I. DEFINITION. Shock is defined as acute circulatory dysfunction resulting in insufficient oxygen and nutrient delivery to the tissues relative to their metabolic demand, leading to cellular dysfunction that may lead to lactic acidosis and if left uncorrected cause cell death. Shock remains an important cause of neonatal mortality and morbidity. Its prognosis depends on the duration and severity of shock and the resultant extent of vital organ damage. Shock can lead to long-term morbidity including severe neurologic compromise because of cerebral ischemia and reperfusion injury. Therefore, recognizing shock promptly and initiating therapy to address the cause of shock and maintaining hemodynamic stability is essential. In the extremely premature newborn, the lowest acceptable blood pressures (BPs) that may be associated with end organ damage are not well established; therefore, its treatment remains controversial.

II. ETIOLOGY. Shock in neonates may be due to lower vascular tone (distributive shock), inadequate blood volume (hypovolemic shock), decreased cardiac function (cardiogenic shock), restricted blood flow (obstructive shock), and inadequate oxygen delivery (dissociative shock). Distributive shock, with or without myocardial dysfunction, is the most frequent cause of hypotension underlying shock, especially in preterm infants.

A. Distributive shock. Changes in vascular tone in neonates can result in decreased flow to tissues due to the following:

1. Impaired vasoregulation from increased or dysregulated endothelial nitric oxide (NO) production in the perinatal transitional period, particularly in the preterm neonate

2. Neurologic injury such as in patients with severe hypoxic-ischemic injury which may affect neurovascular pathways

3. Sepsis-related release of proinflammatory cascades that lead to vasodilation

4. Anaphylactic shock is more common in children and rarely affects neonates.

B. Hypovolemic shock. The following conditions can reduce circulating blood volume:

1. Placental hemorrhage, as in abruptio placentae or placenta previa

2. Fetal-to-maternal hemorrhage

3. Twin-to-twin transfusion

4. Intracranial hemorrhage

5. Massive pulmonary hemorrhage (often associated with patent ductus arteriosus [PDA])

6. Blood loss due to disseminated intravascular coagulation (DIC) or other severe coagulopathies

7. Plasma leak into the extravascular compartment, as seen with low oncotic pressure states or capillary leak syndrome (e.g., sepsis)

8. Dehydration due to excessive insensible water loss or inappropriate diuresis as commonly observed in extremely low birth weight (ELBW) infants

C. Cardiogenic shock due to myocardial dysfunction. Decreased cardiac output either due to poor myocardial function or diverted flow through accessory channels results in cardiogenic shock. Some common causes of neonatal cardiogenic shock include the following:

1. Large PDA in a premature infant diverting left ventricular output to pulmonary circulation when uncompensated by an increase in left ventricular output

2. Intrapartum asphyxia leading to myocardial depression

3. Bacterial or viral myocarditis. Congenital viral infections such as enterovirus are more likely to cause severe myocarditis.

4. Fetal or neonatal arrhythmias compromising cardiac output

5. Large arteriovenous malformations (AVM) such as an intracranial AVM that divert a considerable amount of cardiac output away from the systemic circulation

6. Metabolic abnormalities (e.g., hypoglycemia), or cardiomyopathy seen in infants of diabetic mothers

D. Obstructive shock. Restricted venous inflow or arterial outflow will rapidly decrease cardiac output and lead to profound shock. Types of obstructions to blood flow include the following:

1. Venous obstructions

a. Cardiac anomalies including total anomalous pulmonary venous return, cor triatriatum, tricuspid atresia, mitral atresia

b. Acquired inflow obstructions can occur from intravascular air or thrombotic embolus.

c. Increased intrathoracic pressure caused by high airway pressures, pneumothorax, pneumomediastinum, or pneumopericardium

2. Arterial obstructions

a. Cardiac anomalies including pulmonary stenosis or atresia, aortic stenosis or atresia

b. Vascular anomalies such as coarctation of the aorta or interrupted aortic arch

c. Hypertrophic subaortic stenosis due to ventricular hypertrophy seen in infants of diabetic mothers with compromised left ventricular outflow

III. DIAGNOSIS. At the onset of shock, normal compensatory mechanisms may be able to maintain adequate BP by diverting blood away from the skin, muscles, and other nonessential organs. This compensation allows BP to remain within the normal range and maintain perfusion to vital organs and is aptly named “compensated shock.” During compensated shock, clinical findings may be subtle and include increased systemic vascular resistance (SVR) presenting as decreased peripheral perfusion (cold, pale skin with delayed capillary refill), tachycardia to maintain cardiac output, weak peripheral pulses and narrow pulse pressure (raised diastolic BP), ileus (decreased splanchnic circulation), and oliguria (decreased renal perfusion). If the clinical condition that results in shock remains unabated or if the underlying etiology is severe (e.g., sudden tension pneumothorax), compensatory mechanisms are usually insufficient to maintain BP and systemic hypotension ensues. “Uncompensated shock” refers to the phase of shock when the patient develops hypotension, and its clinical presentation will reflect decreased perfusion to vital organs. Lack of perfusion to the brain may cause changes in consciousness and lethargy. Lack of coronary perfusion increases the risk of cardiac arrest. In preterm infants, the associated decrease in brain blood flow and oxygen supply during hypotension predisposes to intraventricular/cerebral hemorrhages and periventricular leukomalacia with long-term neurodevelopmental abnormalities. In addition, in ELBW infants, the vasculature of the cerebral cortex may respond to transient myocardial dysfunction/shock with vasoconstriction rather than vasodilatation, further diminishing cerebral perfusion and increasing the risk of neurologic injury.