A second-degree atrioventricular (AV) block type II is also known as Mobitz type II second-degree AV block. This bradycardic rhythm is identified through an electrocardiogram (ECG) and is caused by an irregular block of atrioventricular conduction below the AV node. A second-degree AV block type II is an unstable warning rhythm that can lead to a complete heart block (third-degree AV block) or ventricular asystole without proper identification and treatment.
A patient with a second-degree AV block type II may exhibit:
This type of heart block may be attributed to structural damage within the AV conduction system of the heart. Patients exhibiting this heart dysrhythmia often have underlying heart disease.
Potential causes of this rhythm include:
Initial steps the healthcare provider should take in caring for a patient, according to the Advanced Cardiac Life Support (ACLS) Primary Assessment, are to manage the patient's airway, provide supplemental oxygen if needed, determine the patient's cardiac rhythm, and monitor vital signs. Intravenous access should be obtained, as well as a 12-lead ECG, if possible. The healthcare provider should also assess and identify potential causes for the patient’s clinical condition by evaluating H's and T's during this initial phase of care.
In determining that an adult patient is symptomatic with a consistent bradyarrhythmia (such as a second-degree AV block type II) and displays signs and symptoms of poor perfusion (as previously described), the healthcare provider should utilize the ACLS Adult Bradycardia Algorithm to guide treatment. Depending on the patient’s clinical condition and response (or lack thereof) to treatment, it may be necessary to move quickly through the treatment options outlined within the ACLS Adult Bradycardia Algorithm to prevent the patient from experiencing cardiac arrest.
For the patient with poor perfusion due to a consistent second-degree AV block type II, the ACLS Adult Bradycardia Algorithm recommends starting with 1mg of atropine administered intravenously, which can be repeated every 3-5 minutes, up to a total dose of 3mg. Although atropine is indicated as a first-line medication in the ACLS Adult Bradycardia Algorithm, patients with a second-degree AV block type II may not respond to this medication or it may worsen the heart block, increasing the risk for clinical deterioration to a complete heart block or ventricular asystole. If the atropine is not effective, the healthcare provider can administer beta-adrenergic medications while concurrently preparing the patient for pacing.
Dopamine and epinephrine infusions are not considered first-line treatments for unstable bradycardia. However, when the patient does not respond to the administration of atropine, these medications can be used as alternatives. The ACLS Adult Bradycardia Algorithm indicates that dopamine can be administered via IV infusion at 5-20 mcg/kg per minute, or, epinephrine may be administered via IV infusion at 2-10 mcg/minute. These medications can be used as the patient is being prepared for pacing (in an effort to prevent the patient from declining) and should be titrated based on the patient’s response.
Because there is a risk for patients with a cardiac rhythm of a second-degree AV block type II to deteriorate clinically into a complete heart block (third-degree AV block) or ventricular asystole, it is imperative that pacing be considered in patients who are hemodynamically unstable due to bradycardia. Transcutaneous pacing (TCP) provides an external electrical stimulus through electrodes applied to the patient’s skin (often through a defibrillator with a pacing function) to pace the heart. TCP can be performed by ACLS providers and should be urgently considered in symptomatic patients experiencing poor perfusion with a second-degree AV block type II. TCP can be painful for the patient; patients who are conscious should be sedated prior to this intervention, if possible. TCP is generally considered a bridge treatment until the patient can be transferred to a higher level of care for expert consultation and transvenous pacing.
A second-degree AV block type II rhythm is an indication that a patient will likely require a permanent pacemaker; transvenous pacing will be necessary until a permanent pacemaker is placed.
A second-degree AV block type I occurs at the AV node. Each impulse is gradually prolonged until one is unsuccessful at being conducted to the ventricles. With this rhythm, the PR interval lengthens gradually until a QRS complex is dropped; there are more P waves than QRS complexes. There may be a repetitious pattern in a series, with one less QRS complex than P waves (such as a P: QRS ratio within a series of 4:3, 3:2, as examples).
A second-degree AV block type II occurs when AV conduction is intermittently blocked below the AV node. While there are similarly more P waves than QRS complexes in this rhythm, P waves are regular, PR intervals are consistent, and the dropped QRS complexes often occur unexpectedly, and when present, are often wide.
In a second-degree AV block type II, AV conduction is irregularly blocked below the AV node. There are more P waves than QRS complexes, P waves are consistent and regular, PR intervals are uniform, and QRS complexes drop unexpectedly. QRS complexes in this rhythm tend to be wide.
In a third-degree AV block (also known as a complete AV block), atrial impulses cannot initiate ventricular contraction, resulting in an escape rhythm that paces the ventricles at an inherent rate. AV dissociation occurs, wherein atrial and ventricular rhythms are independent of each other. With a third-degree AV block, there will be more P waves than QRS complexes, the P waves will be equidistant with each other, the R waves will be equidistant with each other, and the PR intervals increase with each beat.
Mastery of rhythms such as this are key to passing your ACLS or PALS exam, and being prepared to respond effectively when a patient is experiencing a cardiac emergency.
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