Right-to-Left Shunt

• In a right-to-left shunt, hypoxemia always occurs because a significant fraction of the cardiac output is not delivered to the lungs for oxygenation.
• The low 02 shunted blood dilutes the portion of cardiac output that is delivered to the lungs.
• Hypoxemia caused by a right-to-left shunt cannot be corrected by having the person breathe a high-O2 gas because the shunted blood never goes to the lungs to be oxygenated, diluting the oxygenated blood.
• Breathing 100% O2 is a useful diagnostic tool to estimate the magnitude of the shunt based on the extent of dilution of the oxygenated blood.
• Right-to-left shunts usually do not cause an appreciable increase in Pa CO2 because central chemoreceptors are sensitive to changes in Pa CO2, causing a small increase to produce an increase in ventilation rate, and the extra CO2 is expired.
• Chemoreceptors for O2 are not as sensitive and are not activated until the Pa O2 decreases to less than 60 mm Hg.
• Blood flow through a right-to-left shunt is calculated with the shunt fraction equation, where flow through the shunt is expressed as a fraction of pulmonary blood flow, or cardiac output.

Shunt Fraction Equation

• Qs = Blood flow through right-to-left shunt (L/min).
• QT = Cardiac output (L/min).
• CNormal = O2 content of nonshunted blood.
• CA = O2 content of systemic arterial blood.
• CV = O2 content of mixed venous blood.
• Left-to-right shunts are more common and do not cause hypoxemia.
• Causes of left-to-right shunts include patent ductus arteriosus and traumatic injury.
• Blood shunted from the left side of the heart to the right side results in higher pulmonary blood flow (right-heart cardiac output) than systemic blood flow (left-heart cardiac output).
• Deoxygenated blood that has just returned from the lungs is added directly to the right heart without being delivered to the systemic tissues.
• Because the right side of the heart normally receives mixed venous blood, the P_a in blood on the right side of the heart will be elevated.

Ventilation/Perfusion Ratios (V/Q)

• The V/Q ratio is the ratio of alveolar ventilation (V) to pulmonary blood flow (Q).
• Matching ventilation to perfusion is critically important for ideal gas exchange.
• It is useless for alveoli to be ventilated but not perfused or for alveoli to be perfused but not ventilated.
• The ideal arrangement for gas exchange occurs when ventilated alveoli are nearly perfused pulmonary capillaries.
• Compensatory bronchoconstriction and compensatory hypoxic vasoconstriction maintain V/Q matching.
• Small regions of ventilated alveoli that are not perfused cause local decrease in V/Q, the local decrease in CO2 causes compensatory bronchoconstriction of nearby airways, such that ventilation is re-routed to regions with well-perfused capillaries, preserving V/Q matching.
• When small regions of alveoli are not ventilated they become right-to-left shunt, with a local decrease in V/Q and compensatory hypoxic vasoconstriction of nearby arterioles causes blood flow is re-routed to regions with well-ventilated alveoli, preserving V/Q matching
• The normal value for V/Q is 0.8: alveolar ventilation (V) is 80% of the value for pulmonary blood flow (Q).
• If breathing frequency, tidal volume, and cardiac output all are normal, V will be 0.8.
• If V is normal, then Q will be its normal value of 100 ml/min kg or 20 ml with a tidal volume of 400 ml.
• If V/Q changes due to an alteration of alveoli and the P_a CO2 are all normal P_a CO2 changes, then exchange will be less than ideal requiring alteration to P_a CO2.

Distribution of V/Q in the Lung

• The value of V/Q can change for the entire lung based on the three zones.
• Zone 1 has the lowest perfusion, highest ventilation, and highest value.
• Zone 3 has the smallest deviation among the zones of the lung.
• The bottom zone is a region of perfusion, an alveolar hypoxia due to anemia, suggesting V/Q is lowest, caused by a serious anatomic gradient in the lung.
• V/Q variations cause significant differences in the regional ventilation, including the lung as an accordion that moves vertically.
• Regional differences in V/Q produce regional differences in P_aO_2 and P_CO_2.

Ventilation/Perfusion Defects

• Ventilated alveoli are close to perfused capillaries, and this arrangement provides for ideal gas exchange.
• The average value for the V/Q ratio is about 0.8.
• A mismatch of ventilation and perfusion, called V/Q mismatch, or V/Q defect, results in abnormal gas exchange.
• A V/Q defect can be caused by ventilation of lung regions that are not perfused, perfusion of lung regions that are not ventilated, and every possibility in between.

Dead Space (V/Q = ∞)

• Ventilation of lung regions that are not perfused is wasted
• Because there is no blood flow to receive O2 from alveolar gas or add CO2 to alveolar gas, no gas exchange is possible
  Alveolar gas has the same composition as humidified inspired air: PaO2 is 150 mm Hg and PaCO2 is 0.
• In regions of dead space.

High V/Q

• Regions of high V/Q have high ventilation relative to perfusion, usually because blood flow is decreased.
• Regions have some blood flow; unlike dead space, which has no perfusion.
• Pulmonary capillary blood from these regions has a high PaO2 and a low PaCO2 because ventilation is high relative to perfusion

Low V/Q

• Regions of low V/Q have low ventilation relative to perfusion, usually because ventilation is decreased.
• Low V/Q regions have some ventilation, unlike shunt, which has no ventilation
• Pulmonary capillary blood from these regions has a low PaO2 and high PaCO2 because ventilation is low relative to perfusion.

Right-to-Left Shunt (V/Q = zero)

• Perfusion of lung regions that are not ventilated
• There is no ventilation to deliver O2 to the blood or carry away CO2 from the blood because no gas exchange is possible
• Illustrated by airway obstruction and right-to-left cardiac shunts.
• Pulmonary capillary blood from these regions has the same composition as mixed venous blood: PaO2 is 40 mm Hg, and PaCO2 is 46 mm Hg because no gas exchange can occur

Mixed V/Q Defects (V/Q ranges from 0 to zero)

• Pulmonary diseases with V/Q defects always have a mixture of defects
• For example, if one region becomes dead space (due to blockage of blood flow), then other regions become low V/Q and/or right-to-left shunt because blood flow diverted from dead space has to go somewhere.
• The entire range of V/Q abnormalities can be exhibited at the same time
• Simultaneously, there can be regions of dead space, low V/Q, high V/Q, and right-to-left shunt.
• In a mixed V/Q defect picture there may be no lung regions with normal V/Q matching
• V/Q defects always cause hypoxemia and hypercapnia
• V/Q defects are always a mixed picture in which most of the blood flow goes to regions of low V/Q and right-to-left shunt.
• The blood coming from those regions has low PaO2 and high PaCO2.