Sun21November0437PM 5

Smokers have reduced oxygen delivery to tissues.

What is the single best explanation for this?

(Please select 1 option)

Reduced forced vital capacity

Narrowing of small airways

Reduced forced expiratory volume Incorrect answer selected

Increased closing volume

Left shift of the oxygen dissociation curve This is the correct answer

Explanation

Smoking is a risk factor for significant intra-operative respiratory complications and post-operative respiratory, cardiovascular complications and wound infections. Nicotine and carbon monoxide are the main harmful substances in cigarette smoke.

Smoking affects oxygen transport and delivery. It leads to narrowing of small airways, causing an increase in closing volume.

The incidence of chronic obstructive airway disease is higher in smokers. When pulmonary function tests are done, chronic smokers show an obstructive pattern (with reduced forced expiratory volume and vital capacity) and the passive smokers show evidence of small airways disease as their closing volumes are significantly increased.

The changes in closing volumes, forced expiratory volume, vital capacity and airway diameter are not the main cause of reduced oxygen delivery to the tissues.

Reduced oxygen delivery to the tissues is primarily due to the carboxyhaemoglobin levels present in smokers, with levels of up to 15%.

Carbon monoxide and oxygen both bind to the alpha chain of haemoglobin, but the affinity of carbon monoxide is 250 times greater than oxygen. The result of this is a reduction in the availability of oxygen binding sites and a reduction in oxygen carrying capacity.

Left shift of the oxygen haemoglobin dissociation curve results in reduced oxygen delivery to the tissues.

Carbon monoxide binds with cytochrome oxidase and myoglobin and inactivates mitochondrial enzymes in the cardiac muscle. There is a decrease in the intracellular oxygen transport and utilisation, and a negative inotropic effect leading to chronic tissue hypoxia.

The half life of carboxy haemoglobin depends chiefly on pulmonary ventilation. At rest, the half life is about four to six hours. It has been found that the half life is longer in males than females. Administering 100% oxygen will significantly expedite the removal of carbon monoxide. With 100% oxygen, the half life of carbon monoxide is 40-80 minutes.