Andersons pediatric cardiology 314

Hemodynamic Assessment and Monitoring The consequences of hemodynamic instability include necrotizing enterocolitis, intraventricular hemorrhage, and periventricular leukomalacia, all of which may lead to mortality or adverse neurodevelopmental outcomes Intraventricular hemorrhage occurs in up to three-tenths of infants born with very low weight, most commonly in the first 7 hours of life The origin of the hemorrhage is the germinal matrix, an immature network of capillaries highly susceptible to hypoxemia, hypercapnia, and altered cerebral blood flow Periventricular leukomalacia is a loss of white matter in the watershed areas around the lateral cerebral ventricles secondary to hypoxic-ischemic injury The infant born at very low weight is highly susceptible to such morbidities, necessitating focused cardiovascular monitoring and targeted intervention in a timely fashion Is Blood Pressure a Reliable Measure of Circulatory Stability? The determination of hemodynamic stability in premature infants is fraught with uncertainty, myths, and dogma without scientific validity Suboptimal systemic blood flow is usually suspected on the basis of tachycardia, delayed capillary refill time, hypothermia, oliguria, altered blood pressure, metabolic acidosis, and increased levels of lactate in the plasma Blood pressure readings are readily available at the bedside as a continuous stream of data from indwelling arterial lines, or periodically using a validated oscillometric cuff method.47 The approach to monitoring and guiding therapeutic intervention has relied on using mean arterial pressure as a surrogate of the adequacy of tissue perfusion and oxygenation This reasoning is based on an assumed proportionality between blood pressure and systemic blood flow.48 In 1992 a report from the former British Paediatric Association stressed the importance of monitoring blood pressure to guide early therapeutic intervention and thus prevent adverse neurologic sequelae.49 It was suggested that mean arterial pressure equivalent to the gestational age in weeks is adequate as a minimum value The group also suggested the need to establish accurate normative ranges for systolic and diastolic blood pressure Unfortunately, achieving a mean blood pressure equal to gestational age has now become dogma and the standard on which therapy is based Normative centiles for systolic blood pressure that take into account both gestational age at birth and postnatal age have been developed but are rarely used.50 Monitoring blood pressure in isolation is problematic for a number for reasons First, blood pressure is but one surrogate of circulatory stability Second, the concept of numeric hypotension as mean blood pressure less than gestational age in weeks has not been validated against indexes of perfusion of end organs Third, important changes in either systolic or diastolic blood pressure may be missed The systolic pressure indicates the pressure generated by the left ventricle and is therefore reflective of cardiac output (Fig 15.1A) Diastolic pressure reflects global vascular resistance and local perfusion of the tissues (see Fig 15.1B) In neonates with a hemodynamically significant arterial duct, diastolic pressure is oftentimes compromised in isolation of changes in mean arterial pressure This may have an unappreciated adverse effect on coronary arterial perfusion and subsequently on myocardial performance The relationship between mean blood pressure and cardiac output is also weak (Fig 15.2) The finding of neonates with numeric hypotension but no clinical or biochemical signs of systemic flow, and conversely patients with normal or high blood pressure but signs of circulatory compromise, is not uncommon It must be recognized that blood pressure is but a surrogate of perfusion and not the end point of interest Although mean arterial pressure may be a guide to cardiovascular health, clinicians should pay attention to systolic and diastolic pressure when making decisions regarding the most appropriate intervention FIG 15.1 Complexity and interrelatedness of physiologic contributors to low mean arterial pressure make it unlikely that a single measurement is adequate to illustrate the status of the cardiovascular system The use of systolic (A) and diastolic (B) pressure in addition to other clinical signs (e.g., heart rate, oxygenation) may improve diagnostic accuracy and may provide clearer therapeutic targets ALCAPA, Anomalous origin of left coronary artery from pulmonary artery; BP, blood pressure; CHD, congenital heart disease; HOCM, hypertrophic obstructive cardiomyopathy; MAP, mean arterial pressure; PBF, placental blood flow; PDA, patent ductus arteriosus; PPHN, persistent pulmonary hypertension of the newborn; PV, pulmonary vein; RV, right ventricle; SVT, supraventricular tachycardia; TMI, transmural myocardial infarction; VT, ventricular tachycardia