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Veterinarian Technician June 2013 (Vol 34, No 6)

Direct Blood Pressure Monitoring: Simple and Inexpensive

by Jaime Maher, CVT, VTS (ECC, Anesthesia)

    Would you like to improve your blood pressure (BP) monitoring capabilities? Do you lack a high-tech monitor? Do your multiparameter monitors not have invasive BP capabilities? You can still monitor continuous, invasive mean arterial pressure (MAP) very simply using an inexpensive aneroid manometer, which costs about $15 (FIGURE 1).

    Arterial Catheterization

    BOX 1 lists the supplies for arterial catheterization. Arterial catheters are unique in that they are used strictly for monitoring and blood sampling. Fluids and drugs should never be injected into an arterial catheter; however, small amounts of heparinized saline may be injected to maintain patency. The dorsal pedal artery is the most common site for catheterization (FIGURE 2); other options are the tail (in cats), ear (in bassett hounds and rabbits), and medial plantar or palmar arteries.

    Initiate catheter placement by using your index and middle fingers to palpate the artery. When a strong pulse is detected, insert a traditional, over-the-needle, 24- to 20-gauge (depending on the species and breed) catheter at an angle slightly less than 45°over the site where the pulse is strongest. As the artery is punctured, a strong flash of blood will enter the catheter stylet. Lower the catheter slightly (decrease the angle) to allow advancement. You will feel somewhat more resistance during advancement than when placing a venous catheter, but advancement should be smooth. Remember that when the stylet is removed, significant blood loss can occur if you are not prepared. Seat a T-set primed with heparinized saline in the catheter hub and tape the catheter in place as when securing a venous peripheral catheter (FIGURE 3). The T-set should have a Luer lock to prevent accidental disconnection and hemorrhage. After taping and/or bandaging, label the catheter “Arterial—do not inject”; ART is sufficient if the catheter will remain in place for a short time and all staff members know what this means (FIGURE 4). Flush the catheter with a small amount (e.g., 0.5 to 1 mL) of heparinized saline every 2 hours to maintain patency. Typically, this type of catheter is reserved for use in anesthetized or obtunded patients. Once the patient is mobile, remove the catheter.

    Direct Blood Pressure Monitoring

    Invasive (direct) blood pressure is the gold standard in arterial pressure monitoring.1 This traditionally involves a pressure transducer connected to a catheter that is inserted into an artery. The pressure transducer relays information to a monitor that records the systolic BP, diastolic BP, and MAP (FIGURE 5). Aneroid manometers are typically designed for use with an inflation device and a BP cuff around the circumference of a limb. Systolic arterial pressure is determined by using this traditional technique along with Doppler ultrasound.2 When an aneroid manometer is connected without an inflation device to an arterial catheter, the MAP is continuously indicated.3 While only measuring the MAP, an aneroid manometer can be excellent for indicating trends and sudden changes in a patient’s status. Noninvasive BP monitoring can be performed intermittently using oscillometric or Doppler techniques.

    Technique

    A small piece of rubber tubing, such as a piece cut from a rubber surgical instrument cap, is connected to the tapered end of a manometer. Connect the rubber piece to a three-way stopcock using a 1-mL tuberculin syringe without a plunger and with its winged end cut off. Connect a low-volume, low-compliance extension set (60 inches long with a 1.1-mL priming volume) to the three-way stopcock. The stopcock will be used to prime the extension set with heparinized saline. Saline will also enter the tuberculin syringe, but some air must remain in the syringe to create pressure in the system for resistance against each pulse. So far, this is the nonsterile setup.

    Because manometers are not easy to sterilize, a “buffer” is needed between the manometer and the patient. Connect a second three-way stopcock to the primed, low-volume extension set. To the “male” end of the stopcock, connect a second low-volume extension set. To the “female” end of the stopcock, connect a 12-mL syringe filled with heparinized saline. This part of the system will be used to flush the arterial catheter when necessary. The entire system should be primed with heparinized saline before being connected to the patient. Once the system is primed, set the manometer by manipulating the stopcock closest to the manometer to either aspirate or instill saline until the manometer reads “0 mm Hg” (FIGURE 6).

    With the distal extension set connected to the patient’s arterial catheter, the stopcocks should be “turned off” to the 12-mL syringe and the atmosphere. The manometer needle will rise and then pulse along with the heart rate. The point to which the needle rises and pulses is considered the MAP (FIGURE 7). The human literature states that the manometer should be positioned at the level of the heart, which makes sense because that is where a pressure transducer would be placed if invasive BP were determined via a monitor.4 In practice, I have found that while a patient is in lateral recumbency, the manometer technique accurately measures the MAP when the manometer is placed at the level of the heart (I have confirmed this using oscillometry and invasive BP via a transducer/monitor). However, when a large, deep-chested patient is in dorsal recumbency, the manometer technique appears to be more accurate when the manometer is placed closer to the level of the arterial catheter (FIGURE 8). This may be due to differences in patient size compared with extension-set length and compliance within the system. Even with slight differences in patient positioning, the manometer technique still proves accurate at monitoring trends in continuous, invasive MAP.

    1. Bryant S. Anesthesia for Veterinary Technicians. Ames, IA: Wiley-Blackwell; 2010:105-112.

    2. Bosiack A, Mann FA, Dodam J, et al. Comparison of ultrasonic Doppler flow monitor, oscillometric, and direct arterial blood pressure measurements in ill dogs. J Vet Emerg Crit Care 2010;20(2):207-215.

    3. Haskins SC. Monitoring anesthetized patients. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb and Jones' Veterinary Anesthesia. 4th ed. Ames, IA: Blackwell Publishing; 2007:543-546.

    4. Zorab JSM. Continuous display of the arterial pressure: a simple manometric technique. J Assoc Anaesth Great Br Ir 1969;24-3:431-437.

    References »

    NEXT: Feline Arterial Thromboembolism: A Terrible FATE

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