Incidence and Definitions
Hypotension is a common problem in the neonatal intensive care unit (NICU).1 More than half of neonates admitted to the NICU also carry the diagnosis of hypotension. Although the incidence is higher with lower gestational age at birth, the exact incidence is not known. This is primarily because of the lack of consensus on what constitutes hypotension.2 Indeed, there are many ways to define neonatal hypotension, and the differences among the definitions obviously affect the reported incidence of hypotension and the decision to treat the condition.3,4 As the ultimate goal is to ensure that oxygen delivery matches tissue oxygen demand in all organs, hypotension best can be defined based on physiologic principles, that is, by assessing the effects of decreased perfusion pressure on organ blood flow and oxygen delivery. However, because in the first, “compensated” phase of shock, vital organ (brain, myocardium, adrenal glands) blood flow and blood pressure (BP) are maintained by the neuroendocrine mechanism-driven redistribution of blood flow from nonvital organs, hypotension only presents when shock enters its second, “uncompensated” phase and vital organ blood flow also declines. Therefore, hypotension can be best defined as the BP below which vital organ (eg, brain) blood flow autoregulation is lost and cerebral blood flow (CBF) starts decreasing in proportion to the decrease in BP (the so-called autoregulatory threshold of hypotension; Figure 20-1).5 On further decrease in BP and oxygen delivery, cellular function cannot be appropriately maintained, but structural integrity is not yet significantly affected (the so-called functional threshold of hypotension; Figure 20-1).5 Finally, on further decline in BP and oxygen delivery, structural integrity of tissues becomes affected, resulting in permanent organ damage (the so-called ischemic threshold of hypotension; Figure 20-1).5 Unfortunately, there are few data on the cutoff values, and because the capacity of the cardiovascular and neuroendocrine systems to appropriately compensate is affected by gestational and postnatal age as well as the underlying pathophysiology, these values likely vary in the individual patient and even for the same patient at different times.
Definition of hypotension by 3 pathophysiologic phenomena5 of increasing severity: autoregulatory, functional, and ischemic thresholds of hypotension. The mean blood pressure (MBP) associated with the loss of cerebral blood flow (CBF) autoregulation is the generally accepted definition of hypotension (autoregulatory blood pressure threshold). Preliminary data suggest that the autoregulatory blood pressure threshold might be around 28 to 30 mm Hg in the very low birth weight (VLBW) neonate during the first postnatal days.5 However, at present it remains unclear what to do with this information in clinical practice. If blood pressure continues to fall, it reaches a value at which cerebral function becomes compromised (functional blood pressure threshold). Available preliminary data suggest that the functional blood pressure threshold might be around 22 to 24 mm Hg in the VLBW ...