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Definition

 When performing a loco‐reginal block, significant bleeding due to puncture of a blood vessel or inadvertent intravascular injection of the local anesthetic, either in a vein or an artery, can occur.

Risk factors

 Use of blind techniques

 Lack of knowledge of anatomy of the region

 Injecting the local anesthetic with no previous aspiration

 Significant bleeding may occur in horses with coagulopathies

Pathogenesis

When a nerve is targeted to perform a block, there is always an associated vein and artery nearby; therefore, there is always the potential to puncture a blood vessel and consequently to induce bleeding and hematoma formation in the area. Similarly, there is the potential to inadvertently inject intravenously or intra‐arterially. This can lead to systemic toxicity, which could be even lethal, depending on the dose of local anesthetic administered intravascularly.

In the spinal canal, there are many blood vessels that could be punctured when performing an epidural injection, the most prominent being two venous plexuses at the floor of the canal, that run parallel to the spinal cord on each side. When performing a proximal paravertebral block with the needle inserted paramedially (parallel to the sagittal plane, separated a few centimeters from the spinal canal), there is also risk of arterial or venous puncture, especially if the needle is advanced too far as it could reach the abdominal aorta (left side) or caudal vena cava (right side).

A recent retrospective study in horses looked at the complications associated with loco‐regional anesthesia for dental procedures, and found that hematoma occurred in 5 out of 270 blocks performed, giving a 1.8% incidence rate [1]. There is a report of retrobulbar hematoma formation in a dog following inadvertent puncture of a blood vessel during a maxillary block, which led to exophthalmos, periorbital swelling, extensive scleral hemorrhage and ecchymosis [2]. In humans, bleeding or intravascular cannulation occurred in 0.67% of cases where an epidural technique was performed [3]; however, this article did not report the consequences of this complication.

Systemic toxicity related to local anesthetics injected intravascularly inadvertently usually starts with the development of neurological signs and it is followed by signs of cardiovascular toxicity. There are no published reports of systemic toxicity in horses following regional anesthetic blocks, most likely because the toxic dose is normally higher than the dose administered locally. In small animals, seizures occurred in two medetomidine‐sedated dogs following subcutaneous administration of lidocaine for skin biopsies, although in these animals a very high dose was used and most likely this caused the systemic toxicity and not an inadvertent intravascular injection [4]. Severe cardiovascular depression was reported in an anesthetized cat immediately following mandibular nerve block with bupivacaine and seizure‐like activity upon recovery, which could have been due to inadvertent intravascular injection as the dose administered was low [5]. In the human literature, there are reports of inadvertent intravascular injection during different types of blocks, leading to seizures and/or cardiac arrest; however, the overall incidence of major complications is very low [6].

Prevention

Knowledge of the anatomy, careful needle insertion and avoiding passing the needle repeated times should decrease the risk of puncturing a blood vessel.

Aspiration before injection should be done to ensure no intravascular injection. Once it has been ascertained that the needle is not in a vessel it should not be moved and injection performed. Whenever the needle is repositioned aspiration should be done again before injecting.

Ultrasound‐guided needle insertion can prevent puncturing undesired structures such as blood vessels [7, 8].

The toxic dose of the local anesthetic should be calculated for the individual horse, and the total administered dose should be below this toxic dose.

When performing epidural injections, the risk of puncturing a venous plexus is lower when the needle is in midline and is not advanced to the floor of the canal, which is where the venous sinuses run, on both sides of the spinal cord.

When performing paravertebral blocks, the needle should be advanced carefully until it reaches the transverse process of a vertebra and then “walked off” the process and advanced only one or two more centimeters to avoid reaching the abdomen.

Loco‐regional blocks, especially epidural or paravertebral injections, should be avoided in animals with coagulation defects.

Diagnosis

If blood is observed in the hub of the needle while it is being advanced, it is advisable to reposition the needle until blood flow stops or to abort the procedure and repeat it using a new needle in a slightly different location.

 Inadvertent intravascular injection may just lead to block failure if the total dose was low. But it could also lead to systemic signs of toxicity. The first signs are neurological due to central nervous system toxicity, starting with rapid eye blinking, ataxia, progressing to sedation, muscle twitching, seizures and unconsciousness [9]. When the intravascular dose of local anesthetic is high enough to cause cardiovascular toxicity, the signs may include ventricular premature beats, ventricular tachycardia and/or fibrillation followed by cardiovascular collapse and arrest [10].

 The clinical signs of local anesthetic toxicity are different in conscious and anesthetized animals. Anesthetized animals are more resistant to the central nervous system toxicity and no seizures are observed, while cardiovascular depression might occur at lower doses than in conscious animals [10].

Treatment

Normally no specific treatment is necessary for hemorrhage/hematoma if the horse’s coagulation is normal. If there is a clotting problem or the bleeding is significant, administration of an antifibrinolytic agent could be considered such as tranexamic acid or epsilon‐aminocaproic acid. If the hematoma is big, drainage of the blood may be attempted, as well as application of local cold treatment and local and/or systemic administration of non‐steroidal anti‐inflammatory agents.

When systemic toxicity is noticed, the administration of local anesthetic should be halted. Treatment of systemic toxicity is supportive as there is no reversal agent. If seizures are observed, an anticonvulsant drug such as a benzodiazepine (i.e. diazepam) can be administered, although it may be safer to induce general anesthesia with a barbiturate (i.e. thiopental). Supportive treatment consists of endotracheal intubation, oxygen administration and controlled respiration [11]. Signs of cardiovascular toxicity induced by lidocaine or mepivacaine are usually mild and reversible with positive inotropic drugs such as dobutamine and fluid therapy. Longer acting local anesthetics such as bupivacaine, levobupivacaine or ropivacaine are more cardiotoxic and the cardiac arrhythmias that they produce are usually malignant and refractory to routine treatment (i.e. ventricular tachycardia or fibrillation). In these cases, administration of a low dose of epinephrine (for cardiac arrest), amiodarone (for ventricular tachycardia) or defibrillation (for ventricular fibrillation) are the recommended treatments. An intravenous infusion of a 20% lipid emulsion (“lipid rescue”) is recommended to treat refractory arrhythmias induced by highly lipophilic local anesthetics (i.e. bupivacaine), as it has been shown to be the only effective treatment in different experimental models [12, 13] and in human clinical reports [14, 15].

Expected outcome

The consequences and the prognosis of hemorrhage/hematoma could be serious depending on the location and amount of blood lost. It is likely that this complication occurs commonly in practice but that it does not carry any serious consequence for the animal. An immediate consequence to this complication could be a less effective or an ineffective block, due to the dilution and entrapment of the local anesthetic within the blood/clots.

If significant bleeding occurs within the spinal canal following an epidural injection, this could lead to spinal cord compression, which depending at what level it occurs, it could lead to ataxia and/or recumbency of the animal. Puncture of the caudal vena cava or the aorta when performing a proximal paravertebral block could lead to significant intra‐abdominal bleeding; however, no reports of such complication could be found by the author.

The outcome of local anesthetic systemic toxicity is generally good when only mild central nervous system signs are observed (i.e. muscle fasciculations); however, it could be fatal if seizures occur as the horse may injure itself. When cardiovascular toxicity occurs, this could lead to irreversible cardiac arrest, particularly when using the longer acting local anesthetics (i.e. bupivacaine).