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Veterinarian Technician November 2009 (Vol 30, No 11)

The Fine Art of Arterial Lines

by Heidi Reuss-Lamky, LVT, VTS (Anesthesia)

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    The measurement of blood pressure is becoming increasingly commonplace in veterinary practices. Blood pressure assessment can be a crucial component of patient care during anesthesia, surgery, and the triage phase of nursing care (e.g., blood loss, trauma), as well as during procedures such as chemotherapy. In addition, blood pressure assessment has become part of the routine diagnostic workup for geriatric patients and patients with renal insufficiency, cardiac disease, vision loss (e.g., retinal detachment), or endocrine disorders (e.g., hyperthyroidism, diabetic ketoacidosis).1,2

    Indirect Versus Direct Measurement

    There are two techniques for measuring blood pressure: indirect, noninvasive methods via Doppler or oscillometric devices, and direct, invasive methods. Indirect blood pressure measurement methods have many caveats. Results may vary greatly based on the cuff size and fit, cuff positioning, patient movement, presence of limb edema or significant arrhythmias, patient size, and operator experience.1,3,4 Furthermore, external methods of measuring blood pressure can be less accurate when the results may be most important, such as in hypotensive patients, patients with small vessel sizes, or those with vasoconstriction.5 Additionally, "white coat" phenomenon, which has been well documented in human and veterinary patients, can result in inaccurate blood pressure assessments.3

    Although the heart rate can be reliably and reproducibly measured by noninvasive methods, there is greater variability when diastolic and systolic pressures and mean arterial pressure (MAP) are indirectly measured compared with telemetrically acquired direct blood pressure readings within a given (3- to 4-minute) time frame in a conscious dog.3 In one informal head-to-head study, readings from oscillometric devices ranged 10% to 30% lower than direct, telemetric recordings.3 Experienced personnel using a Doppler device to record systolic pressures obtained readings that were 18% to nearly 28% lower than the reference measurements. The results of this study included the following:

    • In normal dogs, "spot" blood pressure measurements may not reflect the patient's physiologic or pathologic state as accurately as time-averaged blood pressure readings over longer intervals.
    • Although blood pressure trends were accurately predicted by the indirect oscillometric methods tested, each one provided unique values that were not consistently comparable with the validated standard.
    • Compared with the reference measurements, there were wide deviations in recorded blood pressure values when trained, experienced personnel in a clinical setting used oscillometric or Doppler techniques.3

    The inability to accurately gauge blood pressure in veterinary patients using currently available indirect methods led the American College of Veterinary Internal Medicine (ACVIM) Consensus Panel to proclaim that, "for the diagnosis of systemic hypertension, the indirect device used should be one that is commonly employed or designed for veterinary use and has been previously validated in conscious animals in the species of interest. However, no indirect device has met these criteria for use in conscious dogs or cats."4 Therefore, direct arterial blood pressure (DABP) monitoring, which has been proven to be the most accurate method of measuring blood pressure in human and veterinary patients, is considered the "gold standard."

    Considerations

    The use of arterial catheters for DABP monitoring has many advantages in critically ill or high-risk anesthetic patients requiring frequent blood pressure measurements over time. It is also extremely helpful in assessing the progress of fluid resuscitation therapy and inotropic or pressor therapy in patients with hypovolemic or septic shock.5,6 Furthermore, using arterial catheters for periodic assessment of blood gases can be beneficial in patients with respiratory disease or acid-base disorders.7 However, arterial catheters must be used prudently in relatively healthy patients because of the possibility of severe hemorrhage if the catheter is dislodged, chewed, or pulled out. It is strongly advised that all patients with arterial catheters be closely observed at all times.5

    A number of arteries can serve as placement sites for an arterial catheter. The dorsal pedal/metatarsal arteries are the most commonly used and easiest to maintain on a long-term basis. Other sites used in veterinary patients include the radial, brachial, palmar, femoral, auricular, coccygeal (tail), and sublingual (ventral tongue) arteries. Radial and brachial arterial catheters are more technically challenging to place. Palmar, femoral, and brachial catheters are best reserved for unconscious patients, as any change in the patient's position can cause catheter maintenance problems.8 In addition to the dangers of excessive motion, coccygeal arterial catheters can prove problematic if the patient experiences diarrhea. The sublingual artery should be used only in unconscious patients, and hematoma formation is common once the catheter is removed. The femoral artery can be used in smaller patients (e.g., cats, small dogs), and the auricular artery—located on the midline of the dorsal surface of the pinna—can be used in larger dogs with pendulous pinnae (e.g., basset hound, beagle).5 It may be beneficial to position an empty 20-mL syringe case on the ventral surface of the pinna and beneath the cannulated artery to provide a steady surface to which the auricular catheter can be securely taped.

    Another consideration in selecting the site for an arterial catheter is to avoid compromise of the circulation distal to the placement site (e.g., end arteries or other areas with known deficiencies in collateral circulation). Other areas that are not suitable for catheter placement include those that are infected or that have sustained trauma proximal to the proposed insertion site.7

    Preparation and Catheter Placement

    Arterial catheter placement requires the same supplies as peripheral IV access: clippers, alcohol and antiseptic scrub, adhesive tape and sterile dressing materials, heparinized saline solution, 1% to 2% lidocaine (without epinephrine), a Luer-Lok T-set prefilled with heparinized saline solution, and a 20- to 24-gauge over-the-needle catheter (Figure 1). Although the length of the arterial catheter partly depends on personal preference, longer lengths should be selected for long-term use. The 20-gauge, 1.5-inch QuickFlash Radial Artery Catheterization Set (Arrow International; Reading, PA) contains a flexible guidewire that is fed into the artery before insertion of the catheter, thereby assisting passage of the catheter into the artery. Various sizes of Argon arterial catheters are also manufactured by Maxxim Medical (Argon Division; Athens, TX).

    The dorsal pedal artery is catheterized as follows: the patient is placed in lateral recumbency, with the limb to be catheterized in the down position. The hair is clipped and the area prepared as for IV catheterization, using a full surgical scrub. An assistant may stabilize the limb, but it is essential to not apply pressure around the hock. The dorsal pedal artery is carefully palpated and identified along its entire course (Figure 2). This artery usually passes between metatarsals II and III (Figure 3), and the optimal site for catheterization is the most distal point where it can still be palpated.

    The greatest difficulties encountered while placing an arterial catheter are due to arterial spasms or the inability to pass the guidewire or catheter through the artery. When arterial spasms are suspected, further attempts to catheterize the artery should be abandoned and an alternative site selected.7 Injecting a small bleb of 1% to 2% lidocaine over the artery at the planned catheter insertion site can help to prevent arterial spasm during placement.

    The catheter should be placed percutaneously at a 10° to 20° angle, just distal to where the artery can be palpated, with the technician always mindful that the artery is located very superficially. The catheter and needle stylet are inserted slowly (in 1-mm increments) into the artery until a blood flash is observed in the end of the catheter (Figure 4). If difficulty is encountered while the catheter is fed into the artery, it can be repositioned slightly and fed again. If the catheter will not feed, but an arterial pulse is still palpable, the catheter can be left in the vessel and a second catheter inserted proximal to the original site. This technique may prevent hematoma formation, which can make additional attempts to catheterize the same vessel extremely difficult or impossible.9

    Unless the patient is extremely hypotensive, pulsatile blood flow from the catheter should be noted once the needle stylet has been removed. When the catheter has been successfully advanced into the artery, it can be secured firmly in place with adhesive tape and catheter bandaging materials. The Luer-Lok T-set prefilled with heparinized saline solution is then attached snugly, and the catheter is flushed with 1 to 1.5 mL of the solution, with the technician ensuring that no air bubbles are in the line. The catheter should be clearly labeled as an arterial catheter. It can then be connected to a pressure transducer and continuous-flush mechanism if being used for DABP monitoring, or it can remain capped to a closed, Luer-Lok T-set.

    Arterial Catheters

    Once an arterial catheter has been placed, the utmost care must be used to maintain it on a long-term basis. It should be treated like any other catheter, including the use of aseptic technique and daily bandage changes. The heparinized saline bag should be changed every 24 hours, and the high-pressure tubing associated with the transducer should be replaced every 72 hours.1

    A routine three-syringe technique is used to obtain blood samples from the arterial catheter, with the technician first drawing 2.5 mL of arterial blood into a 3-mL syringe containing 0.5 mL of heparinized saline solution. The arterial blood sample is then collected into a syringe that can be heparinized or nonheparinized. Finally, the catheter is flushed with heparinized saline solution and reattached to the blood pressure transducer and either a flush system pressurized to 150 mm Hg that continuously flushes the catheter with heparinized saline solution (1000 U of heparin per L of 0.9% sodium chloride), or a closed, Luer-Lok T-set that is flushed at least once every 1 to 4 hours with 1 to 3 mL of heparinized saline solution. The blood that was first acquired from the arterial catheter can be returned to the patient through a peripheral IV catheter if desired. Arterial catheters should never be used to administer drugs or fluid therapy.9

    Arterial catheters used for DABP measurements must be connected to a continuous-flush system and a pressure transducer that has been placed at the level of the right atrium or sternum via the shortest possible length of noncompliant extension tubing. The most accurate measurements possible can be ensured by eliminating all air bubbles from the entire system. Once the transducer is attached to the monitor, the transducer must be "zeroed" at the level of the heart. This is typically accomplished by opening a three-way stopcock on the transducer so that the pressure is equilibrated with the atmosphere to establish a zero reference point. After this, the monitor will display a series of waveforms and the patient's real-time systolic and diastolic pressures as well as MAP. The rate of inaccuracy associated with DABP measurements is reportedly 2% to 4%, with 1% to 2% of the inaccuracy arising from the transducer and another 1% to 2% from the amplifier.5

    Not all brands of anesthetic monitors can measure DABP. The instruction manual should be consulted to determine the capabilities of a monitor, the type of transducer kit and other equipment it may require, and the correct assembly and operation.

    Waveform Interpretations

    It is important to thoroughly understand the DABP waveform. When used in conjunction with electrocardiography, the DABP waveform can provide valuable information regarding the effects of arterial perfusion on the major organ systems. For example, a state of poor perfusion exists when cardiac arrhythmias (e.g., intermittent ventricular premature contractions) are associated with a dampened waveform appearance in conjunction with an abnormal MAP.

    Evaluation of the waveform is essential for assessing cardiac function, particularly as it relates to left ventricular ejection (Figure 5). Peak ejection occurs during the highest point on the waveform and is associated with systole. The downstroke of the waveform is associated with a drop in pressure. Midway through the downstroke, a notch, called the dicrotic notch, may be visible, indicating closure of the aortic valve. The dicrotic notch also represents the beginning of diastole. The remainder of the waveform's downstroke represents blood flow into the arterial tree, with the lowest point representing diastole.

    A thorough knowledge of the potential problems associated with arterial catheters and their waveform indications can simplify the troubleshooting process and ensure continuous DABP monitoring. One common problem involves waveform dampening or loss. This may be associated with air bubbles, blood clots, excessive blood, or kinks in the catheter or tubing.9 Arterial spasm may also cause waveform dampening or loss. The MAP is generally correct during arterial spasm, even without a good tracing. In all cases of waveform dampening or loss it is important to ensure that the line is not clamped off, and then the line and catheter should be flushed. If the catheter has migrated against the vessel wall, changing the patient's position may resolve the problem.1

    Sometimes the line is flowing and can be aspirated, but the waveform is not visible. In these cases, the cable attachment to the monitor should be confirmed, as should the "on" position of the stopcock.

    In the event of a sudden change in pressure, it is essential to ensure that the transducer has not moved and is still at the level of the heart and that no one is leaning on the patient's line or a major blood vessel. More important, a sudden change in pressure can indicate that cardiac arrest has occurred or is imminent, so the patient's pulse and end-tidal CO2 production should be assessed immediately to make sure this is not the case.

    Inaccurate readings (i.e., lower systolic and higher diastolic values) may occur when the waveform is dampened. Furthermore, inaccurately low readings may occur in patients with severe peripheral vasoconstriction (e.g., due to severe hypovolemia or high-dose pressor agents).5

    Reflections of the waveform from a peripheral catheter may amplify the systolic pressure, resulting in falsely elevated systolic values. Although less common in veterinary medicine, this phenomenon occurs routinely in geriatric human patients when arteries are noncompliant.5

    When the accuracy of DABP measurements is questioned, it is not necessary to spend a great deal of time troubleshooting the system. Instead, assess the patient's cardiovascular status (e.g., respiration, pulse quality, mucous membrane color, etc.) and combine those findings with the indirect blood pressure readings to help ensure timely intervention during a potential cardiovascular crisis. Although indirect blood pressure and DABP measurements may differ, the patient's clinical management may be affected by large disparities.

    Precautions and Complications

    There are many drawbacks associated with arterial cannulation. Although arterial catheters can be used to provide critical information, their use may be impractical in relatively healthy, mobile, conscious patients. Around-the-clock supervision is imperative for managing all patients with an arterial line. The use of arterial catheters increases the risk of iatrogenic hemorrhage (possibly leading to exsanguination), infection, thromboembolism, and hematoma formation.2,5,9 Air bubbles that are flushed into the artery of a distal extremity eventually enter the small capillaries of the foot. The presence of large air bubbles can result in an air embolism, possibly leading to tissue necrosis. Moreover, frequent administration of heparin can lead to iatrogenic coagulation abnormalities, especially in small patients.4 In humans, other reported complications (rare) include temporary or permanent occlusion of the artery, abscessation, cellulitis, nerve paralysis, suppurative thromboarteritis, arteriovenous fistula, and pseudoaneurysm. There is an increased risk of infection and sepsis when an arterial catheter is left in place for longer than 96 hours. Sepsis is also more prevalent in the presence of local inflammation. Caregivers may be a source of contamination, contributing to infection rates while monitoring the system and flushing the infusion lines.10

    Conclusion

    Arterial cannulation is generally considered a safe technique associated with only a small number of serious complications. When used for assessing blood pressure and blood gases, arterial catheterization can be invaluable for managing critically ill patients, high-risk anesthesia patients, and trauma victims as well as for providing objective information on acid-base status and pulmonary function. Arterial catheterization can also help to guide volume replacement and monitor patients with hypovolemic or septic shock during the administration of pressor or inotropic agents (Figure 6). Furthermore, evaluation of the pressure waveform permits clinicians to determine when cardiac arrhythmias may be causing poor pressures or when pulse deficits become detrimental to the patient, allowing timely institution of interventional drugs. Although a thorough understanding of the regional anatomy and an advanced skill level are necessary to place arterial catheters, they can help to provide clinicians with accurate, aggressive monitoring of numerous hemodynamic parameters and help guide resuscitation efforts.

    1. Tefend M. Blood pressure monitoring: what you may not know. Proc ACVIM 2003.

    2. Durham E. Arterial blood pressure measurement. Vet Tech 2005;26(5):325-338.

    3. Cowgill L. Accuracy of methods for blood pressure measurement. Proc ACVIM 2006:658-659.

    4. Atkins C, Brown S, Bagley R, et al. Guidelines for the identifcation, evaluation, and management of systemic hypertension in dogs and cats. JVIM 2007;21(3):542-558.

    5. Waddell L. Blood pressure monitoring for the critically ill. Proc WVC 2004.

    6. Macintire DK, Dobratz KJ, Haskins SC, Saxon WD. Monitoring critical patients. In: Manual of Small Animal Emergency and Critical Care Medicine. Baltimore/Philadelphia: Lippincott Williams & Wilkins; 2004:73-74.

    7. Tegtmeyer K, Brady G, Lai S, et al. Videos in clinical medicine: placement of an arterial line. N Engl J Med 2006;354(15):e13.

    8. Beal MW. Vascular access in the trauma patient. Proc Mich Vet Conf 2007.

    9. Mazzaferro E. Arterial catheterization. Proc IVECCS. 2004.

    10. Scheer B, Perel A, Pfeiffer UJ. Clinical review: complications and risk factors of peripheral arterial catheters used for haemodynamic monitoring in anesthesia and intensive care medicine. Crit Care 2002;6(3):198-204.

    References »

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