Journal of Clinical Neonatology

: 2020  |  Volume : 9  |  Issue : 4  |  Page : 231--234

The Doppler ultrasound: A bedside tool to understand cerebral autoregulation in neonates

Iyer Harohallli Venkatesh 
 Consultant Neonatologist, Department of Neonatology, Manipal Hospital, Bangalore, Karnataka, India

Correspondence Address:
Dr. Iyer Harohallli Venkatesh
Manipal Hospital, Old Airport Road, Bangalore, Karnataka


Cerebral blood flow is a unique circulation in the human body, constituting around 15% of cardiac output with the brain comprising 2% of the total body weight. The cerebral autoregulation is maintained physiologically to keep the oxygen extraction constant from the brain cells. There are different ways to measure cerebral circulation, including near-infrared spectroscopy and diffuse optical spectroscopy. The limitation of using such equipment is because of the cost and nonavailability. The bedside Doppler ultrasound because of its easy availability it is used in the assessment of cerebral blood flow. There are many factors that contribute to cerebral blood flow. Understanding the variation in the cerebral blood flow by calculating the resistive index of play in the cerebral vessels will help the physician to understand the autoregulation – compensated or uncompensated.

How to cite this article:
Venkatesh IH. The Doppler ultrasound: A bedside tool to understand cerebral autoregulation in neonates.J Clin Neonatol 2020;9:231-234

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Venkatesh IH. The Doppler ultrasound: A bedside tool to understand cerebral autoregulation in neonates. J Clin Neonatol [serial online] 2020 [cited 2021 Dec 6 ];9:231-234
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The cerebral autoregulation is maintained physiologically for the normal activities of the brain. The vertebral and carotid arteries merge at the base of the brain to form the circle of Willis.

Many physiological factors such as carbon dioxide, oxygen, hydrogen ion concentration in the blood and adenosine, arachidonic acid, nitric oxide, and potassium ions secreted by astrocytes play a significant role in the cerebral autoregulation.[1]

The carbonic acid formed by carbon dioxide in the brain dissociates and liberates hydrogen ions, in turn, causing cerebral vasodilatation. Subsequently, the hydrogen ion gets washed away due to cerebral vasodilatation preventing depressed neuronal activity that is caused by the ion.

The oxygen is essential to the brain. The oxygen utilization of the brain is around 3.5 ml of oxygen/100 g of the brain/min. Any clinical condition that brings down the level of oxygen in the brain will make the cerebral arteries to dilate to get more blood into the brain making the oxygen available to the cerebral tissue. The normal cerebral tissue PO2 is 35–40 mm of Hg and anything <30 mm of Hg makes the cerebral blood flow to increase.

The astrocytes, ensheathing brain capillaries play a major role in the cerebral circulation. The uptake of excess extracellular potassium and an increase in the intracellular calcium causes cerebral vasodilatation.[2],[3]

Depending upon the activities, the arterial pressure fluctuates, the cerebral autoregulation maintains the brain activity during such changes. When the mean arterial blood pressure drops to the critical level, the cerebral blood flow increases.

The middle cerebral and anterior cerebral arteries are used historically to demonstrate the cerebral hemodynamics through transcranial Doppler with the limitation of defining only the velocity and not the flow.[4],[5],[6],[7],[8],[9]

The cerebral blood flow can be measured by injecting radioactive xenon into the carotid artery. Recently, the near-infrared spectroscopy (NIRS) and diffusion optical correlation spectroscopy (DCS) are used. The oxyhemoglobin optical density using NIRS is utilized to estimate the cerebral blood flow.[10] The utility of NIRS is limited in developing countries.

The bedside Doppler ultrasound in view of easy bedside availability is used to look at the blood flow in the cerebral arteries and to define the systolic and diastolic velocities. The resistive index (RI) is derived from these two velocities.

The RI = peak systolic velocity (PSV) − end-diastolic velocity (EDV)/PSV.

where PSV is peak systolic velocity and EDV is the end-diastolic velocity.

The anterior fontanel [Figure 1] and the transtemporal regions [Figure 2] are used to place the probe with high frequency to demonstrate the anterior cerebral [Figure 3] and middle cerebral artery [Figure 4], respectively.{Figure 1}{Figure 2}{Figure 3}{Figure 4}

Minarik (2000) determined the reference values of the anterior cerebral artery using transcranial Doppler. The normal cerebral artery RI ranges from 0.56 and 0.8 in neonates [Figure 5]. Any value <0.56 [Figure 6] or >0.8 [Figure 7] will indicate increased and decreased blood flow into the brain, respectively. The reversal of diastolic blood flow [Figure 8] indicates the loss of cerebral autoregulation and could be a strong prognosticating factor.{Figure 5}{Figure 6}{Figure 7}{Figure 8}

The prospective study was conducted at a tertiary care hospital catering outborn neonates looking at the RI as a prognosticating marker in neonates with hypoxic-ischemic encephalopathy.[11] The newborns delivered after 37 weeks were enrolled in the study satisfying the criteria of perinatal asphyxia.[12],[13] The RI was evaluated using anterior cerebral artery Doppler in the prediction of adverse neurological outcomes. The RI of the cerebral artery between 0.56 and 0.8 was considered normal.[14],[15] The presence of abnormal resistance index was associated with significantly higher risk of death and abnormal neurodevelopmental outcomes at 6–12 months of age.

 Factors Affecting Cerebral Circulation

There are many clinical conditions such as prematurity, intraventricular hemorrhage, hypoxic-ischemic encephalopathy, and congenital heart disease will affect the cerebral autoregulation. The factors influencing the cerebral blood flow are listed in [Table 1].{Table 1}


The management of sick neonates is a real challenge. The continuous monitoring of these neonates will be done to understand the well-being. The arterial level of carbon dioxide, the oxygen, the blood pressure, and conditions such as patent ductus arteriosus and cerebral pathology have a role in altering the cerebral circulation. Understanding the ongoing cerebral hemodynamics is also crucial in the care of sick neonates and might help in the therapeutic intervention or in prognostication. The bedside ultrasound could be used in the assessment of cerebral autoregulation, where NIRS and DCS would be a dream in most of the centers.

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Conflicts of interest

There are no conflicts of interest.


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