M103 T4 L4 Flashcards
How does pulse oximetry work?
put the finger probe on
light emitting diode and a receiver on the other side
the light goes through the finger and essentially detects the light with the receiver
What is PaO2 measured by?
an abg analyser
What might affect / nullify the results of pulse oximetry?
highly pigmented skin - lower reading than expected
nail varnish - completely nullifies the result
What is Type 1 respiratory failure usually caused by?
some kind of intrinsic lung disease affecting the airways or the vasculature
What is Type 2 respiratory failure usually caused by?
(brain) a reduction in ventilatory drive - this could be caused by an overdose of a sedative medication
(muscle) reduced neuromuscular power - some kind of myopathy
(receptors) resetting of chemoreceptors that drive ventilation
How is the base excess affected in metabolic alkalosis & acidosis?
increases in metabolic alkalosis
decreases in metabolic acidosis
What are the physiological causes of hypoxaemia?
V/Q mismatch Right-to-left shunt Diffusion impairment Hypoventilation Low inspired pO2
equation for a-a gradient
PAO2 = PIO2 – (PaCO2/0.8)
What is the function of the a-a gradient?
indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange
disease / pathology of the alveolocapillary unit widens the gradient
What is hypoxaemia where the a-a gradient widens caused by?
V/Q mismatch, diffusion limitation and shunt
What is the most common cause of hypoxaemia?
V/Q mismatch
How does V/Q change in different parts of the lungs?
V/Q ratio is higher at apex and low at the base
What is the name of the mechanism by which V/Q mismatches are compensated for? (HypPulVas)
hypoxic pulmonary vasoconstriction
What are symptoms of a high V/Q ratio?
when ventilation is in excess of perfusion
What compensation can occur for hypoxaemia?
a compensatory rise in total ventilation
How is hypoxaemia due to V/Q mismatch corrected?
supplemental oxygen
What are the common causative conditions of Hypoxaemia?
(ABCC.PH-ILD)
asthma COPD bronchiectasis cystic fibrosis ILD pulmonary hyptertension
What happens in a pulmonary shunt?
blood from the right side of the heart enters the left side of the heart without taking part in gas exchange
so it misses the pulmonary circulation
has a poor reaction to oxygen therapy
What are the signs of a pulmonary shunt?
A-a gradient is elevated
pCO2 is normal
Poor response to oxygen therapy
What are the common causes of a pulmonary shunt?
pneumonia
pulmonary oedema
acute respiratory distress syndrome
pulmonary arteriovenous malformations
What are the causes of diffusion limitation?
pulmonary fibrosis
hypoxaemia
Why is hypercapnea not usually associated with diffusion limitation?
CO2 is 20x more soluble than O2 and is less likely to be affected by diffusion limitation
What are the signs of diffusion limitation?
shows a good response to oxygen therapy
A-a gradient is elevated
PaCO2 is normal
What does the A-a gradient look like in hypoventilation?
Normal A-a gradient (low PAO2 then subsequent low PaO2)
How does hypoventilation react to oxygen therapy?
If hypoxaemia is present it is easily corrected by oxygen therapy
but hypoventilation and hypercapnoea can persist
What are the causes of hypoventilation?
Impaired central respiratory drive (brain)
a disease of the spinal cord, NMJ or nerve (neural)
a myopathy in respiratory muscles (muscular)
What are the signs of hypoventilation?
shows good response to oxygen therapy
A-a gradient is normal
PaCO2 is high
What does the Beer-Lambert-Bougeur law state?
that the attenuation of light depends on the properties of the materials through which the light is travelling
What causes the VQ mismatch in Type 1 respiratory failure?
the reduction in PaO2 but the lack of change in PaCO2
ventillation issue
What causes underventilation in Type 1 respiratory failure?
the ics in PaCO2 and dcs in PaO2
What physiological states are associated with Type 1 & 2 respiratory failure?
1 - hypoxia
2 - hypoxia with hypercapnia
What group of patients usually experience hypercapnia?
people with COPD
makes it harder for them to breathe
What is the normal pH of a litre of blood in terms of PaCO2?
PaCO2 of 40 mmHg
What is the most common type of sleep apnoea?
obstructive sleep apnoea
What is above normal for obstructive sleep apnoea?
5 or more obstructive apnoeas per hour
What are statistical criteria features of hypopnea?
50% reduction in airflow
4% oxygen desaturation
and/or arousal from sleep
What gases are delivered by a Venturi mask?
specific concentrations of oxygen by mixing oxygen with inhaled air
What apparatus makes up a Venturi mask?
a face mask and reservoir bag device
What mask delivers oxygen?
Hudson mask
uncontrolled and controlled mask
What are the causes of a V/Q mismatch?
reduced ventillation
reduced perfusion
What causes reduced ventilation (VQ mismatch)?
hypoventilation, low FiO2
What causes reduced perfusion (VQ mismatch)?
diffusion impairment or pulmonary shunt (low perfusion)
What does pulse oximetry measure?
oxygen saturation of Hb in the capillary beds
What is SaO2 measured by?
abg analyser and a pulse oximeter
What is the normal blood pH range?
7.35-7.45
What three organs are involved in acid-base homeostasis?
lungs, kidneys and brain
What structures are involved in acid-base homeostasis?
chemical buffers in blood
blood cells (erythrocytes)
the lungs, kidneys and brain
Why isn’t the base excess widely used in metabolic acidosis and alkalosis?
its utility in interpreting blood gas results is controversial
What could cause an impaired central drive, resulting in hypoventilation?
drug overdose, brainstem infarction, primary alveolar hypoventilation
What could cause an impaired central respiratory drive, resulting in hypoventilation?
diseases of the spinal cord, NMJs or nerves
What causes Myasthenia gravis?
antibodies destroying the communication between nerves and muscle
How is the body affected in Myasthenia gravis?
weakness of the skeletal muscles
How is Guillain-Barré syndrome pronounced?
geeyan baray
What is an example of a spinal cord disease that could cause hypoventilation?
ALS
What is an example of a nerve disease that could cause hypoventilation?
Guillian-Barre syndrome
What is an example of an NMJ disease that could cause hypoventilation?
myasthenia gravis
What are the first symptoms of Guillian-Barre syndrome?
weakness and tingling in extremities
How does Guillian-Barre syndrome progress?
weakness and tingling in extremities quickly spread, eventually paralyzing the whole body
What is the cause of Guillian-Barre syndrome?
unknown, but 2/3 of patients report symptoms of an infection in the six weeks before (respiratory, GI infections or zika virus)
What is the effect of hypoventilation in healthy and unhealthy lungs?
healthy lungs - will not cause sufficient hypoxia
lung disease - will cause sufficient hypoxia
What is hypoxaemia where the a-a gradient is normal caused by?
hypoxia and hypoventilation
