Acute Respiratory Failure Flashcards
What is the A-a gradient?
The difference between the pressure of O2 in the alveolus and the arterial blood.
What does A-a gradient tell you?
Whether there is a shunt or diffusion abnormality present.
What does an increased A-a gradient suggest?
Any hypoxaemia is caused by a ventilation perfusion mismatch (usually shunt), or a diffusion abnormlaity.
What is shunt?
Blood flow to non-oxygenated alveoli.
Where gas flow in and out of the alveolus is reduced (due to alveolar collapse, or being filled with pus/blood/oedema), so blood traverses the alveoli without picking up any oxygen.
As a result, poorly oxygenated blood leaves the alveoli, and reducess the overal O2 saturation in the pulmonary vein.
Does increasing FiO2 improve oxygenation in patients with shunt?
Increasing supplemental O2 in patients with shunt often has little effect on hypoxaemia.
In shunt, alveoli are being perfused, but not ventilated due to blood/pus/fluid/collapse. Supplemental O2 cannot get to the shunted alveoli.
Meanwhile, the non-shunted alveoli are already saturating blood 100%.
This is because the O2 will not get
What is the most common pathophysiological cause of hypoxemic respiratory failure?
Shunting
What are some causes of shunting?
Pneumonia (pus)
Pulmonary oedema (fluid)
Pulmonary haemorrhage/contusion (blood)
Atelectasis (collapse)
What is dead space?
Where well ventilated alveoli don’t receive any perfusion.
What are some causes of deadspace?
Hypovolaemia
Pulmonary embolus
Poor cardiac function
High intrathoracic pressures
COPD
Name some causes of breathlessness that have an onset in minutes.
Pneumothorax
Pulmonary embolism
Pulmonary oedema
Name some causes of breathlessness that have an onset of hours.
Asthma
Pneumonia
Pulmonary oedema
Metabolic acidosis
Name some causes of breathlessness that have an onset of days.
Pleural effusion
Exacerbation of COPD
Pneumonia
Why is an SpO2 of 90% an important threshold?
Below SpO2 90%, a small decrease in PaO2 will result in a precipitous drop in Hb saturation.
(And saturation, not PaO2, is the main determinant of O2 content and O2 delivery to tissues)
What are the most common sources of error in pulose-oximetry readings?
How do you identify these sources of error?
Poor perfusion and poor probe placement.
To identify these sources of error, compare the heart rate from the pulse-oximeter to the ECG heart rate.
What are some secondary sources of error in SpO2 readings?
Dark skin (leading to over-reading, and false reassurance)
False nails / nail varnish
Lipaemia
Bright ambient light
Excessive motion
Carboxyhaemoglobin
