physiologic basis for the management of acute respiratory disorders in the newborn marc collin, md...

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