19 Ventilation Strategies in Neuro-ICU

Amandeep S. Dolla and M. Kamran Athar

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):

**Fig. 19.1** Etiological categories of respiratory failure.

**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
Cardiovascular system	Left ventricular failure Biventricular failure Valvular failure Pulmonary embolism
Abbreviations: ARDS, acute respiratory distress syndrome; CNS, central nervous system; COPD, chronic obstructive pulmonary disease.

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.

**Table 19.2** Causes of type 1 and type 2 respiratory failure
Type 1	Causes	Type 2	Causes
Hypoxic respiratory failure	Inadequate oxygenation without hypercapnia Parenchymal diseases causing shunt physiology (pain, edema, ARDS, ILD) Ventilation/perfusion mismatch Diffusion defects Alveolar hypoventilation Decreased inspired oxygen Acute tissue hypoxia	Hypercapnic respiratory failure	Failure of the lungs to adequately remove CO₂ Reduced respiratory drive due to CNS depressants, brain or brainstem lesions (stroke, trauma, tumors), hypothyroidism Increased drive to breathe due to increased metabolic rate (increased CO₂ production), metabolic acidosis, anxiety associated with dyspnea Paralytic disorders (myasthenia gravis, Guillain-Barré syndrome, poliomyelitis, etc.) Paralytic drugs (curare, sarin/nerve gas, succinylcholine, insecticides) Drugs that affect neuromuscular transmission (calcium channel blockers, long-term adrenocorticosteroids, etc.)
Abbreviations: ARDS, acute respiratory distress syndrome; ILD, interstitial lung disease; PNA.

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.

**Table 19.3** Methods of oxygen delivery, flow rate, and percentage of oxygen delivered
Device	Flow rate (in lpm)	Delivered O₂ (%)
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)	6 7 8 9 10–15	60 70 80 90 100
Venturi mask Flow rate depends on color-coded jet adapter	Blue 2 White 4 Orange 6 Yellow 8 Red 10 Green 15	24 28 31 35 40 60

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

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
Sensitivity of front teeth
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)=PaO2\ \left(desired\right)\ \times \ 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 (V_T): 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.
Flow pattern: Selection of flow pattern (Fig. 19‑2) and rate may depend on the patient’s lung condition. Most common flow pattern is descending ramp. Studies have shown that it improves the distribution of gas in the lungs, reduces dead space, and increases oxygenation by increasing mean and plateau airway pressures.
Sensitivity setting: Sensitivity is normally set so that patients can easily flow or pressure-trigger a breath.
- Flow triggering is set in a range of 1 to 10 L/minute below the base flow, depending on the ventilator.
- Pressure sensitivity is commonly set between −1 and −2 cm H₂O.

**Fig. 19.2** Flow patterns. Modified from Pontoppidan H, Geffin B, Lowenstein E. Acute respiratory failure in the adult. 3. N Engl J Med. 1972;287.

Flow triggering is now the preferred method of triggering, because it has a faster response time compared with pressure triggering.

Extrinsic positive end-expiratory pressure (PEEP): It is the application of positive pressure at end exhalation. This prevents pressure from returning to zero, or atmospheric, at the end of the breath. When positive pressure is applied at the end of a mechanical breath, it is referred to as extrinsic PEEP. When positive pressure is applied throughout the spontaneous breathing cycle, it is referred to as CPAP, or continuous positive airway pressure. It increases functional residual capacity (FRC) and improves oxygenation. It recruits collapsed alveoli, splints and distends patent alveoli, and redistributes lung fluid from alveoli to perivascular space.
Intrinsic PEEP or Auto-PEEP: It is a complication of positive pressure ventilation in which air is accidentally trapped in the lung. This occurs in three situations: ⁸
- Strong active expiration, often with normal or even with low lung volumes; e.g., Valsalva maneuver.
- High minute ventilation (>20 L/minute) where expiration time is too short to allow full exhalation or when expiration is inhibited by resistance external to the patient such as a partially obstructed expiratory filter.
- Expiratory air flow limitation due to increased airway resistance, as may occur in patients with chronic obstructive pulmonary disease (COPD) on mechanical ventilation or with small endotracheal (ET) tubes.
  - It is measured by doing inspiratory pause and then subtracting extrinsic PEEP from total PEEP.