Session 5- Respiratory Failure Flashcards
what is respiratory failure
impairment of gas exchange causing hypoxaemia with or without hypercapnia- may be acute or chrinic condition
type 1 resp failure
- low PaO2 < 8kPa or O2 saturation <90% breathing room air at sea level
- pCO2 normal or low
- gas exchange is impaired at the level of alveolar-capillary membrane
type 2 resp failure
- Low pO2 AND high PaCO2 > 6.5kPa breathing room air at sea level
- Reduced ventilatory effort or inability to overcome increased resistance to ventilation entore lung
hypoxaemia
low pO2 in arterial blood
hypoxia
o2 deficiency at tissue level
normal o2 saturation
94-98%
what does central cyanosis indicate
hypoxaemia- occurs when the level of deoxygenated haemoglobin in teh arteries is below 5g/dL with o2 sats below 85%
where is central cyanosis seen
oral mucosa
tongue
lips
what is the main cause of hypoxaemia
ventilation:perfusion mismatch
what compensatatory mechanisms increase oxygen delivery and therefore decrease hypoxia
- increase EPO by kidney to raise Hb
- increase 2,3 DPG - shifts curve to the right so o2 is released more freely
- increased capillary density
what are the consequeces of chronic hypoxic vasoconstriction of pulmonary vessels
pulmonary hypertension
right heart failure
cor pulmonale
what happens when V:Q RATIO IS <1
- PaO2 is low
- PaCo2 rises unless there is compensatory hyperventilation then it will be low or normal
- Hyperventilation induced by peripheral chemoreceptor firing secondary to hypoxaemia
- if lung disease severe hypervetilation may nor be able to compensate for V:Q <1
What happens when V:Q > 1
- PaO2 rises and PaCo2 falls
- resdistribution of pulmonary blood flow
- blood is redirected to unaffected areas
how is Co2 affected by an increase in diffusion distance
CO2 is more soluble there fore Co2 diffusion is lessaffected then diffusion of O2
what type of resp failure do diffusion impairing diseases lead to
type 1
what is shunt in respiratory system
blood that enters the lung but doesnt take part in gas exchange
what is ARDS
acute resp disress syndrome
pathophysiology of ARDS
massive widespread inflammation which damages teh alveoli and results in oedema increased vascular permeability, loss of surfactant and fibrin exudation
results in loss of the hypoxic pulmonary vasoconstriction mechanism and compliance decreases as lungs become stiff and lung volume decreases which creates an intrapulmonary shunt as blood cant get perfused
what does hypoventilation cause
alveolar pO2 falls-> arterial pO2 falls - hypoxaemia
alveolar pCO2 rises-> arterial pCO2 increases -> hypercapnia
type 2 respiratory failure
what can cause acute hypoventilation
opiate overdose
head injury
very severe acute asthma
what can cause chronic hypoventilation
severe COPD
how does kyphoscolisosis affect ventilation
causes disordered movement of the chest wall- respiratory system compliance reduced in kyphoscoliosis primarily due to the reduction in chest wall compliance and to a lesser degree a reduction in ling compliamce due to miscroatelectasis
effects of acute hypercapnia
respiratory acidosis
impaired CNS function - confusion, drowsiness, coma, flapping tremors
peripheral vasodilatation - warm hands, bounding pulse
cerebral vasodilatation- headache
effects of chronic hypercapnia
respiratory acidos compensated by retention of HCO3- by kidney
acclimation to CNS effects - CSF pH normalised
vasodilatation mild but may present
how does chronic CO2 retension effect central chemoreceptors
- CO2 diffuses into CSF- CSF pH drops and stimulates central chemoreceptors
- low CSF pH corrected by choroid plexus cells which absorb (HCO3-) into CSF
- the CSF pH returns back to jormal ans the chemoreceptors are no longer stimulated
- PaCO2 is still high but central chemoreceptors now unresponsive to this- if PaCO2 goes higher the reset value central chemoreceptors will fire
how can treating hypoxaemia worsen hypercapnia
correction of hypoxia removes pulmoney arteriole hypoxic vasoconstriction
- leads to increased perfusion of poorly ventilated alveoli
- diverting blood away from better ventilated alveoli- worsen V:Q mismatch
haldene mechanism
-oxygenated Hb cant carry as much CO2 with oxygen CO2 dissociated from Hb into blood
what type of resp failure does V/Q mismatch cause
type 1
- low pO2 but normal or low pCO2
