Cardiology in Critical Care

Bradycardia

nursebob 07/25/10

There are two types of slow heart rate or bradycardia according to the American Heart Association, absolute bradycardia and relative bradycardia. Absolute bradycardia refers to any heart rate below 60 beats per minute. If the heart rate is to low to maintain normal blood pressure and the patient become symptomatic, or if there’s a decrease in cardiac output devices may be suffering from relative bradycardia. With this condition the patient may become weak and diaphragmatic and show symptoms of heart failure.

With both absolute and relative bradycardia the nurse must be aware of the classic signs and symptoms of hemodynamic compromise. Among these are decreased level of consciousness, shortness of breath, chest pain or discomfort, dizziness, hypotension diaphoresis and pulmonary congestion.

In addition to categorizing bradycardias according to the heart rate, they can be categorized according to that part of the hearts conduction system that is affected. Generally, there are three types of arrhythmias that originated from the hearts conduction system. They are sinus bradycardia, junctional escape rhythms, and ventricular escape rhythms. These blocks are categorized by the degree and sight affected.

Normal cardiac conduction proceeds from the sinoatrial (SA) node, to the atrial ventricular (AV) node and to the bundle of His, which is an extension of the atrial ventricular node. It then travels to the rest of the conduction system including the left and right bundle branches and the Punkinje fibers.

At the cellular level various cardiac disturbances can cause bradycardia is. Normally, one of these is the sinoatrial nodes inability to generate electrical impulses.

Sinus bradycardia originates in the sinoatrial node and is characterized by a heart rate less than 60 beats per minute. The rhythm is usually regular, the P waves are identical, normal and constant and each precedes a QRS complex, which are usually of normal width each following a P wave. Keep in mind that a slow heart rate may be normal in some people, such as athletes.

Junctional escape rhythms originate in the atrial ventricular junction or the atrial ventricular and bundle of His. The rate is usually 40-60 bpm and usually regular. The P waves are usually identical and each are might proceed or follow QRS complexes. They maybe absent if they occurred during the QRS complex. The PR intervals is usually < 0.12 seconds if regularly following QRS complexes, usually < 0.2 seconds.

The QRS is usually > 0.12 seconds wide. This usually occurs when the rate of the patient’s primary pacemaker or the SA node falls below that of the AV junction escape pacemaker.

Ventricular escape rhythms originate in the bundle branches. Purkinje system, or ventricular myocardium. A patient with this arrhythmia will have a hart rate of below than 40 beats per minute. It occurs when the rights for both the patients’ dominant and AV junction escape pacemakers following below the rate of the ventricular escape pacemakers.

Perhaps the greatest challenge to nursing is the correct interpretation of AV blocks. An AV block is a delaying or interruption of the electrical impulse conduction through the AV junction. The difference between this and other forms of bradycardia is that there is no specific heart rate associated with AV blocks. Often these are referred to as intranodal blocks. If these internal blocks occur at the level of the AV a normal QRS complex may be present. On the other hand, an infranodal block occurs below the AV node and Purkinje system, there may be wide QRS complexes.

In addition to the infranodal and intranodal types, AV blocks can be categorized as complete or incomplete. Incomplete AV blocks included first-degree AV block, type I second-degree AV block, and type II second-degree AV block.

In first-degree AV block, electrical impulses from the atria to the ventricles are constantly delayed, usually at the level of the AV node.

The heart rate is normal usually 60-100 beats per minute. It can be associated with sinus bradycardia or tachycardia. The rhythm is usually regular. The pacemaker site is the SA node. The P waves are identical with each proceeding a QRS complex. The QRS complexes as are usually normal with each following a P wave. A normal rhythm and electrocardiogram waveform may be present but there will be a prolonged PR interval (normal, 0.12-0.2 seconds). First-degree AV block usually requires no treatment. It does require close observation since it can progress to a more serious type of AV block.

With a type I second-degree AV block or Wenckebach, the electrical impulses are progressively delayed through the atrial ventricular node until conduction is completely blocked.

The heart rate may be normal are slow. Ventricular rate is greater than the atrial right. The atrial rhythm is usually regular while the ventricular rhythm is irregular. The P waves are identical with each proceeding a QRS complex except when the P wave is not conducted. There will be a progressive lengthening of the PR interval followed by a P wave that is not associated with a QRS complex. There is a progressively lengthening PR interval until the QRS complexes dropped. This block is usually reversible; however, it may progress to a more serious block.

With type II second-degree AV block, the electrical impulses are blocked below the level of the AV node. A dropped beat or nonconducted P wave may occur without prior lengthening of the PR interval. When they appear, QRS complexes are usually wide. Type II second-degree AV blocks commonly occur as a result of extensive myocardial damage, for example, following acute myocardial infarction. Since this block may progress rapidly to third degree AV block, it is more serious than type I.

Third degree AV block commonly called complete AV block occurs when the conduction between the atria and the ventricles is completely interrupted. With third-degree heart block, the P waves have no association with the QRS complexes. This is characterized by a atrial and ventricular rhythms that are independent of each other, this block may occur at the level of the AV note, bundle of His, or bundle branches.

Patients who have a third degree AV block with normal QRS complexes and heart rate of 40-60 beats per minute usually have conduction blocked at the AV node level. This condition is usually reversible and has a more favorable outcome. However, patients who have a third degree AV block with a wide bizarre. QRS complex and a heart rate of 40 beats per minute are less, usually have conduction blocked below the level of the AV node. This arrhythmia is rarely reversible and the patient usually has a poor prognosis. This condition may quickly deteriorate to ventricular asystole. The patient will probably need a permanent pacemaker.

REFERENCES:

Denise Drummond Hayes, RN, CCRN, MSN “Bradycardia Keeping The Current Flowing”, Nursing97, June 1997, 50-56

 

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