Ventilation-perfusion relationships Flashcards
What should ventilation/perfusion be in a healthy person?
Approximately 1 would be ideal
In reality, it is around 0.8
Where must ventilation/perfusion be matched?
At the ALVEOLAR level
- that is where gas exchange occurs
Overall VA/Q may be misleading
What is the V/Q ratio of a patient with a shunt?
Tends towards 0
What is the V/Q ratio of a patient with lots of dead space? (e.g. emphysema)
Tends towards affinity
What are normal right to left shunts?
Venous blood from bronchial veins and a few small veins draining the wall of the LV (Thebesian veins)
- added to left-sided blood without going via lungs
When might you have an abnormal right to left shunt?
Lobar pneumonia
Collapsed lung
Some congenital heart diseases
What are the two types of heart defect that initially cause L to R shunts?
Atrial septal defect
Ventricular septal defect
What happens after time with a ventricular septal defect?
The pressure in the RV increases causing hypertrophy of the RV muscle - becomes a right to left shunt
e.g. Fallot’s tetralogy
What does a moderate fall in O2 content do to the PO2?
Large decrease
What does a moderate rise in CO2 content do to the PO2?
V small increase
What is the effect of increased ventilation in a patient with a right to left shunt?
PO2 will remain low
PCO2 will decrease or even become low due to increased ventilation
Is breathing 100% oxygen helpful for a right to left shunt patient?
Only has a mild effect
This is because it does not reach the shunted blood and the ventilated blood is already near saturation
High Co2 in an asthma patient indicates what?
Ominous finding
Acute respiratory acidosis
What does the kidney do in chronic hypoventilation?
Renal compensation normalises pH (increase HCO3-)
Can high V/Q areas balance out low V/Q areas?
NO
- more blood comes from the low V/Q areas
- high V/Q areas do not have high oxygen content
Where is ventilation greatest in the lung?
At the bottom
- these alveoli are smaller (because of gravity) and have a bigger capacity to expand
Where is V/Q highest in the lung?
Top
Effect on perfusion is larger than ventilation at the bottom of the lung
What accounts for the PO2 A-a difference? (PO2 slightly lower in arterial than alveolar)
- The fact that most of the lung has a low V/Q (effects of gravity on VA/Q
- Physiological shunts
Where in the lung is TB seen?
Top (apices) - highest V/Q ratios, most O2
Obligate aerobes
What is hypoxic pulmonary vasoconstriction?
BVs constrict regionally when exposed to hypoxic conditions
Moves blood to better ventilated parts of the lung
Therefore improves V/Q matching
When is Hypoxic pulmonary vasoconstriction not helpful?
Total hypoxia (hypobaric hypoxia) e.g. at altitude
Or respiratory failure
Can lead to right heart failure
How can you treat hypoxic pulmonary vasoconstriction?
Use a vasodilator (NO) - inhaled (gets to lungs specifically)
How do we assess ventilation-perfusion mismatching?
- Isotope ventilation
- Ventilation/ Perfusion scans
- Measure alveolar dead space/shunt effect
- Measure A-a PO2 gradient
What can pneumonia cause?
Right to left shunt
What are the 5 potential causes of arterial hypoxia? (Low PaO2)
- Low inspired oxygen (hypobaric hypoxia)
- hypoventilation
- Diffusion impairment (fibrosis)
- Right to left shunt
- Ventilation-perfusion mismatch (MOST COMMON)
Why does PO2 decrease when PCO2 increases?
Alveolar gas equation
Which is the only mechanism causing hypoxia that leads to an increased pCO2?
Hypoventilation
Which factors causing hypoxia increase the A-a PO2 gradient?
Diffusion impairment
Right to left shunt
Ventilation-perfusion mismatch
What is the normal A-a difference?
10-15 mmHg
