Ontogeny of cerebrovascular critical closing pressure

Abstract

Background: Premature infants are at risk of vascular neurologic insults. Hypotension and hypertension are considered injurious, but neither condition is defined with consensus. Cerebrovascular critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which cerebral blood flow (CBF) ceases. CrCP may serve to define subject-specific low or high ABP. Our objective was to determine the ontogeny of CrCP. Methods: Premature infants (n = 179) with gestational age (GA) from 23-31 wk had recordings of ABP and middle cerebral artery flow velocity twice daily for 3 d and then daily for the duration of the first week of life. All infants received mechanical ventilation. CrCP was calculated using an impedance-model derivation with Doppler-based estimations of cerebrovascular resistance and compliance. The association between GA and CrCP was determined in a multivariate analysis. results: The median (interquartile range) CrCP for the cohort was 22 mm Hg (19-25 mm Hg). CrCP increased significantly with GA (r = 0.6; slope = 1.4 mm Hg/wk gestation), an association that persisted with multivariate analysis (P < 0.0001). conclusion: CrCP increased significantly from 23 to 31 wk gestation. The low CrCP observed in very premature infants may explain their ability to tolerate low ABP without global cerebral infarct or hemorrhage. c erebrovascular critical closing pressure (CrCP) is the arterial blood pressure (ABP) at which blood flow to the brain ceases due to vascular collapse. Also referred to as collapsing pressure, CrCP can be easily and noninvasively monitored at the bedside using Doppler ultrasound and ABP measurements (1). CrCP is posited to be the sum of vascular wall tension and intracranial pressure (2). CrCP can be conceptualized as a factor for the normalization of ABP to an "effective cerebral perfusion pressure" or "closing margin" (3,4). The "effective cerebral perfusion" or "closing margin" (ABP-CrCP) can be determined for any phase of the cardiac cycle by subtracting the CrCP from systolic, mean, or diastolic ABP.Limited data sets report CrCPs in term and preterm newborns ranging from 24 to 33 mm Hg, which is similar to reported values in mature subjects (5,6). Low ABP in premature infants results in a closing margin that is strikingly low (Figure 1). Premature infants are commonly treated for low values of blood pressure based on mean ABP thresholds related to their gestational age (GA) (7-9), and these thresholds are within the range of published CrCP values (5,6). Moreover, during postnatal transition, a period of time often characterized by shock, premature infants can have CBF limited to the systolic phase of the cardiac cycle (10).Disparity in the definition of hypotension and treatment thresholds for low ABP are likely attributable to a lack of outcome data in premature infants demonstrating a benefit for treating low ABP (11). We propose that an important confounding factor may be the low closing margin relative to the range of CrCP in this population. For example, a difference of 10 mm Hg Cr...

Elevated Diastolic Closing Margin Is Associated with Intraventricular Hemorrhage in Premature Infants

“…17 At term gestation and into adulthood, ABP continues to increase but CrCP plateaus at ~30 mmHg, rendering a progressive increase in the DCM. The wide DCM in term infants and adult patients minimizes the effect of relatively small vascular wall tension and intracranial pressure changes on cerebral perfusion pressure.…”

“…The full derivation of this method has been previously described in detail and validated. 17,21 In brief, CrCP is first estimated from the model by the equation: $$\mathrm{italicCrCP}=\mathrm{italicABP}-\frac{\mathit{ABP}}{\sqrt{{(\mathit{CVR}·Ca·HR·2\pi )}^2+1}}$$ where CVR is cerebral vascular resistance and Ca is cerebral arterial compliance, which are not directly measureable, but can be calculated using CBFV and ABP waveforms. The product of resistance and compliance is termed the time constant τ.…”

“…The reason for this assertion is that the critical closing pressure (CrCP) of the cerebral vasculature of premature infants is at or near resting mean ABP. 17 …”

Neonatal cerebrovascular autoregulation

“…59,60 In premature neonates, low diastolic blood pressures can approach or reach the CrCP with little evident CBF. 2,61 CrCP in HIE and other neonatal neurologic diseases deserves further study. CrCP is distinct from and lower than the autoregulatory plateau.…”

Cerebral Hemodynamics Are Not Affected by the Size of the Patent Ductus Arteriosus

“…Another unique aspect of premature infants' cerebral hemodynamics is the role of the brain's critical closing pressure (CrCP). CrCP is the ABP at which blood flow ceases to the brain, and often the mean ABP is close to or below the CrCP [11]. When this occurs, CBF is present only during systole [11,12].…”