9.5 Surfactant Biology, Products & Dosing
Key Takeaways
- Surfactant is mostly lipid with SP-B/SP-C supporting film function and SP-A/SP-D supporting host defense. Complete SP-B deficiency often causes severe, treatment-refractory neonatal disease, while genotype and phenotype determine course and specialist options.
- Surfactant lowers surface tension as lung volume falls, improving alveolar stability and FRC and reducing—but not necessarily eliminating—atelectasis and intrapulmonary shunt in neonatal RDS.
- Commercial preparations differ by source, concentration, and dosing: Poractant alfa (Curosurf, porcine, 80 mg/mL) is dosed at 200 mg/kg (2.5 mL/kg) initially, then 100 mg/kg (1.25 mL/kg) Q12h; Beractant (Survanta, bovine, 25 mg/mL) is dosed at 100 mg/kg (4.0 mL/kg) Q6h; Calfactant (Infasurf, calf lung, 35 mg/mL) is dosed at 105 mg/kg (3.0 mL/kg) Q12h.
9.5 Surfactant Biology, Products & Dosing
Neonatal Respiratory Distress Syndrome (RDS), historically known as hyaline membrane disease, is the primary cause of respiratory failure in premature neonates. RDS is caused by developmental deficiency in endogenous pulmonary surfactant. The introduction of exogenous surfactant replacement therapy revolutionized neonatal intensive care, drastically decreasing neonatal mortality and pneumothorax rates. The Neonatal/Pediatric Specialist must master surfactant biochemistry, commercial preparations, advanced delivery methods, and the rapid mechanical ventilator adjustments required immediately following administration.
Biophysical Composition & Physiology of Natural Surfactant
Endogenous pulmonary surfactant is synthesized, packaged, and secreted by Type II alveolar pneumocytes. It is stored intracellularly in lamellar bodies and exocytosed into the alveolar liquid hypophase lining the alveolar epithelium.
PULMONARY SURFACTANT BIOCHEMICAL PROFILE
┌────────────────────────────────────────────────────────────────────────┐
│ 90% LIPIDS (Phospholipids) │
├────────────────────────────────────────────────────────────────────────┤
│ • Dipalmitoylphosphatidylcholine (DPPC / Lecithin) ~50% │
│ Amphipathic molecule; tight hydrophobic intermolecular packing │
│ Lowers surface tension to near 0 dynes/cm at low lung volumes │
│ • Phosphatidylglycerol (PG) ~8–10% │
│ Marker of fetal lung maturity; promotes spreading of DPPC monolayer │
│ • Phosphatidylinositol (PI), Phosphatidylethanolamine, Neutral Lipids │
└────────────────────────────────────────────────────────────────────────┘
│
▼
┌────────────────────────────────────────────────────────────────────────┐
│ 10% SURFACTANT PROTEINS (SP) │
├────────────────────────────────────────────────────────────────────────┤
│ HYDROPHILIC COLLECTINS (Host Defense) │
│ • SP-A: Opsonizes bacteria, viruses, and fungi; pathogen clearance │
│ • SP-D: Modulates alveolar inflammation; pathogen clearance │
│ │
│ HYDROPHOBIC APOPROTEINS (Surface Tension & Film Kinetics) │
│ • SP-B: Essential for life; accelerates DPPC monolayer film formation │
│ and stability; complete deficiency often causes severe disease │
│ • SP-C: Enhances lipid insertion; prevents film collapse during breath │
└────────────────────────────────────────────────────────────────────────┘
1. Surfactant Proteins & Congenital Deficiency Syndromes
- SP-A & SP-D (Hydrophilic): Members of the collectin protein family. They possess collagen-like domains and carbohydrate recognition domains that bind microbial pathogens, facilitating macrophage phagocytosis and immune clearance. They play minimal direct roles in lowering surface tension.
- SP-B & SP-C (Hydrophobic): Small, highly lipophilic peptides that integrate directly into the phospholipid bilayer. They accelerate the adsorption of DPPC from lamellar myelin aggregates into a continuous monolayer at the air-liquid interface.
- Congenital SP-B Deficiency:
- Biallelic pathogenic variants in SFTPB can produce inherited surfactant dysfunction.
- Complete deficiency often presents in a term infant with severe diffuse disease that resembles RDS and responds poorly or only transiently to exogenous surfactant.
- Presentation and course vary by variant; genetic testing and specialist evaluation are central, and lung transplantation may be considered for otherwise progressive severe disease.
2. Physical Mechanics: The Law of Laplace
Alveolar mechanics can be modeled using the Law of Laplace for a spherical liquid-air interface:
Where:
-
$P =$ collapsing (inward distending) pressure
-
$\gamma =$ surface tension at the liquid-air interface
-
$r =$ alveolar radius
-
The Surfactant-Deficient Lung: Without surfactant, surface tension ($\gamma$) remains constant at approximately $70\text{ dynes/cm}$ (the surface tension of water). According to Laplace's equation, as alveolar radius ($r$) decreases, collapsing pressure ($P$) increases exponentially. Consequently, smaller alveoli generate high collapsing pressures and empty their gas into adjacent larger alveoli, culminating in widespread microatelectasis, loss of Functional Residual Capacity (FRC), intrapulmonary right-to-left shunting, and severe refractory hypoxemia.
-
The Surfactant-Sufficient Lung: Surfactant forms an amphipathic monolayer. During expiration, as alveolar surface area shrinks, DPPC molecules are tightly compressed together, driving surface tension ($\gamma$) down to $< 2\text{ to }5\text{ dynes/cm}$ (near zero). By reducing $\gamma$ in direct proportion to radius ($r$), the collapsing pressure ($P$) remains low and equal across alveoli of all sizes. Alveoli remain patent at end-expiration, preserving FRC and lung compliance.
Commercial Surfactant Preparations & Dosing
Natural exogenous surfactants derived from animal lungs are standard of care; synthetic protein-free surfactants were discontinued due to inferior efficacy.
Table 9.3.1: Comparison of Commercial Exogenous Surfactant Preparations
| Generic Name | Brand Name | Animal Source | Concentration | Initial Dose | Repeat Dosing & Schedule | Maximum Total Doses |
|---|---|---|---|---|---|---|
| Poractant alfa | Curosurf | Porcine (pig) minced lung extract | $80\text{ mg/mL}$ | $200\text{ mg/kg}$<br>($2.5\text{ mL/kg}$) | $100\text{ mg/kg}$ ($1.25\text{ mL/kg}$)<br>every 12 hours | Up to 3 doses (initial + 2 repeat doses) |
| Beractant | Survanta | Bovine (cow) minced lung extract + additives | $25\text{ mg/mL}$ | $100\text{ mg/kg}$<br>($4.0\text{ mL/kg}$) | $100\text{ mg/kg}$ ($4.0\text{ mL/kg}$)<br>every 6 hours | Up to 4 doses within first 48 hours |
| Calfactant | Infasurf | Calf lung lavage extract | $35\text{ mg/mL}$ | $105\text{ mg/kg}$<br>($3.0\text{ mL/kg}$) | $105\text{ mg/kg}$ ($3.0\text{ mL/kg}$)<br>every 12 hours | Up to 3 doses |
Clinical Pharmacology Nuances
- Poractant Alfa (Curosurf) Advantage: Formulated at a high phospholipid concentration ($80\text{ mg/mL}$). This allows delivery of an initial dose of $200\text{ mg/kg}$ in a small liquid volume of only $2.5\text{ mL/kg}$. Clinical trials demonstrate that an initial $200\text{ mg/kg}$ dose of Curosurf results in faster weaning of $FiO_2$ and lower neonatal mortality compared to $100\text{ mg/kg}$ doses of Survanta or Infasurf. Furthermore, the lower fluid volume significantly decreases the incidence of transient airway obstruction during instillation.
- Preparation & Handling:
- Must be stored refrigerated at $2^\circ\text{C to }8^\circ\text{C}$ and protected from light.
- Before administration, warm slowly to room temperature by allowing the vial to stand at room temperature for at least 20 minutes, or warm in the palm of the hand for at least 8 minutes.
- NEVER use artificial warming devices, microwaves, or hot water baths!
- Inspect the suspension: Curosurf is creamy white to light brown; Survanta is off-white to brown; Infasurf is off-white. Gently invert or swirl the vial to suspend; NEVER shake vigorously, as excessive agitation causes foaming and denatures the hydrophobic apoproteins.
A 28-week gestational age neonate weighing 1.0 kg is intubated in the delivery room for severe respiratory distress syndrome and placed on a pressure-controlled mechanical ventilator with the following settings: PIP 24 cmH2O, PEEP 5 cmH2O, Rate 40 breaths/min, FiO2 0.70. The baseline expired tidal volume is 4.5 mL/kg. Poractant alfa (Curosurf) is administered via endotracheal instillation. Ten minutes post-administration, the bedside respiratory therapist notes that the expired tidal volume has rapidly increased to 9.2 mL/kg and the SpO2 has risen to 99%. What is the most critical and immediate clinical intervention?
A full-term infant born at 39 weeks gestation develops profound hypoxemic respiratory failure within two hours of birth. Chest radiography reveals diffuse, bilateral reticulogranular opacities with prominent air bronchograms indistinguishable from severe respiratory distress syndrome. The infant is intubated and receives two standard doses of Poractant alfa at 200 mg/kg and 100 mg/kg, but shows zero improvement in arterial oxygenation, lung compliance, or ventilator requirements. Genetic testing is ordered. Which of the following pathophysiological conditions best explains this infant's clinical presentation?