Nitrates not used in (DNB Dec 2009)
a. Amylnitrite, and not nitrates are used in cyanide poisoning.
b. This is because it is nirities that cause methemoglobinemia rather than nitrates.
c. Nitrates have occasionally been used to treat ureteric colic but are also used routinely in esophageal spasm and biliary colic.
d. This is because, these drugs have excellent smooth muscle relaxing property.
e. The drug is used routinely in CHF as it is used to reduce preload and afterload. Nitrates to donate their nitro group but this property is much more pronounced in case of nitrites (e.g. amyl nitrite).
f. Hence, amyl nitrite is the drug of choice for cyanide poisoning.
g. Pharmacologically, nitrites donate their nitro group and this combines with hemoglobin to form nitrohemoglobin (that is known as methemoglobin).
h. Normally, insignificant amount of methemoglobin is formed and is of no clinical consequence. However, in large doses (such as those used to treat cyanide poisoning), large amount of met-hemoglobin is formed.
i. This then displaces the cyanide group and forms met-hemoglobin rather than cyanhemoglobin.
j. But too much of met-hemoglobin can also be toxic and that can be reduced to normal hemoglobin by giving sodium thiosulphate.
Figure: Nitrates and their mechanism of action.