Central venous catheters: how, when, why? (Proceedings)

Central venous catheters are catheters that are inserted so that the tip of the catheter rests within a central vein, usually the cranial or caudal vena cava.  These catheters are usually longer catheters, with a larger gauge, and may be inserted either peripherally or centrally.  In the context of hospitalized patients, central catheters allow the sampling of blood without repeated venipuncture, and may be used to infuse hyperosmolar or irritant solutions, or to measure central venous pressure.  Central catheters that are inserted peripherally (ie. saphenous vein) are termed peripherally-introduced central catheters or PICC lines.  Catheters that are inserted into the cranial vena cava via the jugular vein are just termed central venous catheters (CVCs).

The types of catheters that may be used as central catheters are generally of a more robust design than catheters designed for peripheral use, although in small patients, a long peripheral catheter placed in the jugular vein may reach the cranial vena cava.  In larger patients, the catheters of choice are made of polyurethane, which has the additional benefit of stiffness at room temperature that becomes softer as the catheter warms to body temperature.  The catheters range in length from 12 cm. to 30 cm, and some PICC lines may be as long as 55 or 60 cm.  The catheters may also have multiple lumens (usually 2 or 3), which allow the co-administration of incompatible solutions through the same catheter (the lumens are contained within the catheter, but are discrete paths, until they are infused into the patient).  The additional lumens are laid beside the central lumen, and empty out 2-5 cm from the tip.  

Catheters with multiple lumens have a larger diameter than regular peripheral catheters.  Because they are not circular, the width of the catheter is measured in French, rather than gauge.  The smallest catheters with multiple lumens are usually 4 fr (approximately 18 g., which contains two 22 g. lumens), and the one most commonly used in cats and small dogs at the University of Georgia is a 5.5 fr. 13 cm. triple lumen catheter, which has a comparative width of about 15 g.   These catheters are placed primarily in the jugular vein of cats and small dogs.  Larger dogs will usually tolerate a 7 fr. catheter (available as double or triple lumen, approximately 13 g.) placed in the jugular or lateral saphenous veins.  The tip of jugular central catheters should be placed into the cranial vena cava, just proximal to the right atrium.  This distance can be estimated, and is the basis for choosing different lengths of catheters.  If necessary, once placement has occurred, radiographs may be taken to confirm placement.  This location is most important for measuring central venous pressure; if the catheter is only to be used for infusion or sampling, the actual location is less critical (as long as it's not too long and ends up sitting in the right heart).

The placement of a central venous catheter should be performed using aseptic conditions, including a sterile clip and preparation of the area.  Central venous catheters can be placed via percutaneous, facilitated percutaneous (ie. cut down), or surgical cut down techniques.  The individuals placing the catheter should at minimum wear sterile gloves, and consideration should be given to a surgical gown, mask, and cap as well, especially if a cutdown procedure is utilized.  To place a central catheter percutaneously, after the skin over the vein of interest has been aseptically prepared, a small stab incision is made in the skin, and a regular peripheral catheter (preferably one that is 1 ½ to 2 inches long) is inserted into the vein.  Using a modified Seldinger technique, a sterile guide-wire is fed up into the vein, effectively holding the place for the catheter.   Once the wire has been placed into the vein, the peripheral catheter is removed, and a tissue dilator is placed over the wire.  The purpose of the dilator is to clear any fascia or debris from the path that will be occupied by the catheter; the dilators are usually tapered to a similar diameter as the catheter.  Once the vessel has been dilated, the dilator is removed, and the central lumen of the catheter is threaded onto the wire (which is still inside the pet).  The wire will provide a pathway for the catheter to follow as it is inserted into the patient.  Once the catheter is in place, the wire is removed, and the lumens are aspirated of air and flushed with heparinized saline.  The catheter is then secured to the patient.  It is important during this procedure that the person placing the catheter or the assistant always maintains a hand on the guidewire.  This will prevent accidental insertion of the wire into the patient's general circulation.

Because the polyurethane central catheters have a stronger construction than regular peripheral catheters, as long as they are placed in an aseptic manner and cared for with regular (daily) bandage changes, these catheters may stay in place for up to 2-3 weeks.  Because of this, they are the catheter of choice for patients who are anticipated to be in hospital for long periods of time, or who will need regular blood sampling for the duration of hospitalization.  The catheters themselves are more expensive than peripheral catheters, but this initial investment will pay off in terms of secure long-term venous access. 

Uses for central venous catheters

The central venous circulation is fast moving, and irritating substances are quickly diluted by the flowing blood.  For this reason, substances that are hyperosmolar, or that might otherwise carry a risk of thrombophlebitis if administered in a peripheral vein, can be safely infused into the central circulation.  This includes infusions such as total parenteral nutrition (TPN) or diazepam.  As noted above, the presence of multiple lumens in some central venous catheters allows co-infusion of compounds that would otherwise be considered incompatible; they are rapidly diluted at the catheter exit points, and cannot react with the other infusions. 

By virtue of the central circulation, blood sampled from these CVCs is more representative of the overall body than blood dawn from peripheral vessels.  Central catheters may be used to collect anaerobic samples to measure central venous blood gas values.  Specifically of interest is the central venous oxygen saturation (ScvO2), which can give an idea of the oxygen usage by the body; if the ScvO2 is low (<60%), the implication is that the body is extracting a large amount of oxygen from the arterial blood, and may benefit from interventions that will increase systemic oxygen delivery.  By contrast, if the ScvO2 is high (>80%), there is a concern that the body is not appropriately extracting oxygen from the arterial blood, and there may be a shunt or distributive problem (e.g. septic shock).  ScvO2 may be increased if the patient is breathing an increased oxygen percentage, so this should also be taken into account.  Similar to the ScvO2, the measurement of central venous lactate may be more accurate than the measurement of peripheral or arterial lactate.  Sometimes, the action of holding off a peripheral vessel for blood sampling can cause an elevation of lactate in the sample.

The central venous circulation is made up of the venous capacitance vessels, which can expand to store a large amount of the body's blood.  As a patient becomes hypervolemic, due to physiologic problems (e.g. heart failure or acute renal failure) or due to iatrogenic causes (e.g. excessive intravenous fluid administration), the venous side will fill with blood and the pressure on the venous side (usually between 0-5 mm Hg) will start to increase.  Another benefit of having a central venous catheter in place is that it allows for the measurement of this pressure.  The central venous pressure (CVP) is generally thought to be accurate only when using jugular central venous catheters. 

CVP can be measured using either a saline-filled manometer (which will measure in cm H2O), or using an arterial blood pressure transducer (which will measure in mm Hg).  The transducer will require a monitor that is able to read and display the pressure, while the manometer method only requires some extension sets and a bag of saline, and can be read without special equipment.  In either method, the saline is used to create a constant column of fluid from the central vein to the manometer or transducer, by first filling the manometer, and then allowing the saline in the manometer to equilibrate with the patient.  It is important to measure this pressure with the zero point of the manometer level with the patient's right heart.  The pressure in the column will vary with the cardiac cycle, and so an average pressure is used to describe the CVP.  Normal ranges for CVP are 0-8 cm H2O or 0-5 mm Hg.  CVP consistent with hypervolemia will read from 10-15 cm H2O or > 8-10 mm Hg.

Although the measurement of CVP will always generate a number, it is best utilized as a dynamic measure; in the evaluation of a patient, a small fluid bolus (ie. 10-15 mL/kg of crystalloid fluid, or 5 mL/kg of colloid) is administered to the patient, and the changes in CVP are followed.  In normovolemic animals, there will be a transient (10-15 min) rise in CVP by about 3 cm H2­O, which then returns to the baseline.  If the CVP does not change at all, the patient is likely hypovolemic.  If the CVP rises and does not decrease, there may be excessive intravascular volume (or decreased cardiac contractility). 

Although a venous pressure may be measured by PICC catheters with the tip in the caudal vena cava, normal ranges have not been established.  Pressure measurements from these catheters are best used serially, to compare the pressures before and after specific interventions, and, used in this manner, these are valuable numbers for directing patient care.

Complications associated with central venous catheters

Central venous catheters are extremely useful for guiding treatment of critically ill patients.  There are a number of factors that need to be taken into account prior to placement of the catheters, however.  One of the primary determinants for placement of central venous catheters (jugular vs. saphenous) is the presence of coagulopathy.  If a patient is coagulopathic, jugular venipuncture should be avoided, and the catheter should be placed in a vein that is easy to tamponade if hemorrhage occurs.  Other factors that influence placement site are the presence of any skin infections that would preclude percutaneous catheter placement and maintenance, or the presence of central nervous system disease that is associated with high intracranial pressure.  In the latter case, jugular venipuncture and jugular vein catheterization are contraindicated.  Other complications of central catheter placement include incorrect placement, either too far into the heart or too short where the catheter tip does not reach the thoracic cavity.  Both errors should be preventable by measuring the patient ahead of time and choosing the most appropriately sized catheter.  If the catheter is inserted into the heart, it may be gently backed out the appropriate distance until it is located within the cranial vena cava. 

Indwelling catheters may become infected, and may serve as a nidus of infection in animals after placement.  If a fever of unknown origin is detected in a patient with a central catheter, it is generally recommended that the catheter be removed, if other sources of infection are not identified.  There is also a general feeling that jugular vein central catheters may predispose hypercoagulable animals to the development of thromboses, either of the jugular vein, or in the form of a thromboembolism into the pulmonary arterial system (pulmonary thromboembolism, PTE).  It is unknown whether this risk is present for catheters in the caudal vena cava, but in patients that are suspected to be hypercoagulable, the risks and benefits of a central line should be weighed.