffusing capacity (DLCO) are hallmarks of early disease. Indeed they may occur before dyspnea or radiographic changes.63 With time, arterial hypoxemia and a widened (a-a)O2 are found at rest. In 20% of patients, arterial hypoxemia is worse when they are in the upright position and is improved on recumbency.62 This paradoxical pattern is also seen with patent foramen ovale, intrapulmonary arteriovenous malformation, and hepatopulmonary syndrome. Arterial saturation also falls significantly in many patients during REM sleep.66 Sleep-related desaturation is due to the exaggerated effects of normal nocturnal hypoventilation and V̇/Q̇ variance, or to abnormal alveolar hypoventilation from respiratory muscle dysfunction or obstructive sleep apnea.

The importance of an anatomic barrier to the diffusion of oxygen (secondary to a thickened, fibrotic interstitium) has been debated. In eight patients with varying types of interstitial lung disease, multiple inert gas analysis V̇/Q̇ inequality was the principal defect; diffusion limitation contributed to none of the (a-a)O2 at rest and only 19% of the (a-a)O2 during exercise.67 However, in 15 patients with IPF also studied by multiple inert gas elimination, 19% of the (a-a)O2 at rest and 40% of the (a-a)O2 during exercise was attributed to diffusion limitation.68 V̇/Q̇ inequality remained the principal defect, contributing to 81% of the (a-a)O2 at rest, and a combination of low Pv̅O2 from an inadequate cardiac output, diffusion limitation, and high V̇/Q̇ variance accounted for the widening of the (a-a)O2 during exercise. Intrapulmonary shunt was small, averaging 2% of cardiac output at rest and 3% during exercise.

The dead space:tidal volume ratio (Vds/Vt) may exceed 0.4 (normal = <0.3) in end-stage fibrosis.69 This reflects an increase in the volume of alveolar dead space and a decrease in tidal volume. When Vds/Vt is high, greater minute ventilation is required to maintain alveolar ventilation and a normal PaCO2. Patients may surpass these heightened requirements to achieve respiratory alkalosis, perhaps in response to greater afferent stimuli from the fibrotic lung.61 The development of hypercapnia is an ominous sign of imminent death.

Effects on the Pulmonary Circulation

Pulmonary hypertension and cor pulmonale are common in patients with end-stage pulmonary fibrosis, correlating with a DLCO <45% predicted and a VC <50% predicted.70 Pulmonary hypertension occurs when blood vessels are altered by the fibrotic process, microthrombi, or hypoxic pulmonary vasoconstriction. Since polycythemia and high cardiac output are rare in IPF, they rarely contribute to pulmonary hypertension. Supplemental oxygen may alleviate hypoxic pulmonary vasoconstriction, but pulmonary hypertension resulting from destroyed and distorted vasculature is likely irreversible. Pulmonary hypertension has been associated with redistribution of pulmonary blood flow to the upper lobes,62 a pattern that rarely normalizes after corticosteroids.69 Because pulmonary vascular resistance is high, the gradient between the pulmonary artery diastolic pressure and the pulmonary capillary wedge pressure is wide.

Acute Cardiopulmonary Failure

Whether acute deterioration is reversible depends on the severity of pulmonary fibrosis, the extent of comorbidities, the nature and severity of the acute insult, and whether the acute insult accelerates the underlying disease process.

Outcome

ICU management including the use of mechanical ventilation may be appropriate for select patients with pulmonary fibrosis: (1) patients with early/mild disease, particularly in absence of a firm diagnosis, (2) patients with previously mild disease who present with an acute, seemingly reversible insult, (3) patients who have experienced adverse effects of therapy (e.g., drug-induced lung disease), (4) patients who may undergo imminent lung transplant, and (5) patients with pulmonary fibrosis associated with connective tissue disease or vasculitis with ground-glass opacifications or consolidation that may represent a treatable form of pneumonitis or alveolar hemorrhage. Appropriately excluded from this practice are patients with progressive, end-stage disease in whom outcome is invariably poor. In these cases, a prospective discussion and decision not to initiate resuscitative efforts should precede the anticipated terminal event.

The results of several recent studies demonstrate the grim prognosis of patients with IPF admitted to the ICU for acute respiratory failure. Blivet and colleagues described the course of 15 patients admitted to the ICU with IPF and respiratory failure.71 Twelve patients required intubation either at the time of admission or after failure of noninvasive ventilation; three patients received only noninvasive ventilation. Eleven patients died either from respiratory failure or septic shock; four patients were discharged alive from the ICU, but two died shortly thereafter. Stern and colleagues reported their experience in 23 patients with IPF requiring intubation for acute respiratory failure.72 With the exception of one patient who received a single-lung transplant 6 hours after initiating mechanical ventilation, all patients died while receiving mechanical ventilation. Fumeaux and colleagues similarly reported 100% mortality in 14 consecutive patients with IPF admitted to the ICU for mechanical ventilation after a mean of 7.6 days.73 Finally, in the study by Saydain and colleagues of 38 patients with IPF admitted to the ICU mainly for respiratory failure, ICU mortality was 43% and hospital mortality was 61%.74 However, 92% of hospital survivors died a median of 2 months after discharge.

Detecting Reversible Features

Although clinical deterioration may reflect disease progression, the differential diagnosis of an acute change in status is quite broad (Table 42-3).59 In our experience it is often difficult to distinguish disease progression from respiratory tract infection. Frequently the cause of acute respiratory failure is not identified.72 Acute deterioration stemming from IPF progression is essentially an untreatable condition (see above).

Pneumonia in IPF generally results from community-acquired bacteria; opportunistic infection is rare despite the widespread use of immunosuppressive agents.70 However, Pneumocystis carinii pneumonia should be considered in immunosuppressed patients, particularly when ground-glass infiltrates are identified on CT imaging. Mycobacterial infection should also be considered. The incidence of tuberculosis is increased in patients with chronic interstitial lung disease, particularly in those with silicosis;75 atypical mycobacterial superinfection may also underlie clinical deterioration in some cases. When fibrosis is associated with connective tissue disease or vasculitis, an acute deterioration may represent pneumonitis, bronchiolitis obliterans-organizing pneumonia, or alveolar hemorrhage. Patients with diffuse alveolar opacifications not responsive to antibiotics or diuresis should be considered for bronchoalveolar lavage or lung biopsy in selected cases to confirm the diagnosis.

When pneumothorax causes acute deterioration, re-expansion is countered by low parenchymal compliance occasionally requiring high levels of negative pleural pressure for prolonged periods of time.

Clinical deterioration may represent acute right or left heart failure. Right ventricular ischemia or pulmonary embolism may provoke right ventricular failure. Left-sided failure is difficult to establish without echocardiography or hemodynamic measurements because bibasilar crackles are invariably present and jugular venous distention may reflect isolated right ventricular failure.

If deterioration occurs over a period of weeks to months, progressive fibrosis, bronchogenic carcinoma, steroid myopathy, drug toxicity, cor pulmonale, and left ventricular failure should be considered.

Oxygen Therapy

Identification and correction of arterial hypoxemia are vital to the initial evaluation. We recommend titration of supplemental oxygen until 90% saturation is achieved. It is not unusual for patients with fibrosis to require higher flow rates than patients with chronic obstructive pulmonary disease or asthma. Hypoxemia, in addition to its many other adverse effects, causes hypoxic pulmonary vasoconstriction and may precipitate right ventricular failure in patients with pre-existing pulmonary hypertension.

Cardiovascular Management

Evaluation of shock in patients with pulmonary fibrosis is similar to that described elsewhere in this text (see Chaps. 20 and 21). Hypotension with cool and clammy extremities and a narrow pulse pressure suggests an inadequate cardiac output from hypovolemia, left ventricular failure, cor pulmonale, pericardial effusion, or valvular heart disease. Hypotension with a warm and bounding circulation and a wide pulse pressure suggests sepsis; however, patients with pulmonary hypertension may be unable to mount a hyperdynamic response to peripheral vasodilation. Adrenal insufficiency contributing to shock should be excluded, particularly when corticosteroids have been used for therapy.

Ventilator Management

When deemed appropriate, intubation is indicated for cardiopulmonary arrest, refractory hypoxemia, progressive ventilatory failure, mental status changes, and shock. Once the decision to intubate has been made, the goals are to achieve adequate arterial oxygen saturation and avoid ventilator-induced lung injury. Although there are no controlled trials to guide specific recommendations, we advocate an approach to mechanical ventilation that is similar to that recommended in the acute respiratory distress syndrome.76 To avoid excessive tidal volume excursions and alveolar overinflation, we use tidal volumes in the range of 6 to 7 mL/kg combined with respiratory rates of 20 to 30 breaths/min when ventilating patients with pulmonary fibrosis. The goal is to maintain airway plateau pressures below 30 cm H2O in an attempt to avoid overdistention beyond physiologic TLC and hyperinflation-induced lung injury. A consequence of small tidal volume ventilation is reduced alveolar ventilation and hypercapnia. Hypercapnia can be reduced for a given minute ventilation by decreasing CO2 production (i.e., treating fever and agitation and avoiding excessive caloric intake), repleting intravascular volume, and avoiding excessive PEEP to decrease Vds/Vt. Still, in order to achieve desired airway pressures, hypercapnia is unavoidable in many cases. In patients with chronic ventilatory failure, we further desire a minute ventilation that maintains PaCO2 greater than or equal to baseline to avoid alkalemia and bicarbonate wasting.

The use of PEEP avoids tidal collapse of alveoli at low lung volumes and may help recruit fluid-filled and atelectatic lung units. We generally start with 5 cm H2O PEEP and increase in an attempt to achieve 90% saturation of the arterial blood with a FiO2 of 0.6 or less. Though FiO2 of 1.0 is desirable in the peri-intubation period, it should be decreased as quickly as possible (to no greater than 0.6) to avoid oxygen toxicity. The use of sedatives and muscle relaxants to decrease oxygen consumption, PEEP, and increasing Pv̅O2 allows for nontoxic FiO2 in many cases.

High alveolar pressures compress alveolar vessels, diverting blood flow from ventilated units and increasing dead space. Increasing Vds/Vt from 0.4 to 0.6 requires an increase in minute ventilation of 50% to maintain a constant PaCO2.77 Increasing minute ventilation, however, increases alveolar pressure and Vds/Vt further, creating a vicious cycle if minute ventilation is continually increased in a misguided attempt to lower PaCO2. High alveolar and pleural pressures also increase right atrial pressure and thereby decrease venous return to the right atrium, cardiac output, Pv̅O2, and blood pressure. Additionally, diversion of blood flow using high alveolar pressure increases flow to nonventilated units, as in pneumonic consolidation.

Long-Term Management of Idiopathic Pulmonary Fibrosis

Because no drug therapy has clearly been demonstrated to benefit patients with IPF, long-term management is largely supportive.60 We suggest referral to a regional center of expertise for consideration of enrollment in a clinical trial or evaluation for lung transplantation. The poor outcomes in patients with IPF admitted to the ICU underscores the importance of advance directives.