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.
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
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.
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.
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.
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.