Respiratory Control Foetus, Neonate, Infant

Question 1

Main centres responsible for respiratory control?

Answer 1

1 - VRG = efferent area in VLM, contains insp and exp neurones

2. DRG = main relay station for afferents from the vagus and CB
3. Nucleus retrofacialis = site of respiratory oscillator

Question 2

Why do newborns differ from adults

Answer 2

Incomplete lung development, even at term
O2 dissociation curve shifted to left due to HbF
Neonatal chest wall more compliant than adults

Question 3

What is the respiratory activity of the foetus?

Answer 3

Shows some respiratory movements (FBM) first shown in human in 1974.

Seen during REM sleep, absent in NREM due to descending inhibitory pathways from lateral pons or midbrain

Thought to be required for lung maturation and normal growth.

Question 4

Fetal response to CO2 and O2?

Answer 4

Hypoxia leads to cessation of respiratory activity and general body movements - OPPOSITE TO POSTNATAL. Hyperoxia leads to continuous breathing movements.

Increase PCO2 increases freq and TV of respiration during REM sleep (not seen in nREM sleep). Hypocapnia abolishes FBM.

Question 5

What are the proposed mechanisms behind the fetal response to O2 and CO2?

Answer 5

Chemoreceptor input is gated out by central mechanisms, explains hypoxic responses. Neurones in rostral pons activated during hypoxia which suppress breathing by inhibition of DRG and VRG neurones.

Lesion of rostral lateral pons dissociated FBM from EcOG activity and reversed depressive response to hypoxia.

Adenosine also implicated as A1 receptors tonically inhibit fetal inspiratory drive.

Question 6

Properties of neonate breathing?

Answer 6

Apnoeic periods during sleep - maturational effect.

Question 7

Neonatal response to CO2?

Answer 7

Proportional to adult. Response mediated via peripheral and central chemo-R. Periodic breathings abolished by administration 1-2% CO2. Rate of response matures over first 7-10days.

Hypercapnia and hypoxia interact positively to increase magnitude of ventilatory response.

Question 8

Response to CO2 - preterm

Answer 8

Lies between fetus and newborn. The more immature, the flatter the response line. ? due to central unresponsiveness. Threshold for response lower cf adult.

PO2 and PCO2 interact negatively

Question 9

Response to O2 term?

Answer 9

Hyperoxia -> decrease ventilation, followed by an increase. Secondary increase due to paralell increase in PCO2 caused by the initial decrease in VE

Hypoxia - neonates initially increase then decrease, the decrease termed hypoxic ventilatory rolloff.

Question 10

What is hypoxic ventilatory rolloff?

Answer 10

Initial increase is peripherally mediated, response eliminated by CB denervation

Late decrease due to depression of central respiratory neurons = HVR

Question 11

Mechanisms of HVR?

Answer 11

Accumulation of central inhibitory NTs - GABA, adenosine, opiated, PDGF.

Reduced chemo-R responsiveness - species specific. Seen in piglets but not kittens.

Decreased input from CB to NTS

Reduced PaCO2 - increase in VE during hypoxia reduced PaCO2 which causes a fall in VE. However, exposure to CO2 and hypoxia did not prevent the late respiratory depression

Decreased metabolic rate - young animals can markedly reduce their metabolic rate, can temporarily suspend metabolic activity (eg growth) in response to hypoxia

Reduced ventilatory pump efficiency - affect of hypoxia on ventilatory muscles

Question 12

Premature response to O2?

Answer 12

Depends on prematurity

Very premature = fetal = apnoea
Mod premature = mix fetus and term, less initial increase followed by larger decrease

Pattern influenced by sleep state, ie, non REM sleep the decrease in VE is main response observed

Hyperoxia causes decrease in ventilation, but response is slower in <36weeks

Question 13

What nerve are the HB reflex dependent on?

Answer 13

SAR via the Vagus

Question 14

What is HB inflation, expiration, paradoxical and deflation reflexes?

Answer 14

Inspiratory = inhibiton/cessation of ongoing inspiration in response to lung stretch

Expiratory = maintained inflation induced prolonged expiratory muscle contraction

Paradoxical = ongoing inspiration causes more inflation. May mediate first breath? Only seen in prenatal period. Important during resuscitation.

Deflation = rapid lung deflation by suction or pneumothorax initiates a strong, prolonged inspiration. Maintains FRC and prevents acetelectasis

Question 15

What is the intercostal inhibitory reflex?

Answer 15

Distortion of chest wall results in shortened inspiratory efforts.

Question 16

Irritant reflex?

Answer 16

Tactile stimulation, irritant gases and deflation all change frequency and depth of respiration. Pronounced insp effort followed by apnoea.

Question 17

Upper airway reflexes?

Answer 17

May contribute to dive reflex