Neonatal Respiratory Assessment: Oxygenation Indices
You're staring at an ABG from a sick neonate and it looks bad. But how bad? That's where these numbers come in. They tell you whether the kid's going to get better with more oxygen and PEEP, or whether you need the next level of support.
Oxygenation Index: The Gold Standard
OI is what you use when the baby's still on conventional ventilation but you're worried:
OI = (MAP × FiO₂ × 100) / PaO₂
MAP = what you're setting on the vent (cm H₂O), FiO₂ = oxygen concentration, PaO₂ = what you actually got from the ABG.
The interpretation is:
- OI <10: Baby's doing okay. Responsive to conventional support.
- OI 10–20: Getting worse. Consider if you need high-frequency oscillation.
- OI 20–40: Sick. High-frequency vent is on the table.
- OI >40: This is ECMO territory. The kid isn't making it on conventional ventilation.
Watch the trend, not just one number. If OI improves over a day or two, you're winning. If it's creeping up despite you turning everything up, you've got a problem.
Use Oxygenation Index to track this.
P/F Ratio and A-a Gradient
P/F ratio (simply PaO₂ divided by FiO₂) is simpler if you don't have vent settings handy:
- >300: Pretty good
- 200–300: Mild problem
- 100–200: Needs aggressive support
- <100: Really bad
The catch: it ignores how hard the vent is working.
A-a Gradient indicates whether the lungs are the problem or something else is occurring:
A-a Gradient = PAO₂ – PaO₂
- <10 mmHg on room air: Normal for a neonate
- >15–20 mmHg on room air: Pulmonary disease
- >30 mmHg on high O₂: You've got shunting (likely RDS)
Big A-a gradient with infiltrates on chest X-ray = RDS. Hypoxemia but normal A-a = think heart.
Use P/F Ratio and A-a Gradient to interpret these.
Surfactant Administration
Surfactant has significantly improved outcomes. The dosing depends on which formulation you have, and it matters:
- Beractant (Survanta): 4 mL/kg (100 mg/kg) down the tube, can repeat every 6 hours up to 4 times
- Poractant alfa (Curosurf): 2.5 mL/kg (200 mg/kg) first dose, then 1.25 mL/kg (100 mg/kg) for subsequent doses, may repeat every 12 hours
- Calfactant (Infasurf): 3 mL/kg (100 mg/kg) initially, repeat every 12 hours
Timing is important. Current evidence favors early selective surfactant (within the first 2 hours for infants with clinical evidence of RDS) rather than routine prophylactic administration. For extremely preterm infants (<26 weeks) with RDS, earlier administration is associated with better outcomes. The trend is toward non-invasive initial support (CPAP) with rescue surfactant when FiO2 requirements exceed 0.30–0.40, rather than universal early surfactant.
Two ways to do it now:
- INSURE: Intubate, push the surfactant, extubate right back to CPAP. Minimizes vent time.
- MIST: Thin catheter during spontaneous breathing on non-invasive support. Even better if you can pull it off.
Use Surfactant Dosing to verify.
When to Call for ECMO
You talk ECMO when OI stays >40 for 3 hours straight despite you doing everything right on the conventional vent. Some places also use mortality prediction >80% or refractory hypoxemia.
Talk to your team though. A baby with a lethal anomaly, grade III–IV IVH, or septic shock may not be a candidate.
Use Oxygenation Index to track when you're getting close.
Oxygen Content: The Real Story
PaO₂ is just a number. What matters is whether the tissues are getting oxygen. That's oxygen content (CaO₂):
CaO₂ = (1.34 × Hemoglobin × SaO₂) + (0.003 × PaO₂)
A severely anemic baby can have a normal PaO₂ and still be hypoxic at the tissue level. You need the hemoglobin.
Use O₂ Content to do this calculation.
What You Actually Do
- Get ABGs lined up with vent changes
- Calculate OI and A-a gradient at each gas
- Give surfactant per INSURE or MIST
- Call ECMO early if you're trending that direction
- Have the talk with the family about what you're seeing