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:

  Higher than in adults

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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