fatal when at least 200–300 milliliters of air is introduced within a few seconds. the consequences of this include: acute embolic stroke (from air that passes through a patent foramen ovale), pulmonary edema, and acute right heart failure (from trapped air in the right ventricle) which can lead to cardiogenic shock. the clinical presentation of a venous air embolism may be silent. in those who are symptomatic, the most common symptoms are sudden-onset shortness of breath and cough. if the presentation is severe, the patient may become rapidly hypotensive and have an altered level of consciousness due to cardiogenic shock. symptoms of an acute stroke may also be seen. echocardiography can be used to visualize air that has become trapped in the chambers of the heart. if a large air embolism is suspected, a syringe can be attached to the catheter cap and pulled pack in an attempt to remove the air from circulation. the patient can also be placed in the left lateral decubitus position. it is thought that this position helps relieve air that has become trapped in the right ventricle. catheter-related thrombosis catheter-related thrombosis (crt) is the development of a blood clot related to long-term use of cvcs. it mostly occurs in the upper extremities and can lead to further complications, such as pulmonary embolism, post-thrombotic syndrome, and vascular compromise. symptoms include pain, tenderness to palpation, swelling, edema, warmth, erythema, and development of collateral vessels in the surrounding area. however, most crts are asymptomatic, and prior catheter infections increase the risk for developing a crt. routine flushings may help to prevent catheter thrombosis. if there is catheter obstruction, thrombolytic drugs can be used if the obstruction is caused by clots or fibrin deposition. anticoagulant treatment is indicated if the obstruction is caused by thrombus formation. there is inadequate evidence whether heparin saline flush is better than normal saline flush to maintain central venous catheter patency and prevent occlusion. insertion before insertion, the patient is first assessed by reviewing relevant labs and indication for cvc placement, in order to minimize risks and complications of the procedure. next, the area of skin over the planned insertion site is cleaned. a local anesthetic is applied if necessary. the location of the vein is identified by landmarks or with the use of a small ultrasound device. a hollow needle is advanced through the skin until blood is aspirated. the color of the blood and the rate of its flow help distinguish it from arterial blood (suggesting that an artery has been accidentally punctured). within north american and europe, ultrasound use now represents the gold standard for central venous access and skills, with diminishing use of landmark techniques. recent evidence shows that ultrasound-guidance for subclavian vein catheterization leads to a reduction in adverse events. the line is then inserted using the seldinger technique: a blunt guidewire is passed through the needle, then the needle is removed. a dilating device may be passed over the guidewire to expand the tract. finally, the central line itself is then passed over the guidewire, which is then removed. all the lumens of the line are aspirated (to ensure that they are all positioned inside the vein) and flushed with either saline or heparin. a chest x-ray may be performed afterwards to confirm that the line is positioned inside the superior vena cava and no pneumothorax was caused inadvertently. on anteroposterior x-rays, a catheter tip between 55 and 29 mm below the level of the carina is regarded as acceptable placement. electromagnetic tracking can be used to verify tip placement and provide guidance during insertion, obviating the need for the x-ray afterwards. a central venous catheter secured to the skin with suture chest x-ray with catheter in the right subclavian vein the outline of superior vena cava on a chest x-ray is labeled at left. catheter flow hagen–poiseuille equation the hagen–poiseuille equation describes the properties of flow through a rigid tube. the equation is shown below: q = δ p ∗ ( π r 4 / 8 μ l ) /8\mu l)} the equation shows that flow rate (q) through a rigid tube is a function of the inner radius (r), the length of the tube (l), and the viscosity of the fluid (μ). the flow is directly related the fourth power of the inner radius of the tube, and inversely related to the length of the tube and viscosity of the fluid. this equation can be used to understand the following vital observations regarding venous catheters: that the inner radius of a catheter has a much greater impact on flow rate than catheter length or fluid viscosity, and that for rapid infusion, a shorter, large bore catheter is optimal because it will provide the greatest flow rate. types the percutaneous cvc is inserted directly through the skin. the subclavian (left), internal (right) or external jugular, or femoral vein is used. there are several types of central venous catheters; these can be further subdivided by site (where the catheter is inserted into the body) as well as the specific type of catheter used. percutaneous central venous catheter (cvc) a percutaneous central venous catheter, or cvc, is inserted directly through the skin. the internal or external jugular, subclavian, or femoral vein is used. it is most commonly used in critically ill patients. the cvc can be used for days to weeks, and the patient must remain in the hospital. it is usually held in place with sutures or a manufactured securement device. commonly used catheters include quinton catheters. peripherally inserted central catheters (picc) picc line inserted in the upper arm (through the basilic vein)main article: peripherally inserted central catheter a peripherally inserted central catheter, or picc line (pronounced "pick"), is a central venous catheter inserted into a vein in the arm (via the basilic or cephalic veins) rather than a vein in the neck or chest. the basilic vein is usually a better target for cannulation than the cephalic vein because it is larger and runs a straighter course through the arm. the tip of the catheter is positioned in the superior vena cava. picc lines are smaller in diameter than central lines since they are inserted in smaller peripheral veins, and they are much longer than central venous catheters (50–70 cm vs. 15–30 cm). therefore, the rate of fluid flow through picc lines is considerably slower than other central lines, rendering them unsuitable for rapid, large volume fluid resuscitation. piccs can easily occlude and may not be used with phenytoin. picc lines may also result in venous thrombosis and stenosis, and should therefore be used cautiously in patients with chronic kidney disease in case an arteriovenous fistula might one day need to be created for hemodialysis. however, picc lines are desirable for several reasons. they can provide venous access for up to one year. the patient may go home with a picc. they avoid the complications of central line placement (e.g. pneumothorax, accidental arterial cannulation), and they are relatively easy to place under ultrasound guidance and cause less discomfort than central lines. picc lines may be inserted at the bedside, in a home or radiology setting. it is held in place with sutures or a manufactured securement device. subcutaneous or tunneled central venous catheter tunnelled cvc tunneled catheters are passed under the skin from the insertion site to a separate exit site. the catheter and its attachments emerge from underneath the skin. the exit site is typically located in the chest, making the access ports less visible than catheters that protrude directly from the neck. passing the catheter under the skin helps to prevent infection and provides stability. insertion is a surgical procedure, in which the catheter is tunnelled subcutaneously under the skin in the chest area before it enters the svc. commonly used tunneled catheters include hickman, and groshong, or broviac catheters and may be referred to by these names as well. a tunnelled catheter may remain inserted for months to years. these cvcs have a low infection rate due to a dacron cuff, an antimicrobial cuff surrounding the catheter near the entry site, which is coated in antimicrobial solution and holds the catheter in place after two to three weeks of insertion. implanted central venous catheter (icvc, port a cath) main article: port (medical) implanted central venous catheter implanted port gripper needle inserted in port an implanted central venous catheter, also called a port a cath or port a catheter, is similar to a tunneled catheter, but is left entirely under the skin and is accessible via a port . medicines are injected through the skin into the catheter. some implanted ports contain a small reservoir that can be refilled in the same way. after being filled, the reservoir slowly releases the medicine into the bloodstream. surgically implanted infusion ports are placed below the clavicle (infraclavicular fossa), with the catheter threaded into the heart (right atrium) through a large vein. once implanted, the port is accessed via a "gripper" non-coring huber-tipped needle (powerloc is one brand, common sizes are 0.75 and 1 inch (19 and 25 mm) length; 19 and 20 gauge. the needle assembly includes a short length of tubing and cannula) inserted directly through the skin. the clinician and patient may elect to apply a topical anesthetic before accessing the port. ports can be used for medications, chemotherapy, and blood sampling. as ports are located completely under the skin, they are easier to maintain and have a lower risk of infection than cvc or picc catheters. an implanted port is less obtrusive than a tunneled catheter or picc line, requires little daily care, and has less impact on the patient's day-to-day activities. port access requires specialized equipment and training. ports are typically used on patients requiring periodic venous access over an extended course of therapy, then flushed regularly until surgically removed. if venous access is required on a frequent basis over a short period, a catheter having external access is more commonly used. triple-lumen catheter the most commonly used catheter for central venous access is the triple lumen catheter. they are preferred (particularly in the icu) for their three infusion channels that allow for multiple therapies to be administered simultaneously. they are sized using the french scale, with the 7 french size commonly used in adults. these catheters typically have one 16 gauge channel and two 18 gauge channels. contrary to the french scale, the larger the gauge number, the smaller the catheter diameter. although these catheters possess one 16 gauge port, the flow is considerably slower than one would expect through a 16 gauge peripheral iv due to the longer length of the central venous catheter (see section on "catheter flow" above). it is important to note that the use of multiple infusion channels does not increase the risk of catheter-related blood stream infections. hemodialysis catheter hemodialysis catheters are large diameter catheters (up to 16 french or 5.3mm) capable of flow rates of 200–300 ml/min, which is necessary to maintain the high flow rates of hemodialysis. there are two channels: one is used to carry the patient's blood to the dialysis machine, while the other is used to return blood back to the patient. these catheters are typically placed in the internal jugular vein. introducer sheaths introducer sheaths are large catheters (8–9 french) that are typically placed to facilitate the passage of temporary vascular devices such as a pulmonary artery catheter or transvenous pacemaker. the introducer sheath is placed first, and the device is then threaded through the sheath and into the vessel. these catheters can also serve as stand-alone devices for rapid infusion given their large diameter and short length. when paired with a pressurized infusion system, flow rates of 850 ml/min have been achieved. routine catheter care triluminal catheter the catheter is held in place by an adhesive dressing, suture, or staple which is covered by an occlusive dressing. regular flushing with saline or a heparin-containing solution keeps the line open and prevents blood clots. there is no evidence that heparin is better than saline at preventing blood clots. certain lines are impregnated with antibiotics, silver-containing substances (specifically silver sulfadiazine) and/or chlorhexidine to reduce infection risk. specific types of long-term central lines are the hickman catheters, which require clamps to make sure that the valve is closed, and