Canonical Question
Applied Resp – Other Physiology
Master answer
Anatomical and Mechanical Differences
- Relatively larger head
- Prominent occiput
- Short neck
- Neonates do not have dentition
- Relatively larger tongue
- Higher larynx (C3-4 in infant, C4-5 in adult)
- Larynx lies more cephalad and anteriorly
- Epiglottis is narrower, omega shaped, more floppy and relatively larger
- The narrowest part of the airway is the subglottic area at the level of the cricoid cartilage
- Infants are obligatory nasal breathers until 3-5 months of age, the narrow nasal passages can easily be obstructed by secretions
- Shorter compressible trachea
- Increased risk of laryngospasm with inappropriate instrumentation.
- The smaller the patient the harder it is to position a LMA appropriately and maintain the correct position
- More compliant tissues and reduced muscle tone
- Main carina also lies more cephalad
- Small peripheral Airways (50% are < 2 mm diameter)
- ↓↓↓ bronchial muscle → so bronchospasms uncommon and bronchodilator drugs have minimal response
- Alveoli immature, and number <10% of adult total
- Reduced mucociliary clearance
- ↓ % type I muscle fibres (highly oxidative/slow contraction) in diaphragm (25% in neonate vs 55% in adults) and IC muscles (45% vs 65%) → ↑ risk of muscle fatigue
Lung Volumes
- FRC (30 mL/kg) → same as adult (as FRC remains 40% of TLC) BUT ↓ stable and ↑ risk of atelectasis due to following reasons:
- Significant ↓ in outward recoil of CW (as rib is cartilaginous and contains very little respiratory muscles)
- Mild ↓ inward recoil
- TLC (75-80 mL/kg) → same as adult
- VC (45 mL/kg) → lower cf. adult (60 mL/kg)
- TV same as adult (7 mL/kg)
- ↑ CC due to low elastic recoil of lungs → in fact, CC > FRC such that there is small AW closure during tidal ventilation → results in gas trapping and hypoxaemia
Lung Physiology
| Variable | Neonate vs Adult |
| Oxygen consumption | 6mL/min vs 3mL/min in adult |
| O2 flux to tissues | ↑ due to: ↑ O2 carrying capacity(↑Hb, HbF, ↑MV → ↑pO2) ↑ Cardiac output |
| Minute Ventilation | 2x adult (to cope with ↑MRO2) Generated by ↑↑RR → ↑ work of breathing |
| Control of ventilation | Respiratory control less developed CO2 ventilatory response curve shifted to left O2/CO2 ventilatory response easily depressed by hypothermia Peripheral chemoreceptors immature Hering-Breuer reflex (transient apnoea lasting 5 secs evoked by gradual lung inflation) Head’s paradoxical inflation reflex (↑ inspiratory effort evoked by partial inflation of lungs) Periodic respirational (pauses lasting 5-10 sec (up to 6 x per hr during sleep) |
| Chest Wall Compliance | Neonates have ↑ chest wall compliance relative to adult → Prone to collapse on inspiration → ↓ Tv for same respiratory effort → ↑ WOB |
| Tissue Compliance | Initially neonates ↓ relative to adults until a few days of life when surfactant production is fully initiated, then → ↑ WOB in the first few days |
| V/Q matching | Significant V/Q mismatch (V/Q = 0.4) due to shunting from gas trapping/small AW closure |
| Physiological dead space | ↑ physiological dead space further necessitating ↑ MV |
| Work of Breathing | ↑ WOB overall |
Gladwin / JC 2019
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2020A Q06 | Outline how the respiratory system of a neonate differs from that of an adult. | historical_member | — |
| 2013A Q07 | Describe how the respiratory system of a newborn differs from that of an adult. | historical_member | — |