VQ Mismatch

Question 1

Why does VQ scatter decrease arterial oxygenation? (2 reasons)

Answer 1

1. By definition, there is more blood coming from regions with low V/Q ratios
2. Areas of low VQ decrease oxygenation and areas of high VQ cannot adequately compensate because Hb is already near full saturation under conditions of normal ventilation

Question 2

Mechanisms of local regulation of VQ mismatch (2)

Answer 2

1. In lung areas with high V/Q ratios, decreased alveolar PCO2 leads to an increase in local airway resistance, which decreases ventilation; thus, V/Q decreases
2. In lung areas with low V/Q ratios, alveolar PO2 drops leading to hypoxic vasoconstriction and decreased lung perfusion; thus, V/Q goes up

Question 3

True shunt

Answer 3

A situation in which alveolar air spaces are filled with transudate (heart failure) or exudate (pneumonia) and thus do not ventilate but are still being perfused

V/Q = 0

Question 4

Dead space

Answer 4

Ventilation of un-perfused alveoli - represents work of ventilation without benefit

V/Q approaching infinity

Question 5

Anatomic Dead Space

Answer 5

The portion of the respiratory tree that does not directly participate in gas exchange

Trachea, bronchi, bronchioles - 1/3 of VT

Increased by rapid, shallow breathing - most of the tidal volume remains in the conducting airways, causing alveolar hypoventilation

Question 6

Calculation of total dead space

Answer 6

VE = VA + VD

Total minute ventilation = alveolar ventilation + dead space ventilation

Question 7

Ratio of dead space to tidal volume

Answer 7

VD / VT is normally < 30%

Question 8

A-a gradient

Answer 8

PAO2 = [(Pb - PH2O) x FiO2] - PaCO2/R

The difference between PAO2 and PaO2 is usually <10 due to normal regional V/Q mismatch

Question 9

Pulse oximetry

Answer 9

Measures the ratio of deoxy-Hb and oxy-Hb by diffrences in light absorption; reports out % oxy-Hb (SpO2)

*Remember that this is a measure of hemoglobin saturation only; if SpO2 is normal but SaO2 is low, Hb may be bound to something else (i.e. CO)

Question 10

5 causes of hypoxemia

Answer 10

Defined as PaO2 < 80 (sea level) or < 65 (Denver)

1. Low ambient O2 (altitude)
2. Hypoventilation
3. Shunt
4. V/Q mismatch
5. Diffusion limitations

Question 11

How does hypoventilation cause hypoxemia?

Answer 11

Hypoventilation leads to elevated PaCO2 and a concomitant decrease in PAO2; therefore, PaO2 also decreases

A-a gradient is normal because the problem is with ventilation, not gas exchange

Question 12

Causes of hypoventilation

Answer 12

Obesity hypoventilation
Central apnea (respiratory depression)
Neuromuscular disease
Drugs (ipiates, benzos)

Question 13

How does shunt cause hypoxemia?

Answer 13

Blood flowing past unventilated alveoli comes into complete equilibrium with those alveoli and never reaches normal arterial PaO2; de-oxygenated blood mixes with ventilated blood causing a widened A-a gradient

Question 14

Causes of shunt

Answer 14

Pulmonary edema 
ARDS 
Pneumonia
Congenital heart disease (ASD, VSD)
Pulmonary AVM