Ultrasound-guided peripheral and central venous catheterization is commonly used in human emergency and critical care (ECC) settings as it has a higher success rate, lower complication rates, and is faster than blind/landmark techniques, particularly when peripheral vascular access is difficult. The success rates and time to place ultrasound-guided catheters in veterinary medicine have not been well studied. Based on the limited evidence available, the complication rate and time to place ultrasound-guided central lines in dogs under anesthesia is similar to that of blind peripheral catheter placement. The success rate of using ultrasound-guided catheter placement in canine cadavers is very high in situations where edema and hematoma make palpation of landmarks difficult. There is also evidence that ultrasound-guided femoral arterial catheter placement has good success and low complication rates in anesthetized dogs.
Indications for ultrasound-guided catheter placement include hematoma formation, inability to palpate landmarks for peripheral percutaneous catheter placement, edema formation, obesity, and failure to place a percutaneous catheter after 3 attempts (defined as difficult vascular access). Contraindications and complications of ultrasound-guided catheter placement are similar to those of percutaneous vascular access.
An advantage of ultrasound-guided catheter placement is that the depth, radius (which may help choose the catheter size), and patency (presence of thrombus) of the vessel can be assessed prior to placing a catheter. Although peripheral superficial vessels can be challenging to access via ultrasound guidance, human studies demonstrate very high success rates with ultrasound guidance when vessels are only millimeters from the skin surface and as small as 0.3 mm in diameter. Blood sampling, including dorsal pedal arterial access for blood gas analysis, can be performed using ultrasound guidance, which has the advantage of visualizing the vessel of interest to avoid accidental venous sampling.
The two common techniques for ultrasound-guided catheter placement include in-plane and out-of-plane techniques. The authors have the greatest success using an out-of-plane technique and sweeping the transducer along the vessel as the catheter is advanced. The out-of-plane technique works well for accessing vessels when the vessel remains visible and can thus be centered as the transducer is swept and/or fanned to follow the tip of the catheter stylet (i.e., you do not slide off vessels). One should keep the stylet tip visible when using the out-of-plane technique to avoid accidental puncture of the far vessel wall. The catheter tip should be adjusted as needed to keep it centered over the vessel while the tip is still within the subcutaneous tissue (proximal to the vessel wall); such adjustments are easily accomplished with out-of-plane ultrasound-guided catheter techniques. Aseptic technique should be followed, including the use of sterile ultrasound gel applied in combination with isopropyl alcohol, and if necessary, covering the transducer with a sterile glove or sleeve. Sterile standoff ultrasound pads or jelly pads have been used in some human studies to facilitate ultrasound-guided catheterization, although their application in veterinary medicine has not been evaluated.
A major advantage of the out-of-plane technique is that it is more forgiving; the operator does not have to maintain both the catheter and the transducer in the same plane to be able to visualize the stylet tip. It is also possible to adjust the stylet tip location within the subcutaneous tissues to re-center the catheter over the vessel if alignment is slightly off. The disadvantage of out-of-plane technique is the risk of passing the catheter tip beyond the ultrasound beam without realizing this has occurred. A black shadow will often appear below the white dot when this happens.
The difficulty with the in-plane technique is that it requires more practice to keep the catheter perfectly aligned in the plane of the ultrasound beam, as being off plane by even 1–2 degrees, or failing to maintain the catheter directly in the center of the ultrasound transducer, precludes catheter visualization. Achieving this alignment is more difficult with smaller peripheral vessels, particularly given the narrow width (1–2 mm) of the ultrasound beam projected by linear array transducers. An advantage of in-plane technique is the fact that surrounding structures can always be visualized as the catheter is advanced.
Complications are uncommon and occur at a similar rate to blind peripheral catheter insertion techniques. In human medicine reported complications vary depending on which site is used for ultrasound-guided vascular access and include paresthesias, brachial artery puncture, hematoma formation, and IV decannulation.
To Avoid Complications
The best target will be the vessel that is the largest and most superficial, for deep vessels, angle your catheter at a steeper angle than you would for a superficial vessel (35–45 degrees). Ultrasound can be used to verify the state of the vessel before and after the catheterization, in order to remove the venous access before inflammation or infection will be evident. In veterinary medicine a standardized evaluation of the state of vessel has been published by Lodzinska et al. in 2019; in their study the authors put in evidence that wall thickening, decreased compressibility, filling defects consistent with intraluminal thrombus, vessel wall hyperechogenicity, and abnormal color Doppler flow were related to clinical phlebitis. Early identification of phlebitis, verification of venous thrombosis and the correct positioning of a venous or arterial catheter are all valuable applications of the ultrasound in critical care. With ultrasound it is, therefore, possible to check the positioning of the tip inside the vessel and also how much of the catheter is inside the vessel. The inclination of the catheter when entering the skin and the tissues is a key factor when positioning an ultrasound guided venous/arterial access; a small slope will make the needle to enter in the vein more distant in respect to the puncture site in the skin, thus a shorter portion of catheter will enter inside the vessel, increasing the possibility of the catheter to exit from the vein.
Ultrasound guided regional anaesthesia (UGRA) is well documented in veterinary medicine, becoming one of the most popular fields of research in veterinary anaesthesia over the past 10 years. The use of ultrasound guidance to perform nerve blocks allows greater precision with less risk; it allows visualization of the needle as it is advanced through the tissues and ensures the needle tip is precisely located in proximity to the desired nerve/structures prior to final injection of local anaesthetics. The most common technique used is in plane with the nerve visualized in short axis. This allows verification that the needle is in proximity to, but not in contact with or located within the nerve. Moreover, UGRA allows distribution of local anesthetic in real time during the injection process. Use of specific UGRA echogenic needles, that are less sharp than standard hypodermic needles, reduces the risk of nerve damage and increases needle visualization.
The easiest nerve to block using ultrasound guidance is the sciatic nerve, along the lateral aspect of the hindlimb. A successful nerve block at this level provides partial analgesia of the knee and near complete analgesia of the portion of the leg distal to the knee. This block has a high success rate because the sciatic nerve is easily recognizable as two hypoechoic round structures (tibial and peroneal nerves) surrounded by a hyperechoic layer located between the fascia of the biceps femoris, the adductor, and the semimembranosus and semitendinosus muscles. The location of the sciatic nerve (due to its depth within the surrounding muscles) allows good visualization of the needle throughout the procedure when using the lateral approach, reducing the possibility of nerve damage and increasing the rate of success. This block can be performed under light sedation making it very useful when lower dosages of systemic analgesics are desired (e.g., in some trauma patients).
More recently, a new kind of UGRA has started to find its way into practice: interfascial anesthesia. With this approach the target structure is not a specific nerve, but rather an interfascial plane, within which several nerves are located. The technique requires use of higher anaesthetic volumes to block as many nervous structures as possible, within proximity to each other.
Regional interfascial anesthesia in the emergency patient can be very advantageous, as it reduces systemic analgesic requirements, thus reducing the unwanted side effects of sedation, anorexia, nausea, and vomiting. The ultrasound-guided transversus abdominis plane (TAP) block was the first and most famous block within this category; the target is the interfascial plane between the transversus muscle and the internal oblique muscle, where the abdominal nerves run. When successful it provides analgesia of the abdominal wall. It is, therefore, indicated to control pain related to surgical incisions following laparotomy or caudal mastectomy. There is also a small case report (n=3) in which a TAP block was used to successfully manage pain secondary to pancreatitis in dogs; a TAP block is believed to desensitize the parietal peritoneum which, along with the visceral pleura, becomes sensitized secondary to the inflammation associated with pancreatitis. This is a relatively simple block which can be performed in the sedated animal.
Another interesting interfascial block is the quadratus lumborum block. In this case the target is the interfascial plane between the quadratus lumborum muscle and the psoas muscle or the transverse process of the first lumbar vertebra. When successful the spread of local anesthetic covers the ventral branches of the last thoracic and the first lumbar nerves, producing both visceral and somatic analgesia of the abdominal compartment. This block is often used in human medicine, and although results are promising, the majority of veterinary publications are limited to cadaveric studies. This block is more technically challenging and requires greater operator experience to perform for several reasons; the depth and the position of target structures require an inclination of the needle that can limit the visibility of the entire needle during ultrasound guidance, which makes the target structures more challenging to identify, and target structures are located in proximity to several important abdominal structures (kidney, large blood vessels, etc.).
The last but not least interfascial block is the erector spinae (ESP) block. This technique is one of the newest UGRA blocks. The target is the interfascial plane between the transverse processes of the thoracic vertebrae and the erector spinae muscle group, which includes the iliocostalis, spinalis, and longissimus muscles. It thus blocks the dorsal-medial and dorsal-lateral branches of the spinal nerves and provides analgesia to the dorsal portion of the back; however, a systematic review of the human literature demonstrates it is also beneficial for thoracotomies, mastectomies, and abdominal surgeries. The mechanism of action of this block is not clear but a case report in veterinary medicine suggests this block can also be useful for abdominal pain secondary to pancreatitis. An advantage of this block is that it can be done with the animal slightly sedated, and the region of injection is lacking vital structures (e.g., organs, vessels, etc.).
In conclusion, ultrasound is becoming more and more important in clinical veterinary practice and its use can increase the quality of the medical care of veterinary patients.
References
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