Pharmacological properties : תכונות פרמקולוגיות

Pharmacodynamic Properties

5.1 Pharmacodynamic properties
Pharmacotherapeutic Group: Medical Gases
The characteristics of Carbon Dioxide are:
• odourless, colourless gas
• molecular weight 44.00
• sublimation point -78.5oC(at 1 bar) .
• density 1.872 kg/m3 (at 15°C) Carbon Dioxide occurs at approximately 350 vpm in the atmosphere.
The effect of inhaling Carbon Dioxide, or of its accumulation in the body through breathing defects, varies with the tension achieved in the blood, the duration and condition of the exposure and the susceptibility of the individual concerned.
If a normal, conscious individual inhales 5% Carbon Dioxide, the rate and depth of breathing rise and the minute volume increases 2 to 5 fold. The skin becomes pink and warm and there may be sweating and a sense of discomfort. There is no effect on consciousness or mental function, even with long exposures. After a prolonged exposure when the return to breathing air takes place, an "off effect" may develop with malaise, pallor, headache and occasional nausea and vomiting, probably due to the metabolic disturbances as a result of breathing a volatile acid.
As the inspired concentration rises, these effects become exaggerated in proportion to the concentration. At around 8-9% dizziness may develop, and at 10% some subjects become unconscious. Most people will become unconscious at 12.5% and all subjects lose consciousness within 1-2 minutes at 20%. When the concentration is raised to 30% consciousness is lost rapidly, the blood pressure may rise to 27 kpa (200 mm Hg) or higher and there is intense vasoconstriction, a reduction in heart rate to 40-50 heats per minute and ECG changes. All anaesthetic agents reduce these responses to Carbon Dioxide.

Pharmacokinetic Properties

5.2. Pharmacokinetic properties
When inhaled, Carbon Dioxide is rapidly distributed throughout the body.
Physiologically, it regulates the rate and depth of breathing and normally there is a constant tension of 5 kpa (40 mm Hg) in arterial blood. The concentration of Carbon Dioxide in the plasma is three times greater than that in red blood cells. The gas is carried partly in solution (2.4 - 2.7 vol %), but mostly either as bicarbonate (42.9 - 46.7 vol %), or as carbamino compound (3.0 - 3.7 vol %). The relative quantities in solution and as bicarbonate regulate the reaction of the blood and buffer changes in pH produced by stronger organic acids.
Carbon Dioxide produced by metabolism plays an integral part in the supply of oxygen to the tissues, since the amount releases by haemoglobin at any given oxygen tension is directly related to the Carbon Dioxide tension in the blood. This in turn is governed by tissue activity in the concentration inhaled. Thus the rate at which oxygen is given up to the tissues is increased when the Carbon Dioxide tension is raised. When a patient becomes apnoeic, Carbon Dioxide produced in the tissues, accumulates in blood at a rate of about 0.7 kpa (5 mm Hg) per minute
.