Palaniappan
Forum Senior
Topics: 30
Posts: 148
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Which of the following increase,the most,in vagal stimulation?
A.Expiratory flow rate
B.Pulmonary arterial resistance
C.Work of breathing
D.V/Q ratio.
THe answer given was C.
I thought of the following sequence.
Vagal stimulation-->bronchospasm-->decreases air flow-->hypoxia-->Pulmonary vasoconstriction-->Increases pulmonary arterial resitance..So anser B.
Can anyone explain it a little bit more?Please
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| babydoc4usmle
Forum Guru
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Posts: 634
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see, when you develop bronchospasm there is decrease in radius/diameter of bronchus --> increase in resistance (R=1/r4)-->increase in flow (Q=deltaP/R), AND increase in work that you need to expire air that is in alveoli (remember change in resistance that is in series, if R increase, upstream pressure increase). remember asthma? breathing during the attack require ADDITIONAL muscle work during expiration.
besides, vagus does not affect arteriolar tonus, SANS controls it.
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| Palaniappan
Forum Senior
Topics: 30
Posts: 148
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Thanks babydoc...However,I am still not clear about the hypoxia factor..When there is bronchospasm there will be hypoxia..right?-independent of sans or pans..Hypoxia induced vasoconstriction can cause vasoconstriction resulting in increased arterial resistance..?Or else due to increased work of breathing hypoxia is not prominent??
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| babydoc4usmle
Forum Guru
Topics: 18
Posts: 634
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initially Pt wth asthma atack experiencing respiratory ALKALOSIS, if nont treated may pregress to respiratory acidosis and is an indication for intubation in this condition
having just bronchoconstriction you have increase in bronchiolar resistance-->increase in P gases upstream-->there will be no hypoventilation in the beginning,
also, mechanism you are talking about is LOCAL and works REGIONALLY in unit of lung where decrease in PO2 (decrease in blood flow will be matched with alveolar ventilation and V/Q ratio will be closer to N). in the settings we have there is no decrease in PO2 and it's throughout whole lung so it will not work
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| Palaniappan
Forum Senior
Topics: 30
Posts: 148
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tHANKS A LOT BRO:-)tHAT WAS AWESOME
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