ECG Module 7: AV Blocks

Third Degree Block

(also known as AV dissociation) This is also known as complete heart block or AV dissociation. This is a very dangerous rhythm because in this instance, the AV node in not transmitting ANY impulses through to the ventricle. The door that is the AV node is closed and locked!. As a result, there are basically two different rhythms running. The atrial rate and ventricular rates are different and the P waves do not in any way correspond to the QRS complexes *the only heart block where the P waves can be buried*

Causes of a First Degree Block

AV node ischemia, digitalis toxicity or side effect of other medications like beta blockers or calcium channel blockers • Generally causes no symptoms

Anthony is in the CCU following an acute inferior MI. His monitor alarms for slow rate and the nurse notes a second degree AV block type II. What are the nurse's priorities? a. Reassure the patient and turn the monitor alarm off b. Apply 100% oxygen via non rebreather, anticipate using the transcutaneous pacer, notify the physician of the change in patient's condition c. Administer atropine 1 mg IVP immediately per protocol d. Administer adenosine 6 mg IVP immediately based on standing orders

B, For a patient in Second Degree AV block, type II, the potential for cardiovascular collapse is high. The best course of action for these patients is the transcutaneous pacer until a permanent pacer can be placed.

Atropine is used in symptomatic bradycardia and occassionally in second degree AV block. The mechanism of action for atropine is: a. A sympathomimetic (acts like the sympathetic nervous system) b. Anti arrhythmic c. Para sympathetic antagonist (blocker) d. Parasympathetic agonist (enabler)

C, Atropine is a drug that takes off the brakes and allows the heart rate to increase, by blocking the actions of the para sympathetic nervous system.

The nurse is caring for a patient who had a cholecystectomy yesterday. The patient was placed on the telemetry unit overnight to recover because of a history of atrial fibrillation. The physician has written orders for digitalis 0.25mg IVP this morning. Upon assessing the patient, the nurse notes the patient to have a PR interval of 0.28 seconds. What should the nurse do? a. Give the Digitalis as ordered b. Hold the digitalis and let the physician know when he/she rounds on Mrs Nelson c. Give half the dose of the digitalis and note it in the MAR d. Hold the digitalis and call the physician to advise him / her of the change in rhythm.

D, the nurse needs to hold this medication as Digitalis can worsen an AV block. The physician needs to know about this rhythm change early, so that other medications can be changed as well.

Pathophysiology of a Type 1 Second Degree Block

Pathophysiologically speaking, the AV node is ischemic and, as a result, waxes and wanes with the patient's rhythm. So the AV node transmits appropriately, then gets a little fatigued and delays the transmission for the next couple of beats, then just closes the door to a transmission, rests for a beat and starts up again. Generally, the patient is fine hemodynamically. There is no change in blood pressure, level of consciousness or urine output. As practitioners, we do not generally consider Type I second degree block to be particularly malignant. It generally does not convert to a more dangerous rhythm and there are certainly people in the general population who have it and are not aware.

Things that cause the AV node to close and lock

can be ischemia or injury to the tissue surrounding the AV node as a result of coronary artery disease. So remembering again that the activity of the AV node is reflected in the PR interval, you can understand that the PR interval and the relationship of the P wave to the QRS complex is going to be the key to identifying AV blocks.

Nursing Implications of a First Degree Block

careful with medications like beta blockers, digitalis preparations and calcium channel blockers

Treatment for type II second degree block is aggressive

o Atropine* o Transcutaneous pacemaker and possibly o Dopamine infusion if hypotension does not normalize with the pacemaker. *We use atropine very cautiously in patients with high grade blocks (second and third degree) due to increased myocardial oxygen consumption.

Causes of a type 2 Second Degree Block

o ischemia to the AV node. o digitalis type medications o beta blockers. o anti-arrhythmics are potentially pro arrhythmics, meaning that they can cause dysrhythmia. So it is important to watch patients receiving things like amiodarone, lidocaine, procainamide and quinidine for dysrhythmias.

Hemodynamics with 3rd degree Block

the hemodynamic consequences of third degree block is significant. The cardiac output is dramatically compromised and the perfusion suffers. Patients present with altered mental status and hypotension as well as shortness of breath. High flow oxygen with potential endotracheal intubation is usually needed.

Type I Second Degree Block

this rhythm frequently goes unnoticed because it is seems erratic. One must look closely to identify the pattern

Treatment of 3rd Degree Block

transcutaneous pacemaker followed by a permanent pacemaker. Dopamine may be used to support blood pressure in the presence of hypotension that does not resolve with the implementation of the transcutaneous pacemaker

Summarize all Blocks

• First degree block: impulses delayed, but they all get through. There is a P wave for every QRS and a QRS for every P wave • Second degree block, type I: Impulses are delayed sometimes and most, but not all, of the impulses are transmitted through the AV node. The are occassional P waves without QRS's. The PR interval gradually lengthens until the QRS is dropped (or skipped) • Second degree block type II: Some of the impulses get through. There are P waves without QRS complexes. Consistent PR interval • Third degree block: None of the impulses get through. There are P waves without QRS complexes and QRS complexes without P waves. PR interval is very inconsistent

Type 2 Second Degree Block Criteria

• Rate can vary depending on the degree of the blockage. If the P waves are firing at 80 / minute and only 50 percent of the impulses are being transmitted, then the ventricular rate is 40. The atrial rate is always faster than the ventricular rate. • P waves are normal appearing, upright and regular. There are P waves without QRS complexes however. • PR interval is consistent where there is a QRS following the P wave. Generally, the PR interval is also within the normal limits of 0.12 - 0.20 seconds. • QRS complex appears normal, however, there are fewer QRS complexes than P waves • Most of the time type II second degree block is easy to identify because one can easily distinguish isolated P waves between the QRS complexes. Many times about 50% of the P waves have no corresponding QRS complex

Third Degree Block Criteria

• Rate: the ventricular rate is between 20 and 40, so it is very slow. The atrial rate may be 60 or 70. • Regularity: the atrial rate and the ventricular rate will be regular, but they will be dissociated. • P waves look very normal except that they are not followed by a QRS. It may be difficult to find all of the P waves as they may be hidden inside a QRS or the T wave. • PR interval: there is no consistent PR interval • QRS is usually a bit widened (>0.12 seconds). QRS complexes are not preceded by a P wave.

First Degree Block

• This is the least problematic for the patient • This type of block is simply a slight delay of the impulse transmission across the AV node • The rhythm meets all other criteria for sinus rhythm • The PR interval is greater than 0.20 seconds (of 5 small boxes). Remember this is measured from the beginning of the P wave to the beginning of the QRS complex. Normal PR interval is, of course, 0.12 seconds to 0.20 seconds. Often these rhythms will have a PR of 0.24 or perhaps 0.28 seconds. Much longer than that and it is likely that the P wave and the QRS are not "talking" at all. • Can occur at almost any rate, but usually not tachycardic Very regular if underlying rhythm is sinus rhythm (which it usually is) P waves are upright and corresponding P wave and QRS complex PR Interval is > 0.20 seconds • First degree block is considered to be completely benign. Many patients will return from open heart surgery with a mild first degree block that spontaneously resolves. Other patients have had first degree block most of lives and are completely unaware. This rhythm generally has absolutely no impact on hemodynamics or perfusion.

Type 2 Second Degree Block

• This rhythm can be a very dangerous rhythm. In type II block the rhythm is interrupted by a dropped QRS complex without warning. • Only some of the P waves are associated with a QRS complexes. In these cases, the PR interval is consistent and normal ( 0.12 - 0.20 seconds). • However, there are P waves without QRS complexes, which represents an impulse that is not transmitted across the AV node. • This is an indication of a very sick (ischemic) AV node. • The concern is that the AV node could stop transmitting all together and the patient would be left with only P waves. This, of course, is simply a form of asystole and leads to cardiovascular collapse!

Two Types of Second Degree Block

• Type I (also known as Wenckebach) is the less significant block in terms of impact to the patient, but it is the most difficult to identify. • Type II (Also known as Mobitz) has greater significance for the patient but is easier to identify.

Type 1 Second Degree Block Criteria

• a gradually lengthening PR interval over about 3 or 4 cycles until the P wave fires and no QRS complex responds. • The PR interval may be 0.16 seconds for the first cycle, then 0.20 for the second cycle and 0.24 for the third complex, then there will be a P wave and no corresponding QRS complex. • Then the next cycle will look normal starting with the PR interval of 0.16 again. • From a distance one would notice a grouping effect of the QRS complexes. This is your first clue • Rate is different between atrium and ventricle. The atrial rate is greater due to P waves without QRS complexes • Irregular rhythm with groups of QRS complexes evident • P waves are upright and normal. All Ps except the blocked P wave has a corresponding QRS complex • PR interval gradually lengthens until a QRS complex is dropped, then the PR interval returns to normal and gradually lengthens again. • QRS is usually normal except for the missing QRS. The width of the QRS should be < 0.12 seconds.

Causes of 3rd Degree Block

• ischemia to the AV node. This can occur in the presence of an AMI • digitalis • beta blockers • hypoxia

Nursing Implications of a Type 1 Second Degree Block

• watch for worsening blocks • It is very important to be careful about medications like beta blockers, calcium channel blockers and digitalis preparations with this rhythm, they can make AV blocks worse. • If the heart rate is very slow (less than 50) the patient may require atropine, but most patients with type I second degree block do not require intervention