Question

Which of the following gas is used to decrease pulmonary artery pressure in adults?

• A. Nitrous Oxide
• B. Nitrogen Dioxide
• C. Nitric Oxide
• D. Nitrogen

Hint:

Inhaled nitric oxide relaxes pulmonary artery smooth muscle through the cyclic GMP pathway and is inactivated by haemoglobin the moment it enters blood, so its effect stays local to the lung.

Answer 1

The Correct Option is C

Step 1: Rule out the gases with no vascular role.
Nitrous oxide is an anaesthetic and analgesic gas, not a pulmonary vasodilator. Nitrogen is an inert gas with no signalling action in the body. Nitrogen dioxide is a toxic pollutant gas that damages lung tissue rather than treating it. None of these three lower pulmonary artery pressure.

Step 2: Recall the biology of nitric oxide.
Nitric oxide is a naturally made signalling molecule that activates guanylate cyclase in vascular smooth muscle, raising cyclic GMP and causing relaxation of that muscle. In the lung, this relaxes the pulmonary arterioles.

Step 3: See why it works selectively on the lung.
When nitric oxide is given by inhalation, it reaches only the ventilated alveoli and the vessels right next to them. As soon as it enters the blood stream it is bound tightly by haemoglobin and inactivated, so it does not reach the systemic circulation. This gives a vasodilator effect that is local to the lung, without dropping the systemic blood pressure.

Step 4: Connect this to clinical use.
Because of this selective action, inhaled nitric oxide is used in adults and in newborns to lower a raised pulmonary artery pressure, for example in acute respiratory distress syndrome or pulmonary hypertension, while sparing the rest of the circulation.

Final Answer:
\[ \boxed{\text{Nitric Oxide}} \]