During a surgery, the patient had sudden hypotension, which of the following will promptly increase & hence be used intramuscularly to increase the blood pressure in this patient?

Ans. b (Mephenteramine). (Ref. Pharmacology by KD Tripathi 6th/pg.l27)MEPHENTERMINE# It produces both cardiac stimulation and vasoconstriction by directly activating a and (3 adrenergic receptors as wellas by releasing NA.# Cardiac output, systolic and diastolic BP are increased.# The direct positive chronotropic effect on heart is generally counter balanced by vagal stimulation due to rise in meanBP.# Mephentermine is not a substrate for either MAO or COMT: active orally with longer duration of action (2-6 hr).# It crosses blood-brain barrier to some extent-may produce excitatory effects at higher doses.# It is used -- To prevent and treat hypotension due to spinal anaesthesia and surgical procedures,- Shock in myocardial infarction and other hypotensive states.EPHEDRINE# Mainly acts indirectly but has some direct action on a and b receptors also.# Repeated injections produce tachyphylaxis, primarily because the neuronal pool of NA available for displacement is small.# It is resistant to MAO, therefore, effective orally.# It is about 100 times less potent than Adr but longer acting (4-6 hours).# It crosses to brain and causes stimulation, but central: peripheral activity ratio is lower than that of amphetamine.# Ephedrine can be used for a variety of purposes, but it lacks selectivity and efficacy in general is low.# Use is now restricted to that in mild chronic bronchial asthma and for hypotension during spinal anaesthesia; occasionally for postural hypotension; 15-60 mg TDS.PHENYLEPHRINE# It is a selective a1agonist, has negligible b action.# It raises BP by causing vasoconstriction.# Because it has little cardiac action, reflex bradycardia is prominent.# It is used as a mydriatic when cycloplegia is not required: tends to reduce intraocular tension by constricting ciliary body blood vessels.# It is a frequent constituent of orally administered nasal decongestant preparations, as well as used topically for this purpose.# Central effects are not seen with usual clinical doses.METHOXAMINE# Another selective a1 stimulant with no b actions (has weak b blocking action).# Resembles phenylephrine very closely.# Occasionally used as a pressor agent.