Physiology of Blood Pressure in the Neonate
Neonatal Blood Pressure Support and Physiology of Blood Pressure
Caring for a critically ill neonate often requires the provider to consider the physiology, goals, complications, and pharmacologic methods of increasing a newborn patient’s blood pressure. Although various methods and medications are available to increase systemic blood pressure, few human data convincingly support the common practices for blood pressure support in adult intensive care unit, pediatric intensive care unit, or neonatal intensive care unit (NICU).1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 This chapter describes the physiology of systemic blood pressure in the neonate, defines the pharmacologic target receptors, and presents the medications most commonly used to raise blood pressure.
Cardiac Output, Systemic Vascular Resistance, and Blood Pressure
Systemic blood pressure is determined by the product of cardiac output and systemic vascular resistance (SVR). Cardiac output is determined by the product of heart rate and stroke volume; stroke volume is determined by the preload, afterload, and contractility of the myocardium. Derangements in any of these parameters can produce hypotension in the neonatal patient. It should be noted, however, that simply increasing the blood pressure may not achieve the desired physiologic effect if the goal is to increase organ blood flow and oxygen delivery. Blood pressure and systemic blood flow are poorly correlated in the newborn,12,13 reflecting the importance of SVR in the neonate. Importantly, the neonate has unique physiology that must be understood when considering systemic blood pressure. Because of a relatively noncompliant myocardium, neonates are more dependent on heart rate to generate cardiac output than adults.14, 15, 16, and 17 Also unique to neonates are the transitional circulation immediately following birth, less-functional cerebral autoregulation mechanisms, the potential state of prematurity, and the ongoing maturation of the myocardium and vascular bed.18, 19, and 20
Defining Neonatal Hypotension
The definition of hypotension in the newborn continues to challenge the clinician. “Mean arterial pressure greater than or equal to gestational age” frequently is touted as a guideline for minimum acceptable blood pressure for a neonate, but few data exist to support this practice.21, 22, 23, 24, 25, 26, and 27 Clinicians often use both gestational age and birth weight as guidelines for estimating expected blood pressure in a neonate (Figures 82-1 and 82-2). In principle, the blood pressure must be adequate to deliver oxygen to the tissues and support delivery of nutrients to the organs (most importantly the brain and heart) while removing toxic waste products from cells. The inability to achieve adequate tissue oxygen delivery and waste removal defines neonatal shock. Blood pressure is clearly inadequate if organs are poorly ...