VASCULAR ACCESS

INTRODUCTION

He who works with his hands is a laborer.
He who works with his head and his hands is a craftsman.
St. Francis of Assisi

The care of critically ill patients requires one or more pipelines to the vascular system, for both monitoring and interventions. This chapter presents some guidelines for the insertion of vascular catheters, including a brief description of the common percutaneous access routes (1-3). The emphasis here is on the craft of establishing vascular access. The labor of vascular cannulation is a skill learned at the bedside.

PREPARING FOR VASCULAR CANNULATION

HANDWASHING

Handwashing is mandatory (and often overlooked) before the insertion of vascular devices. Scrubbing with antimicrobial cleansing solutions does not reduce the incidence of catheter-related sepsis (4), so a simple soap-and-water scrub is sufficient.

UNIVERSAL PRECAUTIONS

In 1985, the Centers for Disease Control introduced a strategy for blood and body fluid precautions known as universal precautions (5). This strategy is based on the assumption that all patients are potential sources of human immunodeficiency virus (HIV) and other blood-borne pathogens (e.g., hepatitis viruses) until proven otherwise. The following recommendations apply to the insertion of vascular catheters.

Use protective gloves for all vascular cannulations.

Use sterile gloves for all cannulations except those involving the introduction of a short catheter into a peripheral vein.

Caps, gowns, masks, and protective eyewear are not necessary unless splashes of blood are anticipated (e.g., in a trauma victim). These measures do not reduce the incidence of catheter-related sepsis (6).

Avoid needlestick injuries. Do not recap needles or manually remove needles from syringes. Place all sharp instruments in puncture-resistant containers immediately after use.

If a needlestick injury is sustained during the procedure, follow the recommendations in Table 4.1.

Needlestick injuries are reported in up to 80% of medical students and interns (9). Therefore, in patients who are known risks for transmitting HIV or viral hepatitis, vascular cannulation should be performed only by an experienced senior-level resident or fellow.

LATEX ALLERGY

The increased use of rubber gloves (made of latex or vinyl) as protection against HIV infections has resulted in an increased recognition of allergic reactions to latex (10). These reactions can be manifest as a contact dermatitis (urticarial lesions of the hands and face), or as a conjunctivitis, rhinitis, or asthma. The latter three manifestations are reactions to airborne latex particles, and they do not require direct physical contact with gloves. They often appear when the affected individual enters an area where latex gloves are being used. Therefore, a latex allergy should be suspected in any ICU team member who develops atopic symptoms when in the ICU. When this occurs, a switch to vinyl gloves will eliminate the problem. Latex allergy can be manifest as anaphylaxis (10), so the transition to vinyl gloves for suspected latex allergy should not be delayed.

CLEANSING THE SKIN

Agents that reduce skin microflora are called antiseptics, whereas agents that reduce the microflora on inanimate objects are called disinfectants. Common antiseptic agents are listed in Table 4.2 (11,12). The most widely used antiseptic agents are alcohol and iodine, both of which have a broad spectrum of antimicrobial activity. Alcohol (commonly used as a 70% solution) may not work well on dirty skin (that is, it does not have a detergent action), so it is often used in combination with another antiseptic agent. The most popular antiseptic solution currently in use is a povidone-iodine preparation (e.g., Betadine), also known as an iodophor, a water-soluble complex of iodine and a carrier molecule. The iodine is released slowly from the carrier molecule, and this reduces the irritating effects of iodine on the skin. This preparation should be left in contact with the skin for at least 2 minutes to allow sufficient time for iodine to be released from the carrier molecule.

HAIR REMOVAL

Shaving is not recommended for hair removal because it abrades the skin and promotes bacterial colonization. If hair removal is necessary, the hair can be clipped or a depilatory can be applied (6).

CATHETER INSERTION DEVICES

Vascular cannulation can be performed by advancing the catheter over a needle or guidewire that is in contact with the lumen of a blood vessel.

CATHETER-OVER-NEEDLE

A catheter-over-needle device is shown in Figure 4.1. The catheter fits snugly over the insertion needle, and has a tapered end to minimize damage to the catheter tip and soft tissues during insertion. This device can be held like a pencil (i.e., between the thumb and forefinger) as it is inserted through the skin and directed to the target vessel. When the tip of the needle enters the lumen of a patent blood vessel, blood moves up the needle by capillary action and enters the flashback chamber. When this occurs, the catheter is threaded over the needle and into the lumen of the blood vessel as the needle is withdrawn.

The advantage of a catheter-over-needle device is the ability to cannulate vessels in a simple one-step procedure. The disadvantage is the tendency for the catheter tip to become frayed as it passes through the skin and soft tissues, and to subsequently damage the vascular endothelium and promote phlebitis and thrombosis. To minimize this risk, the catheter-over-needle device is usually reserved for cannulation of superficial vessels.

CATHETER-OVER-GUIDEWIRE

Guidewire-assisted vascular cannulation was introduced in the early 1950s and is often called the Seldinger technique, after its inventor. This technique is illustrated in Figure 4.2. A small-bore needle (usually 20 gauge) is used to probe for the target vessel. When the tip of the needle enters the vessel, a thin wire with a flexible tip (called a J-tip because of its shape) is passed through the needle and into the vessel lumen. The needle is then removed, leaving the wire in place to serve as a guide for cannulation of the vessel. When cannulating deep vessels, a rigid dilator catheter is first threaded over the guidewire and removed; this creates a tract that facilitates insertion of the vascular catheter.

The guidewire technique has the presumed advantage of minimizing damage to soft tissues and blood vessels by using a small-bore probe needle. However, the use of a rigid dilator catheter (as explained above) seems to nullify this advantage. Nevertheless, the guidewire technique is currently the preferred method for central venous and arterial cannulation (1,2).

THE CATHETERS

Vascular catheters are composed of plastic polymers impregnated with barium or tungsten salts to enhance radiopacity. Catheters intended for short-term cannulation (days) are usually made of polyurethane; catheters used for long-term venous access (weeks to months) are composed of a more flexible and less thrombogenic derivative of silicone. The silicone catheters (e.g., Hickman and Broviac catheters) are too flexible for routine percutaneous insertion, and are not appropriate for use in the ICU.

HEPARIN BONDING

Some vascular catheters are impregnated or coated with heparin to reduce thrombogenicity. However, this measure has not proven effective in reducing the incidence of catheter-associated thrombosis (13). Because heparin-coated catheters can be a source of heparin-induced thrombocytopenia (see Chapter 45), catheters used in the ICU should not be impregnated or coated with heparin.

CATHETER SIZE

The size of vascular catheters is commonly expressed in terms of the outside diameter, and the units of measurement are shown in Table 4.3. The French size is a metric derivative equivalent to the outside diameter in millimeters multiplied by 3; that is, French size = outside diameter (in mm) ´ 3. The gauge system was developed for wires and needles, and has been adopted for catheters. There is no simple mathematical relationship between gauge size and other units of measurement, and a table of reference values such as Table 4.3 is needed to make the appropriate conversions (14). Gauge sizes usually range from 14 (largest diameter) to 27 (smallest diameter).

As described in Chapter 1, the steady or laminar flow through a rigid tube is influenced most by the radius of the tube (see the Hagen-Poisseuille equation in the section on peripheral blood flow in Chapter 1). The influence of tube diameter on flow rate is demonstrated in Table 4.3 for gravity flow of one unit of packed red blood cells diluted with 250 mL normal saline flowing through catheters of equal length (15). Note that a little more than a doubling of tube diameter (from 0.7 mm to 1.65 mm) is associated with almost a quadrupling of flow rate (from 24.7 to 96.3 mL/minute). Thus, catheter size (diameter) is an important consideration if rapid flow rates are desired.

MULTILUMEN CATHETERS

Multilumen catheters were introduced for clinical use in the early 1980s, and are now used routinely for central venous cannulation. The design of a triple-lumen catheter is shown in Figure 4.3. These catheters have an outside diameter of 2.3 mm (6.9 French) and may have three channels of equal diameter (usually 18 gauge) or may have one larger channel (16 gauge) and two smaller channels of equal diameter (18 gauge). The distal opening of each channel is separated from the other by at least 1 cm to help minimize mixing of infusate solutions.

Multilumen catheters have proven to be valuable aids because they minimize the number of venipunctures needed for monitoring and infusion therapy, yet they do not increase the risk of thrombosis or infection when compared with single-lumen catheters (13).

INTRODUCER CATHETERS

Another valuable addition to the family of vascular catheters is the introducer catheter, shown in Figure 4.3. These large-bore catheters (8 to 9 French) can be used as conduits for insertion and removal of smaller vascular catheters (including multilumen catheters and pulmonary artery catheters) through a single venipuncture. The side-arm infusion port on the catheter provides an additional infusion line, and allows a continuous flush to prevent thrombus formation around smaller catheters that sit in the lumen of the introducer catheter. This side-arm infusion port also allows the introducer catheter to be used as a stand-alone infusion device (a rubber membrane on the hub of the catheter provides an effective seal when fluids are infusing through the side-arm port of the catheter). The large diameter of introducer catheters makes them particularly valuable as infusion devices when rapid infusion rates are necessary (e.g., in the resuscitation of massive hemorrhage).

ACCESS ROUTES

The following is a brief description of common vascular access routes in the arm (antecubital veins and radial artery), the thoracic inlet (subclavian and jugular veins), and the groin (femoral artery and vein).

ANTECUBITAL VEINS

The veins in the antecubital fossa provide rapid and safe vascular access for acute resuscitative therapy. Although long catheters can be inserted into the antecubital veins and advanced into the superior vena cava, such peripherally inserted central venous catheters (PICC devices) are more appropriate for home infusion therapy than for treating critically ill patients (16). Short catheters (5 to 7 cm) are preferred for acute resuscitation via the antecubital veins because they are more easily inserted and allow more rapid infusion rates than the longer PICC catheters.

Anatomy

The surface anatomy of the antecubital veins is shown in Figure 4.4. The basilic vein runs along the medial aspect of the antecubital fossa, and the cephalic vein is situated on the opposite side. The basilic vein is preferred for cannulation because it runs a straighter and less variable course up the arm than the cephalic vein.

Insertion Technique

The patient need not be supine, but the arm should be straight and abducted. The antecubital veins can be distended by tourniquet or by inflating a blood pressure cuff to just above the diastolic pressure (this allows arterial inflow while impeding venous outflow). Once the veins are visible or palpable, a catheter-over-needle device is used to insert a short 16- or 18-gauge catheter into the basilic or cephalic vein.

Blind Insertion

If the antecubital veins are neither visible nor palpable, palpate the brachial artery pulse at a point 1 inch above the antecubital crease. The basilic vein (or brachial vein) should lie just medial to the palpated pulse at this point, and can be entered by inserting the catheter-over-needle device through the skin at a 35° to 45° angle and advancing the needle until blood return is noted. This approach has a reported success rate of 80% (17). Injury to the median nerve (which is also medial to the artery, but deep to the veins) can occur with excessive movement of the probe needle.

Comment

Cannulation of the antecubital veins is recommended (18,19).

For rapid venous access (e.g., cardiopulmonary resuscitation)

For thrombolytic therapy in acute myocardial infarction

For trauma victims who require thoracotomy

Remember that the shorter the catheter, the more rapid the flow rate through the catheter (see Chapter 1). Thus, insertion of short catheters into the antecubital veins permits more rapid volume resuscitation than insertion of the longer central venous catheters.

RADIAL ARTERY

The radial artery is a favored site for arterial cannulation because the vessel is superficial and accessible and the insertion site is easy to keep clean. The major disadvantage of the radial artery is its small size, which limits the success rate of cannulation and promotes vascular occlusion.

Anatomy

The surface anatomy of the radial and ulnar arteries is shown in Figure 4.4. The radial artery is usually palpable at a point just medial to the styloid process of the radius. The ulnar artery is on the opposite (medial) side of the wrist, just lateral to the pisiform bone. Although the radial artery is preferred, the ulnar artery is the larger of the two arteries and should be easier to cannulate (20).

The Allen Test

The Allen test evaluates the capacity of the ulnar artery to supply blood to the digits when the radial artery is occluded. The test is performed by first occluding the radial and ulnar arteries with the thumb and index finger. The patient is then instructed to raise the wrist above the head and to make a fist repeatedly until the fingers turn white. The ulnar artery is then released, and the time required for return of the normal color to the fingers is recorded. A normal response time is 7 seconds or less, and a delay of 14 seconds or greater is evidence of insufficient flow in the ulnar artery.

Although a positive Allen test (i.e., 14 seconds or longer for return of color to the digits) is often stated as a contraindication to radial artery cannulation, in numerous instances the Allen test has indicated inadequate flow in the ulnar artery, yet subsequent radial artery cannulation has been uneventful (2,21). Thus, a positive Allen test is not a contraindication to radial artery cannulation. Another limitation is the need for patient cooperation to perform the test. Therefore, this test is not worth the time it takes.

Insertion Technique

The wrist should be hyperextended to bring the artery closer to the surface. A short 20-gauge catheter is appropriate, and can be inserted by a catheter-over-needle device or by the guidewire technique. When using a catheter-over-needle device, the following through-and-through technique is recommended: When the needle tip first punctures the artery (and blood appears in the flashback chamber), the tip of the catheter is just outside the vessel. To position the catheter tip in the lumen of the vessel, the needle is passed completely through the artery and then withdrawn until blood returns again through the needle. At this point, the catheter tip should be in the lumen of the artery, and the catheter can be advanced while the needle is retracted. If two attempts at cannulation are unsuccessful, switch to an alternative site (to reduce trauma to the vessel).

Comment

Arterial occlusion occurs in as many as 25% of radial artery cannulations, but digital ischemia is rare (2,22). Despite being well tolerated in most patients, cannulation of the radial artery (or any artery) should be reserved for monitoring blood pressure, and is not to be used as a convenience measure for monitoring blood gases or other blood components (23).

THE SUBCLAVIAN VEIN

More than 3 million central venous cannulations are performed yearly in the United States (24), and a majority of these procedures are performed via the subclavian vein (25). The subclavian vein is well suited for cannulation because it is a large vessel (about 20 mm in diameter) and is prevented from collapsing by its surrounding structures. The immediate risks of subclavian vein cannulation include pneumothorax (1% to 2%) and hemothorax ( less than 1%) (25). The incidence of bleeding is no different in the presence or absence of a coagulopathy (26); that is, the presence of a coagulation disorder is not a contraindication to subclavian vein cannulation.

Anatomy

The subclavian vein is a continuation of the axillary vein as it passes over the first rib, and the apical pleura lies about 5 mm deep to the vein at its point of origin. As shown in Figure 4.5, the subclavian vein runs most of its course along the underside of the clavicle. The vein runs along the outer surface of the anterior scalene muscle, which separates the vein from its companion artery on the underbelly of the muscle. At the thoracic inlet, the subclavian vein meets the internal jugular vein to form the brachiocephalic vein. The convergence of the right and left brachiocephalic veins forms the superior vena cava.

Anatomic Distances

The lengths of the vascular segments involved in subclavian (and internal jugular) vein cannulation are shown in Table 4.4. The average distance from venipuncture site to the right atrium is 14.5 cm and 18.5 cm for right-sided and left-sided cannulations, respectively. These distances are far shorter than catheter lengths recommended for right-sided (20 cm) and left-sided (30 cm) central venous cannulations, and are more consistent with a recent report showing that the average distance to the right atrium is 16.5 cm in central venous cannulation from either side in adults (27). Therefore, to avoid placing catheter tips in the right atrium (which can lead to cardiac perforation and fatal cardiac tamponade), all central venous catheters should be no longer than 15 or 16 cm in length (27).

INSERTION TECHNIQUE

The patient is placed supine, with arms at the sides and head faced away from the insertion site. A towel roll can be placed between the shoulder blades, but this is uncomfortable and unnecessary. Identify the clavicular insertion of the sternocleidomastoid muscle. The subclavian vein lies just underneath the clavicle where the muscle inserts onto the clavicle. The vein can be entered from either side of the clavicle.

Infraclavicular Approach (Insertion Site 1 in Figure 4.5). Identify the lateral margin of the sternocleidomastoid muscle as it inserts on the clavicle. The catheter is inserted in line with this margin, but below the clavicle. Insert the probing needle (18 or 20 gauge) with the bevel pointing upward (toward the ceiling) and advance the needle along the underside of the clavicle and toward the suprasternal notch. The path of the needle should be parallel to the patient's back. When the vein is entered, turn the bevel of the needle to 3 o'clock so the guidewire threads in the direction of the superior vena cava.

Supraclavicular Approach (Insertion Site 2 in Figure 4.5). Identify the angle formed by the lateral margin of the sternocleidomastoid muscle and the clavicle. The probe needle is inserted so that it bisects this angle. Keep the bevel of the needle facing upward and direct the needle under the clavicle in the direction of the opposite nipple. The vein should be entered at a distance of 1 to 2 cm from the skin surface (the subclavian vein is more superficial in the supraclavicular approach). When the vein is entered, turn the bevel of the needle to 9 o'clock so the guidewire threads in the direction of the superior vena cava.

Comment

Patient comfort and ease of insertion are the most compelling reasons to select the subclavian vein for central venous access. Selection of the infraclavicular versus supraclavicular approach is largely a matter of personal preference. Some recommend avoiding the subclavian vein in ventilator-dependent patients because of the risk of pneumothorax. However, the risk of pneumothorax is too small to justify abandoning the subclavian vein in patients with respiratory failure.

THE INTERNAL JUGULAR VEIN

Cannulation of the internal jugular vein reduces (but does not eliminate) the risk of pneumothorax, but introduces new risks (e.g., carotid artery puncture and thoracic duct injury).

Anatomy

The internal jugular vein is located under the sternocleidomastoid muscle in the neck and, as shown in Figure 4.5, the vein follows an oblique course as it runs down the neck. When the head is turned to the opposite side, the vein forms a straight line from the pinna of the ear to the sternoclavicular joint. Near the base of the neck, the internal jugular vein becomes the most lateral structure in the carotid sheath (which contains the carotid artery sandwiched between the vein laterally and the vagus nerve medially).

Insertion Technique

The right side is preferred because the vessels run a straighter course to the right atrium. The patient is placed in a supine or Trendelenburg position, with the head turned to the opposite side. The internal jugular vein can be entered from an anterior or posterior approach.

The Anterior Approach (Insertion Site 4 in Figure 4.5). The anterior approach is through a triangular region created by two heads of the sternocleidomastoid muscle. The carotid artery is palpated in the triangle and retracted medially. The probe needle is inserted at the apex of the triangle with the bevel facing up, and the needle is advanced toward the ipsilateral nipple, at a 45° angle with the skin surface. If the vein is not encountered by a depth of 5 cm, the needle is withdrawn 4 cm and advanced again in a more lateral direction. When a vessel is entered, look for pulsations. If the blood is red and pulsating, you have entered the carotid artery. In this situation, remove the needle and tamponade the area for 5 to 10 minutes. When the carotid artery has been punctured, no further attempts should be made on either side because puncture of both arteries can have serious consequences.

The Posterior Approach (Insertion Site 3 in Figure 4.5). The insertion site for this approach is 1 centimeter superior to the point where the external jugular vein crosses over the lateral edge of the sternocleidomastoid muscle. The probe needle is inserted with the bevel positioned at 3 o'clock. The needle is advanced along the underbelly of the muscle in a direction pointing to the suprasternal notch. The internal jugular vein should be encountered 5 to 6 cm from the skin surface with this approach (28).

Carotid Artery Puncture. If the carotid artery has been punctured with a probing needle, the needle should be removed and pressure should be applied to the site for at least 5 minutes (10 minutes is recommended for patients with a coagulopathy). No further attempts should be made to cannulate the internal jugular vein on either side, to avoid puncture of both carotid arteries. If the carotid artery has been inadvertently cannulated, the catheter should not be removed, as this could provoke serious hemorrhage. In this situation, a vascular surgeon should be consulted immediately.

Comment

As with the subclavian vein, cannulation of the internal jugular vein is safe and effective when performed by skilled operators. However, several disadvantages of internal jugular cannulation deserve mention. (a) Accidental puncture of the carotid artery is reported in 2 to 10% of attempted cannulations (28). (b) Awake patients often complain of the limited neck mobility when the internal jugular vein is cannulated. (c) In agitated patients, inappropriate neck flexion can result in thrombotic occlusion of the catheter and vein. (d) In patients with tracheostomies, the insertion site can be exposed to infected secretions that drain from the tracheal stoma.

THE EXTERNAL JUGULAR VEIN

Cannulation of the external jugular vein has two advantages: (a) There is no risk of pneumothorax, and (b) hemorrhage is easily controlled. The major drawback is difficulty advancing the catheter.

Anatomy

The external jugular vein runs along a line extending from the angle of the jaw to a point midway along the clavicle. The vein runs obliquely across the surface of the sternocleidomastoid muscle and joins the subclavian vein at an acute angle. This acute angle is the major impediment to advancing catheters that have been inserted into the external jugular vein.

Insertion Technique

The patient is placed in the supine or Trendelenburg position, with the head turned away from the insertion site. If necessary, the vein can be occluded just above the clavicle (with the forefinger of the nondominant hand) to engorge the entry site. As many as 15% of patients so not have an identifiable external jugular vein, even under optimal conditions of vein engorgement (28).

The external jugular vein has little support from surrounding structures, so the vein should be anchored between the thumb and forefinger when the needle is inserted. The bevel of the needle should be pointing upward when it enters the vein. The recommended insertion point is midway between the angle of the jaw and the clavicle (see Fig. 4.5). Use a 16-gauge single-lumen catheter that is 10 to 15 cm in length. If the catheter does not advance easily, do not force it, as this may result in vascular perforation at the junction between the external jugular and subclavian veins.

Comment

This approach is best reserved for temporary access in patients with a severe coagulopathy, particularly when the operator is inexperienced and does not feel comfortable cannulating the subclavian or internal jugular veins. Contrary to popular belief, cannulation of the external jugular is not always easier to accomplish than central venous cannulation because of the difficulty in advancing catheters past the acute angle at the junction of the subclavian vein.

THE FEMORAL VEIN

The femoral vein is the easiest of the large veins to cannulate and also does not carry a risk of pneumothorax. The disadvantages associated with this route are venous thrombosis (10%), femoral artery puncture (5%), and limited ability to flex the hip (which can be bothersome for awake patients). Contrary to popular belief, the infection rate with femoral vein catheters is no different from that of subclavian or internal jugular vein catheters (28).

Anatomy

The anatomy of the femoral sheath is shown in Figure 4.6. The femoral vein is the most medial structure in the femoral sheath and is situated just medial to the femoral artery. At the inguinal ligament, the femoral vessels are just a few centimeters below the skin surface.

Insertion Technique

Palpate the femoral artery just below the inguinal crease and insert the needle (bevel up) 1 to 2 cm medial to the palpated pulse. Advance the needle at a 45° angle to the skin surface, entering the vein at a depth of 2 to 4 cm. Once in the vessel, if the catheter or guidewire will not pass beyond the tip of the needle, tilt the needle so that it is more parallel to the skin surface (this may move the needle tip away from the far side of the vessel wall and into more direct contact with the lumen of the vessel). Femoral vein catheters should be at least 15 cm long.

Blind Insertion

If the femoral artery pulse is not palpable, draw an imaginary line from the anterior superior iliac crest to the pubic tubercle, and divide the line into three equal segments. The femoral artery lies at the junction between the middle and most medial segment, and the femoral vein is 1 to 2 cm medial to this point. This method of locating the femoral vein has a reported success rate of over 90% (29).

Comment

Femoral vein cannulation is usually reserved for patients who are paralyzed or comatose and immobile. This approach is not recommended for cardiopulmonary resuscitation (because of the delayed transit times for bolus drug injections) (18) or in patients with bleeding disorders (28).

THE FEMORAL ARTERY

Cannulation of the femoral artery is usually reserved for situations where radial artery cannulation is unsuccessful or contraindicated. Despite its reserve status, the femoral artery is larger than the radial artery, and is easier to cannulate. The complications of femoral artery cannulation are the same as for radial artery cannulation (thrombosis, bleeding, and infection). The incidence of infection is the same with radial and femoral artery catheters, and the incidence of thrombosis is lower with femoral artery cannulation (2). Thrombosis of the femoral artery, like that in the radial artery, only rarely results in troublesome ischemia in the distal extremity (2).

Localization and cannulation of the femoral artery proceeds as described in the section on femoral vein cannulation. The Seldinger technique is preferred for catheter insertion, and catheters should be 18 gauge in diameter and 15 to 20 cm long.

Comment

Femoral artery cannulation is a viable alternative and may be preferable to radial artery cannulation in patients who are paralyzed or otherwise immobile, unless they have a significant coagulopathy (in which case the radial artery is preferred). The incidence of thrombotic complications is lower in femoral artery cannulations, and the pressure in the femoral artery more closely approximates the pressure in the aorta than does the pressure in the radial artery (see Chapter 8).

IMMEDIATE CONCERNS

VENOUS AIR EMBOLISM

Inadvertent air entry is one of the most feared complications of central venous cannulation. The importance of maintaining a closed system during insertion is highlighted by the following statement:

A pressure gradient of 4 mm Hg along a 14-gauge catheter can entrain air at a rate of 90 mL/second and can produce a fatal air embolus in 1 second (30).

Preventive Measures

Prevention is the hallmark of reducing the morbidity and mortality of venous air embolism. The most effective method of preventing air entry is to keep the venous pressure more positive than atmospheric pressure. This is facilitated by placing the patient in the Trendelenburg position with the head 15° below the horizontal plane. Remember that the Trendelenburg position does not prevent venous air entry because patients still generate negative intrathoracic pressures while in the Trendelenburg position. When changing connections in a central venous line, a temporary positive pressure can be created by having the patient hum audibly. This not only produces a positive intrathoracic pressure, but allows clinicians to hear when the intrathoracic pressure is positive. In ventilator-dependent patients, the nurse or respiratory therapist should initiate a mechanical lung inflation when changing connections.

Clinical Presentation

The usual presentation is acute onset of dyspnea that occurs during the procedure. Hypotension and cardiac arrest can develop rapidly. Air can pass across a patent foramen ovale and obstruct the cerebral circulation, producing an acute ischemic stroke. A characteristic "mill wheel" murmur can be heard over the right heart, but this murmur may be fleeting.

Therapeutic Maneuvers

If a venous air embolism is suspected, immediately place the patient with the left side down, and attempt to aspirate air directly from the venous line. In dire circumstances, a needle should be inserted through the chest wall and into the right ventricle to aspirate the air. Unfortunately, the mortality in severe cases of venous air embolism remains high despite these maneuvers.

PNEUMOTHORAX

Pneumothorax is a concern primarily with subclavian vein cannulation but can also complicate jugular vein cannulation (2,30). This is one reason that postinsertion chest films are recommended after all central venous cannulations (or attempts). If possible, postinsertion films should be obtained in the upright position and during expiration. Expiratory films facilitate the detection of small pneumothoraxes because expiration decreases the volume of air in the lungs, but not the volume of air in the pleural space. Thus, during expiration, the volume of air in the pleural space is a larger fraction of the total volume of the hemithorax, thereby magnifying the radiographic appearance of the pneumothorax (31).

Upright films are not always possible in ICU patients. When supine films are necessary, remember that pleural air does not often collect at the apex of the lung when the patient is in the supine position (32,33). In this situation, pleural air tends to collect in the subpulmonic recess and along the anteromedial border of the mediastinum (see Chapter 28).

Delayed Pneumothorax

Pneumothoraxes may not be radiographically evident until 24 to 48 hours after central venous cannulation (31,33). Therefore, the absence of a pneumothorax on an immediate postinsertion chest film does not absolutely exclude the possibility of a catheter-induced pneumothorax. This is an important consideration in patients who develop dyspnea or other signs of pneumothorax in the first few days after central venous cannulation. In the absence of signs and symptoms, there is little justification for serial chest films following central venous catheter placement.

CATHETER TIP POSITION

The properly placed central venous catheter should run parallel to the superior vena cava, and the tip of the catheter should be positioned above the junction of the superior vena cava and right atrium. The following conditions warrant corrective measures.

Tip Against the Wall of the Vena Cava

Catheters inserted from the left side must make an acute turn downward when they reach the superior vena cava. If they fail to make this turn, the catheters can end up in a position like the one shown in Figure 4.7. The tip of the catheter is up against the lateral wall of the vena cava, and in this position, the catheter tip can stab the vessel wall and perforate the vena cava. Therefore, catheters that abut the wall of the vena cava should be repositioned as soon as possible. (The problem of vascular perforation is discussed in more detail in Chapter 5.)

Tip in the Right Atrium

The Food and Drug Administration has issued a strong warning about the risk of cardiac perforation from catheter tips that are advanced into the heart (24). However, cardiac perforation is a rare complication of central venous cannulation (27), even though over half of central venous catheters may be misplaced in the right atrium (27). Nevertheless, tamponade is often fatal, so cardiac placement of catheters should be avoided. A few measures help to minimize the risk of cardiac perforation. The most effective measure is to use shorter catheters, as recommended earlier. The tip of indwelling catheters should be above the third right costal cartilage (this is the level where the vena cava meets the right atrium). If the anterior portion of the third rib cannot be visualized, keep the catheter tip at or above the tracheal carina.