Lecture 10 Flashcards
What is the actual value of hypoxemia?
PaO2
What is V/Q?
ventilation/perfusion ratio
What are the 5 five major causes of hypoxemia
1 Hypoventilation
2 Reduced PIO2 - altitude, anesthetic machine malfunction
3 Diffusion impairment – exercising race horse
4 V/Q mismatch
5 Shunt
- What is the normal V/Q?
- How is that number arrived at?
- Normal V/Q = 0.8
- 4 Liters Ventilation (Air) / 5 Liters Perfusion (Blood) 4/5 = 0.8
What does V/Q ratio determine?
Determines the adequacy of gas exchange
What can cause an ventilation-perfusion mismatch
- Differences in airway geometry and lung expansion produce uneven regional ventilation.
- Differences in vascular geometry and hydrostatic pressures produce uneven regional blood flow.
- Pathologies that lead to regional changes in ventilation (e.g. bronchoconstriction) or changes in perfusion (e.g. pulmonary embolism) will cause a V/Q mismatch.
Are V/Q ratios the same across normal healthy lung
vary across regions within normal healthy lung.
What is the scale range for V/Q ratio
0 to infinity
- What would the idea situation for V/Q be?
- Does this always happen in the normal body?
- Millions of alveoli would receive air and blood in amounts that are equal and optimal for gas exchange
- Never occurs – always some V/Q mismatch
- Branching patterns of bronchi and vessels
- Gravity – to some extent
What is the significance of a low V/Q ratio?
What are the causes of a low V/Q ratio?
- obstructed airway
- alveoli filled with exudate
- local blood flow obstructions
- What is the normal PO2?
- when there is a low V/Q ratio what will happen to PO2
- high V/Q ratio what will happen to PO2
- 100 mm Hg
- decreased
- increased
- What is diagram saying
- is it a high or low V/Q ratio?
- Both alveoli are being perfused equally but low V/Q (bronchiole is narrowed) will decrease amount of ventilation entering alveolus
- The resulting mixture will still be hypoxemic
- There is a low V/Q ratio
If one alveolus has a normal V/Q while another has a low V/Q will the mixture of blood be a 50:50 of oxygen?
no!
- What are the pink spots?
- red?
- what will happen to V/Q?
- Pink is normal lung parenchyma
- Red, abnormal, alveoli filled with exudate
- Will lead to a low V/Q ratio
Can a high V/Q compensate for a low V/Q where oxygen is concerned?
High V/Q does not compensate for low V/Q where oxygen is concerned
If you mix two equal quantities of blood how do you calculate PO2
- Take average of the 2
- 20+17 = 37/2 = 18.5
- Then draw line over and find the PAO2
- Generally what is happening when V/Q = 0,
- Is there perfusion?
- What will the body do to try and rectify?
- what is this called?
- No ventilation to alveoli (atelectasis)
- Perfusion still exists
- Hopefully, hypoxic pulmonary vasoconstriction has reduced the perfusion
- shunting
- When V/Q is high what has happened tp perfusion?
- does ventilation still exist?
- does it cause hypoxemia?
- No perfusion, maybe pulmonary embolism
- Ventilation still exists,
- Does not cause hypoxemia
- what is a right to left shunt
- Right Ventricle outflow bypasses ventilated lung and returns to the left atrium
- Give examples of anatomic shunts
- physiologic shunts
- Anatomic- Right to left shunt =Foramen ovale, between atria,
- left to right shunt= PDA (patent ductus arteriosus) between pulmonary artery and aorta
- Physiologic shunts – complete airway obstruction,
- alveoli filled with exudate
- atelectasis (collapsed alveoli)
- What is happening at 1
- ?
- ?
- atelectasis
- Thicker septum of alveoli
- exudate
What will you see if you place an animal with a shunt on oxygen (increase FIO2)
- Shunts are unresponsive to increasing FIO2
Explain why an animal with hypoxemia due to shunts will not improve with the administration of oxygen
- in a shunt V=0
- so even if you increase Q, the result will still be 0
- Cannot get oxygen into a collapsed or diseased alveolus, so will not improve
Describe the significance of a V/Q ratio = ∞
- Inspired air that does not participate in gas exchange
- Anatomic dead space
- Alveolar (physiologic) dead space
- high V/Q
- Regions with V/Q > 1.0 do not contribute to hypoxemia.
regarding the 5 causes of hypoxemia, hypoventilation
- What will you see with PACO2 (increase, normal, decrease)
- (A-a) O2 difference (increase, normal)
- Will the patient be responsive to increased FIO2 (giving oxygen)
regarding the 5 causes of hypoxemia, Low PIO2
- What will you see with PACO2 (dincrease, normal, decrease)
- (A-a) O2 difference (increase, normal)
- Will the patient be responsive to increase FIO2 (giving oxygen)
regarding the 5 causes of hypoxemia, shunt
- What will you see with PACO2 (increase, normal, decrease)
- (A-a) O2 difference (increase, normal)
- Will the patient be responsive to increased FIO2 (giving oxygen)
regarding the 5 causes of hypoxemia, V/Q mismatch
- What will you see with PACO2 (increase, normal, decrease)
- (A-a) O2 difference (increase, normal)
- Will the patient be responsive to increased FIO2 (giving oxygen)
regarding the 5 causes of hypoxemia, Diffusion limitation
- What will you see with PACO2 (increase, normal, decrease)
- (A-a) O2 difference (increase, normal)
- Will the patient be responsive to increased FIO2 (giving oxygen)
Describe the Diagnostic Hypoxemia Diagnostic Flow Chart
- If there is no A-a O2 difference
- Want to see if they are not breathing (hypoventilation)
- Increase in altitude or not enough oxygen in anesthesia machine (low PIO2
- If there is an increased A-a O2 difference
- want to put on oxygen
- If responsive there is a diffusion impairment or a low ventilation perfusion ration
- If they still are unresponsive there is a shunt going on
- want to put on oxygen
