Physiologic Basis for the Management of Acute

Respiratory Disorders in the Newborn

Marc Collin, MD18 November 2003

Developmental Anatomy

• Alveoli-developed by 25th week -increase in # until 8 yr. -from 20 to 300 million -surface area: 2.8 m2 @ birth 32 m2 @ 8 yr.

75 m2 @ adulthood -diameter: 150- 300 um(NB-Adult)

Developmental Anatomy

• Airways- cartilaginous - relatively weak in infancy - dynamic compression - bronchiolitis (RSV)

- RAD - crying!

Developmental Anatomy

– airways enlarge in diameter/length– distal airways lag in first 5 yr.– high peripheral resistance in infancy

– Resistance = 1/R4

Pulmonary Physiology

• Compliance = Change in Volume Change in Pressure

Static Lung Volumes

Mechanics of Infant v. Adult Lung

Pulmonary Physiology

• Alveoli at birth• fluid-filled v. air-filled v. air-liquid interface

• pressures up to 80 cm H2O @ birth

• alveolar rupture

Pressure-Volume Curves after Air v. Liquid Lung Expansion

Pulmonary Physiology

LaPlace relationship:

P = 2T/R

P= distending pressure

T= wall tension

R= radius (alveolar)

Pressure-Volume Curves of First 3 Breaths

Developmental Biochemistry of Alveoli

• History: Avery & Mead-1959 - RDS secondary to surfactant deficiency - Treatment: CPAP

Surfactant

• Phospholipids - phosphatidylcholine

- phosphatidylglycerol

• Surfactant proteins - A, B, C

Surfactant Components

Surfactant

• Type II alveolar epithelial cells-responsible for synthesis,

storage, secretion, and reuptake

• Lamellar bodies -intracellular storage form of surfactant -secreted via exocytosis -forms tubular myelin in extracellular space

Surfactant and Type II Cells

Surfactant

• Inactivation by: - alveolar-capillary leak - pulmonary edema - hemorrhage (hemoglobin) - alveolar cell injury - meconium

Surfactant

• Recycling - spent forms taken up/reused by Type II cells. - process facilitated by SP-A, B, and C - half-life = 3.5 days

RDS

• US incidence: 30,000/yr.

• Inversely related to gestational age

• Onset-shortly after birth

• Signs-grunting, flaring,retracting

• Duration-1 week

RDS

RDS

• Progressive atelectasis

• V/Q mismatch

• Decreased FRC

• Impaired ventilation (weak respiratory m’s, compliant chest wall)

• Increased PVR due to hypoxia, acidosis

RDS

• Right to left shunting leading to further hypoxemia

• Left to right shunting leading to pulmonary edema

Exogenous Surfactants

• Replacement therapy/Fujiwara, Japan, 1980

• Human (from C/S)

• Artificial (Exosurf)

• Bovine (Survanta)

• Calf (Infasurf)

• Pig (Curosurf)

Compliance Before and After Surfactant

Before surfactant

After surfactant

VOLUME

PRESSURE

Air Leaks

• Pulmonary interstitial emphysema (PIE)

• Pneumomediastinum

• Pneumothorax

• Pneumopericardium

• Pneumoperitoneum

Subtle left pneumothorax

Left pneumothorax now more obvious

Left pneumothorax?

pneumothorax

Transillumination of left pneumothorax

pneumomediastinum

Pneumopericardium (note air under heart)

Air Leaks

• initiating factor: PIE (alveolar rupture into perivascular and peribronchial spaces)

• dissection into mediastinum

• further dissection into pleural, pericardial space

• rupture from surface blebs

• direct lung rupture-VERY rare

Air Leak Risk Factors

• RDS: 12-26%

• MAS/other aspirations

• Spontaneous

Air Leak Management

• early recognition (esp. in preterms)

• nitrogen wash-out (term/near-term)

• needle aspiration v. tube thoracotomy

• limit barotrauma

• HFOV

• positioning

• selective ET intubation

Meconium Aspiration Syndrome (MAS)

• GI secretions, cellular debris, bile, pancreatic juice, mucus, lanugo hairs, vernix; blood.

• incidence: ~15% (30% @ >42 wks)

• cause v. result of ‘asphyxia’

MAS

• Asphyxia intestinal ischemia

anal sphincter relaxation

meconium passage

MAS

• Asphyxia fetal gasping

enhanced meconium entry into respiratory tract

MAS-Presentation

• Respiratory distress

- tachypnea

- prolonged expiratory phase - hypoxemia

• Increased A-P diameter (‘barrel’ chest)

• Pulmonary hypertension

MAS-Radiographic Findings

• coarse alveolar infiltrates

• consolidation/hyperaeration

• pleural effusion (30%)

• pneumothorax/pneumomediastinum

Meconium aspiration syndrome

Meconium aspiration syndrome

MAS-Pathophysiology

• Acute small airway obstruction -increased expiratory resistance -increased FRC -regional atelectasis -V/Q mismatching

MAS-Pathophysiology

• Surfactant inactivation -decreased compliance -hypoxia

• Pulmonary hypertension

MAS-Treatment

• Intubation/tracheal suction @ delivery

• Saline lavage?

• Surfactant therapy

MAS-Ventilatory Support

• CPAP/PEEP (be careful)

• Air leak due to ball-valve phenomenon

• Decreased I/E ratio (more E time)

• Hyperventilation (CMV)

• HFOV

• iNO

• ECMO

Persistent Pulmonary Hypertension of the Newborn

(PPHN)

• Etiology: Primary v. Secondary

• Failure of transition from high to low PVR after birth

• PFO and PDA rightleft shunting

• Intrapulmonary shunting, esp. w/ pulmonary parenchymal disease

PPHN

• PVR decreases secondary to:

• -mechanical distention of pulmonary vascular bed

• improved oxygenation of pulmonary vascular bed

• prostacyclin and NO production

PPHN

• Remodeling of pulmonary vascular musculature

• Normally, fully muscularized preacinar arteries extend to terminal bronchiolar level.

• Muscularization begins to decrease w/in days, complete w/in months.

• Regression process delayed by hypoxia

• Chronic hypoxia stimulates further muscularization

PPHN

• Differential Diagnosis:

- Primary (chronic hypoxia) - Parenchymal disease (MAS, pneumonia, RDS, hemorrhage) - Cyanotic heart disease (TGV, critical PS, HLHS, severe coarctation) - Pulmonary hypoplasia (Potter’s S., Oligohydramnios, CDH, CCAM)

Congenital cystic adenomatoid malformation

Congenital diaphragmatic hernia

Thoracic hypoplasia

Hypoplastic right lung

Hypoplastic lungs

PPHN-Treatment/Medical

• Intravascular volume

• Correct metabolic acidosis

• Pressors (be careful!)

• Sedation (for lability) v. paralysis

PPHN-Treatment/Respiratory

• induction of respiratory alkalosis

• pressure support/barotrauma risk depending on etiology (compliance)

• very labile….SLOW wean (maintain relative HYPERoxia, if possible)

• iNO

• ECMO