7.4 Initial Settings & ABG-Guided Titration
Key Takeaways
- Conventional pediatric initiation uses tidal volumes of 5 to 8 mL/kg of ideal body weight with PEEP 5 to 8 cmH2O; rates run about 25 to 35 breaths/min for infants 6 to 12 months and 20 to 28 breaths/min for children 2 to 8 years, with inspiratory times of 0.50 to 0.80 seconds.
- Ventilation (PaCO2) is managed by altering minute ventilation via respiratory rate or driving pressure (PIP - PEEP), whereas oxygenation (PaO2) is dictated by FiO2 and Mean Airway Pressure (mPAW).
- The desired-rate formula New Rate = (Current Rate x Current PaCO2) / Desired PaCO2 converts a rate of 20 at a PaCO2 of 56 mmHg into 28 breaths/min to target 40 mmHg.
- Mean airway pressure equals [(PIP x Ti) + (PEEP x Te)] / (Ti + Te), so raising PEEP or lengthening Ti improves oxygenation without directly changing CO2 clearance.
7.4 Initial Settings & ABG-Guided Titration
Flow Waveform Dynamics: Decelerating vs. Square Flow
Modern pediatric and neonatal ventilators deliver inspiratory gas via distinct flow profiles:
- Decelerating flow: Common in pressure-targeted breaths and many adaptive modes. Flow is high early and tapers as the pressure gradient falls. Exact waveform and cycling depend on the ventilator, mode, rise time, leak compensation, and patient effort; it does not uniformly improve oxygenation in every patient.
- Square (Constant) Flow Pattern: Flow remains constant throughout the entire inspiratory cycle. Square flow generates sharp initial airway resistive pressure spikes (P_resistive = Flow x Raw) and higher peak inspiratory pressures, historically predisposing patients to barotrauma.
Arterial Blood Gas Adjustments & Titration Formulas
Correcting blood gas derangements requires understanding the distinct physiological determinants of ventilation versus oxygenation.
1. Ventilation (PaCO2 Clearance)
Alveolar ventilation and carbon dioxide clearance are governed by Minute Ventilation (Ve):
Minute Ventilation (Ve) = Vt x Respiratory Rate
- In Volume Control: Adjust target Vt or Respiratory Rate.
- In Pressure Control: Adjust Driving Pressure (Delta P = PIP - PEEP) or Respiratory Rate. Increasing PIP widens Delta P, increasing delivered Vt and blowing off CO2.
- Desired Rate Formula:
New Rate = (Current Rate x Current PaCO2) / Desired PaCO2
- Desired Tidal Volume Formula:
New Vt = (Current Vt x Current PaCO2) / Desired PaCO2
2. Oxygenation (PaO2 / SpO2 Regulation)
Arterial oxygenation is determined by FiO2 and Mean Airway Pressure (mPAW). Mean airway pressure represents the average pressure exerted on the lungs across the entire respiratory cycle:
mPAW = [(PIP x Ti) + (PEEP x Te)] / (Ti + Te)
- To increase PaO2: Increase PEEP (primary tool for alveolar recruitment and FRC restoration), increase PIP, prolong Ti, or increase FiO2.
- PEEP and FiO2 are the primary clinical knobs adjusted for hypoxemia.
Table 7.2.1: Conventional Mechanical Ventilation Initial Settings Matrix
| Parameter | Preterm Neonate (<1.5 kg) | Term Neonate (3–4 kg) | Pediatric Infant (6–12 mo) | Pediatric Child (2–8 yr) |
|---|---|---|---|---|
| Mode of Choice | PC-VG / PRVC or PC-SIMV | PC-VG / PRVC or PC-SIMV | PRVC / VC-SIMV / PC | PRVC / VC-SIMV / PC |
| Tidal Volume (Vt) | 4.0–6.0 mL/kg | 5.0–6.0 mL/kg | 5.0–8.0 mL/kg | 5.0–8.0 mL/kg (IBW) |
| PIP (if PC) | 18–22 cmH2O | 20–24 cmH2O | 20–26 cmH2O | 20–28 cmH2O |
| PEEP | 5–6 cmH2O | 5–7 cmH2O | 5–7 cmH2O | 5–8 cmH2O |
| Respiratory Rate | 40–50 bpm | 30–40 bpm | 25–35 bpm | 20–28 bpm |
| Inspiratory Time (Ti) | 0.30–0.35 s | 0.35–0.45 s | 0.50–0.65 s | 0.60–0.80 s |
| Flow Trigger | 0.1–0.3 L/min | 0.2–0.5 L/min | 0.5–1.0 L/min | 0.8–1.5 L/min |
Worked Clinical Calculation: Respiratory Acidosis Management
Clinical Scenario
A 6-year-old child (Ideal Body Weight = 20 kg) intubated for severe aspiration pneumonia is mechanically ventilated in Volume Control (VC-SIMV) with initial settings:
- Set Vt = 120 mL (6 mL/kg)
- Set Rate = 20 breaths/min
- PEEP = 6 cmH2O
- FiO2 = 0.40
Arterial Blood Gas (ABG) after 30 minutes:
- pH = 7.28
- PaCO2 = 56 mmHg
- PaO2 = 88 mmHg
- HCO3- = 25 mEq/L
Objective & Step-by-Step Execution
The physician requests adjusting the respiratory rate to normalize the PaCO2 to 40 mmHg.
- Apply the Desired Rate Equation:
New Rate = (Current Rate x Current PaCO2) / Desired PaCO2
- Insert the Clinical Values:
New Rate = (20 bpm x 56 mmHg) / 40 mmHg = 1,120 / 40 = 28 breaths/min
- Clinical Verification: Increasing the rate from 20 to 28 bpm increases minute ventilation from 2.4 L/min to 3.36 L/min (120 mL x 28), effectively blowing off excess CO2 and shifting pH into the normal range (7.35–7.45) without increasing tidal volume above safe lung-protective thresholds (6 mL/kg). If Ti is 0.7 s, a rate of 28 bpm gives a total cycle time of 60 / 28 = 2.14 s, providing an adequate expiratory time of 2.14 - 0.7 = 1.44 s (I:E ratio approximately 1:2).
NPS Exam Traps Callout Box: Conventional Modes & Initiation
[!WARNING] NPS Exam Trap 1: The Post-Surfactant Compliance Surge A favorite NBRC exam question presents a preterm infant on Pressure Control who receives surfactant. Forty minutes later, blood gas shows pH 7.58 and PaCO2 22 mmHg, with chest excursions bouncing vigorously. The student who selects "increase rate" or "switch to square flow" fails! The correct response is to reduce excessive delivered ventilation in measured steps—often pressure or target tidal volume when volume rose after surfactant, and rate when appropriate—then reassess exhaled volume and CO2.
NPS Exam Trap 2: Neonatal Inspiratory Time (Ti) Trap Preterm infants with RDS often need a short inspiratory time, but 0.40 seconds is not a universal ceiling. Choose Ti from gestation, mechanics, rate, waveform, synchrony, delivered volume, and expiratory flow. A Ti that is too long can reduce expiratory time and worsen air trapping or air leak; one that is too short can impair delivery and recruitment.
NPS Exam Trap 3: Fixing Hypoxemia vs. Hypercapnia When PaO2 is low and PaCO2 is normal, do NOT increase tidal volume or respiratory rate. Oxygenation is fixed by adjusting FiO2 or PEEP (increasing mean airway pressure). Changing rate or driving pressure alters CO2, not primary oxygenation.
A 4-year-old pediatric patient (Ideal Body Weight = 16 kg) is intubated and mechanically ventilated in Volume Control (VC-SIMV) following abdominal surgery. Current ventilator settings are: set Vt 100 mL (6.25 mL/kg), respiratory rate 22 breaths/min, PEEP 5 cmH2O, and FiO2 0.30. An arterial blood gas reveals: pH 7.29, PaCO2 54 mmHg, PaO2 92 mmHg, and HCO3- 25 mEq/L. The attending physician asks the respiratory therapist to adjust the respiratory rate to achieve a target PaCO2 of 40 mmHg while keeping tidal volume constant. What new respiratory rate should be set?