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Mechanical ventilation in neurological and neurosurgical patients
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.181585
Approximately 20% of all patients requiring mechanical ventilation suffer from neurological dysfunction. It is imperative in the ventilatory management of such patients to have a thorough understanding of the disease pathology that may require institution of mechanical ventilation as well as in realizing its effects on the injured brain. These patients have unique challenges pertaining to the assessment and securing of the airway, maintenance of mechanical ventilation, as well as weaning and extubation readiness. This manuscript aims to present the current evidence in ventilatory management of the important subset of patients with neuronal injury. The indications for ventilatory management include both neurological and neurosurgical causes. Keywords: Mechanical ventilation; neurological patient; ventilatory management; weaning
The association of mechanical ventilation with neurological afflictions can be historically traced to the advent of mechanical ventilation in clinical practice in the 1950s, when it was used a life-saving measure in the polio epidemic in Europe.[1] Mechanical ventilation has since evolved in tandem with the ongoing technological advancements and is being increasingly used across different subspecialties, including in those patients suffering from neurosurgical and neurological ailments. Approximately 20% of all patients requiring mechanical ventilation suffer from neurological dysfunction.[2] Hence, there is a recent trend to treat neurocritical care as a separate subspecialty.[3] The major subsets of patients suffering from neurological afflictions and requiring ventilatory support can be categorized into those suffering from either neurosurgical or neurological disorders. While patients with traumatic brain injury (TBI) and spinal cord injury (SCI) are the major subset of patients in the neurosurgical population, patients suffering from disorders such as Guillain–Barre syndrome (GBS) and myasthenia gravis (MG) form the major bulk among neurological patients requiring mechanical ventilation.[3] It is important to have a thorough knowledge of the diseases that may require mechanical ventilation. An understanding of its effects on the injured neuronal tissue is also mandated.
The indications for mechanical ventilation in neurologically injured patients could either be one of the four pathologies or their combinations that include loss of respiratory drive, dysfunction of lung compliance, low Glasgow Coma Scale (GCS) that is hampering gas exchange, and ventilatory failure due to disorders of the neuromuscular junction [Table 1]. The loss of consciousness and the resultant airway obstruction caused by the falling back of flaccid tongue on the posterior pharyngeal wall in neurologically obtunded patients is the major indication for airway management in such patients.[4] In addition, worsening of the upper airway muscle tone and protective reflexes predispose such patients to life-threatening sequelae such as pneumonia and acute respiratory distress syndrome (ARDS). GCS, a universally used score to measure consciousness, has been shown to have a direct correlation with the incidence of aspiration in neurologically injured patients such as those with spinal cord injury and stroke.[5],[6],[7]
Disruption of the ventilatory control centers originating in the medulla oblongata as a result of CNS injury is another pathological mechanism requiring mechanical ventilation. Such insults to the respiratory drive mechanisms are commonly seen in a variety of surgical pathologies, such as traumatic brain injury (TBI), intracranial space-occupying lesions (ICSOLs), subarachnoid hemorrhage (SAH), medical illnesses such as stroke and morbid obesity. They may also be seen as the side effect of commonly administered drugs such as opioids and sedative agents. Disorders of lung compliance (which may be due to aspiration pneumonitis, neurogenic pulmonary edema, and pneumothorax or a flail chest in patients with traumatic injury) are also major indications for mechanical ventilation in the neurosurgical population.[8] While understanding the pathophysiology in patients with neurological injury requiring mechanical ventilation, it is also pertinent to note the neurological ramifications of impairment in ventilation. Hypercapnia and hypoxia, as a consequence of ventilatory dysfunction, result in an increase in cerebral blood volume. The consequent rise in intracranial pressure (ICP) often causes a concern in neurological patients with decreased intracranial compliance. Another significant aspect in the cohort of neurologically injured patients is the occurrence of systemic compromise, especially the pulmonary complications, which cause further difficulty in the ventilatory management of such patients.[9]
The initiation of mechanical ventilation in neurosurgical/neurological patients involves appropriate decision making in identifying the patients at risk, followed by prompt assessment of their airway, and culminating in the safe securing of the airway. The clinical features signifying respiratory failure in such patients include a disorder of respiratory rate (RR <6 or >30 breaths per minute), use of accessory muscles of respiration, nasal flaring, tracheal tug, paradoxical breathing, and periods of apnea. These should be used in conjunction with laboratory parameters such as vital capacity (VC), negative inspiratory force (NIF), forced expiratory volume (FEV), and minute volume (MV).[10] Assessment of the airway has a critical role in the initiation of mechanical ventilation in such patients. The established predictors of difficult mask ventilation and laryngoscopy in the context of emergency scenarios should be carefully evaluated.[11],[12] There is an omnipresent risk of aggravation of the pre-existing cervical spine injury in patients who have suffered from major trauma and have an underlying cervical pathology. Hence, a careful perusal of the different modes of airway management is desirable.[13],[14],[15] Direct laryngoscopy is the gold standard of airway management. Direct laryngoscopy, with and without manual in-line stabilization (MILS), while offering the advantages of familiarity, accessibility, and availability, has been shown to cause significant undesirable craniocervical movement.[16],[17],[18],[19],[20] A variety of video laryngoscopes have been studied for cervical spine movement. They have shown lesser cervical spine movement in comparison to the classical blades.[21],[22],[23] The application of video laryngoscopes in day-to-day practice is limited by the availability issues and the fact that tracheal cannulation is a demonstrably arduous process with these devices.[24] While fiber optic intubation has been shown to be a reliable and safe method of securing the airway in patients with suspected cervical spine injury, it requires training and may not be a viable alternative in the emergency setting, especially in the presence of hemorrhage and swelling of the airway.[25],[26] Common adjuncts such as the jaw thrust maneuver and devices such as oral and nasal airways as well as laryngeal mask airways (LMAs) play a vital role in the maintenance of oxygenation. They act as a bridge for facilitating oxygenation before the placement of a definitive airway in the form of an endotracheal tube (ETT) or a tracheostomy tube. Patients undergoing early tracheostomy have demonstrated advantages in terms of a reduction in the overall duration of mechanical ventilation and an early weaning when compared with ETT, without any significant difference in neurological outcome.[27],[28],[29] The use of pharmacological adjuncts in the form of sedatives, analgesics, and muscle relaxants is common in neurological patients requiring mechanical ventilation. Their use, however, should be tempered with the knowledge about their effects on the brain and other organ systems. One should, therefore, take precautions so that the use of these medications does not impede neurological monitoring and weaning in neurocritical patients [Table 2].
Appropriate delivery of mechanical ventilation to the neurological/neurosurgical patients requires a precise understanding of the various modes as well as the effects of mechanical ventilation on the injured brain and spinal cord. Modes of ventilation Despite the abundance of various modes of ventilation in critical care, a basic understanding of volume, pressure, timing, and trigger should suffice to aid in the proper selection of ventilatory modes in different subsets of neurological patients. The choice of the ventilatory modes in these patients is largely governed by the site of the lesion and the severity of ventilatory impairment caused by it [Table 3].
Brain stem and cortex Cortical pathologies such as TBI, ICSOLs and hemorrhage as well as lesions of the brain stem cause an impairment of the respiratory drive and result in aberrant respiratory patterns such as hypo- and hyperventilation syndromes, apnea, and Cheyne–Stokes breathing. They usually require controlled ventilation as the initial mode, followed by a switching over to the assisted modes when there is a progressive improvement in the act of spontaneous breathing.[30],[31] Spinal cord Lesions of the spinal cord, more commonly spinal cord injury (SCI), present with unique ventilatory concerns. SCI at C5 and above usually causes complete paralysis of the phrenic and intercostal nerves and consequently requires long periods of controlled ventilation and an early tracheostomy.[32] Lesions below C5 show a variable course with a high incidence of delayed-onset respiratory failure that is commonly attributed to a variety of causes such as muscle fatigue, aspiration pneumonia, atelectasis, and pooling of secretions.[33],[34] The proposed mechanisms for the sudden deterioration include a rapid expansion of the lesion and cellular degeneration, but conclusive evidence is lacking.[35] An improvement in the respiratory function in patients with cervical SCI is also observed a few weeks after the primary insult as a result of development of spasticity in the intercostal muscles, which has a favorable effect on the lung mechanics.[36] Thus, patients with spinal cord injury might require either controlled or assisted modes of ventilation depending on the site and extent of the lesion, the amount of respiratory muscle compromise, the degree of respiratory drive present, and the duration of injury. Neuromuscular diseases Ventilatory requirements in patients with neuromuscular diseases (NMDs) such as Guillain-Barre syndrome (GBS) and myasthenia gravis (MG) are very specific as they have to compensate for a slow affliction of the respiratory system affecting oxygenation of the patient. In these conditions, an initial compensatory phase of ventilation precedes the ongoing respiratory compromise. A high index of suspicion as well as a continuous assessment using bedside pulmonary function tests (PFTs) will help the clinician in the assessment of these patients and in instituting elective intubation [at a vital capacity (VC) <10–15 mL/kg and a negative inspiratory force (NIF) <20g].[37] Noninvasive ventilation had been shown to have a limited application in patients with MG that is responsive to pharmacologic therapy and is largely redundant in the GBS population.[38],[39],[40],[41] Brain protection versus lung protection In patients receiving mechanical ventilation, the components such as volutrauma, atelectrauma, oxygen toxicity, and biotrauma have been found to contribute to ventilator-initiated lung injury (VILI).[42],[43],[44],[45],[46],[47] Protection from VILI has had a significant effect on outcome. This has been unequivocally demonstrated in mechanically ventilated patients who are managed based on the principles of institution of a minimal tidal volume, utilization of high levels of positive end expiratory pressure (PEEP), and avoidance of high inspired oxygen concentrations.[48],[49] These principles may occasionally act as a hindrance in the adequate management of neurosurgical patients, and the challenge is to administer lung protective ventilation without compromising the basic tenets of brain protection. Role of hyperventilation and tidal volume Hyperventilation and the consequent hypocapnia (PCO2 <25 mm Hg) has been postulated as a method for decreasing cerebral blood flow (CBF) and has been used in patients with a refractory increase in the intracranial pressure (ICP). The advantages should be weighed against the harmful effects of cerebral ischemia on the injured brain as has been demonstrated in various types of brain insults, and in particular, TBI.[50],[51],[52],[53],[54] On the basis of the available evidence, hyperventilation should be used only as a treatment adjunct in the cases of acute deterioration in the neurological status due to refractory causes of raised ICP, with a target PCO2 between 30 and 35 mm Hg.[55] There has been a rising incidence in the use of small-tidal-volume protocols in the ventilatory management of critical care patients with the perceived advantage of avoiding volutrauma while still achieving eucapnia.[56] However, achievement of the desirable PCO2 levels in brain-injured patients with coexisting dysfunction in the intracranial compliance often presents a clinical conundrum. We hereby present a flow chart [Figure 1] for management of ventilation in terms of the desired PCO2 titrated to the intracranial compliance.
Positive end expiratory pressure (PEEP), oxygen therapy, and newer modalities The inherent concerns of using PEEP in brain-injured patients that includesits proposed effect in decreasing the cerebral blood flow and causing a consequent raised in ICP as well as a decrease in the mean arterial pressure has not been reflected in the clinical trials. Indeed, the application of mild-to-moderate levels of PEEP does not seem to result in any clinically relevant effects on ICP.[57],[58],[59],[60],[61] Another significant and favorable observation is the demonstration of either no change or minimal change in the ICP in patients with decreased lung compliance. This is the particular subset of patients where the use of lung recruitment properties of PEEP are of benefit.[62],[63] Hyperoxia therapy has not been shown to improve the outcome in patients with neurologic injury and is currently not recommended.[64],[65] Newer modalities such as prone positioning, high frequency oscillatory ventilation, high frequency percussive ventilation, and extracorporeal lung assist have only recently been investigated in the neurocritical patients and may be considered in patients refractory to the standard modalities of ventilatory management.[66],[67],[68],[69]
In addition to the routine monitoring of critical care patients (that includes monitoring of the vital signs, hemodynamics, arterial blood gases, and neurological status), certain specific parameters such as ICP, brain tissue oxygenation, and electrical activity of the brain are being investigated as part of the multimodality monitoring in brain-injured patients.[70] Continuous invasive monitoring of the ICP has been extensively studied as a guide to instituting treatment and in decision making, especially in TBI patients, even though its beneficial effects on the mortality and other outcome measures are yet to be proven.[71] Noninvasive measures of ICP such as optic nerve sheath diameter (ONSD) have recently been investigated in neurocritical patients and have shown a good level of diagnostic accuracy for detecting intracranial hypertension.[72],[73] Brain tissue oxygen monitoring (jugular venous oximetry) offers the advantage of being a more direct measure of tissue hypoxia. It has shown encouraging signs as a monitoring tool with the potential of influencing decision making and outcome in neurologically injured patients (especially in patients with TBI).[74],[75],[76],[77] Available evidence is presently lacking to support the routine use of sedation scores such as the Ramsay score, Richmond agitation–sedation score, and sedation–agitation score as well as electroencephalography (EEG)-derived parameters such as entropy and bispectral index (BIS) in the neurocritical care of patients receiving sedation.[78],[79]
Weaning and extubation in neurocritical patients poses unique challenges, and, indeed, the incidence of extubation failure and reintubation in such patients has been observed to be approximately 10%–15%.[80] Extubation failure in such patients has been associated with a longer hospital and critical care stay as well as poor functional outcomes.[81],[82] The causes of extubation failure in these patients include weakening of protective airway reflexes, neuromuscular weakness, deteriorating consciousness, inappropriate intravenous fluid administration, and deranged respiratory mechanics.[83],[84] The traditional weaning parameters have not been found to be good predictors of a successful outcome in neurocritical care patients.[85] Weaning in these patients should be initiated only after amelioration of the disease pathology and careful consideration of the risks and benefits of continuing the ventilatory support. Although there are a number of protocols to assess the readiness for ventilator withdrawal in non-neurological patients, there are surprisingly very few studies addressing this issue in neurocritical cohorts.[86],[87],[88] The interruption of sedation and weaning according to set protocols has been shown to facilitate shorter durations of ventilation and is desirable in neurologically injured populations.[89] Spontaneous breathing trials have been shown to have a significant impact on decision making in weaning of patients from mechanical ventilation, especially when used in conjunction with ancillary measures such as rapid shallow breathing index (RSBI).[90],[91],[92] Numerous studies have shown that a GCS <8, absence of spontaneous cough, the presence of copious secretions, and the existence of lesions in the posterior fossa and upper cervical spine are associated with difficulties in maintenance of airway and the consequent extubation failure.[93] While a recent meta-analysis does not show any advantages in employing the cuff leak test prior to extubation in neurocritical patients, recent data suggest the usefulness of employing corticosteroids, especially low doses of methyl prednisolone (15–20 mg/kg), before planned extubation of difficult-to-wean patients.[94],[95] Weaning of patients with neuromuscular disorders such as GBS and MG should be attempted only after the disease has run its full course and immunological recovery is complete. The predictors of successful extubation in such patients are nonspecific, and the use of noninvasive ventilation has generally proven to be ineffective, especially in the MG population.[96],[97],[98]
Patients with neurological and neurosurgical diseases requiring mechanical ventilation present with unique considerations because of the complex interplay of the systemic effects of neuronal injury and the effects of mechanical ventilation. The management of such patients requires a multidisciplinary approach incorporating the practices of lung protective ventilation, rigorous monitoring, adequate care of the injured brain, as well as comprehensive support of other affected body systems. The advent of neurocritical care as a distinct subspecialty would go a long way in fostering research into contentious areas such as weaning and extubation of these patients. Focus in the future should be on generating further evidence to improve patient outcomes with an optimum utilization of the existing health care resources. Financial support and sponsorship No financial support or grants received. Conflicts of interest There are no conflicts of interest.
[Figure 1]
[Table 1], [Table 2], [Table 3]
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