Hemodynamic Aspects of DCSFs

C O N T E N T S

10.5Hemodynamics and Pathophysiology in CSFs 297

10.6Flow Velocity and Pressure Measurements in DCSFs 300

Comments 303

References 305

10.1 Introduction

The study of blood flow in arteriovenous shunting lesions goes back to Shenkin et al. (1948), who were able to demonstrate a flow velocity elevated by a factor of three from the ICA into the IJV in patients with arteriovenous malformations (AVMs). Murphy (1954) defined the “steal phenomenon” associated with clinical findings in AVMs such as seizures or psychic alterations. He concluded that the AVM shunt perfusion works at the expense of cerebral tissue perfusion. Further research by several investigators focused mainly on measure-

ments of regional blood flow (Feindel et al. 1967;

Haeggendal et al. 1965; Ingvar and Lassen 1972;

Lassen et al. 1963).

Nornes (1979, 1980) pioneered the use of Doppler ultrasound to measure flow velocity in cerebral arteries and measured intraoperatively elevated perfusion pressures up to 50%. The use of Doppler techniques for more detailed assessment of blood flow in AVMs is extensively described in the monograph by Hassler (1986). He studied flow characteristics in AVMs using transcranial Doppler sonography (TCD) as well as intravascular probes by measuring flow and pressure intraoperatively before and after removal. Nornes (1972) also used Doppler measurements in the management of five patients with CSFs (four traumatic, one spontaneous). He found “steal flow” ranging from 90– 975 ml/min, forward flow rates in the ICA between 40–170 ml/min and reverse flow between 35–60 ml/ min. Nornes used the ratio of reverse flow/forward flow as an indicator of sufficient collateral capacity of the cerebral circulation in the case of permanent ICA occlusion.

Many studies on hemodynamics of CSFs utilized transophthalmic ultrasound for detecting and monitoring patients with AV fistulas (Kawaguchi et al. 2002; de Keizer 1982, 1986) or duplex sonography of the ICA and ECA flow (Lin et al. 1994; Chen et al. 2000). Lack of proper imaging tools is a reason for the scarce literature on AV shunting flow in CSFs that mainly focused on flow pattern on the arterial side preand post embolization. Very little is known about arteriovenous shunt flow and pressure in DAVFs in general, or in DCSFs in particular. Data on intrasinus flow and pressure is mostly lacking. Criteria for hemodynamic classifications are mainly based on the speed of contrast filling in angiograms, and thus are to a large degree subjective and impossible to quantify (see Sect. 4.3).

10.2

Basic Hemodynamic Principles

Hagen-Poiseuille’s law is a third cornerstone law and is written as:

Blood flow through vessels follows the laws and principles of fluid dynamics, whose key parameters are as follows:

Vessel radius: r

Vessel cross-sectional area: A

Velocity: ν

Flux of fluid: Q

Viscosity: μ

Density: ρ

Resistance: Rs

Pressure gradient: P

L = Tube length: L

ρνL

Reynolds number: Re = μ

The flow rate Q depends on the difference in pressure between both ends of a tube and the resistance to that flow. Only 2% of the heart action is transformed into kinetic energy, while 98% is used to overcome frictional force (Nornes and Grip 1980).

The flow rate varies with the fourth power of its radius and is inversely proportional to its length and to the viscosity. Thus, the vessel’s diameter is a critical factor for regulating blood flow in the human body. This law, however, is only strictly applicable to rigid, straight tubes with constant diameter and homogenous fluids under laminar (as opposed to turbulent) flow conditions. In reality blood is inhomogeneous and flows through compliant vessels with changing diameters and curvatures. It has a variable viscosity changing with shear rate and the concentration of red blood cells (hematocrit). Under normal temperatures and pressures and a 40% hematocrit, blood viscosity is approximately four times that of water. Very low flow shear rates may result in relative values of more than 1000 or “prestasis” (Hassler 1986).

Flow and resistance (Rs) are inversely proportional to each other (Eq. 2) and for Hagen-Poi- seuille’s flow, the resistance to the flow is written as:

Conservation of mass embodied in the continuity equation is one of the most basic concepts in fluid dynamics and thus in hemodynamics (SECCA and GOULAO 1998). To maintain constant flow, when the diameter of the tube gets smaller, the velocity will increase. More precisely,

Conservation of energy is another key concept in the physics of flow and can be expressed in Bernoulli’s equation:

This equation is valid only for fluids without viscosity, and therefore does not directly apply to blood flow. However, it has been usefully applied to the estimation of pressure drops in larger arteries (carotid, aorta). Viscosity of blood is inversely related to temperature.

In a closed vessel under constant pressure gradient, the flow will increase with a larger radius, decreasing length and a lower viscosity (Secca and

Goulao 1998).

In arteriovenous shunting lesions velocity is increased and the flow may become unsteady or turbulent, especially when the Reynolds number Re exceeds 400. The flow becomes less laminar and develops marginal swirls; laminar flow completely disappears for a Re above 2,000 (Hassler 1986). While studies on hemodynamics in brain AVMs have been of interest for a number of investigators (Nornes and Grip 1979, 1980; Miyasaka et al. 1994; Norbash et al. 1994; Hassler and Thron 1994; Duckwiler et al. 1990), there is only scant research of this type in DAVFs or DCSFs.

10.3

Invasive Assessment of Hemodynamics

In order to study hemodynamics in intracranial arteriovenous shunting lesions, direct invasive measurements of arterial and venous flow velocities were performed by the author during embolizations of AVMs, DAVFs and DCSFs (Benndorf et al. 1994a,b, 1995). In addition, simultaneous pressure measurements in draining sinuses of AVMs (Benndorf et al. 1994b) or DCSFs (Benndorf and Wellnhofer 2002) were recorded.

For assessments of flow and pressure, two different sensor-tipped micro-guidewires were used:

1.The FloWire system from Cardiometrics (Mountain View, CA) was developed in the 1990s for invasive studies of coronary blood flow velocities, preand post angioplasty (Serruys et al. 1993; Di Mario et al. 1995; Labovitz et al. 1993; Wellnhofer et al. 1997). A few investigators utilized this technology for hemodynamic studies in other vascular territories, mainly the cerebrovascular circulation (Benndorf et al. 1994a, 1995, 1997; Murayama et al. 1996; Henkes et al.

1993).

This Doppler guidewire consists of a 0.014s (0.036 mm) micro-guidewire, onto which a Doppler probe is mounted with a sample volume of 5 mm, provided by a 10-MHz transducer. The “SmartWire” was initially a modified version of the FloWire (Doppler Wire) with a more flexible tip for use in the tortuous cerebral circulation (Fig. 10.1). The SmartMap enabled real-time display and recording of flow velocity and flow pattern. The system is currently manufactured by Volcano (Laguna Hill, CA) and meanwhile offers a new Smart-II Wire for pressure measurements and a ComboWire (ComboMap) for assessment of coronary flow reserve (CFR) and fractional flow reserve (FFR). The latest wire versions have a special core for better trackability, a PTFE coating and are compatible with standard 18-microcatheters. The following parameters can be recorded:

MPV = Maximum (over pulsatile cycle) peak velocity

APV = Average (over pulsatile cycle) peak velocity

PI (CPI) = Pulsatility index (PI) = (IPV max-IPV mean) / APV

IPV = Instantaneous peak velocity

2.The PressureWire (Radi Medical Systems AB, Uppsala Sweden) is a guidewire-mounted high-

fidelity fiberoptic pressure sensor, located 3 cm proximal to the shapeable radiopaque tip (Fig. 10.1). Similar to the FloWire the PressureWire was also initially developed for intracoronary use (Di Mario et al. 1993, 1995; Gorge et al. 1993) and applied to other territories to only some degree (Benndorf et al. 1994b; Abildgaard et al. 1995). It was intended for use with a PGA interface that provided pressure values, but no curve. The latest version (Certus) comes as a hydrophilic-coated 0.014s(0.036 mm) guidewire in 175 cm/300 cm length and measures pressures between 30 and 300 mmHg. Using the principle of thermodilution, monitoring intravascular temperature is possible and flow velocities can be calculated, although a waveform is not obtained. The wire can also be advanced through various standard microcatheters (e.g. Tacker-Excel). The initial version of Radi’s PressureWire was relatively stiff compared to the FloWire and could not be advanced through the carotid siphon. Flexibility may be improved with the latest versions, but has not been tested by the author for this purpose.

The above described wire configurations with two sensors have only recently become available. In earlier studies, two separate wires (one for flow, another for pressure) had to be used. Both systems are employed to measure pressure gradients, intracoronary flow and the fractional flow reserve (FFR) that is currently used as a standard diagnostic tool in cardiac catheterization laboratories (Wellnhofer et al. 1997; Abildgaard et al. 1995; Marques et al. 2002; Nisanci et al. 2002; Alfonso et al. 2000; Briguori et al. 2001).

A few reports describe non-coronary interventions (Mahmud et al. 2006; Cavendish et al. 2008) measurements of carotid artery pressure during angiography (Kanazawa et al. 2008) and even assessment of cerebrospinal fluid pressure in Chiari I animal models (Turk et al. 2006).

In order to obtain data on venous flow and pressure in AV shunting lesions, the author used the two sensor-tipped guidewires, which were simultaneously or alternatingly advanced into the great dural cerebral sinuses, such as the sigmoid sinus (SS), transverse sinus (TS), straight sinus (StS), superior sagittal sinus (SSS) or the CS.

Fig. 10.1 a–h. Sensor-tipped guidewires for invasive measurements of pressure and flow. a Original 0.014s FloWire®/SmartWire® (arrow, Cardiometrics) carrying a miniaturized 10 Mhz Doppler probe next to a standard 0.014s guidewire, here introduced into 0.018s microcatheters as used by the author. b A 5 mm sample volume of the Doppler probe. c, d Magnified views of the currently available ComboWire® with two sensors: Doppler probe at the tip and pressure sensor either with 1.5 cm offset or next to the flow sensor (arrows, Volcano Therapeutics Inc., Laguna Hills, CA). e, f Scheme of the current SmartWire® Floppy and the SmartMap® (ComboMap®) for real time monitoring of pressure and flow. g, h Recent version of the 0.014s (0.36 mm) PressureWire® (Radi Medical Systems AB, Sweden) with the opening for the sensor 3 cm proximal to the tip, and the RadiAnalyzer. Using thermodilution, flow data can also be estimated

g

h