19 Ventilation Strategies in Neuro-ICU
19.1 Introduction
Mechanical ventilation has become the cornerstone of modern intensive care unit (ICU) care. The term “ventilate” is derived from Latin word “ventus” meaning wind. Its history dates back to biblical times.1,2 This chapter describes the basic concept of positive pressure ventilation (PPV), the initial settings on a ventilator, and various indications for initiating mechanical ventilation. We will learn how to troubleshoot commonly encountered errors during mechanical ventilation. Lastly, we will discuss the liberation strategies from mechanical ventilation and various factors which can hamper vent liberation.
19.2 Respiratory Failure
It is not uncommon to find patients intubated in the neuro-intensive care unit (neuro-ICU). There are multiple causes of respiratory failure usually broken up into categories based on the system affected (▶ Table 19.1). The causes can be broken up to categories (▶ Fig. 19.1):
There are two main types of respiratory failure:
• Type I: Hypoxic respiratory failure defined as PaO2 < 60 mm Hg without hypercapnia
• Type II: Hypercapnic respiratory failure defined as PaCO2 > 50 mm Hg
Some of the main causes of respiratory failure are listed in ▶ Table 19.2.
19.2.1 Noninvasive Oxygenation and Ventilation
Not all patients with respiratory failure require mechanical ventilation. Many patients can be successfully managed using supplemental oxygen. Nasal cannula, non-rebreathers, and Venturi masks are considered low-flow devices with a limit of 15 lpm. However, supplemental oxygen via nasal cannula or face mask is limited by its flow rate, inability to provide humidity/heat, and delivery of O2 will be lowered when mixed with inspired room air.3 The patient’s tolerance will be impacted by these limitations and also by the method of delivery, nasal cannula, or face mask. ▶ Table 19.3 lists different methods of oxygen delivery that can be utilized in the neuro-ICU.
Noninvasive systems to provide humidified high-flow oxygen or positive airway pressure ventilation are additional methods which can be utilized as a bridge to intubation or post-extubation in the appropriate patient. Humidified high-flow devices provide a mechanism to deliver O2 flow up to 60 lpm depending on the device, thereby increasing the FiO2 to nearly 100%. The flow rate can be set to match the severity of the patient’s respiratory distress/inspiratory demand.4
Table 19.1 Causes of respiratory failure
Location | Cause |
CNS | Brainstem stroke Central hypoventilation Drug overdose Anoxic brain injury Subarachnoid hemorrhage Intracranial hemorrhage Bulbar poliomyelitis Meningitis Encephalitis Status epilepticus |
Spinal cord anterior horn cell | Acute spinal cord injury Multiple sclerosis/transverse myelitis Amyotrophic lateral sclerosis (ALS) Poliomyelitis |
Neuromuscular system motor nerves muscle | Myasthenia gravis Guillain-Barré syndrome Neuromuscular blockade Muscular dystrophy Critical illness myopathy Tetanus/botulism/toxins Hypokalemia period paralysis |
Thoracic cage and pleura | Pneumothorax Large pleural effusion Pulmonary fibrosis Flail chest Morbid obesity Kyphoscoliosis |
Upper airway | Vocal cord paralysis Epiglottitis Laryngotracheitis Post-extubation airway edema Tracheal obstruction Obstructive sleep apnea |
Lower airway | Pneumonia Asthma Aspiration ARDS COPD Atelectasis Interstitial lung disease Traumatic pulmonary contusion |
Left ventricular failure Biventricular failure Valvular failure Pulmonary embolism | |
Abbreviations: ARDS, acute respiratory distress syndrome; CNS, central nervous system; COPD, chronic obstructive pulmonary disease. | |
Fig. 19.1 Etiological categories of respiratory failure.
Table 19.3 Methods of oxygen delivery, flow rate, and percentage of oxygen delivered
Device | Delivered O2 (%) | Flow rate (in lpm) |
Nasal cannula | 1 2 3 4 5 6 | 21–24 25–28 29–32 33–36 37–40 41–44 |
Simple face mask | 6–10 | 35–60 |
Face mask with O2 reservoir (non-rebreather) | 60 70 80 90 100 | 6 7 8 9 10–15 |
Venturi mask Flow rate depends on color-coded jet adapter | 24 28 31 35 40 60 | Blue 2 White 4 Orange 6 Yellow 8 Red 10 Green 15 |
Benefits from humidified high-flow nasal cannula:
• Improve oxygenation
• Improve ventilation
• Decrease work of breathing
• Improve tachypnea
• Can provide positive airway pressure in the pharynx of up to 8 cm H2O.4
CPAP—continuous positive airway pressure
BiPAP—provides inspiratory and expiratory pressure
Venturi mask—constant flow of oxygen through various port size
HHFNC—Humidified High-Flow Nasal Cannula: Allows for high-flow oxygen of up to 60 L/minute to be given via nasal cannula.
Problems:
• Air leak from poor seal
• Pressure sores
• Mucosal dryness
• Claustrophobia
Not all patients are suitable for noninvasive ventilation (NIV). Patients who have a poor mental status, bulbar weakness, and hemiplegia/paresis are unable to clear secretions or have copious secretions, and facial fracture/deformity have a higher risk of aspiration and NIV may be contraindicated.
19.2.2 Invasive Mechanical Ventilation
Indications for Initiating Mechanical Ventilation
Roughly 5% of oxygen (VO2) is utilized for work of breathing.5 In a critically ill patient this may rise to more than 20%.5 Invasive mechanical ventilation eliminates the metabolic cost of breathing.
• Type I respiratory failure with PaO2 < 60 mm Hg with FiO2 > 50%
• Type II respiratory failure with PaCO2 > 55 mm Hg with progressive acidosis
• Progressive acidosis, pH < 7.3
• Hyperventilation for a central nervous system (CNS) event (to rapidly reduce intracranial pressure)
• Tachypnea, paradoxical breathing, and use of accessory muscles
• Upper airway obstruction
• Glasgow coma score < 8
• Bulbar weakness or inability to clear oral secretions
• Weakness of neck flexor/extensors
• Failure of noninvasive methods of oxygenation and ventilation
• Ventilatory mechanics:
◦ Vital capacity: < 15 mL/kg
◦ Negative inspiratory force < −20 cm H2O
◦ Respiratory rate > 35 bpm
Basic Ventilator Parameters
• Fractional concentration of inspired oxygen delivered (FiO2): Expressed as a percentage (%) (21–100). The goal is to keep the FiO2 below 50% as much as possible
◦ Desired FiO2 × PaO2 (desired) × FiO2 (known)=PaO2 (known)6,7
• Respiratory rate (f): The number of times inspiration is initiated in 1 minute (breaths per minute or bpm).
• Tidal volume (VT): The amount of gas that is delivered during inspiration expressed in milliliters (mL) or liters (L). Inspired or exhaled.
• Flow: The velocity of gas flow or volume of gas per minute. Typical flow rate is 60 L/minute (40–80 L/minute). Minimum flow of at least two times the minute ventilation volume is required. High-flow rate may increase the risk of alveolar rupture.
