Respiratory failure and VQ matching Flashcards
up to slide 12
Normal arterial values
what are the normal limits for PaO2 in arteries, in kPa and mmHg
- 11-15 kPa
- 90-113 mmHg
what is the usual partial pressure of oxygen in the veins?
40mm Hg
normal arterial values
what are the normal limits for PaCO2 in arteries, in kPa and mmHg
- 4.6-6.4 kPa
- 33-46 mm Hg
what is the deficit between PO2 in alveoli (PAO2) and PO2 in arteries (PaO2) known as, and if the deficit is greater than 15 mmHg, what does it indicate?
- known as the PA-aO2 difference
- deficit greater than 15mmHg indicates oxygen is being obstructed in the blood
normal arterial values
what can Hypercapnia mean for mechanical breathing mechanisms
it means your mechanical breathing mechanisms aren’t working correctly
normal arterial values
what can being outside normal arterial values of PaO2 and PaCO2 mean
can indicate respiratory failure
respiratory failure
define respiratory failure and what is it characterised by?
a failure to maintain adequate gas exchange, characterised by abnormalities in arterial blood gas partial pressures
respiratory failure
what is type 1 respiratory failure ( include values in you answer
hypoxaemia (< 9kPa) with normal or low CO2
respiratory failure
what is type 2 respiratory failure (include values in your answer) and what does this mean for breathing rate?
hypoxaemia with HIGH CO2 (>6 kPa) so breathing rate cannot keep up.
respiratory failure: pathophysiological causes
what are 4 main causes of Respiratory failure
- alveolar hypoventilation
- diffusion deficit
- shunts
- ventilation - perfusion (VQ) mismatch
define ventilation
ventilation = frequency of each breath X size of each breath
hypoventilation
define hypoventilation
decrease in minute ventilation leading to CO2 retention and less O2 absorption
hypoventilation
what does hypoventilation increase regarding ventilation
increases proportion of dead space ventilation
hypoventilation
what can hypoventilation cause regarding respiratory failure and give one example as to how ?
causes Type II respiratory failure due to respiratory muscle fatigue
Diffusion deficit
define diffusion deficit
can’t get oxygen into gas exchange zones
what is fick’s Principle/law?
the greater the concentration gradient, the more readily gases diffuse down their concentration gradient
diffusion deficit
what is the equation of Fick’s Law
Vgas ∝ A/T x Diffusion coefficient of gas x (P1 - P2)
where:
- A = surface area
- T = tissue thickness
- D = diffusion coefficient of gas
- P1-P2 = partial pressure gradient
diffusion deficit
what is the equation for the diffusion coefficient of gas
D ∝ Gas solubility / √gas molecular weight
shunts
what are shunts
shunts are when venous blood mixes with arterial blood, so oxygen isnt reaching gas exchange centres
eg: tissues
shunts
what are extra-pulmonary shunts
when venous and arterial blood mix but completely bypass the lungs
shunts
what are intra-pulmonary shunts
- blood is transported through the lungs without undergoing gas exchange first.
shunts
what are the 2 common causes of intrapulmonary shunts
- alveolar fillings (pus, oedema,blood, tumour)
- atelectasis (lung collapse)
shunts
True or False: providing Oxygen can correct pure shunt hypoxia in intra-pulmonary shunts
False, pure oxygen CANNOT correct pure shunt hypoxia. The cause of the intra-pulmonary shunts needs to be addressed first
What does H.A.S.H and what is this used for
H - hypoxia hypoxemia (when oxygen is not available in the environment)
A - anaemia
S - stagnant ( i.e embolism/ ischemia)
H - histotoxic (I.e poison of he or blood cells such as with carbon monoxide)
Used to remember what causes O2 insufficiency
what are 4 factors about haemoglobin
- haemoglobin differences (eg fetal vs normal)
- acidosis
- temperature change
- 23-DPG
what is 2 examples of when partial pressure gradient can increase or decrease diffusion
Increase:
- increased metabolism
- 100% O2
decrease:
- altitude
what is an example of when gas physical properties effect rate of diffusion
- heavier gas or less soluble gas decreases rate
what are 4 examples of when the alveolar-capillary (tissue thickness) membrane can affect rate of diffusion
- pulmonary fibrosis
- oedema
- asbestosis
- fibrosis
what are 2 examples of when reduced gas exchange area affects rate of diffusion
- emphysema
- Pneumonia
what is the realistic value for Ventilation (air/min) and perfursion (blood/min) in litres and what is the realistic whole lung V/Q ratio because of this?
Ventilation:
- 5L air/min
Perfusion:
- 6L blood/min
whole lung V/Q ratio:
- 0.8:1
where ventilation is 0.8 and perfusion is 1
what are the values of ventilation and perfusion at the top, middle and bottom of the lungs and the consequent V/Q ratio at each level because of this.
TOP:
- ventilation: 1.2L/min
- perfusion: 0.4L/min
- V/Q ratio: 3.0
MIDDLE:
- ventilation: 1.8L/min
- perfusion: 2.0L/min
- V/Q ratio: 0.9
BOTTOM:
- ventilation: 2.1L/min
- perfusion: 3.4 L/min
- V/Q ratio: 0.6
how many times greater is ventilation AND perfusion at the bottom of the lungs compared to the top?
ventilation at the bottom is 2x greater at the bottom compared to the top
perfusion is 5x greater at the bottom compared to the top
how can you measure ventilation at the top middle or base of the lungs?
- use radioactive xenon (1,2 or 3) as a respiratory ventilation marker
