Causes and Mechanisms of Hypoxemia Flashcards
What is hypoxia?
A condition in which all or specific regions of the body do not receive adequate oxygen for aerobic metabolism
What can cause hypoxia?
Hypoxemia (low level of oxygen in arterial blood)
Anemia
- An arterial blood gas analysis directly measures the concentration of what form of oxygen?
- An arterial blood gas analysis can be use ed to calculate the concentration of what form of oxygen?
- What does a pulse oximeter measure directly?
Oxygen that is dissolved in the blood (PaO2)
Oxygen bound to Hb
Oxygen bound to Hb
- What are the causes of hypoxemia? (5
- In which of these causes will the A-a gradient appear normal or decreased? In which will it appear elevated?
- Low inspired oxygen normal or decreased
- Hypoventiliation normal or decreased
- Diffusion impairment elevated
- V/Q mismatch elevated
- Shunt elevated
How do you calculate the A - a gradient?
PAO2 - PaO2
The tidal volume of an average adult during normal quiet breathing is […] mL. Of this volume, […] mL actually reaches the alveoli, while […] mL is dead space that remains in the conduction zone of the respiratory tract.
500
350
150
What is the equation to calculate the normal expected A-a gradient?
(Patient’s age / 4) + 4
This expected normal gradient would be compared to the measured A-a gradient.
What is V/Q matching vs. V/Q mismatching?
Matching: process by which areas of the lung BEST ventilated receive HIGHEST blood flow, ventilation and perfusion equality with V/Q of 1
Mismatch: V/Q for various parts of the lung that average out to something other than 0.8
In Zone 1 of the lung, ventilation is more efficient than perfusion. As such there is a high V/Q ratio. If an extreme condition develops in this part of the lung, it is therefore most likely to result in development of a […].
All ventilation, no perfusion –> physiological dead space
In Zone 3 of the lung, perfusion is more efficient than ventilation. As such, the V/Q ratio is low. If an extreme condition develops in this part of the lung, the result is likely a […].
All perfusion no ventilation –> Shunt
Describe the effect of chronic bronchitis on the V/Q ratio and the A-a gradient.
Chronic bronchitis narrows the patient’s airways and thus reduces ventilation, decreasing V/Q ratio. This leads to a high A-a gradient because there is insufficient gas exchange occuring so the PAO2 will be higher than in a normal patient and this will drive the A-a gradient up.
Describe the effect of pneumonia on the V/Q ratio and the A-a gradient.
Pneumonia is an accumulation of fluid in the lungs and this reduces ventilation by impairing gas exchange at alveolar membranes, decreasing V/Q ratio. This leads to a high A-a gradient because there is insufficient gas exchange occuring so the PAO2 will be higher than in a normal patient and this will drive the A-a gradient up.
Describe the effect of pulmonary edema on the V/Q ratio and the A-a gradient.
Pulmonay edema leads to entire filling of alveoli with fluid thus totally preventing ventilation. This makes V = 0 and thus V/Q = 0 and this represents the situation of a shunt (perfusion is OK but ventilation is 0). This increases the A-a gradient b/c no gas is being exchanged with the blood so there is a high PAO2 relative to the PaO2.
Describe the effect of pulmonary embolism on the V/Q ratio and the A-a gradient.
Pulmonary embolism blocks the flow of blood in the pulmonary capillaries thus preventing gas exchange. This will lead to Q being 0 and V/Q being infinite. The PAO2 will be >> PaO2 so the A-a gradient will be elevated.
What is atelectasis?
The collapse or closure of a lung or lung segment resulting in reduced or absent gas exchange
