Pul 4 - Oxygenation Flashcards
How do we measure dissolved blood gas and how do we estimate O2 saturation?
Measuring dissolved blood gas: Arterial blood gas.
Estimate O2 saturation: Pulse oxymeter.
How does anemia affect the blood-oxygen content? What physiological parameters change and what stay the same?
When you are anemic, hemoglobin decreases, in turn, causing the fall of oxygen. It decreased total O2 content but the O2 saturation does NOT change. Arterial PO2 does NOT change.
What are three big causes of anemia?
- Decreased RBC synthesis.
- Increased RBC destruction.
- Loss of RBC.
What is the equation of the total blood O2 content in the body?
The total O2 content of the blood (in mL/dL) is calculated using the following equation:
Oxygen content = ([Hgb] x 1.34 x % saturation) + (0.0031 x PaO2)
[Hgb] = concentration of hemoglobin in the blood (g/dL)
% saturation = refers to the percent saturation of hemoglobin in the blood with O2 (expressed as a decimal)
PaO2 = refers to the total pressure of O2 dissolved in the blood in mmHg/dL
Note: The “1.34” in the equation refers to the fact that 1 g/dL of hemoglobin can carry 1.34 mL of O2
How does COPD affect blood-oxygen content?
- It decreased arterial PO2.
- It decreases blood O2 content: Lung tissue can shunt blood away from hypoxic lung tissue by clamping down on those pulmonary vessels: this is called physiologic shunt, which decreases O2 extraction ratio.
What does exercise and activity have on the blood-oxygen content?
- No change in arterial O2 content.
- Decreased venous PO2.
- Right shift of O2-hemoglobin dissociation curve.
What is the alveolar gas equation? What is the simpler equation?
PAO2= PIO2 - (PaCO2/R)
PA: Partial Alveolar
Pa: Partial arterial (small a for arteries because arteries are smaller than lungs)
PIO2: Partial Oxygen in Inspired air.
PAO2= 150 - (PaCO2/ 0.8)
- 150 because atmospheric air pressure at sea level (760) and subtract it by partial pressure of water vapor (47) which gives 713. Then multiply it by FIO2 (fraction of inspired oxygen which is 21%): this gives 150.
- 0.8 is an estimate of Resistance.
What is the A-a gradient?
PAO2- PaO2: Partial Oxygen at alveoli minus partial oxygen at arteries. Normally, the A-a gradient is small: 10-15.
What are things that can increase the A-a gradient?
- Shunting or V/Q mismatch: When blood isn’t making it to the alveoli.
- Pulmonary fibrosis: It thickens that diffusion barrier b/w type 1 pneumocytes and basement membrane and endothelial cells, making diffusion more difficult.
- Increase in FIO2.
- Advancing age.
How do we calculate alveolar O2? And what are some important values?
We use the ratio PaO2/FiO2.
- Normal value is around 300-500 mmHg
- A value less than 300 indicates gas exchange deficit.
- A value less than 200 indicates severe hypoxia like in ARDS.
What are three ways to be oxygen deprived?
- Inadequate PaO2: Hypoximia (low oxygen in the blood).
- Inadequate O2 delivery: tissue hypoxia.
- Inadequate perfusion: Isquemia.
What is another name for inadequate PaO2 and what are some of the causes?
It is hypoxemia:
- High altitudes.
- Hypoventilation.
- Right-to-left cardiac shunt.
- V/Q mismatch.
Reasons 3-5 also cause high A-a gradient.
What is another name for inadequate O2 delivery to organs/tissues and what are some of its caues?
It is hypoxia:
- Hypoxemia.
- Anemia.
- CO poisoning.
- Low cardiac output.
What is another name for inadequate perfusion and what are the causes?
It is ischemia:
- Obstruction of arterial flow (MI or stroke).
- Reduced venous drainage.
What is the normal value for an A-a gradient?
It is 10 to 15 mmHg.
