R.S. has smoked for many years and has developed chronic bronchitis, a chronic o

Question

R.S. has smoked for many years and has developed chronic bronchitis, a chronic obstructive pulmonary disease (COPD). He also has a history of coronary artery disease and peripheral arterial vascular disease. His arterial blood gas (ABG) values are pH = 7.32, PaCO2 = 60 mm Hg, PaO2 = 50 mm Hg, HCO3- = 30 mEq/L. His hematocrit is 52% with normal red cell indices. He is using an inhaled ß2 agonist and theophylline to manage his respiratory disease. At this clinic visit, it is noted on a chest x-ray that R.S. has an area of consolidation in his right lower lobe that is thought to be consistent with pneumonia.

Please answer in depth

1.Considering both his COPD and pneumonia, in what position would RS have the worst ventilation-perfusion matching?

Explanation / Answer

Ventilation signifies the amount of air that moves in to lung during respiration, this air is the source of new O2 to blood.

Perfusion signifies blood flow, as lung receives blood from heart via pulmonary artery .

The ratio of ventilation and perfusion(V/Q) is important to understand pulmonary pathophysiology of many diseases.

Normally in the whole lung V/Q is NOT evenly matched and the physiological value of V/Q is 0.8

Explanation:

Perfusion or blood flow to various areas of lungs occur under a low pressure circuit/Right heart system, where the active systolic blood pressure is at the range of 20 mm Hg. Therefore gravity plays a strong role here as lower zones of lung achieve more blood(High Q) than the upper zones(Low Q)

air is also differentrially distributed through out lung zones, upper lobes getting more ventilation and lower lobes getting less(However variability of zone wise ventilation is of lesser degree than the variability of perfusion)

considering and stratifying all these factors the average V/Q ratio across whole lung(all zones stratified) appears to be 0.8.(Individual areas V/Q is different)

now in case there is widespread destruction of alveolar wall and thereby capillary membranes available blood flow becomes less leading to decreased Q, so increased V/Q as seen in emphysema.

whereas in areas of consolidation or local fluid collection,there is less ventilation or no ventilation, however perfusion continues so V is very low, Q value is significant leading to decreased V/Q. In such areas there is a formation of intrapulmonary shunt or physiological dead space.

Now coming to our candidate Mr R.S.

Mr R.S is a chronic smoker, so he must be having emphysematous changes all over the lung fields.

he has developed chronic bronchitis, thereby there is increased mucus plug,broncho constriction,out flow obstruction leading to decreased Ventilation at various parts of lung.

Acid base values showing respiratory acidosis, however this PCO2 value is not alarming in COPD patients. He also has hypoxemia. HCO3 is increased due to renal compensatory effects.

Using B2 inhalers are causing brochodilation therby it will improve Ventilation in areas of lung.

Now as per recent CXR he has developed an area of consolidation at RLL of lung which is consistent with pneumonia.

In this situation at this area of consolidation, ventilation is very low , considering his COPD and Bronchtis whole of lung is having less V,

Perfusion is high in areas of consolidation but blood passing through this zone can't take new O2 thereby saturation drops.

Research has found that during acute phases(CONSIDERING ABOUT THIS PNEUMONIA) hypoxic vasoconstriction mechanisms in lung fail to some extent(Which is largely due to acute phase reactants, local metabolites and vasodilator)

so blood saturation decreases and when this blood passes through other areas of Ventilation compromised lungs, O2 saturation does NOT improve leading to Hypoxemia.

This V/Q mismatch is dependent upon body position too.

If he is nursed with diseased side dependent way(below), therefore Left side up. his overall ventilation perfusion matching will be better.

worst matching is at the consolidation zone which can even deteriorate if Mr RS is placed in a Right side UP decubitus.

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