STEPTRACKERRESPIRATORY | PATHOLOGY + PHYSIOLOGY | HIGH-YIELD REVIEW

ARDS & Neonatal Respiratory Distress

Two lungs full of hyaline membranes: adults after diffuse alveolar damage, and premature infants without enough surfactant.

FOCUS ON
ARDS is noncardiogenic edema: bilateral infiltrates, hypoxemia that doesn't fix with O₂, normal wedge pressure, hyaline membranes. Sepsis is the top cause. Ventilate with low tidal volumes. NRDS: surfactant deficiency in preemies; risk ↑ with maternal diabetes and C-section; L/S ratio < 2; antenatal steroids prevent it.
TOPIC
DETAIL
MECHANISM
CLUE
ARDS — definition
Within 1 week of an insult: bilateral infiltrates + PaO₂/FiO₂ ≤ 300, not explained by heart failure.
Diffuse alveolar damage → noncardiogenic pulmonary edema
Wedge pressure normal
ARDS — causes
Sepsis (most common), pneumonia, aspiration, trauma, shock, pancreatitis, TRALI, overdose, near-drowning, toxic inhalation, burns, amniotic fluid embolism.
Direct (lung) or indirect (systemic) injury
Pancreatitis + sudden hypoxemia
ARDS — mechanism
Neutrophils activated by IL-1, TNF, IL-8 release proteases and ROS.
Endothelium and type I/II pneumocytes damaged → protein-rich exudate → hyaline membranes; surfactant lost → collapse → shunt
Refractory hypoxemia from shunt
ARDS — course & care
Exudative (week 1) → proliferative (type II pneumocyte hyperplasia) → ± fibrotic.
Low tidal volume (6 mL/kg), PEEP, prone positioning, conservative fluids; treat the cause
High mortality; survivors may have fibrosis
NRDS — mechanism
Too little surfactant (dipalmitoylphosphatidylcholine from type II pneumocytes; adequate after ~35 weeks).
↑ surface tension → alveoli collapse on expiration (Laplace)
Hyaline membranes in the newborn lung
NRDS — risk & signs
Prematurity, maternal diabetes (fetal insulin ↓ surfactant), C-section (no labor steroid surge), male sex.
Tachypnea, grunting, nasal flaring, retractions, cyanosis soon after birth
Ground-glass CXR with air bronchograms
NRDS — testing & prevention
Lecithin:sphingomyelin ratio < 2 in amniotic fluid = immature lungs.
Antenatal betamethasone/dexamethasone (24–34 weeks); exogenous surfactant; CPAP
O₂ toxicity → retinopathy of prematurity, bronchopulmonary dysplasia
NRDS — other complications
Intraventricular hemorrhage, PDA (hypoxia keeps it open), necrotizing enterocolitis.
Prematurity + hypoxia
—
Look-alikes in newborns
Transient tachypnea (term C-section, retained lung fluid; resolves in 1–2 days) · meconium aspiration (post-term; chemical pneumonitis, persistent pulmonary HTN).
Fluid in fissures · patchy infiltrates + air trapping
Green amniotic fluid = meconium

Mechanism — two roads to hyaline membranes

Sepsis, trauma, aspiration
→
Neutrophil activationProteases, ROS
→
Alveolar-capillary leakProtein-rich fluid
→
Hyaline membranesShunt, stiff lung
Premature lung
→
Surfactant ↓
→
Alveolar collapse↑ work of breathing
→
Injury + exudateHyaline membranes

ARDS vs cardiogenic pulmonary edema

FEATURE
ARDS
CARDIOGENIC EDEMA
Cause
Alveolar-capillary injury
↑ hydrostatic pressure (LV failure)
Fluid
Protein-rich exudate
Protein-poor transudate
Wedge pressure
Normal (< 18)
Raised
BNP, heart size
Normal
Raised, enlarged
Response to O₂
Poor (shunt)
Better
Histology
Hyaline membranes
Edema, heart-failure cells

How it's tested

COMMON CONFUSIONBoth ARDS and cardiogenic edema give bilateral infiltrates. A normal wedge pressure and normal BNP point to ARDS.
THE TRAPBig tidal volumes in ARDS overstretch the remaining good alveoli (volutrauma) and raise mortality. The protective strategy is low volumes with PEEP.
YOUR APPROACHAdult: find the insult, exclude heart failure, ventilate protectively. Newborn: gestational age + maternal factors + L/S ratio → steroids before birth, surfactant after.

If you see… think…

Septic patient, then severe hypoxemia, bilateral infiltrates, normal wedge → ARDS
Hyaline membranes lining adult alveoli → diffuse alveolar damage
Pancreatitis followed by hypoxemia unresponsive to O₂ → ARDS
Respiratory failure within 6 hours of a plasma transfusion → TRALI
30-week preemie: grunting, retractions, ground-glass CXR → NRDS
L/S ratio 1.5 at 32 weeks → immature lungs — antenatal betamethasone
Infant of a diabetic mother with respiratory distress → insulin suppresses surfactant
Preemie on prolonged high O₂ → retinopathy of prematurity, BPD
Term C-section baby with mild tachypnea resolving by day 2 → transient tachypnea
Post-term baby, green fluid, respiratory distress → meconium aspiration