Chapter 89: Arterial Line Monitoring and Placement

Arterial catheterization is one of the most frequently performed invasive procedures performed on critically ill patients. It is generally considered to be a safe procedure with few serious complications and a major complication rate ranging between 1% and 5%.^1,2,3,4 Although arterial catheterization was traditionally performed by physicians, contemporary practice in many organizations allows credentialing for this procedure to be performed routinely by nonphysician providers including nurse practitioners, certified registered nurse anesthetists, and physician assistants. Arterial line placement remains a readily acceptable intervention for unstable patients requiring continuous monitoring of blood pressure, frequent blood sampling, and blood gas analysis.^1,3,4,5 Newer technologies for hemodynamic monitoring such as measurement of stroke volume variation and cardiac output are also facilitated by the presence of an arterial line. This chapter will review general principles of arterial line placement, monitoring, and care.

In the majority of hospitalized patients, non-invasive indirect monitoring of blood pressure by auscultation of Korotkoff sounds is sufficient. However, in critically ill and hemodynamically unstable patients indirect techniques may underestimate blood pressure¹; thus the need for more intensive blood pressure monitoring via arterial catheterization may be beneficial. Historically, the indications for placement of arterial lines included: (1) continuous beat-to-beat monitoring of blood pressure; (2) frequent sampling of blood for laboratory analysis and monitoring of ventilatory impairment; (3) arterial administration of drugs such as thrombolytics; and (4) use of an intra-aortic balloon pump.^1,3 These remain compelling indications for placement of arterial catheters, however technological advances in contemporary design of catheter and monitoring systems now allow arterial lines to be used for more advanced hemodynamic monitoring, including real-time calculation of cardiac output, stroke volume, and evaluation of fluid responsiveness in suspected hypovolemic states.¹ The modern practitioner requires adequate knowledge of new technologies and data interpretation in order to effectively use these new modalities to enhance patient care and delivery.

The waveform seen on bedside monitors is a visual representation of intravascular fluid dynamics as a result of rhythmic pulsation of blood generated by cardiac systole. Changes in intravascular pressure are transmitted through rigid, fluid-filled tubing that propagates the pressure wave to a transducer. This transducer converts the pressure wave from a mechanical process (displacement of fluid) into an electrical signal that is, in turn, amplified, processed, and represented on the monitor as a readily recognizable and characteristic wave. As a result of different pressures through arteries of varying circumference and distance from the heart, the visual representation of the waveform on the monitor will be different based on which artery the catheter has been placed (see Figure 89–1).

The basic equipment needed for the placement of an arterial catheter includes (1) a flexible catheter, which selection (long vs short) will depend on site selection (femoral vs radial vs axillary); (2) sterile gown and gloves, hair cap, mask, and drape; (3) sterile connector tubing to attach to the monitoring system; (4) a 2.0 silk suture or tape; (5) a clear biocclusive dressing; and (6) a monitoring system with pressure transduction tubing. A bedside ultrasound device may be used to identify vessels prior and during insertion of the arterial catheter. Ultrasound guidance may be beneficial in technically challenging procedures, or if there is known or suspected anatomic deviation.

Commercially available arterial catheter kits are present in most organizations. These kits are customizable and contain the equipment routinely used in arterial catheter insertion. The advantages of using customized kits include efficient storing of supplies used for arterial cannulation and avoidance of the need for the operator to gather all the supplies independently. These commercially available kits usually offer supplies needed for placement via in-line guidewire/catheter systems, as well as via the modified Seldinger technique described below.

Proper monitoring of arterial waveforms requires positioning, calibration, and zeroing of the transducer system in order to prevent false elevations in blood pressure measurement or artificial dampening of the waveform. Zeroing of the transducer is accomplished by opening a stopcock located proximal to the transducer to ambient air, followed by pressing the “zero” button on the bedside monitor. This provides the transducer with a pressure reference value (atmospheric pressure) against which intravascular pressure can be measured. Once this is done, the pressure tracing should rest on the zero line of the monitor and a pressure value of zero should be demonstrated. Errors in zeroing the transducer will not result in the desired pressure equilibration; this may occur from technical difficulty related to user error or from electronic difficulty due to the phenomenon of “zero drift.” Zero drift is, literally, electronic malfunction of the transducer, transduction cable attached to the monitor, or of the monitor itself, which results in artificial offset of the arterial waveform from the zero line. Sequential manual replacement of each element is indicated to systematically troubleshoot the electronic components.

The arterial transducer system must be calibrated to a point where the monitor accurately reflects the mechanical displacement of blood through the artery. If the system is over- or under-responsive to the amplitude of the pulse wave, it will give a falsely elevated or damped waveform.¹ The test most commonly used to determine the accuracy of the damping coefficient and resonant frequency of the tubing-transducer-monitor system is the fast-flush test.¹ This is performed by briefly flushing the system using the manual flush device and observing a square wave while the flush is in progress, followed by a return to the arterial waveform with one or two discrepant waveforms that may vary in amplitude.¹ A larger number of irregular waveforms corresponds to an underdamped or overdamped system that will provide inaccurate arterial pressure monitoring (Figure 89–2).

The transducer system must be leveled to a point parallel with the midaxillary line of the patient. This is easily estimated by visual inspection, limits technical challenge, and is approximate to the level of the patient’s heart.¹ This plane allows for accurate measurement of hydrostatic pressure within the heart. Additionally, this allows for correlation with other measurements of cardiac filling pressures obtained from devices with catheter tips in the great vessels or intracardiac chambers,¹ such as central venous pressure measurement and hemodynamic measurements obtained from a pulmonary artery catheter. Failure to level the catheter to the desired plane being monitored may generate spuriously low or high pressure readings based on whether the transducer is lower or higher than the desired position, with a degree of inaccuracy proportional to the height offset (Figure 89–3).

Site selection is the first consideration for arterial cannulation. Common sites for placement include the radial, brachial, axillary, pedal, and femoral arteries; the radial, femoral, and axillary sites are the most frequently cannulated.^3,4 All of these arteries, in the absence of specific patient complications, are of suitable circumference to hold the arterial catheter. However, each of these sites has advantages and disadvantages related to patient comfort during the insertion and once the catheter is in place. For instance, radial catheterization requires little in the way of positioning for insertion, but may leave the affected hand with limited mobility due to the presence of the catheter and tubing. Axillary cannulation is comfortable for the patient, but requires the arm to be immobilized in an unnatural position throughout the procedure. The femoral artery site is arguably the easiest to cannulate and provides an easy access in an emergent situation, but carries the highest risk for infection. Additionally, femoral catheterization severely limits mobility and may prevent ambulation in the alert patient.

The cannulation of deep arteries is frequently achieved using the modified Seldinger technique. This involves the use of a large, hollow introducer needle that is inserted into the artery. The angle, depth, and technique of insertion vary depending on the specific location. A 3-milliliter syringe is attached to the needle prior to insertion. Once the needle penetrates the skin, the syringe is aspirated while the needle is slowly advanced. The operator will recognize that the needle has entered the artery when brisk, pulsatile flow of bright red blood has been obtained. The syringe is then unscrewed while the needle is stabilized with the nondominant hand, and pulsatile flow is seen from the needle. A guidewire is then inserted through the needle, after which the needle is removed. The catheter is then passed over the guidewire, which is then subsequently removed.

Regardless of location, it is vital to maintain visualization of the guidewire while it remains in the patient in order to prevent inadvertent loss within the vessel. Once the catheter is successfully placed in the artery, it should be attached to the tubing–transducer system. Confirmation of an arterial waveform should be noted on the bedside monitor. The catheter should be secured with a suture or tape and an occlusive dressing with antimicrobial properties should be placed over the insertion site.

A flexible board or roll of gauze is placed under the wrist in order to obtain dorsiflexion before the arm is abducted and the hand is secured to a flat surface for stability and immobilization with tape. Local anesthesia is achieved with 1% lidocaine infiltrated laterally and medially to the pulsation of the artery. Administration of lidocaine directly into the artery will result in vasospasm, which may preclude placement. Following topical anesthesia, the radial pulse is palpated with either the index or middle finger of the non-dominant hand until the maximal pulsation is felt. The needle is then inserted at a 15° to 30° angle and advanced slowly until return of bright red, pulsatile blood is noted. If using a commercially prepared needle with in-line guidewire and catheter, the guidewire is then advanced into the artery, and the catheter advanced over the wire. The needle–wire device is then removed and the catheter is attached to the tubing and transducer. Alternatively, the modified Seldinger technique can be used in a similar fashion.

The femoral artery is the preferential site for emergent arterial access due to both its large size and central location relative to other potential cannulation sites. These same attributes make the femoral artery the preferred choice for vascular access for surgical and interventional procedures. Thus, the patient’s procedural history should be reviewed, and caution must be taken if the femoral vascular system has been previously manipulated. The leg should be placed in a fully extended and abducted position, which can be achieved by laying the patient supine and dangling the lower leg off the edge of the bed. The introducer needle should be inserted at a 45° angle to the skin, bevel up and facing the umbilicus, and distal to the crease of the hip. Subsequent steps for cannulation follow the modified Seldinger technique, as described above.

Similar to the femoral site, the axillary artery is cannulated using the modified Seldinger technique. The arm is properly positioned in a position of abduction, external rotation, flexed at the elbow, and raised; commonly it is suspended above the head by use of a makeshift sling affixed to the head of the bed or IV pole. Successful cannulation is achieved by palpating the artery at the top of a near to the concavity of the axilla. The introducer needle is inserted at a 15° to 30° angle to the skin, aiming for the point where the pulse is most strongly palpable. Once pulsatile blood is obtained, the procedure follows that as described in the Seldinger technique above.

The brachial artery can be cannulated using either the Seldinger technique as described for the femoral or axillary approach, or by the use of a catheter-over-wire apparatus as described for radial artery catheterization. The artery is access by extending the arm completely and palpating the pulse within the antecubital fossa. Potential disadvantages of this site include distal ischemia and patient discomfort from maintaining the arm in the extended position. Flexing the arm will kink the catheter within the antecubital fossa and preclude proper catheter function.

Several studies have shown a reduction in complications and failure rate, as well as an increase in first-pass success with the use of ultrasound guidance during central venous catheter placement compared to traditional landmark technique.^2,4,5,6,7 As a result more ICUs are now equipped with various bedside ultrasound machines and practitioners are becoming more comfortable with its use, especially for insertion of invasive catheters. The use of ultrasound for arterial line placement was initially used as salvage therapy when conventional methods had failed. However, in recent years, the use of ultrasound guidance for radial catheter placement has increased.

Initial ultrasound methodology was based on Doppler techniques, whereas current ultrasound systems use more advanced modes such as B-mode which creates a two-dimensional cross-section of the tissue being imaged.^2,6,7 Other types of images can be displayed to assist the clinician including blood flow. Clinicians use a hand-held probe, typically called a transducer which is placed directly over the area to be imaged. Specific application of ultrasound to arterial cannulation includes differentiating between artery (pulsatile) and vein (nonpulsatile), as well as between blood vessels which appear dark (hypoechoic) in contrast to soft tissue which appears gray (isoechoic). In patients with small arteries or who may be hypotensive, direct visualization of the artery can at times be difficult. In this instance, practitioners can use color flow Doppler to confirm the presence of pulsatile flow within the artery.

Probe selection is also a key component to the proper use of an ultrasound machine. Higher frequency probes (7.5-15 MHz) are used most often for vascular procedures; however, lower frequency probes (5 MHz) may be necessary for deep vessels or obese patients. The ultrasound machine should ideally be positioned on the contralateral side of the patient with the operator on the ipsilateral side. The transducer should be held in the operator nondominant hand and held low on the probe. Various views can also be used during catheter insertion. While direct visualization of the needle at all times can only be accomplished using the longitudinal (long access) view, the transverse (short access) view allows for visualization of smaller and/or more tortous arteries and remains the preferred method for radial artery catheterization.⁷

A meta-analysis of four trials (n = 311) by Shiloh et al compared radial artery catheterization using the conventional palpation method (152 patients) versus ultrasound guidance (159 patients). A 71% improvement (relative risk, 1.71; 95% CI, 1.25-2.32) in the likelihood of first attempt success was noted in the group using ultrasound guidance during radial artery catheterization.² In a separate study by Levin et al, 69 patients undergoing elective surgery and requiring arterial catheter placement were randomized into two groups: ultrasound guidance versus palpation alone. There was a significantly higher first pass success rate using ultrasound guidance (62%) versus palpation alone (34%).⁸ Several other studies have also shown increased first attempt success rates when comparing conventional palpation methods to ultrasound-guided insertion techniques.^2,4,5,6,7,8

Although arterial cannulation is a generally safe procedure, complications can occur. The total complication rate is estimated to range from 15% to 40% of procedures, although clinically significant occurrences are limited to < 5%.^1,2,3,4,7,9 Of these, some of the more common incidents include thrombosis and arterial occlusion, embolization and organ ischemia, infection, bleeding, and/or hematoma formation. Less severe but still common complications include vasospasm, diagnostic blood loss, and pain. Heparin-induced thrombocytopenia is also a problem as a result of the heparinized solution sometimes used in continuous flush systems.

Thrombosis

Thrombosis is the most common complication associated with catheter placement.^3,9 It is far more common in the narrow vessels of the distal circulation than in the larger central arteries. In addition to site selection, the incidence of thrombosis increases with duration of indwelling catheter use, length and gauge of arterial catheter selected, and predisposing hypercoagulable state.⁹ It is mitigated by use of a continuous flush system, which works to limit stagnation or turbulence of blood flow through the catheter. Although thrombosis may occur, it is usually not a serious complication in that it rarely results in clinically significant ischemia. Furthermore, ischemia usually resolves with catheter removal, and the thrombus is resorbed within several weeks of catheter removal. Clinically significant ischemia is rare, occurring in < 1% of arterial catheter placements, and usually develops in the setting of preexisting or concurrent circulatory alterations. However, when surgical intervention for ischemia is required, partial to total amputation of the affected extremity is frequently necessary.

Embolization

Cerebral embolization occurs as a result of either air being externally introduced into the systemic circulation, or via dislodgment of a thrombus at the catheter site. It is frequently associated with peripheral cannulation at radial and brachial sites, although has the potential to occur with any catheter. Since gas travels up a fluid-filled system, air will travel up to the cerebral circulation in a sitting or nonrecumbent patient. The rate of instillation of air into the circulation will also predispose to higher rates of embolization. Manual flushing of the arterial catheter with a syringe as opposed to use of the flush valve can cause higher volumes of air to be introduced. Clinical relevance, if any, depends on the site of embolization, the volume of air involved, and the extent of vessel occlusion.

Infection

As with any percutaneous procedure, there is a risk of infection associated with arterial catheterization. The most common routes of arterial infection include contamination with skin flora during catheter insertion, contaminated sterile flush/infusate system, and introduction of bacteria during blood drawing or opening of the tubing–stopcock system to the ambient environment. Common practices to mitigate infection include the use of chlorhexidine solution prior to catheter insertion, use of sterile technique during insertion (including mask, sterile gown and gloves, and hair cap if necessary), and covering stopcocks with diaphragms instead of caps.¹ Routine changing of the tubing/transducer system varies across institutions; 96 hours is a common practice. Routine changing of the arterial catheter itself is infrequently performed as arterial catheterization results in a very low rate of bacteremia (0%-5%),¹ and is rarely the cause of fever. However, repeat cannulation at a new site may be indicated if all other sources of sepsis are ruled out. The most common bacterial isolate from arterial catheters sent for microbial analysis is Staphylococcus epidermidis. If bacteremia from the arterial catheter is confirmed, treatment with appropriate antimicrobial agents is indicated.

Hemodynamically Significant Retroperitoneal Bleeding

The femoral artery is a large vessel that is frequently selected in emergent situations due to ease of cannulation. However, improper technique can result in transection of the artery and resultant bleeding into the retroperitoneal space. Large amounts of occult bleeding into the retroperitoneum can occur. Unexplained hemodynamic instability and pallor after femoral arterial catheterization should be promptly evaluated radiographically if hematoma or bleeding is suspected.

Hematoma

Percutaneous puncture of smaller, superficial arteries may result in smaller, visible hematomas; these are more frequently seen at the radial, brachial, and dorsalis pedis sites, but can be seen with axillary puncture. If superficial hematoma develops, direct manual pressure should be held until the hematoma is reduced and the area is soft. The procedure should be aborted, and a new site selected.

Vasospasm

Vasospasm may occur under similar conditions to local hematoma formation. The small, superficial radial, brachial, and dorsalis pedis arteries may become vasospastic after cannulated. If the catheter is unable to be placed due to obstruction or inability to advance the guidewire, the operator may notice diminution of a palpable pulse. In such circumstance, the procedure should be aborted and a new site selected, as further attempts at cannulation of the artery are less likely to be successful and may result in unnecessary patient discomfort.

Diagnostic Blood Loss

Significant blood loss can occur from frequent arterial blood sampling as a result of the need to draw intraarterial blood that is not contaminated by saline diluent or heparinized flush from the transducer system. In order to assure that pure blood is taken, 3 to 5 ml of blood is extracted prior to obtaining the sample for analysis. This can aggregately lead to an increased need for transfusion (with associated morbidity risks). Mitigation of blood loss can be achieved through use of pediatric tubing (smaller volumes), utilization of tubing systems that incorporate a reservoir, and point of care rather than traditional chemical analysis.

Heparin-Induced Thrombocytopenia (HIT)

Some institutions use small amounts of heparin in the arterial flush solution. In critically ill patients with new thrombocytopenia (platelet count decrease of 50% of preheparin levels or absolute platelet count of < 100,000/ml) but no clear etiology, HIT should be considered. If heparin is considered to be a likely cause of thrombocytopenia, all use of heparin in the flush solution should be discontinued. Alternatives include sodium citrate, lactated Ringer’s, or 0.9% saline solution.

Arterial line placement has become a commonly accepted procedure for continuous monitoring of blood pressure and as a reliable access for frequent blood samplings in critical care settings. Although generally considered a safe procedure with few serious complications, consideration of appropriate site selection, contraindications, and potential complications are important prior to insertion of an arterial line.¹⁰ Once the site is selected, use of ultrasound evaluation of the vessel should be considered. Complications associated with arterial catheterization include arterial spasm, thrombosis, embolization and distal ischemia, infection, bleeding and/or hematoma formation. Once accurately placed, continued necessity of the arterial catheter should be evaluated on an ongoing basis, and the catheter should be discontinued as early as possible once the patient is stabilized.