Cerebrovascular Time Constant in Patients with Head Injury

Abstract

The cerebrovascular time constant (τ) theoretically estimates how fast the cerebral arterial bed is filled by blood volume after a sudden change in arterial blood pressure during one cardiac cycle. The aim of this study was to assess the time constant of the cerebral arterial bed in patients with traumatic brain injury (TBI) with and without intracranial hematomas (IH). We examined 116 patients with severe TBI (mean 35 ± 15 years, 61 men, 55 women). The first group included 58 patients without IH and the second group included 58 patients with epidural (7), subdural (48), and multiple (3) hematomas. Perfusion computed tomography (PCT) was performed 1-12 days after TBI in the first group and 2-8 days after surgical evacuation of the hematoma in the second group. Arteriovenous amplitude of regional cerebral blood volume oscillation was calculated as the difference between arterial and venous blood volume in the "region of interest" of 1 cm(2). Mean arterial pressure was measured and the flow rate of the middle cerebral artery was recorded with transcranial Doppler ultrasound after PCT. The time constant was calculated by the formula modified by Kasprowicz. The τ was shorter (p = 0.05) in both groups 1 and 2 in comparison with normal data. The time constant in group 2 was shorter than in group 1, both on the side of the former hematoma (р = 0.012) and on the contralateral side (р = 0.044). The results indicate failure of autoregulation of cerebral capillary blood flow in severe TBI, which increases in patients with polytrauma and traumatic IH.

Hypocapnia after traumatic brain injury: how does it affect the time constant of the cerebral circulation?

“…However, the change in CVR predominated, resulting in a change in tau in the same direction as CVR (increase) 0.26) seconds for young normal subjects] [1], but it has also been expressed as a percentage between 20-30% of the cardiac cycle in normal volunteers [1,4]. It has been demonstrated that the time constant gets longer during hypocapnia in healthy volunteers through an increase in CVR [1], and shortens in patients with traumatic brain injury through a decrease in Ca [6].…”

“…Its calculation has been described using mathematical transformations of arterial blood pressure (ABP) and transcranial Doppler (TCD) cerebral blood flow velocity waveforms, and as a product of brain arterial compliance (Ca) and resistance (CVR) [1,2]. The time constant has been studied in different situations: vasospasm in subarachnoid haemorrhage [3], hydrocephalus [4] carotid stenosis [2], and hypercapnia [5]; it has also been studied in TBI patients [6] and compared in different vascular brain territories in normal volunteers [7].…”

Transcranial Doppler: a stethoscope for the brain‐neurocritical care use

“…In a recent study (Trofimov, Kalentiev, Gribkov, Voennov, & Grigoryeva, ), the cerebrovascular time constant was assessed in patients with TBI, with and without intracranial hematomas (epidural, subdural, and multiple hematomas). CTAU was shorter ( p = .05) in both groups in comparison with normal data, but in patients with intracranial hematomas, the time constant was even shorter.…”

Neurocritical Patient in ICU: Transcranial Doppler (TCD/TCCS) as the Brain Stethoscope

“…In a recent study [23], TAU was assessed in patients with traumatic brain injury (TBI) with and without intracranial hematomas (epidural, subdural, and multiple hematomas). Tau was shorter in both groups in comparison with normal data, but in patients with intracranial hematomas, the time constant was even shorter, indicating a failure of autoregulation of cerebral capillary blood flow after severe TBI occurs.…”