The effects of increased intracranial pressure on flow through major cerebral arteries in vitro

Evaluation of transcranial Doppler flow velocity changes in intracerebral hemorrhage rats using ultrasonography

This study investigated the blood flow velocity changes in seven major arteries in rat brain before and after intracerebral hemorrhage (ICH) using high frequency transcranial Doppler (TCD) ultrasonography (13-4 MHz). Eighteen adult Sprague Dawley rats received either the collagenase-injection surgery (ICH, n=12) or the saline-injection surgery (control, n=6) after baseline TCD flow velocity values were recorded. The TCD flow velocity changes were measured at 0.5h after the surgery and daily for the following 8 days in seven major cerebral arteries, including bilateral internal carotid arteries, bilateral middle cerebral arteries (MCAs), bilateral posterior cerebral arteries (PCAs), and basilar artery. The results showed a significant decrease of TCD flow velocity in the right MCA at 0.5 h and 24 h after the collagenase-injection surgery, and in the right PCA at 0.5 h in the ICH group rats. The TCD flow velocities in these two arteries gradually increased and then returned to the baseline values in the following days. The control group rats did not show significant changes in TCD flow velocity in all monitored arteries. This study demonstrates the feasibility and reliability of monitoring TCD flow velocity in cerebral arteries using ultrasonography technique in a rat ICH model. The results of this study extend our knowledge in the cerebrovascular changes during intracranial hemorrhage and suggest a possibility of clinical application of TCD ultrasonography in studying the dynamic cerebral circulation after strokes. Moreover, this method could be extensively applied in further studies using potential neuroprotective treatments that affect the cerebral dynamics in the intracerebral hemorrhage.

Arterial transducer placement and cerebral perfusion pressure monitoring: a discussion

AIM

To discuss existing disparity of practice and clinical implications of measuring cerebral perfusion pressure (CPP) from differing reference points thus highlighting the need for standardized benchmarks.

BACKGROUND

When managing traumatic brain injury (TBI), the arterial transducer level is a key to an accurate CPP reading; however, there is a lack of national standards about where to zero arterial transducers when monitoring CPP.

METHODS

A systematized search using the Cochrane library database, Pubmed database, Medline, British Library on line, CINAHL and PROQUEST using key search terms was used to identify articles that could form a basis for a discussion. Papers published between 2000 and 2008 were included. Papers that did not discuss arterial transducer level placement and CPP were excluded. The Brian Trauma Guidelines 2007 were scrutinized for recommendations.

RESULTS

Of 57 empirical studies accessed, none reported or explored the placement of the arterial transducer during CPP measurement. Conflicting opinions were identified within the literature and there were no recommendations made for practice within the Brain Trauma Foundation Guidelines 2007.

DISCUSSION

At the present time, there is insufficient evidence for recommending standard placement for mean arterial pressure (MAP) measurements for patients with TBI. There are implications to consider as the treatment prescribed will differ depending on where the arterial transducer is placed because the MAP and CPP displayed will fall by 15 mm Hg at a head elevation of 30 degrees. This poses a number of questions: is the CPP underestimated with the arterial transducer placed at head level? Is the CPP overestimated if the transducer is placed at mid axilla level?

RECOMMENDATIONS

Further research is recommended. However, studies would be difficult to power as head-injured patients constitute a heterogeneous population. Professional consensus should be applied and standardized benchmarks agreed.

[Biomechanics and intracranial hypertension]

Biomechanics can be defined as the physical concepts exploring the effects of mechanical forces in biology. Biomechanics explores the consequences of an alteration in the steady-state normally existing between living tissues and their immediate environment. The application of this science in the field of brain physiopathology and intracranial hypertension is essential to understanding the sequence of events triggered during an alteration of the intracranial volume such as observed with intracranial pathologies. Furthermore, biomechanics allows the development of a logical and rational therapeutic approach, better adapted to the various intracerebral problems anaesthesiologists might be confronted with.

Cerebrospinal fluid pulse waveform as an indicator of cerebral autoregulation

Systems analysis of the systemic arterial (SAPW), cerebrospinal fluid (CSFPW), and sagittal sinus (SSPW) pulse waves was carried out in 13 dogs during hypercapnia (5% CO2), intracranial normotension (inhalation of 100% O2), and intracranial hypertension (inhalation of 100% O2 plus an intraventricular infusion). Power amplitude and phase spectra were determined for each wave, and the power amplitude and phase transfer functions calculated between the cerebrospinal fluid (CSF) pressure and systemic arterial pressures, and between the sagittal sinus pressure and CSF pressure. The study indicates that the CSFPW and SSPW were virtually identical when impedance between the cerebral veins and sagittal sinus was minimal, which argues that the CSF pulse was derived from the cerebral venous bed. During inhalation of 100% O2, transmission of the SAPW across the precapillary resistance vessels into the cerebral venous pulse (as represented by the CSFPW) was nonlinear, while transmission across the lateral lacunae into the sagittal sinus was linear. During intracranial hypertension, wave transmission across the precapillary resistance vessels was linear, and across the lateral lacunae was nonlinear. During hypercapnia, wave transmission across the precapillary resistance vessels and the lateral lacunae was linear. When the wave transmission was nonlinear, there was also suppression in transmission of the lower harmonics, particularly the fundamental frequency, and a more positive phase transfer function, suggesting an inertial effect or decrease in acceleration of the pulse. Conversion from a nonlinear to linear transmission across the precapillary resistance vessels is evidence of loss of vasomotor tone, and is accompanied by rounding of the CSFPW. A vascular model which encompasses the above data and is based on flow in collapsible tubes and changes in vasomotor tone is posited to explain control of pulsatile flow and pulse waveform changes in the cerebrovascular bed. The model helps to clarify the strong interrelationship between intracranial pressure, cerebral blood flow, and cerebral autoregulation.

Chronic cerebral arterial spasm. The role of intracranial pressure

The author used isolated rabbit common carotid and femoral arteries perfused at a constant pressure of 90 mm Hg to examine the variation of flow (F) with transmural pressure (TMP). When the TMP was reduced below 50 to 60 mm Hg in arteries with normal smooth muscle tone, arterial resistance increased significantly causing a reduction in flow. It is suggested that the diffuse arterial narrowing that occurs in patients with severe intracranial hypertension may be the result of a similar reduction in TMP. In the presence of active vasoconstriction, any increase in extraluminal (intracranial) pressure (ICP) resulted in a substantial increase in arterial resistance and subsequent reduction of flow. This F-TMP relationship depended only on the initial degree of constriction and was independent of the vasoconstrictor used to achieve this constriction and of the artery in which this constriction was produced. A review of the literature suggests that human cerebral arteries normally exhibit only mild constrictions in response to subarachnoid blood during the chronic phase of spasm. In the present study, a mild constriction in the absence of increased ICP or a moderate increase in ICP (45 mm Hg) in the absence of constriction produced minor reductions in arterial diameter and an average flow reduction of only 5% to 10%. However, when ICP was increased to 45 mm Hg in the presence of a mild constriction, severe arterial narrowing resulted and flow was reduced by 50%. Therefore, it is suggested that chronic arterial spasm is the result of a mild constriction which is amplified by the simultaneous occurrence of increased ICP. Phenoxybenzamine was found to be effective in reversing and preventing these contractions. The improvement in flow produced by phenoxybenzamine decreased as the TMP was reduced below 60 mm Hg. The effects of both diffuse and local spasm on cerebral blood flow are discussed.

Effects of increased arterial pressure on blood flow in the damaged brain

The effect of induced arterial hypertension on cerebral blood flow and intracranial pressure was measured before and after the production of a standard cryogenic brain lesion in 10 anaesthetized, ventilated baboons. Before injury the animals were divided into a group with intact autoregulation, having more than 20% increase in cerebrovascular resistance during arterial hypertension, and a group with impaired autoregulation, in which the change in cerebrovascular resistance was much less. The cryogenic injury produced a rapid rise in intracranial pressure and a reduction of cerebral blood flow in the affected hemisphere. Despite this, there was an increase in cerebrovascular resistance during arterial hypertension in all animals after brain injury, accompanied by a further significant rise in intracranial pressure. It is suggested that this response is unlikely to represent normal physiological autoregulation and caution should be exercised in interpreting it as such in the course of studies of cerebral blood flow in patients with acute brain damage.