Ventilation-Perfusion Matching Flashcards
Normal V/Q
V/Q = (4 L/min)/(5 L/min) = 0.8
Pulmonary embolism/pulmonary dead space
Lung area is ventilated but not perfused –> V/Q = infinity
Shunt
Lung area is perfused but not ventilated (bronchial obstruction) –> V/Q = 0
Ex: pneumonia
Equation to calculate vascular resistance
R = P/Q (from V = IR)
SVR = (Paorta - Pra)/QdotC
PVR = (Ppa - Pla)/QdotC
Remember QdotC is CO
Fick Principle equation
VdotO2 = QdotC x (CaO2 - CvbarO2)
Main idea: difference between what is flowing in and what is flowing out is what is taken up by the lung
Mixed venous and arterial O2 content
CvbarO2 = 150 ml/L
CaO2 = 200 ml/L
Factors determining fluid movement across pulmonary capillaries
Hydrostatic pressure
Oncotic pressure
Alveolar pressure
How do we get pulmonary edema?
1) Increased mean pulmonary capillary pressure (left heart failure)
2) Reduced plasma oncotic pressure (hypoproteinemia)
3) Leaky alveolar-capillary membranes which allow plasma proteins out of capillaries
What are some things that can prevent edema?
Lymphatic drainage keeps lung dry
Positive end-expiratory pressure (PEEP) which generates positive alveolar pressure that turns into positive interstitial space pressure
Ventilation and perfusion in 3 zones of the lung
Zone 1: PA > Pa > PV; least overall ventilation and blood flow; highest Vdot/Qdot; capillaries will collapse
Zone 2: Pa > PA > PV; blood flow driven by difference between Pa and PA, not arterial and venous pressure like usual; some capillaries may collapse
Zone 3: Pa > PV > PA; most overall ventilation and blood flow; lowest Vdot/Qdot; blood flow driven by difference between arterial and venous pressure
Note: blood flow is more affected by gravity than ventilation
What happens to PO2 and CaO2 at low V/Q regions and high V/Q regions?
This is all because of sigmoid shape of Hb-O2 dissociation curve
Low V/Q regions: low PO2 and low O2 content
High V/Q regions: high PO2 but only slightly increased O2 content because dissociation curve is FLAT in this range
What happens to PCO2 in high and low V/Q regions?
CO2 dissociation curve is linear in the physiological range
Hyperventilatory effects of high V/Q regions counterbalance hypoventilatory effects of low V/Q regions, so PaCO2 and PACO2 are not affected (?)
What happens when you have low V/Q?
Hypoxic pulmonary vasoconstriction
Low PAO2 triggers reflex vasoconstriction to close blood vessels there
This is good because you’re not getting ventilation so want to shunt blood into regions that ARE getting ventilation
This increases pulmonary arterial pressures
(Note: NO is involved in this vasoconstriction)
What happens when you have high V/Q?
Reflex bronchoconstriction
Low PACO2 causes airways to restrict
This is good because want to get ventilation to areas that are more perfused
Shunt fraction
Physiological shunt calculation: some mixed venous blood goes through areas of lung that are perfused but not ventilated
QdotShunt/QdotTotal = (Cc’O2 - CaO2)/(Cc’O2 - CvbarO2)
Can calculate Cc’O2 from PAO2 and assuming a lung exchanges gases ideally
Cc’O2 is blood that goes through part of lung with full ventilation and CvbarO2 is before and CaO2 is after
How can you distinguish V/Q mismatch from shunt?
Give person 100% O2:
V/Q mismatch person has SOME ventilation, so their PO2 would increase
Shunt person would have NO ventilation, so PO2 would not increase
What is an extra-pulmonary shunt? (right to left shunt)
1) Ventricular atrial septal defect–goes right heart to left heart (skips lungs!), introducing deoxygenated blood into systemic circulation
2) Thebesian veins drain myocardium by draining venous blood into left heart
3) Pulmonary artery to vein fistula
Why is it easy to compensate for increased CO2 but hard to compensate for decreased O2 that results from physiological shunt?
With CO2, places in the lung with high V/Q (lower CO2 content) can make up for areas of low V/Q (higher CO2 content)
With O2, nonlinear curve. Areas of low V/Q (low O2 content) can’t be compensated by overventilating high V/Q areas because hemoglobin is already fully saturated, and blood only carries a TINY proportion of dissolved O2 (low solubility)
4 causes of hypoxemia (low PaO2)
Alveolar hypoventilation (normal PA-aO2 because low PA also!)
Diffusion impairment
Maldistribution of V/Q
Right to left shunt
