V/Q mismatch

emphysema will result in

lost V and Q

distribution of ventilation (V)

- distribution is lower at apex of lung - distribution is greater at base of lung *due to pleural pressure and compliance *change in pressure upon inspiration causes greater volume change in basilar alveoli than apical alveoli

low V/Q ratio: what happens to partial pressures in arterial blood and saturation?

*pressure gradient of oxygen is not so big because bad ventilation: PaO2 will be smaller than 100 *pressure gradient of CO2 will not be as big so PaCO2 will be greater than 40

how do we image shunt?

- inject dye into vein and watch for appearance of bubbles in left atrium in case of atrial septal defect or left ventricular septal defect - intrapulmonary shunt use CT with contrast

when do you measure shunt clinically

- pt with hypoxemia out of proportion to lung disease or without lung disease

V

alveolar minute ventilation

shunt

*wasted perfusion Shunt: blood flows which do not pass ventilated alveoli and therefore cannot participate in gas exchange

resting normal V/Q

0.8

how to determine cause of hypoxemia

if low PaO2, could be: - low alveolar oxygen - V/Q mismatch or shunt So: calculate A-a gradient if PAO2 is low and approximates PaO2 then low alveolar oxygen is cause of hypoxemia if A-a gradient is high, then V/Q mismatch or shunt is likely

does PCO2 increase or decrease in individual with high V/Q?

in one alveolus decreased due to over ventilation, but in the lung there is now more dead space, and alveolar ventilation decreases making PCO2 rise elsewhere *basically blood rerouted and lung can't keep up with getting rid of the CO2

hypoventilation

inadequate ventilation for blood flow PCO2 > 40 or 45

individual alveoli vs. lung V/Q ratio

individual alveoli are far from normal 0.8 value based on flow at region and other issues *lung is more of an average

pathological causes of V/Q mismatch

- infection - bronchospasm - chronic obstructive lung disease - pulmonary embolism

which of the two molecules has the greater content change per partial pressure change?

CO2

high V/Q pressures in arteries

PaO2 approaches 150 or PO2 in air PaCO2 approaches zero *oxygen content, however, reaches the same value as normally ventilated alveoli due to the sigmoidal saturation curve topping out *PaCO2 approaches zero due to overventilation *dead space in this scenario due to ventilation without results

shunt equation

Qs/Qt = (Cc'O2 - CaO2) / (Cc'O2 - CvO2) Qs: shunted flow Qt: total flow CaO2: content of oxygen in arterial blood Cc'O2: content oxygen in alveolar capillary blood that has been ventilated CvO2: content of oxygen in mixed venous blood

venous admixture

addition of venous blood to arterial blood *happens when blood does not participate in gas exchange and continues on to left atrium

which areas of lung are more likely to be hyperventilated? Not perfused at all?

areas at apex of lung, can be areas of dead space

also, increasing PO2 dissolved does not increase oxygen content. Why?

because of multiplying it by 0.003 so this basically does not add a lot of content

what do we use shunt equation for?

calculate fraction of blood that resembles shunt (mixed venous blood entering arterial circulation without exposure to ventilated alveoli) *only when pt is on 100% oxygen, which eliminates hypoxemia due to V/Q mismatch

normal conditions causes of V/Q mismatch

changes of position or breathing patterns can affect ventilation and perfusion

true shunt

even increasing FiO2 will not saturate blood passing by alveoli acting as true shunt because no gas gets through

hyperventilation

excess ventilation for the blood flow PCO2 < 40 or 35

what is the shunt equation really measuring?

how much true shunt would account for drop in O2 sat, which is why 100% oxygen is so important

two types of V/Q mismatch

hypoventilation hyperventilation

low V/Q will result in

hypoxemia

saturation in regional lung disease

if hypoxic pulmonary vasoconstriction response fails, then blood flow continues toward alveoli with low PO2, shunting!

high V/Q

low perfusion relative to ventilation *alveolar gas tensions approach inspired air

what inhibits Hypoxic pulmonary vasoconstriction?

nitroglycerin, nitroprusside, and isoproterenol inhibit hypoxic pulmonary vasoconstriction *however, will not affect areas that are not ventilated if inhaled...

can we compensate for poor ventilation in low V/Q situation in terms of oxygen content?

nope, there is an upper limit to saturation of oxygen that is reached quickly, so there will not be extra oxygen to compensate for dead space *oxygen saturation curve tops out

under what circumstances would you have normal V/Q ratio?

when ventilation and perfusion are both standard, and normal gas tensions are sent into arterial blood

Q

pulmonary blood flow

what happens when you mix low V/Q unit with high V/Q unit?

*normal PaCO2 but low PaO2 (remember, can't compensate as oxyhemoglobin curve tops out)

V/Q

ratio between alveolar minute ventilation to pulmonary blood flow

Bronchitis and asthma have low or high V/Q ? What does this result in?

low, results in hypoxemia

measurement of regional V/Q

- radioactive xenon (not soluble in water or blood) - will travel to lung and diffuse into alveolar gas - measure by radiation counter

measurement of venous admixture

- use 100% inspired oxygen - alveoli will fill up the blood with O2 to high saturation - addition of blood with saturation in 70's will cause a large decline in PO2 due to flat slope of O2 dissociation curve above PO2 of 90 - we cannot know how much venous admixture is from anatomic shunt or alveolar hypoventilation (V/Q mismatch)

causes of right to left shunts

1. tetralogy of fallot: congenital heart defect 2. eisenmenger syndrome: large ventricular septal defect, bidirectional flow when pulmonary hypertension kicks in 3. valsalva maneuver 4. left and right heart pressures during diastole aren't diff so flow can be bidirectional 5. "open door" effect: if there is flow from left to right, there can be some flow right to left causing hypoxemia 6. large septal defects

normal values if V/Q ratio is normal

PaO2 = 95-100 PaCO2 = 40 SaO2 = 97%

how do we distinguish between V/Q mismatch and intrapulmonary shunt?

V/Q mismatch, if some ventilation occurs, will improve with supplemental oxygen *intrapulm shunt will not improve with supplemental oxygen because no ventilation occurs

low V/Q ratio

low ventilation but normal blood flow (perfusion) *alveolar gas tensions approach venous blood gas