Hypoxemia:Hypoxia Flashcards
Hypoxemia: define
Low partial pressure of oxygen in the arterial blood
Hypoxia: define
low PO2 systemically or at tissue level
Interpret the measurement, PaO2 < 60mmHg. Hypoxemic or nah?
Hypoxemic
Hyperventilation: what are the carbon dioxide levels like?
LOW! PaCO2 <35mmHg
Hypoventilation: what are the carbon dioxide levels like?
HIGH! PaCO2 > 45mmHg
What’s the difference between SaO2 and SpO2?
While they both measure the saturation of oxyhemoglobin, one does so via arterial blood while the other does so via pulse ox.
Regardless, they should be the same!
FiO2…what’s this?
the inspiratory oxygen concentration
What are the 5 mechanisms of hypoxemia?
1) Decreased FiO2 or decreased PAO2
2) Hypoventilation
3) Right => Left Shunt
4) Diffusion impairment
5) Ventilation Perfusion V/Q mismatch
What are the 3 of the 5 mechanisms of hypoxemia that constitute “venous admixture”?
3) Right => Left Shunt
4) Diffusion impairment
5) Ventilation Perfusion V/Q mismatch
What IS venous admixture?
Venous blood mixing up with saturated arterial blood as a result of a R:L shunt, diffusion impairment from some lung disease or even V/Q mismatch which is common in horses.
Hypoventilation, a mechanism of hypoxemia, can be due to two things…
drugs
damage to chest wall/respiratory muscles
With Hypoventilation, is alveolar ventilation increased or decreased?
decreased
Does Hypoventilation increase or decrease the PC02?
Increase it!
What does Hypoventilation do to PO2?
Decreases the PO2 unless additional O2 is inspired
Is Hypoxaemia caused by hypoventilation reversible?
Yea it’s easy with the addition of O2.
What’s the connection between shunts and hypoventilation?
Usually from R:L, blood is shunted away from entering the oxygenated parts of the lungs so it enters the arterial system with insufficient O2.
Do shunt patients respond well to additional O2?
Nerp.
What are the 4 advantages to using a pulse ox?
- Beat‐to‐beat assessment of oxygenation
- Practical
- Simple
- Non‐invasive
What are the 3 disadvantages to using a pulse ox?
- Beat‐to‐beat assessment of oxygenation
- Practical
- Simple
- Non‐invasive
T/F
Pulse ox can be used to assess oxygenation with and without supplemented O2
True
Arterial Blood Gas Analysis vs Pulse Ox—what do they measure?
Arterial Blood Gas Analysis measures dissolved O2 in the blood
Pulse Ox measures saturation of hemoglobin with oxygen in the blood.
PaO2 is related to which measurement?
SpO2 via the oxyhemoglobin dissociation curve
REMEMBER: PaO2 60mmHg = SpO2 90%
How will low Hb concentration affect O2 content?
will also decrease
Rule of 4
pH 7.4 +/‐ 0.04
pCO2 40 +/‐4 mmHg
HCO3 24 +/‐ 4 mmol/l
BE ‐4 to +4 mmol/l
Alveolar Gas Equation
PAO2 = PIO2 – (PACO2/0.8)
Can we tell based off PaO2 alone if a patient has pulmonary dysfunction?
Nope. Based on PaO2, it will just tell us how well the patient is oxygenating.
Why is Arterial Blood Gas(ABG) analysis critical to assessment of pulmonary function?
Because ventilation influences oxygenation in the wake animal but not so much in the anesthetized animal.
Be sure to factor in PaCO2
How do we calculate the relationship between fall in PO2 and the rise in PCO2 that occurs in hypoventilation?
use the alveolar gas equation
***We use the A-a gradient/Alveolar gas equation to figure out what?
***Why the animal is hypoxemic/pulmonary dysfunction
Normal A‐a should be
<15mmHg
anything greater than this indicates pulmonary dysfunction, some hindrance to ventilation…gas exchange
T/F
You can be hypoxemic but maintain pulmonary function.
True
The hypoxemia could be a result of hypoventilation
The equation we use to check on pulmonary function?
A-a gradient
This equation identifies the presence or absence of a pulmonary gas exchange abnormality
A-a gradient
Your ability to ventilate affects…?
oxygenation
Does a normal A-a gradient mean the patient is 100% in the clear?
No. A normal A-a gradient only lets you know there isn’t an issue with pulmonary gas exchange. It does not rule out pulmonary pathology.
What’s another way we can measure the efficiency of gas exchange?
PaO2 : FiO2 Ratio
The use of this PaO2:FiO2 is good for two conditions:
• Useful for patients receiving FiO2>0.21 (21%)
• Useful for patients where FiO2 is known
– Room air
– Anesthesia
PaO2:FiO2 normal range
350‐500
What’s a limitation to PaO2:FiO2?
– Difficult to quantify FiO2 in non‐ intubated patients
P(A – a)O2 in room air—how do you interpret results?
- <15 mm Hg is normal
- 15‐30 mm Hg is mild to moderate impairment of oxygen exchange
- 30 mm Hg severe impairment of O2 exchange
PaO2/FiO—how do you interpret results?
- PaO 2/FiO2 ratio > 400: normal
- PaO 2/FiO2 ratio < 300: O2 exchange impairment
- PaO 2/FiO2 ratio < 200: severe lung disease