Ppt 13 Flashcards
Normal alveolar ventilation is
4L/min (ventilation)
Normal pulmonary capillary blood flow is
5 L/min (perfusion)
Average ratio of ventilation to perfusion or V/Q ratio
4:5 or .8
The alveoli in the upper apices of lungs receive more ventilation and less blood flow so the v/q is
Higher than .8 ex.( V6/Q4= 1.5)
Lower regions of the lung have lower ventilation and increased blood flow so v/q ratio is
Lower than .8 (example V4/Q6=0.67
How does the ratio(V/Q ) change from top of lung to bottom?
Decreases from top to bottom
PAO2 determined by
Amount of oxygen entering the alveoli(ventilation)
It’s removal by capillary blood flow(perfusion)
PACO2 determines by
Amount of carbon dioxide that diffuses into the alveoli from capillary blood(perfusion)
It’s removal from alveoli by ventilation(ventilation)
High V/Q >.8 is
Increase in ventilation or decrease in perfusion.
PAO2 increases
PACO2 decreases
When the V/Q is high the PAO2 rises because
It doesn’t diffuse quickly as it’s being delivered via ventilation
PACO2 decreases because ventilation is increased blowing off the CO2
Low V/Q < 0.8
Decrease in ventilation or increase in perfusion
PAO2 decreases
PACO2 increases
With low V/Q the PAO2 drops because
It’s diffusion into the capillaries faster than being ventilated
The PACO2 rises because it’s diffusion go to the alveoli faster than it’s washed out by ventilation
Mixing of PcO2 and PcCO2 occurs
In the pulmonary veins
Higher the v/q in upper lungs has what ph
Higher more alkalotic
Lower v/q in lower lungs has what kind of ph
Lower or more acidotic
Increase in v/q leads to
Dead space ventilation
Dead space= ventilated but not perfumed
Factors that cause low perfusion but normal ventilation or high v/q
Pulmonary embolism
Low cardiac output
Blockage in pulmonary artery
Pressure on pulmonary vessel
Emphysema
Cardiac arrest
Pulmonary htn
Factors that decrease v/q very little ventilation but normal blood flow
Shunt-perfused but not ventilated
Obstructive disorders- bronchitis, asthma
Restrictive- pneumonia, silicosis, pulmonary fibrosis
Hypoventilation
VA/QC Ratio Hypothetical Extremes
- If blood flow to an alveolus ceases, hypothetically PAO2 until equals PIO2 (i.e., O2 cannot be taken up by alveolus and CO2 is not removed):
- If ventilation to alveolus is blocked, hypothetically PAO2 until equals PvO2
Absolute shunt (v/q= 0)
No ventilation but there is perfusion
Relative shunt
V/Q is greater than 0 but less than 1
Shunting diseases =
Decrease in V/Q =P(A-a)O2
Absolute dead space
Normal ventilation but no perfusion
V\Q = Infinity
High V/Q
Blood flow is low ventilation is high
Low V/Q
Ventilation is low blood flow is high
Absolute shunt
Right to left shunt(venous admixture)
Anatomical
Atelectasis or pneumonia( Intrapulmonary)
Ventilation perfusion mismatch
Most common cause of hypoxemia
Responds to O2
Causes of dead space
High ventilation
Low blood flow ( shock, embolism, over-distended alveoli)
Shunt( low PAO2)
Pulmonary vasoconstriction
Dead space ( low PaCO2) upper lungs
Alveolar duct constriction
Increased airway resistance
P(A-a)O2. ( A-a gradient)
Most common index of oxygen transfer efficiency
Increased A-a gradient= increased physiological shunting
PAO2/PAO2( a-A ratio)
More stable than A-a when fio2 changes
Normal is 0.75 to 0.95
Used to predict fio2 to achieve desired PaO2
Capnogram
Waveform produced
Capnometry
Numerical display of ETCO2 value
Capnogram provides
Direct measurement of partial pressure of CO2( PcO2) eliminated by lungs
I direct measurement of mixed venous blood( pvco2) from tissue to lungs