2013A11 Compare and contrast lung function in the neonate with that of an adult

 

List:

·         Definitions

·         Anatomy

·         Lung volumes

·         Resistance and compliance

·         Ventilation and perfusion

·         Gas exchange

·         Control of ventilation

·         Other

 

Definitions:

Neonate: <4/52 old, born 37-40/40

Adult: >18 years

 

Anatomy:

Anatomical feature

Clinical implication

Upper airways:

 

Narrow upper airways:

·   Narrow nasal passages

·   Large tongue

·   Narrow pharynx

↑ Resistance

·   Provides auto-PEEP

·   Any obstruction = dangerous

Obligate nose breather because of

·      Big tongue

·      High laryngeal inlet (C3-4 cf. C6)

·   Any obstruction = dangerous

Prominent occiput

·   Tendency for flexion -> obstruction

·   No need for pillow for bag & mask, intubation

 

 

Lower airways:

 

Narrower lower airways

·   Narrower larynx, trachea and bronchi

·   Note narrowest point cricoid ring (cf. glottis in adults)

 

·   Obstruction = dangerous

·   Use uncuffed ETT

·   Epiglottis long, U shape, floppy

·   Use straight blade

·   Laryngeal inlet is high C3-4 and anterior

·   Different intubation mechanics

·   Short trachea

·   Endobronchial intubation common

 

 

Chest wall

 

·   Horizontal, cartilaginous ribs

·   ↓Outward recoil

·   ↓Bucket handle motion

·   ↓AP and lateral expansion

·   Almost all diaphragmatic breathing

·   Abdo constriction dangerous

·   ↑Airway collapse in expiration

·   FRC < closing capacity

Resp muscle: ↓type 1 fibres

·   Diaphragm: 20% diaphragm cf. 55% adult

·   Intercostals: 45% diaphragm cf. 65% adult

·   ↓ reserve

·   ↑ risk type 2 respiratory failure

 

Lung volumes:

Volume

Implication

·  VT 7mL/kg in both

 

·  VD 2.2mL/kg in both

 

·  FRC 30mL/kg supine in both.

 

·  *Note FRC < closing capacity in neonates

·  Gas trapping

·  *Note FRC ↓↓ under GA

·  (muscle relaxation -> outward recoil)

·  Rapid desaturation (also because ↑metabolic rate)

·  FVC cannot accurately be measured in the neonate

 

·  TLC 50mL/kg neonate cf. 70mL/kg adult

 

 

Resistance and compliance

Parameter

Implications:

Static compliance:

·  1.5-6mL/cmH2O neonate

·  100mL/cmH2O adult

Specific lung compliance: no change

Chest wall compliance:

·  ↓ Due to ↓outward recoil

Neonate ↑RR for ↓work of breathing

·  Resistance a bit higher

·  Compliance a lot lower

Dynamic compliance: neonate > adult

·  ↑AWR neonate

Specific compliance = static compliance/FRC:

·  Same in neonate and adult

Airway resistance:

·  25cmH2O/L/s on day 2, cf. adult 2cmH2O/L/s

 

Ventilation and perfusion:

Parameter

Implication

·  RR 12 in adult, 40 in neonate

·  VT 5-8mL/kg both

·  ↓Reserve for ↑RR

·  MV 220mL/kg/min neonate, 110mL/kg/min adults

·  VA 140mL/kg/min in neonate, 70mL/kg/min adult

·  Due to 2x ↑VO2 (7-8mL/kg/min)

·  ↑Work of breathing

·  ↑Cardiac output 140mL/kg/min

·  ↑pulmonary blood flow

 

Gas exchange:

Difference:

Implication

(1)↑ O2 consumption:

7-8mL/kg/min in neonates, 3-4mL/kg/min in in adults

·  Rapid desaturation in apnoeic neonates

(2)↓ PaO2: 50-70mmHg neonate, 100mmHg adult

Due to:

·  VQ mismatch: 0.4 neonate (due to FRC <CC), 0.8 adult

·  R->L shunt: 10% neonate, 2-3% adult (??)

·  ↓CaO2

·  ↓Reserve in illness e.g. pneumonia

(3)↑O2 carrying capacity

·  [Hb] 17-18g/dL cf. 12-14g/dL

·  HbF p50 19mmHg cf. HbA p50 27mmHg

·  Partly offsets ↓PaO2 such that CaO2 16mL/100mL c.f. 20mL/100mL

·  Increases blood viscosity hence afterload

 

Control of ventilation:

Difference

Implication

·  Immature respiratory centre in neonates

·  Depressed by hypothermia

·  Frequent mild apneas

·  ↑Frequent ↑duration post-op

Acid-base neonate:

·  CO2-MV response curve left shifted

·  Compensated resp alkalosis

·  ↓Reserve for ↑ventilation

 

Other:

Neonate specific oxygen toxicity

·  Retinopathy of prematurity

·  Bronchopulmonary dysplasia

·  Necrotising enterocolitis

Non-respiratory functions

·  Innate immunity e.g. mucocilicary escalator present in neonate and adult

·  Adaptive immunity: passive (from breast milk) vs endogenous. No endogenous IgG or IgA in neonatal alveolar lining fluid

Response to stress

·  ↓PaO2, ↑PaCO2, ↓pH -> ↑PVR -> reversion to foetal circulation -> hypoxia

 

 

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