Cardiac Drugs: Vasopressors and Inotropic Agents

Why it is worth your twenty-two minutes

Almost nothing you know about vasoactive drugs in adults transfers cleanly to a newborn. The neonatal myocardium has fewer contractile elements and more connective tissue, so stroke volume is close to fixed and cardiac output depends on rate — which is why bradycardia is the emergency and why a rate-slowing agent is rarely your friend. Calcium handling relies more on transmembrane flux than on a mature sarcoplasmic reticulum, so these hearts are unusually sensitive to ionised calcium. Adrenergic receptor populations are still developing and are relatively down-regulated in the preterm infant, so the dose–response relationship you expect may simply not be there. And all of it is happening inside a circulation that is still transitioning: pulmonary vascular resistance falling by the hour, a duct that may be open, and shunts that can reverse when you change the systemic pressure. An agent that raises blood pressure by raising systemic resistance may reduce output; an agent that lowers pulmonary resistance may steal from the systemic circuit. The blood pressure you are treating and the perfusion you actually want are not the same variable.

The agents divide roughly by what you think has failed. Dopamine remains the most-used first-line drug and raises blood pressure reliably in the preterm infant, but it does so substantially through vasoconstriction, and at higher doses it raises pulmonary vascular resistance too — which is a problem precisely where pulmonary hypertension is the issue. Dobutamine is the better choice where the problem is myocardial dysfunction rather than vasodilatation: it raises output, but the pressure may not follow. Adrenaline covers both alpha and beta territory and is useful in refractory hypotension, at the cost of lactataemia and hyperglycaemia that will then confuse your monitoring. Noradrenaline has a particular place in septic shock and in persistent pulmonary hypertension, where it tends to raise systemic resistance more than pulmonary. Vasopressin is the catecholamine-sparing option in refractory shock and is relatively kind to the pulmonary circulation. Milrinone sits apart as an inodilator — it improves contractility and relaxation while lowering both afterloads, which makes it valuable in pulmonary hypertension and in the low-output state after ductal ligation, provided you can tolerate the vasodilatation. And hydrocortisone is the one people reach for too late: relative adrenal insufficiency is common in the sick preterm infant, and steroid often restores catecholamine responsiveness when nothing else will.

The uncomfortable part, and the reason this topic keeps returning, is that nobody has satisfactorily defined what we are treating. The working rule that mean arterial pressure should exceed gestational age in weeks is a convention of convenience rather than a threshold validated against outcome, and a substantial proportion of infants who meet the definition of hypotension have entirely adequate organ perfusion. Meanwhile the evidence that treating the number improves anything that matters — survival, neurodevelopment, intraventricular haemorrhage — remains thin, and there is a real possibility that some of the harm we attribute to hypotension is in fact caused by the swings we induce treating it. Which turns the practical question into a clinical one rather than a pharmacological one: capillary refill, lactate, urine output, conscious state, and increasingly functional echocardiography and near-infrared spectroscopy, in preference to a single number on a chart. Know the drugs well. Be considerably less certain about when to start them.