January 2025

In this VETgirl online veterinary continuing education blog, Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia) reviews the common arrhythmia – often seen under general anesthesia – atrioventricular (AV) block. This blog will review 1st, 2nd and 3rd AV degree in veterinary medicine.

By Amanda M. Shelby, RVT, VTS (Anesthesia & Analgesia)

AV Block in the 1st, 2nd, or 3rd Degree

Atrioventricular (AV) block can be a common occurrence in patients under sedation and/or anesthesia. All atrioventricular blocks result from varying degrees of electrical delay or disturbance between the sinoatrial node and the atrioventricular nodes within the heart. The degree of AV block is associated with characteristic changes reflected on the electrocardiogram (ECG). Learning to identify the appropriate AV block assists toward understanding causes and delivering appropriate treatments. The first step to identifying any arrhythmia is being able to recognize components of a normal ECG in the species being evaluated (see Figure 1). The next step is to ask yourself 3 simple questions:

1. What is the heart rate? Fast (tachycardia), normal, or slow (bradycardia)
2. Is there a P-wave associated with every QRS complex? And is there a QRS complex associated with every P-wave?
3. Is the rhythm predictable or regular?

Based on the answer to these three questions, the veterinary professional will likely be able to identify the arrhythmia. AV blocks are a type of bradyarrhythmia—meaning the answer to question (1) is usually the patient has a slow heart rate. Not all AV blocks are pathologic or negatively impact the patient. Let’s explore the differences between AV blocks to the first, second, and third degree.

Figure 1: Major ECG Components Identified; Photo courtesy of Amanda M. Shelby

First Degree AV Block

Figure 2: First Degree AV Block in a Horse; Photo courtesy of Amanda M. Shelby

First-degree AV block is characterized by a prolongation of the P-R interval generally seen with slower than normal heart rates. First-degree AV block can be seen in patients with high vagal tone. It is also observed following the administration of drugs, e.g. opioids and alpha2 adrenergic agonists, which cause bradycardia and increase the likelihood of bradyarrhythmia.(1, 2) First-degree AV block is commonly unnoticed, which can be demonstrated by addressing the three systematic questions (see above) for which the anesthetist would produce the answers (1) slow or normal heart rate, (2) yes there is a P wave for every QRS and QRS for every P wave, and (3) yes, the rhythm looks regular. These answers would be unlikely to prompt further investigation. Fortunately, first-degree AV block rarely requires treatment. If hypotension accompanies first-degree AV block, the veterinary care team may consider increasing heart rate by either reducing the level of anesthetic depth, reducing, or reversing the causative drug agent, and/or increasing heart rate with an anticholinergic. Reducing opioid, pressor or alpha2 agonist constant rate infusions (CRI) would be ideal over reversing the opioid or alpha2 agonist. Exercise caution when using a reversal agent as the patient may experience pain and/or rapid arousal.

Second-Degree Atrioventricular Block

Figure 3: Second-Degree AV Block in Dog; Photo courtesy of Amanda M. Shelby

Second-degree AV block (Figure 3) is best recognized by the characteristic p-wave with corresponding absent QRS complex. Patients with second-degree AV block generally have a slower than normal heart rate. Like first-degree AV block, second-degree can occur in patients with high vagal tone and following the administration of opioids, α-2 adrenergic agonists, pressors, and low doses of anticholinergics, although occurrence is generally transient.(1, 2) Second-degree AV block can also be an incidental finding in athletic patients such as racing greyhounds and performance horses. Second-degree AV block can be a progression of untreated first-degree AV block. There are two subtypes of second-degree AV block, Mobitz Type I and Type II.

Mobitz Type I is characterized by a prolonged P-R interval in the P-QRS-T complex, leading up to the eventual complete atrioventricular block complex that is characteristic of a second-degree AV block, which consists of a P-wave with an absent QRS complex. The next complete complex will have a shorter P-R interval than the complete complex preceding the block, or a P-wave with missing QRS complex. Second-degree AV block Mobitz Type I is a progression of first-degree AV block. The QRS complex morphology is generally narrow or ‘normal’ in appearance.

Mobitz Type II is different in that there is not a prolongation of the P-R interval before the complete AV block, i.e., P-wave with missing QRS complex. QRS complexes in Mobitz Type II can be narrow like ‘normal’ QRS complexes or appear wide and bizarre like that of escape beats. With Mobitz Type II, the P-R interval remains the same in the complexes throughout the ECG strip.

Second-degree AV blocks are described as low grade when two P-waves are present to missing QRS complexes (2:1) and high grade when three or more P-waves are present to missing QRS complexes (3:1). High-grade Mobitz Type II blocks accompanied by cardiac instability, syncopal episodes and hypotension, are high-risk anesthetic candidates where pacemaker implantation is often recommended treatment. Otherwise, treatment of a second-degree AV block is based on the bradyarrhythmia’s impact on blood pressure and contributing cause. If the patient is normo- or hypertensive because of administration of a drug with vasoconstrictive properties (e.g., dexmedetomidine or phenylephrine) the anesthetist often elects to wait out the duration of the vasoactive drug. If severe, some elect to administer an antagonist if available, as in the case of dexmedetomidine. However, caution should be exercised as the administration of an α-2 antagonist reverses the sedative and analgesic properties of the dexmedetomidine, and rapid patient arousal and pain may be experienced. If the patient is hypotensive while experiencing second-degree AV block, often increasing the heart rate with an anticholinergic will increase cardiac output and blood pressure and resolve the AV block.

It is worth noting that low doses of anticholinergics (e.g. atropine and glycopyrrolate) can cause paradoxical, transient first- and second-degree AV block. Generally, the AV block secondary to a low dose of anticholinergic resolves as the onset of the anticholinergic saturates the pre- and post-synaptic muscarinic receptors or the anesthetist delivers another dose. See this in action in the short video below, which is captured immediately following the administration of glycopyrrolate to a bradycardic dog.

Figure 4: Second-Degree AV Block Following Administration of Glycopyrrolate 0.005 mg/kg IV; Video courtesy of Amanda M. Shelby

Third-Degree AV Block
One would assume that third-degree AV block is the clinical progression of first to second to third AV block however, this is not always the case. Third-degree AV block is characterized by a complete dissociation between the sinoatrial (SA) node responsible for the initiation of the cardiac electrical cycle by generation of the P-wave and the atrioventricular (AV) node, responsible for the QRS complex of the heart’s electrical complex cycle. Third-degree AV block is also sometimes referred to as AV dissociation. As shown in Figure 5, the SA node produces a consistent atrial rate of about 100 beats per minute. However, the ventricular rate is slower at about 30 beats per minute. There is no association between the P-waves and QRS complexes, hence a complete dissociation between the SA and AV nodes. QRS complexes often appear wide and bizarre and are commonly referred to as an escape beat. However, the QRS complexes in third-degree AV block can also appear quite narrow and normal. The morphology of the QRS complex will depend on the location of origin for the electrical depolarization of the ventricular beat—if closer to the AV node, the QRS complex will be narrower and more normal in appearance.

Figure 5. Third Degree AV Block in a Dog; Photo courtesy of Dr. Jane Quandt, MS, DACVAA, DACVECC

Causes for third-degree AV block are unknown. There are some breeds that are more predisposed to the occurrence including chow chows, cocker spaniels and Labrador retrievers.(3) It is important to recognize that a third-degree AV block does not commonly occur ‘during’ anesthesia as a complication related to the anesthetic event or drugs. It is more likely the patient has undiagnosed third-degree AV block before the anesthetic event. While the heart rate is commonly characteristically low, it’s possible a patient is clinically asymptomatic and the arrhythmia is missed during the pre-anesthetic assessment but found once an ECG is placed. Cats are less likely to be symptomatic than dogs in third-degree AV block. Effective treatment for patients with third-degree AV block involves a pacemaker implantation. If discovered during the peri-anesthetic period, the procedure should be aborted or delayed if possible. If the anesthetic event cannot be rescheduled the owner should be notified of the patient’s increased anesthetic risk and an atropine dose should be attempted. It is common for third-degree AV block to be unresponsive to anticholinergics. It is recommended the patient be referred to a cardiologist for pacemaker implantation. It is uncommon for primary care facilities to have access to isoproterenol, a non-selective beta-adrenoceptor agonist. Isoproterenol is used to temporarily increase heart rate. Alternatively, if the pet’s owners elect to continue a procedure in light of the incidental finding of third-degree AV block, temporary pacing of the heart can be performed by some referral hospitals.

Atrioventricular blocks can be identified during the peri-anesthetic period in our patients. While first-degree goes commonly unnoticed, second-degree and third-degree can increase the anesthetist’s blood pressure. In anesthetic plans including opioids and α-2 adrenergic agonists, second-degree AV block is commonly experienced. Third-degree AV block is never benign, therefore anesthesia should be delayed and patients should be transferred to an appropriate referral hospital. Do not be discouraged, AV block in the first, second, or third degree can be easily identified by ECG interpretation by an appropriately trained veterinary professional. In most scenarios, AV blocks can be quickly resolved to optimize patient outcomes during sedative or anesthetic procedures.

References:
1. Uilenreef JJ, Murrell JC, McKusick BC, Hellebrekers LJ. Dexmedetomidine continuous rate infusion during isoflurane anaesthesia in canine surgical patients. Vet Anaesth and Analg 2008;35(1):1-12.
2. Keating S, Fries R, Kling K, et al. Effect of Methadone or Hydromorphone on Cardiac Conductivity in Dogs Before and During Sevoflurane Anesthesia. Front Vet Sci 2020;7:573706.
3. Schrope DP, Kelch WJ. Signalment, clinical signs, and prognostic indicators associated with high-grade second- or third-degree atrioventricular block in dogs: 124 cases (January 1, 1997-December 31, 1997). J Am Vet Med Assoc 2006;228(11):1710-7.

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