Vasopressors and inotropes: choosing and titrating

Why examiners ask this

"The pressure is 70/40, what are you going to give?" is the most common intraoperative question on the exam. The examiner is not asking for a drug; they are asking whether you know why the pressure is low and whether your drug matches the physiology. It tests application of knowledge (receptor pharmacology) and judgment (when a pressor is the wrong answer).

The 60-second answer

Decision points and the "why"

• Phenylephrine versus ephedrine — phenylephrine is pure alpha-1 and slows the heart by baroreflex, which I want in aortic stenosis, HOCM and tachycardia; ephedrine adds indirect beta stimulation, which I want when the heart rate is slow. Ephedrine shows tachyphylaxis because it depletes norepinephrine stores.
• Norepinephrine as the default infusion — alpha with modest beta-1, it raises MAP while preserving cardiac output; it is first-line in septic shock, and starting it peripherally is preferred to delaying for central access.¹
• Vasopressin — V1 vasoconstriction independent of catecholamine receptors, so it works in acidosis and catecholamine-resistant states (sepsis, ACE-inhibitor vasoplegia, post-bypass). It does not raise pulmonary vascular resistance much, which makes it useful in right heart failure.
• Epinephrine — beta at low doses and alpha at high doses; first-line for anaphylaxis and cardiac arrest, and a strong inotrope after bypass. Cost: tachycardia, arrhythmia and lactate rise from beta-2 glycolysis.
• Dobutamine and milrinone — inodilators for a low-output state with adequate pressure. Milrinone is a PDE-3 inhibitor, works despite beta-blockade or downregulation, lowers PVR, but drops SVR and is renally cleared.
• Methylene blue and angiotensin II — rescue agents for refractory vasoplegia; methylene blue 1–2 mg/kg blocks nitric oxide–guanylate cyclase and carries a boxed warning for serotonin syndrome with serotonergic drugs and opioids;² angiotensin II starts at 20 ng/kg/min, at most 80 in the first 3 hours.³
• Stress-dose steroids — hydrocortisone 50 mg every 6 hours once norepinephrine has been at least 0.25 mcg/kg/min for 4 hours in septic shock,¹ because relative adrenal insufficiency blunts pressor response.

The complication they will inject

After cardiopulmonary bypass the MAP is 55 on norepinephrine 0.2 mcg/kg/min, the CVP is 18, the PA pressures are high and the TEE shows a dilated, hypokinetic right ventricle with a small, underfilled left ventricle. More norepinephrine will not fix this. I tell the surgeon and perfusionist, check for a coronary air or graft problem, start epinephrine or milrinone for inotropy, add inhaled nitric oxide or epoprostenol to unload the right ventricle, keep the MAP up with vasopressin so the right coronary stays perfused, and correct hypoxia, hypercarbia and acidosis, which all raise PVR. I avoid volume because the right ventricle is already distended. If it does not respond, I discuss returning to bypass and mechanical support.

Pitfalls that cost points

• Naming a pressor before naming the cause of hypotension.
• Giving phenylephrine to a patient whose problem is a failing ventricle; it raises afterload and cuts output further.
• Forgetting vasopressin and methylene blue in vasoplegia.
• Treating a number without perfusion endpoints: lactate, urine output, mental status, SvO2.
• Using dopamine as a first choice; it causes about twice the arrhythmias of norepinephrine in septic shock.¹

Rapid-why

Q: Why does phenylephrine cause bradycardia? A: The rise in pressure triggers the baroreceptor reflex, increasing vagal tone.

Q: Why add vasopressin rather than escalate norepinephrine? A: The Surviving Sepsis Campaign suggests adding it over escalating norepinephrine;¹ it acts on a different receptor, spares catecholamine dose and arrhythmias, and replaces a relative vasopressin deficiency in sepsis.

Q: Why does milrinone work in a beta-blocked patient? A: It raises cAMP by blocking its breakdown, downstream of the beta receptor.

Q: What is your MAP target? A: 65 mmHg in most patients, the septic shock target;¹ higher in chronic hypertension, raised intracranial pressure or spinal cord perfusion concerns.

Numbers to know

Links

• Intraoperative hypotension: differential and management, Sepsis and septic shock, Heart failure and reduced ejection fraction (coming), Pulmonary hypertension and right ventricular failure (coming), Cardiopulmonary bypass: conduct and separation (coming)
• Atlas: Ventricular function and wall motion
• Surviving Sepsis Campaign international guidelines (2021)