Pressor agents should be avoided, if possible, in low cardiac output states as they may further compromise the circulation.

Methoxamine and phenylephrine are the 'purest' pressor agents; other a-adrenergic agents have inotropic properties to greater or lesser degrees. Ephedrine is similar to epinephrine but its effects are more prolonged as it is not metabolised by monoamine oxidase.

Effects of pressor agents on splanchnic, renal and cerebral circulations are variable and unpredictable. Pulmonary vascular resistance is also raised by these agents.

Methylthioninium chloride (methylene blue) inhibits the NO-cGMP pathway. It is currently unlicensed as a pressor agent and its use has only been reported in a few small case series. A multicentre study of a NO synthase inhibitor (L-NMMA) was prematurely discontinued due to adverse outcomes.

Vasopressin (short half-life, infusion needed) and terlipressin (longer half-life, can be given by bolus) may be effective in treating catecholamine-resistant vasodilatory shock. Paradoxically, such patients respond to small doses that have no pressor effect in healthy people. Multicentre outcome studies are ongoing.

Excessive dosing of any pressor agent may lead to regional ischaemia, e.g. cardiac, splanchnic. Digital ischaemia may respond to prompt administration of intravenous prostanoids (e.g. PGE1, PGI2).


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