Peripherally Inserted Central Venous Catheters

© Springer-Verlag Italia 2014

Sergio Sandrucci and Baudolino Mussa (eds.)Peripherally Inserted Central Venous Catheters10.1007/978-88-470-5665-7_2

2. Which Material and Device? The Choice of PICC

Enrico de Lutio¹  

(1)

Via P. Carleni, 2, Amelia (TR), 05022, Italy

Enrico de LutioConsultant

Email: [email protected]

2.1 Introduction

Until a few years ago, the medium- to long-term central venous catheters (CVC) were essentially silicone (SR) catheters. Silicone was the material of choice for the vascular access devices (VADs) including PICCs.

Nowadays, the newest polyurethanes (PUR) are available, and their features increase therapy options accomplishable with these devices with indubitable advantages for patient’s outcome.

It is fair to say that there are no data from clinical studies that prove the superiority of one of those two materials in all the situations, but there are circumstances in which it makes sense to choose one of the materials for its superiority with regard to particular aspects.

So, therefore, the clinical implications of material properties can certainly influence vascular access device (VAD) selection with regard to ease of insertion, mechanical phlebitis, flow rates, infusate compatibility, extravasation of infusates, catheter occlusion, clotting and thrombosis, catheter weakening and embolization, vascular damage, stability and durability, and catheter maintenance requirements.

2.2 PICC Structure and Materials

Every venous catheter is made up of two parts: the intravascular part and the extravascular one.

The intravascular segment is usually made of one of the two abovementioned materials, while the extravascular segment can have different materials due to the several components like clamps and the hub.

The knowledge of the material of such components is important because it can be damaged by the liquids used for skin disinfection, for the scrubbing of the needleless valved connectors, etc.

There is a huge variety of materials (Table 2.1

Table 2.1

Materials

) that must be taken into right consideration.

2.3 Silicone (SR)

Silicone materials have been used in medicine for almost six decades. Available in a variety of material types, they have unique chemical and physical properties that manifest in excellent biocompatibility and biodurability for many applications [1].

Their high biocompatibility is explained with their remarkable chemical stability that enables biocompatibility in many long-term implant applications.

Silicones are a general category of synthetic polymers whose backbone is made of repeating silicon to oxygen bonds. In addition to their links to oxygen to form the polymeric chain, the silicone atoms are also bonded to organic groups, typically methyl groups. This is the basis for the name silicones, which was assigned by Kipping based on their similarity with ketones, because in most cases, there is on average one silicone atom for one oxygen and two methyl groups [2]. Later, as these materials and their applications flourished, more specific nomenclature was developed. The basic repeating unit became known as siloxane, and the most common silicone is polydimethylsiloxane, abbreviated as PMDS (Fig. 2.1):

Fig. 2.1

Silicone and PUR formulation

Polysiloxanes are polymers made up of a chain of small repeating units with an end group. They tend to be chemically inert due to the strength of the silicon-oxygen bond.

Furthermore, surface-active additives can be mixed with the polymer, or added to the ends of the polymer chain, to modify its surface properties [3].

For medical applications, the silicone must be medical grade. Medical-grade silicone has special requirements that are different from industrial-grade silicone. The major difference is that the silicone manufacturers have a special silicone grade that they manufacture for medical application. These silicones are tested for biocompatibility and meet the necessary regulatory body requirements. Any additives such as color must also be medical grade. Medical silicone applications are divided into two classes: restricted and unrestricted. Normally, restricted is referred to as short-term implantable and unrestricted is referred to as long-term implantable.

2.4 Polyurethane (PUR)

Otto (Friedrich) Bayer (1902–1982) and coworkers discovered and patented the chemistry of polyurethanes in 1937, and the first true medical-grade elastomeric polyurethanes were patented in1960 by du Pont (Pierce et al.) called Biomer® (trade name Lycra) [3].

Today, polyurethanes (PURs) are a class of polymer, which has achieved industrial relevance due to their rough and elastomeric properties and good fatigue resistance and has been employed in biomaterial applications such as artificial pacemaker lead insulation, catheters, and vascular grafts [1].

Polyurethanes consist of a class of materials with widely varying physical and chemical properties.

The commonality is the urethane linkage between polymer chains made up of isocyanate (hard segment, aromatic, or aliphatic), macroglycol (soft segment, polyesters; polyethers or polycarbonates), and chain extender (diols; diamines) (Fig. 2.2).

Fig. 2.2

Uretahne linkage

The hard segments are glassy or crystalline with the use of temperature, while the other segment – the soft segment – is rubbery.

Polyurethane hard segments consist of either aliphatic (carbon polymer backbones with single bonds between Cs) or aromatic (backbones with single and double bonds) di-isocyanates.

The isocyanate monomers are toxic, and their removal is essential for use in PURs for biomedical applications.

With regard to soft segments, for catheter applications, polyether and polycarbonate soft segments are used.

PURs with polycarbonate soft segments are more resistant to attack by biological enzymes and hydrolysis than those made with polyethers.

Polyurethane properties are notoriously difficult to control because of the number of components in the polymer and the requirement for tight control of polymerization conditions.

Some catheter manufacturers require prefiltering of all components to assure purity of monomers.

The final polymer is also filtered to assure a consistent molecular weight and tight molecular weight distribution.

Each lot is individually inspected to assure conformance with specifications and is only released after review of all documentation.

PUR materials are varied by altering the composition of the hard and soft segments, as well as the length of the segment chains.

The processing of the polymer also plays a role in determining the properties of the finished catheter.

Examples of commercially available PUs used in medical applications are in the Table 2.2

Table 2.2

PUR brand names

.

2.5 SR vs. PUR Catheter Material Properties

Once showed the physical and chemical differences between silicones and PUs, we can now see what such differences implicate in terms of functions of all CVCs including PICCs.

First of all, let us look at the physical tests and material properties which are used to evaluate and describe the strength and resilience of catheter tubing to certain environmental conditions [3].

Some of the most common of these are:

Tensile strength

Burst strength

Flow rates

Kink resistance

Durometer (hardness)

Flexural modulus (stiffness)

Environmental stress cracking

Solvent resistance

Drug compatibility

2.5.1 Tensile Strength

The ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking [1, 2, 4, 5].

In the case of catheters, a measure of the maximum force that can be applied to the catheter before it breaks.

Silicone catheters have much less tensile strength than polyurethane catheters with identical dimensions.

2.5.2 Burst Strength

Pressure at which a film or sheet (e.g., of paper or plastic) will burst. Used as a measure of resistance to rupture, burst strength depends largely on the tensile strength and extensibility of the material.

In the case of catheters, the burst strength is the pressure applied to a catheter lumen of a closed catheter that causes it to leak.

Silicone catheters have lower burst strength than polyurethane catheters with identical dimensions.

2.5.3 Flow Rates

Because SR has lower tensile and burst strength than PUR catheters of equal dimensions, the wall thickness of SR catheters is increased to provide adequate strength.

Consequently, for the same catheter French size, SR catheters have a smaller lumen and lower flow rate than PUR catheters.

Flow is proportional to r4, so very small changes in inside diameter dimension – especially of small-diameter catheters – have a very large effect on flow rates.

2.5.4 Kink Resistance

Kink resistance is the ability of the catheter to maintain an open lumen when bent.

SR catheters bend more easily than polyurethane and, in general, can be bent to larger angles before kinking but kink with less applied force than PUR catheters. SR catheters also recover more readily or are not permanently deformed as easily as are PUR catheters.

2.5.5 Durometer (Hardness)

Durometer is one of several measures of the hardness of a material.

Hardness may be defined as a material’s resistance to permanent indentation. The durometer scale was defined by Albert F. Shore, who developed a measurement device called a durometer in the 1920s. The term durometer is often used to refer to the measurement, as well as the instrument itself. Durometer is typically used as a measure of hardness in polymers, elastomers, and rubbers.

Useful because it is easily determined and reflects catheter feel – i.e., whether it is soft and pliable or stiff.

SR catheters, in general, are made of materials with lower durometer than

PUR catheters and are, therefore, floppier, even though they have greater wall thickness.

2.5.6 Flexural Modulus (Stiffness)

A measure of the ease with which a catheter can be bent.

SR materials have lower flexural moduli than PUR materials, and consequently SR catheters are usually less stiff.

The easiest way to compare is to let equal lengths of catheters extend over a table and see which one bends more under the force of gravity.

2.5.7 Environmental Stress Cracking

When subject to repeated cycles of chemical and mechanical stress, cracks can form in the surface of a material and grow into the bulk, eventually leading to failure.

SR is less prone to stress cracking than PUR because it is cross-linked and is also more resistant to attack by common antiseptic and cleaning preparations (Fig. 2.3).

Fig. 2.3

Materials and their resistance to various chemical environments

2.5.8 Solvent Resistance

Solvent resistance is a measure of the ability of a material to retain its properties when exposed to chemicals.

SR is more resistant to solvents in general, because it is cross-linked. SR catheters swell, but do not break in most solvents. SR’s hydrophobicity limits its attack by water.

2.5.9 Drug Compatibility

It is important to know that catheters are not attacked by drugs, but by the solvents necessary to put them into solution or to preserve them [3]. Therefore, drug compatibility is based upon the solvent or carrier compatibility with the polymer.

Drugs diffuse through SR catheters to a lesser extent than most PUR catheters (dependent on chemical composition and structure of the PUR), because drugs can only diffuse while in solution (Table 2.3

Table 2.3

Chemicals and their impact on PUR

).

2.5.10 Radiopacity

Radiopacity is a function of the amount of radiopaque material in the fluoroscopic image of the catheter. Smaller-diameter catheters or catheters loaded with a lower concentration of radiopaque agent have less presence and present therefore appear dimmer.

Radiopacity is achieved thanks to the addition of radiopaque agents (i.e., BaSO4) to the SR or PUR, but they weaken catheter materials by increasing their rigidity.

Imaging of thicker-walled catheters is better than that of thinner-walled catheters if loading is the same.

2.5.11 Biocompatibility

Biocompatibility is related to the behavior of biomaterials in various contexts.

There are several definitions of biocompatibility:

The ability of a material to perform with an appropriate host response in a specific application, Williams’ definition or The quality of not having toxic or injurious effects on biological systems [6, 7].

Comparison of the tissue response produced through the close association of the implanted candidate material to its implant site within the host animal to that tissue response recognized and established as suitable with control materials. ASTM (American Society for Testing and Materials)

Refers to the ability of a biomaterial to perform its desired function with respect to a medical therapy, without eliciting any undesirable local or systemic effects in the recipient or beneficiary of that therapy, but generating the most appropriate beneficial cellular or tissue response in that specific situation, and optimizing the clinically relevant performance of that therapy [8].

Biocompatibility is the capability of a prosthesis implanted in the body to exist in harmony with tissue without causing deleterious changes.

With regard to venous catheters, the main biological issue for catheters is hemocompatibility and, to a lesser extent, compatibility with tissue contacted to access the vessel lumen.

Hemocompatibility is a complex issue. Depending on how it is defined, on the patient population, disease state, catheter entrance site, and other factors (phase of the moon?), one catheter material can be said to perform better, or worse, than another.

For short-term applications, in general, there are no noticeable differences between PUR and SR catheters.

For longer-term applications, durability may be more important than biocompatibility.

Once the differences between silicone and polyurethanes in terms of material properties are seen, let us take a look at the following figure that recaps those differences (Table 2.4

Table 2.4

Differences between materials and properties

).

2.6 Clinical Implications of Material Properties That Influence Catheter Selection

For the ultimate purpose of this book, the simple listing of the properties of the SR vs. PUR catheter material would be pretty useless if those properties were not associated with their clinical implications, assessed in terms of:

Ease of insertion

Mechanical phlebitis

Flow rates

Infusate compatibility

Extravasation of infusates

Catheter occlusion

Clotting and thrombosis

Catheter weakening and embolization

Vascular damage

Stability/durability

Catheter maintenance requirements

2.6.1 Ease of Insertion

The ease of insertion is influenced by catheter stiffness, wall thickness, and frictional properties of catheter surface.

In general, SR catheters are more difficult to advance over the guidewire than PUR catheters of same dimensions because they have higher surface friction to the guidewire.

For other catheters than venous accesses or PICCs, it can be compensated for by applying hydrophilic coating on the guidewire or jacketing guidewires in Teflon®.

The PICCs are inserted with the modified Seldinger technique in which the guidewire is only used for a few centimeters and just for the introduction of the Peel-Away introducer and therefore the abovementioned possible issues are not a