□ Both agents are predominantly alpha-agonists. Available evidence in sepsis suggests that phenylephrine has a very similar physiologic effect compared to norepinephrine.Alternatively, for a patient who is preload-responsive with a stronger ejection fraction, phenylephrine could cause a net increase in cardiac output. For example, in a patient with systolic heart failure and volume overload, added preload won't help, whereas the heart may be unable to tolerate afterload – so the net effect is to reduce the cardiac output. Effect on cardiac output depends on preload-responsiveness versus ability of the heart to handle increased afterload.Venoconstriction increases the preload.Increased systemic vascular resistance (SVR). Mechanism: Pure alpha-agonist, causes arterial and venous vasoconstriction.Vasopressin can cause digital ischemia, especially when combined with norepinephrine – must pay careful attention to perfusion of hands and feet shut off vasopressin at first sign of ischemic digits.Vasopressin shouldn't generally be given peripherally (if it extravasates, there is no antidote).It may cause some pulmonary vasodilation, which can be helpful in the context of pulmonary hypertension.Vasopressin may preferentially cause vasoconstriction of post-glomerular arterioles in the kidney, causing improvement in renal function.
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