Paediatric Physiology

Question 1

What ages correspond to neonates, infants, and children?

Answer 1

Neonates up to 1 month
Infant 1 month - 1 year
Child up to 16
Adolescent between 13-16

Question 2

What airway factors complicate intubation of young children & babies?

Answer 2

Bigger head, more prominent occiput
Small pharynx & larynx, with anterior, higher larynx at C3-4
Cricoid is narrowest point, not glottis
Short, narrow trachea with higher risk of endobronchial intubation
Soft floppy tissues make airway collapsable
Large floppy epiglottis
Larger tongue with smaller receding chin
Prominent lymph nodes & tonsils

Smaller FRC & higher BMR - faster desaturation, less intubating time

Question 3

Describe the physiological differences seen in paediatrics compared with adults.

Answer 3

Unlike adults, children need to enable growth, rather than homeostasis.

Respiratory
Closing capacity exceeds FRC below 6yrs
Increased RR with poor ability to increase TV (horizontal ribs)
WoB higher, over 10% of O2 consumption
Ventilation mainly diaphragmatic with nasal breathing

Cardiovascular
Complex transition from foetal to adult circulation
Increased HR with CO relatively 40% higher
Minimal ability to increase SV, CO dependent on HR
Small blood volume, minimal reserve

Metabolic
High BMR & O₂ consumption
High surface area to volume ratio (rapid heat loss)
Risk of hypoglycaemia

Neurological
Spinal cord terminates at L3
Sympathetic system underdeveloped
Myelination of peripheral nerves not complete unti 6yrs
Immature BBB

GI & Hepatic
Immature liver enzymes

Renal
Extracellular compartment drastically higher in infants (effects volume of distribution)
Reduced GFR & renal tubular function (high renal vascular resistance)
Reduced ability to concentrate urine

Haematological
RBCs have up to 80% HbF at delivery, dropping to 5% by 3 months
Hb elevated at birth, nadirs at 3 months
Reduced Vit-K dependent clotting factors

Musculoskeletal
Fatigue quickly

Question 4

How is fluid deficit calculated in an infant?

Answer 4

Deficit = 10 x body weight in kg x % dehydration

% dehydration calculated by measuring peripheral temperature

New fasting guidelines (Sip-till-send) allow clear fluids orally until 1 hour pre-op, in low risk elective settings.