Calculation of physiologic dead space
Its a functional measurement .
Defined as the volume of the lung that does not participate in gas exchange .
Is approximately equal to the anatomic dead space in NORMAL lung ( 150ml).
May be greater than the anatomic dead space in LUNG DISEASE in which there are V/Q defect .
VD ----- physiologic dead space
VD= Anatomic vd + Alveolar Vd
Anatomic vd---------------------conducting airways (no gas exchange)
Alveolar Vd----------------------Alveoli with poor blood flow (ventilated but not perfused)
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Now lets calculate physiologic Dead space in normal lung
Physiologic Vd --------------------Anatomic vd + Alveolar Vd
Physiologic vd = 150 +0
Physiologic dead space in normal lung = anatomic dead space = 150 ml
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lets calculate the physiologic dead space in lung with V/Q mismatch
Here we need to calculate the alveolar ventilation and alveolar dead space
The adequacy of alveolar ventilation can be determined from the concentration of expired carbon dioxide
All expired CO2 come from alveolar gas NOT from dead space gas, there for the fraction shows the dilution of CO2 by the dead space
Total ventilation (VE) = Tidal volume(VT)❌ Respiratory rate (F)
Alveolar ventilation (VA) = ( Tidal volume (VT) - Dead space (VD) )❌ F
TO calculate the physiologic dead space
VD = VT (PACO2-PECO2/PACO2 )
VD /VT = PACO2-PECO2/PACO2
Where
VT ------- tidal volume
VD --------- ventilation dead space ( physiologic dead space)
PA CO2 ------PCO2 OF Alveolar gas which = PCO2 of arterial blood
PE CO2 -------PCO2 of Expired air
I hope this is helpful
. I thought the deadline was this evening while I just found out it already expired this morning. Have about 200+Q to revise. Have to resubscribe.