4.2 Mechanical Ventilation Strategies

Mechanical Ventilation Strategies

Clinical Indications for Invasive Ventilation

Invasive mechanical ventilation is indicated when non-invasive respiratory support fails to maintain adequate gas exchange. Objective criteria include severe respiratory acidosis (pH < 7.20, PaCO2 > 60 to 65 mmHg), refractory hypoxemia (FiO2 > 0.50 to 0.60 on CPAP of 8 cmH2O), persistent apnea of prematurity unresponsive to pharmacological therapy, clinical signs of severe exhaustion (severe retractions, grunting), and cardiovascular instability or shock.

Conventional Mechanical Ventilation Modes

Selecting the correct ventilation mode is crucial to balance gas exchange with the prevention of ventilator-induced lung injury (VILI).

Assist/Control (A/C) or Continuous Mandatory Ventilation (CMV)

In A/C mode, the ventilator delivers a set backup rate of breaths. Additionally, every time the patient initiates a breath that crosses the trigger threshold, the ventilator delivers a full mandatory breath using the preset pressure or volume parameters.

• Clinical Advantage: Reduces work of breathing because all breaths receive full ventilator support.
• Exam Trap/Risk: If the infant is tachypneic due to pain, agitation, or neurological issues, they may trigger excessive breaths, leading to severe hyperventilation, hypocapnia, and air leak syndromes.

Synchronized Intermittent Mandatory Ventilation (SIMV)

SIMV delivers a set number of mandatory breaths synchronized with the patient's inspiratory efforts. Spontaneous breaths that exceed the set backup rate are unsupported and occur at the patient's own effort, unless Pressure Support (PS) is added.

• Clinical Advantage: Allows the patient to perform some work of breathing, helping to prevent diaphragmatic atrophy and facilitate weaning.
• Clinical Disadvantage: Spontaneous breaths against the resistance of a narrow endotracheal tube without support increase work of breathing and lead to muscle fatigue. Therefore, modern practice combines SIMV with Pressure Support Ventilation (PSV) to assist spontaneous breaths.

Volume Guarantee (VG) & Volume-Targeted Ventilation (VTV)

Volume Guarantee is a patient-protective strategy combined with pressure-regulated modes. The clinician sets a target tidal volume (VT), typically 4 to 6 mL/kg (sometimes up to 7 mL/kg in chronic lung disease). The ventilator measures the expired tidal volume from the previous breath and dynamically adjusts the Peak Inspiratory Pressure (PIP) on the next breath to deliver the target VT.

• Pathophysiologic Benefit: Limits volutrauma (over-stretching of alveoli) and reduces the risk of hypocapnia, which is a major driver of periventricular leukomalacia (PVL). As the infant's lung compliance improves (e.g., after surfactant administration), the ventilator automatically reduces the PIP to maintain the set VT, preventing accidental over-pressurization.

Key Conventional Ventilator Parameters

Conventional ventilation adjustments are guided by the patient's clinical status and blood gases:

• Peak Inspiratory Pressure (PIP): The maximum pressure reached during inspiration. It directly drives tidal volume. Elevated PIP increases the risk of barotrauma and air leaks.
• Positive End-Expiratory Pressure (PEEP): The continuous pressure maintained during expiration. PEEP recruits collapsed alveoli, increases Functional Residual Capacity (FRC), and improves ventilation-perfusion (V/Q) matching. Standard PEEP is 4 to 6 cmH2O.
• Mean Airway Pressure (MAP): The average pressure applied to the lungs over the entire respiratory cycle. MAP is the primary determinant of oxygenation. It is influenced by PEEP, PIP, Inspiratory Time, and respiratory rate.
• Inspiratory Time (Ti): The duration of the inspiratory phase. In preterm infants, it is typically set to 0.3 to 0.4 seconds. Setting a Ti too long can cause gas trapping (auto-PEEP) and reduce time for expiration.

High-Frequency Ventilation (HFV) Modes

HFV modes utilize extremely rapid respiratory rates and small tidal volumes (often less than the anatomic dead space) to maintain lung volume while minimizing cyclical alveolar stretch.

High-Frequency Oscillatory Ventilation (HFOV)

HFOV employs a piston or diaphragm that creates an active inspiratory and active expiratory phase.

• Oxygenation: Adjusted by changing the Mean Airway Pressure (MAP) and FiO2. MAP is set high initially to recruit alveoli and is then weaned.
• Ventilation (CO2 Elimination): Controlled by Amplitude (Delta P) and Frequency (Hertz).
• The Hertz (Hz) Paradox: Hertz is frequency (1 Hz = 60 breaths/minute). Preterm infants are typically started at 10 to 15 Hz (600 to 900 breaths/min). On the exam, remember that lower frequencies (e.g., 8 Hz) increase CO2 elimination, while higher frequencies (e.g., 15 Hz) decrease CO2 elimination. A lower frequency allows the piston to move a larger distance, increasing the delivered tidal volume, which has a quadratic effect on CO2 clearance.

High-Frequency Jet Ventilation (HFJV)

HFJV delivers short, high-velocity "jets" of gas down the endotracheal tube. Expiration is passive. It must be paired with a conventional background ventilator to supply PEEP and sigh breaths.

• Settings: Key parameters include Jet PIP, Jet rate (usually 240 to 420 bpm), and a short, fixed Inspiratory Time of 0.02 seconds.
• Clinical Utility: HFJV is the treatment of choice for Pulmonary Interstitial Emphysema (PIE) and active pneumothoraces. The extremely short Ti and passive exhalations allow the leaking airways/alveoli to rest and heal while maintaining adequate gas exchange.

Surfactant Replacement Therapy

Surfactant reduces alveolar surface tension, preventing collapse at the end of expiration.

Clinical Indications

Indicated for premature infants with Respiratory Distress Syndrome (RDS) who have an increasing oxygen requirement (FiO2 > 0.30 to 0.35 on CPAP of >= 6 cmH2O) or are intubated for respiratory failure. Chest X-ray showing diffuse reticulogranular patterns (ground-glass appearance) and air bronchograms supports the diagnosis.

Administration Techniques

• Endotracheal Tube (ETT) Instillation: The traditional method where surfactant is warmed to room temperature, gently swirled (never shaken, to prevent foaming), and instilled directly down the ETT in aliquots or as a single bolus. The infant is monitored closely during administration.
• LISA / MIST: Less Invasive Surfactant Administration / Minimally Invasive Surfactant Therapy involves placing a thin catheter through the vocal cords under direct visualization while the infant remains on non-invasive CPAP. Surfactant is slowly infused, allowing the infant to distribute it using their own spontaneous breathing, reducing the need for intubation and positive pressure ventilation.

Nursing Care and Safety

During administration, the nurse must monitor for transient bradycardia, desaturation, ETT obstruction, and reflux of surfactant. If these occur, the instillation must be paused, and the infant's airway cleared or supported.

• Post-Administration Weaning: Following surfactant delivery, lung compliance can improve rapidly. This increases tidal volume (in volume-controlled modes) or chest rise (in pressure-controlled modes). The nurse must immediately collaborate with the provider to wean the PIP, tidal volume targets, and FiO2 to prevent severe hypocapnia, pneumothorax, or pulmonary hemorrhage.