7.3 Conventional Ventilation Physics & Modes

7.3 Conventional Ventilation Physics & Modes

When noninvasive respiratory support fails or is clinically contraindicated, invasive conventional mechanical ventilation provides definitive gas exchange support. However, immature neonatal and pediatric lungs are exquisitely vulnerable to ventilator-induced lung injury (VILI), including barotrauma (pressure-induced damage), volutrauma (alveolar overdistension), atelectotrauma (repetitive shear stress from alveolar collapse and reopening), and biotrauma (inflammatory cytokine release). On the NBRC NPS exam, candidates must master conventional ventilatory modes, age-specific initial parameter selection, flow waveform dynamics, and blood gas titration.

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Mechanical Ventilation Physics & Tubing Circuit Dynamics

In neonatal ventilation, circuit dynamics exert a profound influence on delivered therapy:

• Compressible Circuit Volume: Ventilator circuits expand during positive pressure delivery, compressing a portion of the tidal volume within the tubing rather than delivering it to the patient. The compressible volume equals circuit compliance multiplied by driving pressure: Vc = C_circuit x (PIP - PEEP). In small neonates whose tidal volume is only 4 to 10 mL, an uncompensated circuit compliance of 1.0 mL/cmH2O can absorb over 50% of the set breath.
• Proximal Flow Sensors: To overcome compression volume artifacts and variable endotracheal tube (ETT) peritubular leaks, modern neonatal ventilators place an ultra-lightweight flow sensor directly between the ETT hub and the circuit Y-piece. This measures true inspired and expired tidal volume (Vti and Vte) at the airway opening.

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Conventional Ventilator Modes

Mechanical ventilation modes are defined by their control variable (pressure or volume), phase variables (trigger, limit, cycle), and baseline variable (PEEP).

1. Pressure Control Ventilation (PC-CMV / PC-SIMV)

• Operational Mechanics: Pressure-limited and time-cycled. The clinician sets the Peak Inspiratory Pressure (PIP), PEEP, Inspiratory Time (Ti), and Respiratory Rate (RR). Flow is decelerating.
• Clinical feature: The ventilator targets a set inspiratory pressure, so delivered tidal volume changes with mechanics. Measured airway pressure can still transiently overshoot or be affected by patient effort, circuit events, and alarm configuration; a pressure limit does not eliminate volutrauma, atelectrauma, or lung stress.
• Critical Physiological Limitation: Delivered tidal volume is variable and unstable. By the equation Vt = Delta P x C_L (where Delta P = PIP - PEEP and C_L is dynamic lung compliance), any change in respiratory system compliance or airway resistance alters tidal volume.
• After surfactant: Compliance can improve quickly. On fixed pressure control, delivered Vt may rise and PaCO2 may fall, increasing overdistension and hypocapnia risk. Monitor exhaled Vt, chest movement, pressure, gases or CO2 trend, and oxygen need closely; reduce driving pressure as needed rather than assuming a fixed magnitude or timing of change.

2. Volume Control Ventilation (VC-CMV / VC-SIMV)

• Operational Mechanics: Volume-limited and volume- or time-cycled. The clinician sets the target tidal volume (Vt), PEEP, RR, and inspiratory flow or Ti.
• Clinical feature: Volume-targeted ventilation aims for a set volume, but exhaled volume and alveolar ventilation still vary with leak, compression, dead space, spontaneous effort, and pressure limits. CO2 clearance is not guaranteed.
• Clinical limitations: Leaks and circuit compression are especially important in neonates. With worsening mechanics, the pressure required for the target volume rises; use appropriate pressure alarms or limits and reassess the target rather than allowing escalating stress.

3. Pressure-Regulated Volume Control (PRVC) & Volume Guarantee (VG)

• Operational Mechanics: Dual-control, time-cycled mode that combines the lung-protective pressure profile of Pressure Control with the volume consistency of Volume Control.
• Breath-by-Breath Microprocessor Regulation: The clinician sets a target expired tidal volume (Vte), PEEP, Ti, backup rate, and a maximum pressure limit (Pmax or PIP limit, typically set 5 cmH2O above baseline PIP).
• The ventilator measures exhaled tidal volume at the proximal flow sensor. If the measured Vte is below the target, the microprocessor automatically steps up the PIP by 1 to 3 cmH2O on the subsequent breath. Conversely, if Vte exceeds the target, the ventilator steps down the PIP by 1 to 3 cmH2O.
• Adaptive pressure: When measured exhaled volume rises above target, PRVC/VG usually reduces pressure over subsequent breaths. Continue close monitoring after surfactant because adaptation is bounded by ventilator algorithms and can be distorted by leak or sensor placement.

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Initial Ventilator Settings: Neonates vs. Pediatrics

1. Neonatal Ventilator Initiation (Preterm vs. Term)

• Peak Inspiratory Pressure (PIP): Initial 18 to 24 cmH2O in Pressure Control, titrated to achieve gentle, visible bilateral chest rise and target tidal volume.
• Positive End-Expiratory Pressure (PEEP): 5–7 cmH2O is a common starting range for neonatal RDS. Titrate to recruitment, oxygenation, mechanics, hemodynamics, and the disease; lower or higher values may be appropriate.
• Respiratory Rate: 30 to 50 breaths/min (preterm infants typically 40–50 bpm; term neonates 30–40 bpm).
• Target Tidal Volume (Vt):
  • Preterm infants (<34 weeks): 4.0 to 6.0 mL/kg.
  • Term infants: 5.0 to 6.0 mL/kg.
• Inspiratory Time (Ti):
  • Preterm infants: 0.30 to 0.40 seconds.
  • Term infants: 0.40 to 0.50 seconds.
  • Neonatal Time Constant Science: The respiratory time constant (tau) is the product of airway resistance and compliance: tau = Raw x C_L. In premature infants with RDS, lung compliance is exceptionally low, yielding very short time constants (tau approximately 0.05 to 0.10 s). Because 95% of alveolar filling or emptying occurs within 3 time constants (0.15 to 0.30 s), setting an excessive Ti (>0.45 s in an extremely low birth weight infant) provides no additional alveolar volume, curtails expiratory time (Te), and induces air trapping and auto-PEEP.
• Flow Trigger: 0.1 to 0.5 L/min (highly sensitive to detect spontaneous efforts without auto-triggering).

2. Pediatric Ventilator Initiation (Infant to Adolescent)

• Tidal Volume (Vt): 5 to 8 mL/kg based on Ideal Body Weight (IBW). In pediatric acute respiratory distress syndrome (PARDS), a lung-protective target of 4 to 6 mL/kg is utilized.
• PEEP: 5 to 8 cmH2O (titrated up to 10–15 cmH2O in PARDS).
• Respiratory Rate (Age-Stratified):
  • Infants (<1 year): 25 to 35 breaths/min.
  • Toddlers and Preschool (1–5 years): 20 to 28 breaths/min.
  • School-Age and Adolescents (>5 years): 16 to 22 breaths/min.
• Inspiratory Time (Ti): 0.5 to 0.8 seconds (up to 0.8–1.0 s in adolescents).
• Flow Trigger: 0.5 to 1.5 L/min.

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