Ventilation/Perfusion relationships Flashcards
What is the most important factor in regulating pulmonary blood flow?
partial pressure of O2 in alveolar gas - PAO2
If the partial pressure of O2 in alveolar gas drops, what impact is there on the pulmonary vasculature?
leads to vasoconstriction (hypoxic vasoconstriction)
Hypoxic vasoconstriction can seem counterintuitive, since in most vascular beds a drop in PO2 leads to vasodilation. Why is this actually a positive compensatory factor in pulmonary circulation?
In the lungs, hypoxic vasoconstriction reduces pulmonary blood flow to poorly ventilated areas where the blood flow would be “wasted”.
Thus, pulmonary blood flow is directed away from poorly ventilated areas and toward well ventilated regions where gas exchange will be better.
Hypoxic vasoconstriction serves as a compensatory mechanism in a number of lung diseases. In what cases does it fail?
Widespread lung disease - such as multilobar pneumonia
insufficient areas of well ventilated alveoli
Hypoxic vasoconstriction can operate locally by directing blood flow to better ventilated portions of the lung. It can also act globally, in what way does this happen?
Producing an increases in overall vascular resistance. For example, a person at high altitude or persons breathing low O2 mixtures the PAO2 is reduced throughout the lungs, thus an increase in pulmonary resistance occurs globally and in response pulmonary arterial pressure increases.
In chronic hypoxia the heart ends up working harder. What causes this, and what is the long term outcome?
The increased pulmonary resitance due to hypoxic vasoconstriction leads to increased afterload for the right ventricle to overcome. Thus it hypertrophys.
What impact does thromboxane A2 have on arterioles and veins?
Powerful vasoconstrictor
What impact do prostacylins (prostaglandin I2) have on vasculature?
Potent local vasodilator
What effect do leukotrienes have on the lungs?
Airway constriction
A shunt refers to a portion of the cardiac output or blood flow that is diverted or rerouted. Forexample normally a small fraction of pulmonary blood flow bypasses the alveoli which is called a physiological shunt.
Several abnormal siturations may also occur where there is a shunting of blood through septal defects in the left and right hearts. What type of cardiac shunt is most common?
Left to right
Right to left shunts can occur in the heart. What is the consequence of this?
Up to 50% of cardiac output can be routed from the right ventricle directly to the left ventricle and never be pumped to the lungs for arterialization.
Consequently hypoxemia always occurs because a significant fraction of CO is not oxygenated, and the portion that is delivered to the lungs for oxygenation is diluted by the low O2 shunted blood.
What is the defining characteristic of hypoxemia caused by a right to left shunt?
In what way can we exploit this characteristic?
It cannot be corrected by administering high O2 gas, because the shunted blood never goes to the lungs to be oxygenated.
This characteristic can be used to identify the magnitude of shunting from the extent of dilution of the oxygenated blood.
Right to left shunt usually does not cause an increase in PaCO2, why not?
Central chemoreceptors are sensitive to changes in PacO2 and a small increase will cause an increase in ventilation rate, expiring the extra CO2.
At what point do chemoreceptors activate in response to decreases in PaO2?
When it decreases to less than 60 mmHg
Do left to right shunts cause hypoxemia?
No
