The measurement of blood pressure has become commonplace in most 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 and trauma cases), as well as during procedures such as chemotherapy. It has also become part of the routine diagnostic work up for geriatric patients or patients suffering from renal insufficiency, cardiac disease, vision loss (e.g., retinal detachment) or endocrine disorders (e.g., hyperthyroidism, diabetic ketoacidosis).There are 2 techniques available for measuring blood pressure; indirectly using non-invasive methods (via Doppler or oscillometric devices such as Cardell, Dinamap and petMAP), or by using direct, invasive methods. There are many caveats associated with indirect blood pressure measurement methods. Results may vary greatly based on the selection of the cuff size and fit, cuff positioning, patient movement, presence of limb edema or significant arrhythmias, patient size, and the experience of the operator. Furthermore, all external methods of obtaining blood pressure measurements are the least accurate when results would be most revered, as in hypotensive patients, or those with small vessel sizes or vasoconstriction.Additionally, the 'white coat' phenomenon has been well documented in human and veterinary patients and can also lead to inaccurate blood pressure assessments.
It has been demonstrated that although the heart rate can be reliably and repeatedly predicted, there was great variability regarding the indirectly obtained measurements of the diastolic, systolic and mean arterial pressures (MAP) as compared to the telemetrically acquired direct blood pressure (Data Sciences International, DSI) readings obtained within a given (3 to 4 minute) time frame in a conscious dog. In general, all brands compared in one head-to-head study (using Cardell 9401 and Dinamap 1846 SX oscillometric devices) ranged consistently 10% (or as much as 20% to 30%!) lower than the direct, telemetric recordings.Experienced personnel using Doppler (Parks Model 811-BTS) methods to record systolic pressures ranged from 18% to nearly 28% lower than the referenced measurements, demonstrating the lowest level of accuracy. The results of this study found that 1) in normal dogs, "spot" blood pressure measurements may not accurately reflect the patients' physiologic or pathologic state as accurately as time-averaged blood pressure readings over larger segments of the day, 2) although blood pressure trends were accurately predicted among the indirect oscillometric methods tested, each one provided unique values that were not consistently comparable to the validated standard, and 3) as compared to the referenced measurements, there were wide deviations in recorded blood pressure values when trained and experienced personnel in a clinical setting used oscillometry or Doppler techniques. Due to the apparent inability to accurately gauge blood pressure in veterinary patients using currently available indirect methods led the ACVIM Consensus Panel to proclaim "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."These are just a few examples as to why direct arterial blood pressure (DABP) monitoring, proven to be the most accurate method of measuring blood pressure in both human and veterinary patients, is considered the 'gold standard.'
The use of arterial catheters for DABP monitoring has many advantages when used in critically ill or high-risk anesthetic patients requiring frequent blood pressure measurements over a given time period. It is also extremely helpful in assessing the progress of fluid resuscitation, or inotropic or pressor therapy in patients with hypovolemic or septic shock.Furthermore, using arterial catheters for periodic assessment of blood gases can be especially beneficial in patients with respiratory disease or acid / base disorders.Always use arterial catheters prudently in relatively healthy patients due to the possibility of severe hemorrhage if the catheter should become accidentally dislodged, chewed or pulled out. It is strongly advised that all patients receiving arterial catheters be closely observed at all times.
There are numerous arteries that may be selected as the placement site for an arterial catheter. The dorsal pedal / metatarsal artery is most commonly used, and is one of the easiest to maintain on a long-term basis. Other arteries that may be used in veterinary patients include the radial, brachial, palmar, femoral, auricular, coccygeal (tail) and sublingual (ventral tongue) arteries. The radial and brachial arterial catheters are technically more challenging to place, and the palmar, femoral, and brachial arteries are best used in unconscious patients only, as any change in the patient's position can cause catheter maintenance problems. In addition to excessive motion, coccygeal artery catheters can prove problematic if the patient experiences diarrhea. The sublingual artery is obviously only to be used in unconscious patients, and beware that hematoma formation is common once the catheter is removed. The femoral artery can be used in smaller patients such as felines and small canines, and the auricular artery (located on midline of the dorsal surface of the pinna) can be utilized in larger canines with big, pendulous pinnae (e.g., Basset hound, Beagle).It may be beneficial to position an empty 20 cc syringe case on the ventral surface of the pinna and beneath the cannulated artery, thereby offering a steady surface to which the auricular cannula can be securely taped into place on. Considerations when selecting the site for an arterial catheter include avoiding compromise of the circulation distal to the placement site (e.g., end arteries or other areas with known deficiencies in collateral circulation), and avoid infected areas or areas that have sustained trauma proximal to the proposed insertion site.
Preparation and Catheter Placement
The supplies necessary to place an arterial catheter are the same as are necessary for peripheral intravenous access and include clippers, alcohol and antiseptic scrub, adhesive tape and sterile dressing materials, heparinized saline, 2% lidocaine (without epinephrine), a luer-lock T-set pre-filled with heparinized saline, and a 20-24g over-the-needle catheter. Although the length of the arterial catheter placed is somewhat of a personal preference, longer lengths should be selected for long-term use. Alternatively, Arrow® International (Reading, PA) manufactures a 20g, 1.5" QuickFlash® Radial Artery Catheterization Set containing a flexible guide wire that is fed into the artery prior to the insertion of the catheter, thereby easing the passage of the catheter into the artery. Maxxim Medical (Argon Division in Athens, TX) also manufactures various sizes of argon arterial catheters.
Catheterize the Dorsal Pedal Artery as Follows
Place the patient in lateral recumbency, with the limb to be catheterized in the down position. Clip the hair and prepare the area the same as for intravenous catheterization, using a full surgical scrub. An assistant may stabilize the limb to be catheterized, but do not apply pressure around the hock. Carefully palpate the dorsal pedal artery and identify it along its entire course. The dorsal pedal artery usually dives in between metatarsals II and III (See Figure 1) and the optimal site for catheterization will be the most distal site where the artery can still be palpated. The greatest difficulties encountered during the placement of an arterial catheter occur secondary to an inability to pass the guide wire or catheter through the artery, or due to arterial spasms. When arterial spasms are suspected, further attempts at catheterizing that particular artery should be abandoned and an alternative site should be selected.Injecting a small bleb of 2% lidocaine over the artery at the catheter insertion site can help to prevent a spasm of the artery during the catheterization procedure. Insert the catheter percutaneously at a 10-20o angle, just distal to where the artery can be palpated. Remember that the artery is located very superficially! Slowly (in 1 mm increments) insert the catheter and needle stylet into the artery until a blood flash is observed in the end of the catheter. If difficulty is encountered while the catheter is fed into the artery try to reposition it slightly, and then reattempt to feed the catheter. If an arterial pulse is still palpable yet the catheter simply will not feed, leave that catheter in the vessel and make another attempt using a 2nd catheter proximal to the original site. This technique may prevent hematoma formation, which causes additional attempts at catheterizing the same vessel extremely difficult if not impossible.Unless the patient is extremely hypotensive, pulsatile blood flow should be noted from the catheter once the needle stylet has been removed. Once the catheter has been successfully advanced into the artery secure it firmly in place with adhesive tape and a sterile dressing, snugly attach the luer-lock T-set pre-filled with heparinized saline, and flush the catheter with 1-1.5 ml of air bubble free heparinized saline, then clearly label it as an arterial catheter. The arterial catheter can then be connected to a pressure transducer and continuous flush mechanism if being used for DABP monitoring, or remain capped-off to a closed, luer-lock T-set.
|Figure 1. Dorsal pedal arterial anatomy--left hind limb.|
Utilizing Arterial Catheters
Once an arterial catheter has been established, the utmost care must be used to maintain it on a long-term basis. It should be treated as any other catheter using a sterile 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 once every 72 hours.
Use a routine 3-syringe technique to obtain blood samples from an arterial catheter. First draw 2.5 mls of arterial blood into a 3.0 ml syringe containing 0.5 ml of heparinized saline. Then collect the arterial blood sample into a heparinized or non-heparinized syringe. Finally, flush the catheter with heparinized saline and reattach it to the blood pressure transducer, a flush system pressurized to 150 mmHg that continuously flushes the catheter with heparinized saline (1000 units of heparin per liter of 0.9% sodium chloride), or a closed, luer-lock T-set that is flushed at least once every 1-4 hours with 1 to 3 ml of heparinized saline. The blood that was first acquired from the arterial catheter should be given back to the patient through a peripheral intravenous catheter, if desired. Arterial catheters should NEVER be used for administering drugs or fluid therapy!
Arterial catheters used for DABP measurements must be connected to a continuous flush system and pressure transducer (that is placed at the level of the right atrium or sternum) via the shortest possible length of non-compliant extension tubing. Ensure the most accurate measurements by eliminating all air bubbles in the entire set up. 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 3-way stopcock on the transducer to the air, to equilibrate with atmospheric pressure and establish a zero reference point. After the completion of this step, the monitor will then display the real-time systolic, diastolic, and mean arterial pressure values for the patient and display a series of waveforms. There is a reported 2-4% level of inaccuracy associated with DABP measurements. One to 2% of the inaccuracy arises from the transducers, and another 1-2% occurs from the amplifier.
DABP Waveform Interpretation
It is important to have a thorough understanding of the DABP waveform. When used in conjunction with an ECG, it can provide valuable information regarding arterial perfusion as it affects the major organ systems of the body. For example, a state of poor perfusion exists when cardiac arrhythmias (e.g., intermittent premature ventricular contractions) are associated with a dampened waveform appearance in conjunction with an abnormal MAP.
Evaluation of the waveform is essential for evaluating cardiac function, particularly as it relates to left ventricular ejection. Peak ejection occurs during the highest point on the waveform and is associated with systole. The down stroke of the waveform is associated with a drop in pressure. Midway through the down stroke a notch (called the dicrotic notch) may be visible and indicates closure of the aortic valve. The dicrotic notch also represents the beginning of diastole. The remainder of the waveform down stroke represents the flow of blood into the arterial tree, with the lowest point of the waveform representing diastole.
A thorough knowledge of the potential complications and limitations associated with arterial catheters can simplify the troubleshooting process and ensure their continued use for DABP monitoring. Some of the most common problems encountered include:
Waveform dampening or loss: This may be associated with air bubbles or blood clots, or excessive blood or kinks present in the catheter or tubing lines.Additionally, the artery may be spasm. During arterial spasms, the MAP is generally correct, even without a good tracing. In all cases flush the line and catheter, after ensuring the line has not been clamped off. If the catheter has migrated up against the vessel wall, a change of the patient's position may resolve the problem.
The line flows and can be aspirated, but the waveform is not visible: Ensure that the cable is still attached to the monitor, and that the stopcock is not in the 'OFF' position towards the patient.
Sudden change in pressure: Ensure the transducer has not moved and is still at the level of the heart, and that the surgeon is not leaning on the patient line or a major blood vessel. More importantly, a sudden change in pressure can indicate that cardiac arrest has occurred or is imminent: Assess the patient's pulse and monitor the production of end-tidal carbon dioxide.
Inaccurate readings: Lower systolic and higher diastolic values may be present whenever the waveform becomes dampened. Furthermore, inaccurately low readings may be evident in patients with severe peripheral vasoconstriction (e.g., severe hypovolemia or due to higher doses of pressor agents.)
Waveform amplification: 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 non-compliant.
Do not waste unnecessary time with troubleshooting techniques when there are doubts regarding the accuracy of DABP. Always perform a thorough exam and assessment of the patient first, and combine those findings with an indirect blood pressure reading. Although differences will be evident between indirect and DABP measurements, the patient's clinical management may be affected by large disparities.
Precautions and Complications
There are certainly many drawbacks associated with arterial cannulation. Although they can be used to provide critical information, managing arterial catheters may be less than practical in relatively healthy or mobile, conscious patients. Around-the-clock supervision is imperative for the management of all patients with an arterial line. In addition to iatrogenic hemorrhage (possibly leading to exsanguination), there is an increased risk of infection, thromboembolism, and hematoma formation associated with arterial catheter usage. Air bubbles that are flushed into the arteries of the distal extremity eventually enter the small capillaries of the foot. The presence of large air bubbles can result in an air embolism, which may lead to tissue necrosis. Moreover, frequent administration of heparin can lead to iatrogenic coagulation abnormalities, especially in smaller patients.In humans, other (rare) reported complications include temporary (or infrequently, permanent) occlusion of the artery, abscessation, cellulitis, nerve paralysis, suppurative thromboarteritis, AV fistulas and pseudoaneurysm. There is an increased risk of infection and sepsis when the arterial catheter is left in place for greater than 96 hours. Sepsis is also more prevalent in the presence of local inflammation. Beware that caregivers may pose a potential source of contamination and contribute to infection rates while monitoring the system and flushing the infusion lines.
Arterial cannulation is generally considered a safe technique that is associated with only a small number of serious complications. Although a thorough understanding of the regional anatomy and an advanced skill level are necessary to place arterial catheters, they can help to provide the veterinarian with essential information enabling accurate, aggressive monitoring of numerous hemodynamic parameters and guiding resuscitation efforts.
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