Key points

Introduction

Mechanical ventilation is used in the intensive care unit. It is estimated that among patients admitted to medical surgical intensive care units, the primary indication for mechanical ventilation is neurologic in 20% of cases. This statistic is much higher in a dedicated Neurocritical Care Unit (NCCU), in which as many as 80% of patients are intubated for a primary neurologic injury. Patients with acute neurologic injuries usually require mechanical ventilation for reasons other than direct injury to the lungs. Many patients with acute brain injury are intubated to protect the airway in the setting of altered mental status. Even after recovery from acute brain injury, there is a subset of neurologically injured patients who fail estuation from mechanical ventilation because of neurologic respiratory insufficiency without injury to the lungs or increased work of breathing.

There are special considerations for the management of mechanical ventilation in patients with neurologic injuries. Clinicians must be aware of clinical issues surrounding ventilator management in these patients and must focus on strategies to enhance neurologic recovery and facilitate extubation. Included is a list of neurologic disorders seen in the NCCU commonly requiring intubation and ventilator support ( Box 1 ).

Patients with brain injury who are comatose or obtunded often present with the concern of airway compromise caused by altered mental status. With decreased levels of consciousness, there is reduced tone of the oropharyngeal muscles leading to posterior displacement of the tongue often causing airway obstruction. Combined with impaired swallowing mechanisms and inhibition of the cough and gag reflexes, these patients are at risk for aspiration. If the state of their altered mental status is rapidly reversible, the patient may not require immediate intubation. However, the patient should be in a monitored setting where he or she can be easily intubated if necessary. If the patient will be neurologically impaired for a long period of time, with absent cough and gag reflexes, intubation should be considered for airway protection.

In certain clinical circumstances, it may be prudent to institute mechanical ventilation based on the anticipation of neurologic deterioration during the progression of the underlying condition. In patients with aneurysmal subarachnoid hemorrhage with severe vasospasm, the best strategy may be to intubate and initiate mechanical ventilation, to insure adequate pulmonary gas exchange in the setting of hemodynamic augmentation and subsequent pulmonary edema and hypoxemia. Patients with hemispheric strokes and malignant cerebral edema may require early intubation in anticipation of the need for transient hyperventilation, hypoxemia, and surgical intervention.

Brain injury always causes dysregulation of the respiratory drive and/or altered pulmonary mechanical function. Neural control of respiration depends on both conscious and automatic inputs integrated in the pons and medulla. Automatic control of respiration is located in areas of the dorsolateral tegmentum of the pons as well as the medulla, specifically the nucleus tractus solitarus and retroambigualis. The descending pathways of the ventrolateral columns of the spinal cord allow for conscious input from the cortex.

Automatic respiration is a homeostatic mechanism through which pulmonary function is controlled by regulatory centers in the brain stem. These centers act to regulate acid-base status and to meet oxygen demand. Central chemoreceptors in the medulla monitor the pH associated with CO ₂ levels within the cerebrospinal fluid in the fourth ventricle. Peripheral chemoreceptors located in the carotid bodies and aortic bodies monitor the P co ₂ , pH, and P o ₂ of arterial blood while relaying the information to the respiratory centers via the vagus and glossopharyngeal nerves. An increase in the P co ₂ in the blood decreases the pH, thereby stimulating the respiratory centers to increase ventilation and improve CO ₂ elimination. The peripheral chemoreceptors also respond to a drop in arterial P o ₂ less than 60 mm Hg, stimulating the respiratory centers to increase ventilation to achieve appropriate oxygenation. Hypercapnea is a more sensitive respiratory stimulus than hypoxemia in most people except those who have compensated for chronic CO ₂ retention, such as in chronic obstructive pulmonary disease.

Human respiration is most strongly affected by conscious inputs originating from the cortex. These outputs represent most of the stimuli affecting respiration. In the healthy human these outputs often occur beyond awareness. In patients who are comatose with brain injury, conscious input from the cortex is eliminated. In this setting the architecture of respiration is controlled almost entirely by automatic input originating from the brain stem. It is in this setting that classical patterns of respiration associated with regional brain injury become apparent.

The most commonly observed patterns of breathing in patients with brain injury are tachypnea and hyperventilation. These patterns are frequently seen as a result of diffuse cortical and subcortical injury and can be seen even in patients who seem to be neurologically intact. This pattern is a consequence of inhibition of conscious input from the cortex and increased dependency on the Pa co ₂ as a trigger for respiratory drive from the automatic centers in the brain stem. In patients with cortical injury a dysnchrony between the normal cortically initiated cues for respiration and the automatic regulation of respiration exists. Thus the coordination between conscious inputs and automatic inputs is disrupted with an increased dependence on automatic regulation and suppression of cortical output.

There are classic patterns of abnormal breathing associated with specific neurologic lesions in different locations in the brain. These patterns are seen in patients with intracranial mass lesions and elevated intracranial pressure. Cheynes-Stokes respiration is the most common pattern seen with brain injury. It is characterized by a regular cyclic crescendo-decrescendo pattern of variable respiratory rate and tidal volumes. It is associated with disruptions between the bilateral cortical hemispheres and dysfunction of the medial forebrain structures. Apneustic breathing is characterized by prolonged inspiratory pause and is associated with lesions of the lower tegmentum of the pons. Cluster breathing is irregular quick breaths regularly separated by long pauses. It is associated with lesions of the lower pons or upper medulla. Ataxic breathing is similar to cluster breathing except that the there is a complete loss of rhythmicity of breathing with irregularly timed breaths with variable tidal volumes usually of smaller sizes. It is often called the atrial fibrillation of breathing and occurs lesions of the medulla. Patients can be observed to have these abnormal breathing patterns in succession over minutes to hours with elevation of intracranial pressure and progressive downward herniation as brain function is compromised in a rostral to caudal fashion.

Introduction

Mechanical ventilation is used in the intensive care unit. It is estimated that among patients admitted to medical surgical intensive care units, the primary indication for mechanical ventilation is neurologic in 20% of cases. This statistic is much higher in a dedicated Neurocritical Care Unit (NCCU), in which as many as 80% of patients are intubated for a primary neurologic injury. Patients with acute neurologic injuries usually require mechanical ventilation for reasons other than direct injury to the lungs. Many patients with acute brain injury are intubated to protect the airway in the setting of altered mental status. Even after recovery from acute brain injury, there is a subset of neurologically injured patients who fail estuation from mechanical ventilation because of neurologic respiratory insufficiency without injury to the lungs or increased work of breathing.

There are special considerations for the management of mechanical ventilation in patients with neurologic injuries. Clinicians must be aware of clinical issues surrounding ventilator management in these patients and must focus on strategies to enhance neurologic recovery and facilitate extubation. Included is a list of neurologic disorders seen in the NCCU commonly requiring intubation and ventilator support ( Box 1 ).

Patients with brain injury who are comatose or obtunded often present with the concern of airway compromise caused by altered mental status. With decreased levels of consciousness, there is reduced tone of the oropharyngeal muscles leading to posterior displacement of the tongue often causing airway obstruction. Combined with impaired swallowing mechanisms and inhibition of the cough and gag reflexes, these patients are at risk for aspiration. If the state of their altered mental status is rapidly reversible, the patient may not require immediate intubation. However, the patient should be in a monitored setting where he or she can be easily intubated if necessary. If the patient will be neurologically impaired for a long period of time, with absent cough and gag reflexes, intubation should be considered for airway protection.

In certain clinical circumstances, it may be prudent to institute mechanical ventilation based on the anticipation of neurologic deterioration during the progression of the underlying condition. In patients with aneurysmal subarachnoid hemorrhage with severe vasospasm, the best strategy may be to intubate and initiate mechanical ventilation, to insure adequate pulmonary gas exchange in the setting of hemodynamic augmentation and subsequent pulmonary edema and hypoxemia. Patients with hemispheric strokes and malignant cerebral edema may require early intubation in anticipation of the need for transient hyperventilation, hypoxemia, and surgical intervention.

Brain injury always causes dysregulation of the respiratory drive and/or altered pulmonary mechanical function. Neural control of respiration depends on both conscious and automatic inputs integrated in the pons and medulla. Automatic control of respiration is located in areas of the dorsolateral tegmentum of the pons as well as the medulla, specifically the nucleus tractus solitarus and retroambigualis. The descending pathways of the ventrolateral columns of the spinal cord allow for conscious input from the cortex.

Automatic respiration is a homeostatic mechanism through which pulmonary function is controlled by regulatory centers in the brain stem. These centers act to regulate acid-base status and to meet oxygen demand. Central chemoreceptors in the medulla monitor the pH associated with CO ₂ levels within the cerebrospinal fluid in the fourth ventricle. Peripheral chemoreceptors located in the carotid bodies and aortic bodies monitor the P co ₂ , pH, and P o ₂ of arterial blood while relaying the information to the respiratory centers via the vagus and glossopharyngeal nerves. An increase in the P co ₂ in the blood decreases the pH, thereby stimulating the respiratory centers to increase ventilation and improve CO ₂ elimination. The peripheral chemoreceptors also respond to a drop in arterial P o ₂ less than 60 mm Hg, stimulating the respiratory centers to increase ventilation to achieve appropriate oxygenation. Hypercapnea is a more sensitive respiratory stimulus than hypoxemia in most people except those who have compensated for chronic CO ₂ retention, such as in chronic obstructive pulmonary disease.

Human respiration is most strongly affected by conscious inputs originating from the cortex. These outputs represent most of the stimuli affecting respiration. In the healthy human these outputs often occur beyond awareness. In patients who are comatose with brain injury, conscious input from the cortex is eliminated. In this setting the architecture of respiration is controlled almost entirely by automatic input originating from the brain stem. It is in this setting that classical patterns of respiration associated with regional brain injury become apparent.

The most commonly observed patterns of breathing in patients with brain injury are tachypnea and hyperventilation. These patterns are frequently seen as a result of diffuse cortical and subcortical injury and can be seen even in patients who seem to be neurologically intact. This pattern is a consequence of inhibition of conscious input from the cortex and increased dependency on the Pa co ₂ as a trigger for respiratory drive from the automatic centers in the brain stem. In patients with cortical injury a dysnchrony between the normal cortically initiated cues for respiration and the automatic regulation of respiration exists. Thus the coordination between conscious inputs and automatic inputs is disrupted with an increased dependence on automatic regulation and suppression of cortical output.

There are classic patterns of abnormal breathing associated with specific neurologic lesions in different locations in the brain. These patterns are seen in patients with intracranial mass lesions and elevated intracranial pressure. Cheynes-Stokes respiration is the most common pattern seen with brain injury. It is characterized by a regular cyclic crescendo-decrescendo pattern of variable respiratory rate and tidal volumes. It is associated with disruptions between the bilateral cortical hemispheres and dysfunction of the medial forebrain structures. Apneustic breathing is characterized by prolonged inspiratory pause and is associated with lesions of the lower tegmentum of the pons. Cluster breathing is irregular quick breaths regularly separated by long pauses. It is associated with lesions of the lower pons or upper medulla. Ataxic breathing is similar to cluster breathing except that the there is a complete loss of rhythmicity of breathing with irregularly timed breaths with variable tidal volumes usually of smaller sizes. It is often called the atrial fibrillation of breathing and occurs lesions of the medulla. Patients can be observed to have these abnormal breathing patterns in succession over minutes to hours with elevation of intracranial pressure and progressive downward herniation as brain function is compromised in a rostral to caudal fashion.

Effects of neurologic injury on the pulmonary system

There is a subset of patients with neurologic disease who experience specific pulmonary complications caused by their neurologic illness. These associated pulmonary conditions are neurogenic pulmonary edema (NPE) and pulmonary edema from stunned myocardium. NPE has been reported extensively in the setting of acute neurologic injury, including seizures, traumatic brain injury (TBI), as well as cervical spinal cord injury associated with hanging. This disorder can occur rapidly with onset of initial neurologic injury or it can occur at later stages of illness. Reports suggest an incidence of 40% of neurogenic pulmonary edema for all head injury subtypes and a 90% incidence in the setting of intracranial hemorrhages. The use of supportive measures, such as positive end-expiratory pressure (PEEP), to maintain sufficient blood and brain oxygenation are usually quite effective in this disorder. Case reports also suggest that patients with this disorder may be particularly responsive to prone positioning, although such positioning is difficult in patients who have intraventricular drains or other monitors.

NPE is strongly associated with lesions in specific regions of the brain. Several experimental models have demonstrated that focused damage to the nucleus of the tractus solitarus in the medulla of the human and experimentally in rodents causes NPE. It has been studied extensively in humans since the 1950s, at which time an association between high cervical cord injury and the immediate onset of pulmonary edema was observed. Since then it has been studied in the setting of armed conflicts as well as in civilian emergency rooms. It is caused by the extravasation of a proteinaceous fluid across the alveolar membrane of the lungs secondary to injury from the catecholamine storm associated with severe neurologic injury. It is different from acute respiratory distress syndrome (ARDS), acute lung injury (ALI), and transfusion-associated lung injury (TRALI) in that the mechanism of injury in ARDS, ALI, and TRALI are the result of an inflammatory reaction to lung injury and the alveolar fluid is produced from the pneumocytes within the alveolar wall.

The diagnosis of NPE is often difficult to separate from other forms of lung injury. Other causes of pulmonary edema commonly seen in the setting of acute neurologic injury include pulmonary edema from congestive heart failure and stunned myocardium, ARDs, ALI, and TRALI. In general, NPE is different from pulmonary edema from heart failure, ARDS, ALI, and TRALI in that the wedge pressure usually is not elevated and the echocardiogram is usually normal. It has a rapid onset at the time of neurologic injury and often involves only one lung field. NPE is usually temporary in duration and often exquisitely PEEP responsive. Treatment involves supportive measures including intubation, elevated PEEP, elevated Fi o ₂ , and diuresis if necessary.

Acute neurologic injury has long been recognized as a primary cause of stunned myocardium. It has been used extensively in subarachnoid hemorrhage and has been associated with several neurologic diseases including brain tumors, seizures, ischemic stroke, hemorrhagic stroke of all types, and peripheral nerve injuries, such as Guillain-Barre syndrome. It has been documented to occur in up to 40% of all brain injuries and 90% of all ICHs. It is synonymous with Takotsubo’s cardiomyopathy, typified as a syndrome of global hypokinesis associated with an increase in serum catecholamines with damage to the myocardium appearing as contraction band necrosis located in the fibers of the myocardium. It is usually a reversible and temporary phenomenon lasting only a few days.

Myocardial stunning in the setting of acute neurologic injury often results in acute fulminant pulmonary edema. In subarachnoid hemorrhage patients with cardiogenic shock due to stunned myocardium, aggressive support with inotropic agents to maintain adequate brain perfusion to avoid focal ischemic from vasospasm is sometimes required. Intraaortic balloon counterpulsation has also been used as a measure to facilitate cardiac output in this situation. In the ventilated patient with severe neurologic injury, this entity can often be confused with other syndromes, such as NPE, ALI, ARDS, or TRALI. This issue is usually easily resolved with the use of echocardiography.

Diagnosis focuses on the detection of increased serum troponins and creatine kinase MB, with troponin levels disproportionately elevated in comparison to creatine kinase MB. However, overall the cardiac enzymes are only minimally elevated as compared with occlusive cardiac disease. Electrocardiogram changes are typified by nonspecific ST changes in an apical distribution. Global as opposed to segmental hypokinesis is observed on echocardiogram usually with an apical pattern. Early detection of this disorder and appropriate treatment with fluid management and inotropic support are essential to avoid potentially fatal outcomes. Intubation and appropriate ventilatory maneuvers to avoid hypoxia are often required to support the patient while the patient is treated for underlying myocardial dysfunction and neurologic injury.