Mi Ok Kim

Central Pulsatile Pressure and Flow Relationship in the Time and Frequency Domain to Characterise Hydraulic Input to the Brain and Cerebral Vascular Impedance

In the time domain, pulsatile flow and pressure can be characterised as the ratio of the late systolic boost of flow or pressure to the pulse amplitude so as to estimate the hydraulic input to the brain. While vascular impedance has been widely used to represent the load presented to the heart by the systemic circulation, it has not been applied to the cerebral circulation.We set out to study the relationship between the pressure and the flow augmentation index (AIx) in the time domain and to determine cerebral vascular impedance using aortic blood pressure and cerebral blood flow waveforms in the frequency domain. Twenty-four young subjects (aged 21-39 years) were recruited; aortic pressure was derived using SphygmoCor from radial pressure. Flow waveforms were recorded from the middle cerebral artery. In three subjects, we performed the Valsalva manoeuvre to investigate their response to physiological intervention. There was a linear relationship between flow and pressure AIx, and cerebral impedance values were similar to those estimated for low resistance vascular beds. Substantial change in pressure and flow wave contour was observed during the Valsalva manoeuvre; however, the relationship in both the time and the frequency domains were unchanged. This confirms that aortic pressure and cerebral flow waveform can be used to study cerebral impedance.

Intracranial Pressure Waveforms are More Closely Related to Central Aortic than Radial Pressure Waveforms: Implications for Pathophysiology and Therapy

In patients with subarachnoid haemorrhage, pulsatile intracranial pressure (ICP) is more strongly associated with adverse events than mean ICP. Furthermore, patients with idiopathic normal-pressure hydrocephalus (iNPH), and pulsatile ICP of 5 mmHg or more, gain more benefit from cerebrospinal fluid (CSF) shunting than those whose pulsatile ICP is lower than 5 mmHg.Our study aims to investigate the morphological relationship between ICP pulsations, aortic pressure pulsations and radial artery pulsations. Central aortic pulse pressure has been known to be the best predictor of adverse cardiac events, whereas radial artery pulse pressure is generally measured and displayed in intensive care environments.We studied 10 patients with iNPH, and their ICP and aortic and radial pressures were digitised, ensemble-averaged and compared in the time and frequency domains. The ICP wave contour was quite different to the radial pressure waveform. By contrast, the ICP waveform was similar to the aortic pressure wave contour. The ICP amplitude averaged <10 % of aortic pulse pressure. In the frequency domain, the relative amplitude of the first three harmonics was similar for the ICP and aortic pressure. Hence, monitoring central aortic pressure through derivation from the radial pressure wave is superior to measurement of radial pressure alone.

Cerebral Haemodynamics: Effects of Systemic Arterial Pulsatile Function and Hypertension

Purpose of ReviewConcepts of pulsatile arterial haemodynamics, including relationships between oscillatory blood pressure and flow in systemic arteries, arterial stiffness and wave propagation phenomena have provided basic understanding of underlying haemodynamic mechanisms associated with elevated arterial blood pressure as a major factor of cardiovascular risk, particularly the deleterious effects of isolated systolic hypertension in the elderly. This topical review assesses the effects of pulsatility of blood pressure and flow in the systemic arteries on the brain. The review builds on the emerging notion of the “pulsating brain”, taking into account the high throughput of blood flow in the cerebral circulation in the presence of mechanisms involved in ensuring efficient and regulated cerebral perfusion.Recent FindingsRecent studies have provided evidence of the relevance of pulsatility and hypertension in the following areas: (i) pressure and flow pulsatility and regulation of cerebral blood flow, (ii) cerebral and systemic haemodynamics, hypertension and brain pathologies (cognitive impairment, dementia, Alzheimer’s disease), (iii) stroke and cerebral small vessel disease, (iv) cerebral haemodynamics and noninvasive estimation of cerebral vascular impedance, (v) cerebral and systemic pulsatile haemodynamics and intracranial pressure, (iv) response of brain endothelial cells to cyclic mechanical stretch and increase in amyloid burden.SummaryStudies to date, producing increasing epidemiological, clinical and experimental evidence, suggest a potentially significant role of systemic haemodynamic pulsatility on structure and function of the brain.

Frequency dependent transmission characteristics between arterial blood pressure and intracranial pressure in rats

The pulsatile energy transmission between arterial blood pressure (BP) and intracranial pressure (ICP) is affected by cerebrospinal fluid (CSF) and brain tissue. Studies in dogs have shown that the transfer function (TF) between BP and ICP shows damping of pulsatile energy around heart rate frequency (1-3Hz) with notch filter characteristics, and the amount of damping is sensitive to cerebral compliance. This investigation aimed to assess whether this notch filter characteristic is an intrinsic property of the brain enclosed in a rigid skull and therefore applies across species with a large difference in body size. This was done by determining the TF between BP and ICP in rats with corresponding significantly smaller body size and higher heart rate (5-7 Hz) compared to dogs. Arterial BP and ICP waveforms were recorded in 8 anaesthetized (urethane) adult male Sprague-Dawley rats with solid state micro-sensor transducer catheters. The TF was computed as the ratio of ICP and arterial BP waveform amplitudes for the first 4 harmonics. Arterial BP and ICP signals were normalized for pulse amplitude such that attenuation or amplification is detected for any TF values significantly different to unity. Mean cardiac frequency was 5.72 Hz (range 4.6 - 7.11 Hz). Of the 4 harmonics only the heart rate frequency band showed a statistically significant attenuation of 17%, while the higher harmonics showed a progressive amplification. Findings show that the rat brain acts as a selective frequency pulsation absorber of energy centered at heart rate frequency. This similarity with larger animals indicates a possible allometric mechanism underlying this phenomenon, with notch filter characteristic frequency scaled to body size. This study suggests that the TF between arterial BP and ICP is an intrinsic property of the brain tissue and CSF enclosed in a rigid compartment and can be used to assess changes in cerebral compliance due to abnormal CSF pressure and flow as occur in hydrocephalus.

346 PULSATILE PRESSURE/FLOW RELATIONS IN HUMAN CEREBRAL ARTERIES, DESCRIBED IN TIME AND FREQUENCY DOMAIN AS VASCULAR IMPEDANCE

Background: The concept of vascular impedance is widely used to characterise hydraulic load presented by a vascular bed, and so to interpret properties of the bed. No studies have been reported in apparently normal humans. Methods: Data were obtained from analysing flow waveforms in the Middle Cerebral (MCA), Anterior Cerebral (ACA), and Basilar (BA) Arteries in 200 subjects undergoing outpatient 24-hour blood pressure monitoring for exclusion of hypertensive disease. Flow waves in each artery were related to ascending aortic pressure waveform (AP), generated by SphygmoCorTM from the radial artery by applanation tonometry. Corresponding harmonic components of pressure ÷ flow waves provide graphs of impedance modulus. Results: Flow waveforms were similar to those seen in the renal arteries, with patterns attributable to low resistance and low reflection from the cerebral vascular bed. As in renal arteries, cerebral flow persisted throughout diastole in all and the flow wave resembled the pressure wave. Impedance phase was negative and low (0 to -0.5 radians) up to the 7th harmonic. Amplitude of the flow waves was relatively low in relation to mean flow. Resistance averaged 5.1*103 d.s.cm−3 and characteristic impedance 1.7*103 d.s.cm−3, so that reflection coefficient was 0.52 for MCA, ACA and VA combined. Conclusions: Cerebral vascular impedance in human subjects can be determined non-invasively. Patterns of pressure and flow waveform, and of impedance indicate that the cerebral vasculature, as in the kidney, acts predominantly as a low resistance bed, and that secondary pulsations in the waves arise from elsewhere in the systemic circulation.