Arterial Puncture and Cannulation

Arterial Versus Venous Analysis

Arterial sampling has been the traditional approach to evaluating acid-base abnormalities in critically ill patients, especially those being maintained on a ventilator. In most ED settings, however, venous blood gas analysis will suffice. Studies have demonstrated that analysis of venous blood (especially central venous blood) for pH, bicarbonate, lactate, base excess, and carbon dioxide pressure (P co ₂ ) are within 95% limits of agreement with arterial sampling and can safely supplant it. On the other hand, arterial blood sampling is still required for accurate analysis of oxygen pressure (P o ₂ ).

Arterial Puncture With a Needle/Syringe

To obtain a single sample of arterial blood by the percutaneous method, attach a 3-mL syringe (preferred and most common) to a needle. Select the needle size based on puncture location and patient size and age. For an adult, use a 20-gauge, 2.5-inch needle for a femoral sample and a 22-gauge, 1.25-inch needle for a radial artery puncture. For pediatric arterial sampling, use a needle with a slightly shorter length in the range of 22- to 24-gauge at the same sites as in adults.

Precoated blood gas plastic syringes (with dry lithium heparin) are commonly used and allow a longer shelf life and ready use ( Fig. 20.1 ). Such devices are designed to minimize sampling error as a result of heparin. If necessary, prepare a regular syringe with 1 or 2 mL of a heparinized saline solution (1000 International Units [IU]/mL) drawn into the syringe to coat the barrel and needle. Fully eject the heparin through the needle immediately before skin puncture to minimize heparin-related errors. Although the syringe may appear devoid of heparin, enough heparin remains in the needle and syringe to provide anticoagulation. Even dry heparin may produce abnormalities in ABG results because of a heparin-induced dilutional effect.

The latest blood gas and chemistry analyzers require only 0.2 mL of whole blood for accuracy, and some point-of-care devices can perform analyses on single drops of blood. However, sample sizes of less than 1.0 mL of blood aspirated into heparin-coated syringes may result in a heparin-related error on ABG values.

Stored heparin solution has higher P o ₂ and lower P co ₂ values than blood does. A dilutional effect from heparin would mean that the addition of 0.4 mL of heparin solution to a 2-mL sample of blood (dilution of 20%) will lower P co ₂ by 16%. Proper technique with dry lithium heparin-prefilled syringes or full ejection of excess heparin will prevent such problems if more than 2 mL of blood is collected. A falsely low P co ₂ is the most clinically significant change caused by excess heparin. Neither P o ₂ nor pH levels are significantly altered by the addition of heparin in most instances, although a slight increase in P o ₂ and a minimal decrease in pH may occur if high concentrations of heparin (25,000 IU/mL) are used. If 2 to 3 mL of blood is collected, heparin-related effects are likely to be clinically inconsequential.

Continuous Monitoring via Arterial Catheter

The fluid-filled recording systems used with arterial cannulation have a great influence on the accuracy of pressure measurements. The frequency responses of tubing, transducers, and other components of the monitoring system influence the accuracy of systolic and diastolic pressure measurement. Failure to recognize recording system artifacts will lead to errors in interpretation of the pressure.

Various catheter types have demonstrated similar frequency-response characteristics, but some studies have found different complication rates. Teflon catheters may carry an increased rate of thrombosis. Another contributing element leading to thrombosis is catheter diameter; the incidence of thrombosis is inversely related to the ratio of vessel lumen to catheter diameter. Thus the risk for thrombosis increases as the diameter of the catheter decreases. The incidence of thrombosis also increases with increased duration of catheter placement. In contrast, a higher risk for thrombosis was seen in the femoral artery than in the radial artery in a study involving a pediatric population. Catheters coated with a combination of chlorhexidine and silver sulfadiazine have produced lower infection rates.

Preparation for Arterial Cannulation

Box 20.1 lists the usual equipment for arterial cannulation, although the majority of prepackaged kits contain the supplies most needed (see also Review Box 20.1 ). Shorter catheters are ideal for peripheral artery cannulation, whereas use of a longer catheter and the Seldinger technique is preferable for the femoral artery.

For arterial cannulation in adults, use a 16- to 18-gauge catheter for the femoral artery and a 20-gauge catheter for the radial artery ( Fig. 20.2 ). Small children and infants require a 22- to 24-gauge catheter, which may need to be inserted percutaneously via the Seldinger technique or through a femoral cutdown. Based on patient size, older pediatric patients usually require 20- to 22-gauge catheters.

The tubing that connects the catheter to the pressure transducer has a significant effect on accuracy of the monitoring system. The higher the frequency response of the entire system, the more accurate the determination of systolic and diastolic pressure; however, artifact also becomes more of a problem. Use stiff, low-capacitance plastic tubing for arterial catheterization and monitoring. Place the electronic pressure transducer connection as close as possible to the patient and zero it appropriately because the frequency response of a tube is inversely related to its length.

The pressure wave produced with each contraction is transmitted from the artery through the catheter and connecting tubing to a measuring device. The arterial fluid wave is received by an electromechanical transducer that changes the mechanical pressure wave into an electrical signal that can be displayed on the monitor. The most basic technique for obtaining blood pressure values involves the use of a simple manometer. This system can be assembled quickly if the material is available.

A continuous method of flushing the pressure tubing is required to maintain patency of the catheter lumen during intraarterial pressure monitoring. A three-way stopcock through which the tubing is intermittently flushed with saline (a minimum of every 15 to 30 minutes) is a simple, effective method. Continuous flush devices push a set amount of fluid (usually 2 to 3 mL/hr) through the line. A typical monitoring system that includes this device is shown in Fig. 20.3 . The pressure transducer must be mounted at the level of the patient's heart. Current pressure-monitoring setups include not only built-in stopcocks but also in-line flushing plungers to facilitate clearance of blood after sampling.

Intravascular transducers were initially seen as an improvement over the external electromechanical transducers in use since the mid-1970s. Many of the numerous brands are fragile, temperature sensitive, of variable quality, and much more difficult to place in vessels than catheters are. Despite anecdotal reports of fibrin deposition on these devices, no increased incidence of thrombus formation has been noted. The most important advantages of intravascular transducers are the ability to continuously monitor ABG values and to eliminate potential errors induced by catheters, stopcocks, and connecting tubing.