4) anaphylactic drug reactions

[Editor]

Anaphylactic drug reactions

a) The severity of cutaneous manifestations correlates well with cardiovascular changes.

b) When a reaction occurs after a barbiturate and a muscle relaxant have been given intravenously in rapid sequence, it is more likely that the barbiturate is at fault.

c) Tachycardia after a suspected drug allergic reaction should be treated with a beta-blocker.

d) The immediate treatment of choice is intravenous hydrocortisone.

e) Reducing the speed of administration of a drug attenuates the effects
of a possible drug reaction.

[Editor]

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Correct answers: F F F F T

Explanation

Cutaneous manifestations frequently occur due to histamine release, or due
to other causes without causing cardiovascular changes.

When a reaction occurs after rapid administration of a barbiturate and a relaxant, it is seven times as likely to be caused by the relaxant. Allergic reactions to barbiturates occur but are very rare. When they occur they are usually very severe possible because the compensatory mechanisms of increased cardiac output are hampered by the depressant effects of barbiturates.

Histamine has a short half life and its effects last up to 5-10 minutes. Giving a
drug over several half lives of histamine in incremental doses rather than by an immediate bolus can significantly attenuate the effects.

The immediate treatment of choice in a severe reaction is adrenaline (0.2, 0.4
mg in an adult). Hydrocortisone takes about 6-8 hours to exert its maximum effect.

Histamine causes an inotropic and chronotopic effect on the heart and a tachycardia may occur. The fall in BP after an allergic reaction is due to massive vasodilation. Histamine is also a potent releaser of adrenaline and noradrenaline and if B-blockers are given to reduce the tachycardia. beneficial effects of adrenaline are counteracted. This is one of the few cases where a severe tachvcardia mandates the use of adrenaline.

[benagoodman]

FFFFF

It is true that

"Histamine has a short half life and its effects last up to 5-10 minutes. Giving a drug over several half lives of histamine in incremental doses rather than by an immediate bolus can significantly attenuate the effects."

Giving a drug slowly which has a side effect of histamine release (e.g. morphine) will therefore lessen its effects.

However, the stem refers to anaphylaxis, which is an on/off state. It can be triggered by minute doses of the causative agent, and the rate of administration should not affect this.