A central line is defined by the presence of the tip of the catheter in the thoracic part of the cranial or caudal vena cava. The most common access point for a centrally inserted venous line (CVL) in dogs and cats is the external jugular vein. A peripherally inserted central line (PICC) is placed via the medial (cat) or lateral (dog) saphenic vein.

There are several indications and advantages to placement of a central line:

 A CVL offers ease of vascular access and its access point and fixation materials can be kept clean more easily than with a peripheral line.

 It offers the possibility to collect blood. This is especially of interest when repeated sampling is necessary.

 Hypertonic fluids cause phlebitis. If a fluid has an osmolality > 600 mOsm/l this is considered a good reason to use a central line. Therefore, total parenteral nutrition must be administered via a dedicated central line.

 A central line offers the opportunity to measure central venous pressure.

 It offers a better opportunity to make use of multilumen catheters and the advantage of separate access lines with dedicated 'tasks'.

 Central lines are better tolerated by the patient than peripheral catheters.

Risks and Contraindications

Instead of 'absolute' and 'relative' contraindications, the author prefers to think of benefits and risks as a balance that can tip to either side in different situations: a risk-benefit analysis is mandatory. The formation of a clot and emboli is a serious risk. It often occurs unnoticed but the consequences can be potentially disastrous. The risk of clot formation depends on how hypercoagulable the patient is.

Much has been said about the 'safe' time period for which a catheter may be in place before there is a high risk that a catheter infection may develop. No such period exists and the choice is an arbitrary one. Preparation of the insertion site, catheter insertion, handling and maintenance are all important factors that can have an effect on this 'safe' period.

Several other risks and complications exist, such as bleeding and air embolism, but overall these problems are relatively rare or with little consequence.

Placement

For an uncomplicated and smooth placement it is important that all equipment and disposables are present and 'ready to go' prior to the start of the procedure. A checklist with a description of materials and their preparation should be part of the protocol.

Catheters differ in material, design, thrombogenicity, diameter, length, etc. 'Through-the-needle' catheters or modifications of these with introduction needles that split length-wise ('peel-away') are long enough to position the catheter tip in the thoracic vena cava and are often used in veterinary medicine.

The guide wire catheter or Seldinger technique has become more popular in recent years although the aseptic placement of this catheter is more cumbersome and time consuming, often necessitating sedation or general anaesthesia.

There are four major advantages of using the Seldinger technique:

 Ideal for smaller veins

 Available as multilumen catheters

 No leakage of blood from the venous puncture site

 Easy exchange of the catheter

'Over-the-needle' catheters can be used in small patients and offer an easy alternative for a guide wire catheter. Central venous pressure (CVP) measurement and repeated blood collection is often impossible. Its relative rigidity can cause problems such as kinking, vessel wall damage and clot formation.

Catheters can be held in place by the use of tape, tape and sutures (butterfly technique), or suture alone (Chinese finger trap). The technique used depends on the catheter and personal preference.

Central Venous Pressure Measurement

The central venous pressure (CVP) is the pressure measured in the thoracic vena cava and is a reflection of the end-diastolic right ventricular pressure/volume. CVP is primarily measured to guide and monitor fluid therapy. It is influenced by blood volume, venous tone and compliance, cardiac function and intrathoracic pressure. The value of a singular measurement is limited, but repeated measurement of CVP to establish trends has merit when monitoring fluid therapy.

There are two methods to measure CVP:

 The water manometer system is cost effective and easy. Besides a central line, only a ruler, a piece of infusion tubing, a three-way stopcock and a syringe with an electrolyte solution are necessary.

 Via a blood pressure monitor, a dedicated infusion set with a piezoelectric sensor that functions as a pressure transducer, and a pressurised fluid bag.

Regardless of the method used, the position of the patient and the zero reference point have to be the same with every measurement to establish trends. With the water manometer method the extension tubing is connected to the catheter, filled with solution and placed in an upright position along the ruler, starting at the bottom. The water column in the extension tubing will drop until it is in equilibrium with the pressure at the tip of the catheter. Larger fluctuations in the meniscus of the water column are associated with ventilation and small, more frequent fluctuations result from the heart action. Measurement of CVP is performed at the end-expiratory pause.

Values in hypovolaemic patients may range from -5 to +3 cmH₂O. A normal CVP range is 0–5 cmH₂O with a measurement of 7–10 often indicating adequate fluid loading in the absence of cardiac failure. An increased CVP can be caused by volume overload, cardiac failure, pleural or pericardial effusion, pneumothorax, pulmonary oedema, pulmonary thromboembolism and pulmonary hypertension.

Acknowledgement

The author acknowledges Chiara Valtolina DipACVECC.

References

1.  Anonymous. Monitoring critical patients. I: Cardiovascular monitoring. D: Central venous pressure (CVP). In: Macintire DK, Drobatz KJ, Haskins SC, Saxon WD, eds. Manual of Small Animal Emergency and Critical Care. Philadelphia: Lippincott Williams and Wilkins, 2005:74.

2.  Davis H. Central venous catheterization. In: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. St. Louis, MI: Saunders Elsevier, 2009:267–270.