Flashcards in Respiratory Control Foetus, Neonate, Infant Deck (17)
Main centres responsible for respiratory control?
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
Why do newborns differ from adults
Incomplete lung development, even at term
O2 dissociation curve shifted to left due to HbF
Neonatal chest wall more compliant than adults
What is the respiratory activity of the foetus?
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.
Fetal response to CO2 and O2?
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.
What are the proposed mechanisms behind the fetal response to O2 and CO2?
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.
Properties of neonate breathing?
Apnoeic periods during sleep - maturational effect.
Neonatal response to CO2?
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.
Response to CO2 - preterm
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
Response to O2 term?
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.
What is hypoxic ventilatory rolloff?
Initial increase is peripherally mediated, response eliminated by CB denervation
Late decrease due to depression of central respiratory neurons = HVR
Mechanisms of HVR?
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
Premature response to O2?
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
What nerve are the HB reflex dependent on?
SAR via the Vagus
What is HB inflation, expiration, paradoxical and deflation reflexes?
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
What is the intercostal inhibitory reflex?
Distortion of chest wall results in shortened inspiratory efforts.
Tactile stimulation, irritant gases and deflation all change frequency and depth of respiration. Pronounced insp effort followed by apnoea.