B P7 C68 Bradyarrhythmias and AV Blocks Flashcards
Based on large population studies of healthy individuals, the lower limit of normal resting heart rate is defined as ___ beats/min.
50 bpm
Frequently, bradyarrhythmias are physiologic, as in _____. In other cases, bradyarrhythmias can be pathologic
Well-conditioned athletes with low resting heart rates
Type I atrioventricular (AV) block during sleep
Sinus bradycardia is diagnosed in an adult when the sinus node discharges at a rate less than ___ beats/min. P waves have a normal contour, and are usually upright in leads I, II, and aVF, and occur before each QRS complex, usually with a constant PR interval longer than 120 msec. Sinus arrhythmia often coexists.
< 50 bpm
Sinus bradycardia can result from _____. In most cases, symptomatic sinus bradycardia is caused or worsened by the effects of medication.
Excessive vagal or decreased sympathetic tone
Effect of medications
Anatomic changes in the sinus node
During sleep, the normal heart rate can fall to _____ beats/min, especially in adolescents and young adults, with marked sinus arrhythmia sometimes producing pauses of ___ seconds or longer
35 to 40 bpm
Sinus arrhythmia 2s or longer
Causes of sinus bradycardia
Eye surgery
Coronary arteriography
Meningitis
Intracranial tumors
Increased intracranial pressure
Cervical and mediastinal tumors
Certain disease states (e.g., severe hypoxia, myxedema, hypothermia, fibrodegenerative changes, convalescence from some infections, gram-negative sepsis, mental depression
Vomiting
Vasovagal syncope
Carotid sinus stimulation
Administration of parasympathomimetic drugs, lithium, amiodarone, beta adrenoceptor–blocking drugs, clonidine, propafenone, ivabradine, or calcium antagonists
In most cases, sinus bradycardia is a benign arrhythmia that can actually be beneficial by producing a _____, especially in heart failure patients. Conversely, it can be associated with syncope caused by an abnormal autonomic reflex
Longer period of diastole and increasing ventricular filling time
Sinus bradycardia occurs in _____% of patients with acute myocardial infarction (MI) and may be even more prevalent when patients are seen in the early hours of infarction. Unless it is accompanied by hemodynamic decompensation or arrhythmias, sinus bradycardia is generally associated with _____ outcome after MI than sinus tachycardia.
10% to 15%
More favorable
SB is usually transient and occurs more commonly during _____than during anterior MI; sinus bradycardia has also been noted during reperfusion with thrombolytic agents.
Bradycardia that follows resuscitation from cardiac arrest is associated with a _____.
Inferior
Poor prognosis: SB after arrest
Sinus arrhythmia is characterized by a phasic variation in sinus cycle length during which the maximum sinus cycle length minus the minimum sinus cycle length exceeds ____ msec or the maximum sinus cycle length minus the minimum sinus cycle length divided by the minimum sinus cycle length exceeds ____%
> 120 ms (CLmax-CLmin)
> 10% (CLmax-CLmin/CLmin)
Treatment of sinus bradycardia is not usually necessary unless cardiac output is inadequate or arrhythmias result from the slow rate.
_____ (0.5 mg intravenously as an initial dose, repeated if necessary) is generally acutely effective; lower doses, particularly given SC or IM, can exert an initial parasympathomimetic effect, possibly by a central action.
For recurrent symptomatic episodes, _____ may be needed.
Although _____ can be used to increase the sinus rate, as a general rule, no drugs are available that increase the heart rate reliably and safely during long periods without undesirable side effects.
Atropine
Recurrent: temporary or permanent pacing
Theophylline and terbutaline
It is the most frequent form of arrhythmia and is physiologically normal. P wave morphology does not usually vary, and the PR interval exceeds 120 msec and remains unchanged because the focus of discharge remains relatively fixed within the sinus node. On occasion, the pacemaker focus can wander within the sinus node, or its exit to the atrium may change and produce P waves of a slightly different contour (although not retrograde) and a slightly changing PR interval that exceeds 120 msec.
Sinus arrhythmia
Nonrespiratory sinus arrhythmia is characterized by a phasic variation in the P-P interval unrelated to the respiratory cycle and can be the result of _____
Digitalis intoxication
In the respiratory form, the P-P interval cyclically shortens during _____, primarily as a result of reflex inhibition of vagal tone, and slows during _____; breath-holding eliminates the variation in cycle length
Inspiration: Shorter P-P
Expiration: Slowing
Loss of sinus rhythm variability is a risk factor for _____
SCD
Marked sinus arrhythmia can produce a sinus pause sufficiently long to cause syncope if it is not accompanied by an escape rhythm. Treatment is usually unnecessary.
Increasing the heart rate by _____ generally abolishes sinus arrhythmia. Symptomatic individuals may experience relief from palpitations with _____, as for the treatment of sinus bradycardia.
Exercise or drugs
Sedatives, tranquilizers, atropine, ephedrine, or isoproterenol administration
The P-P interval delimiting the pause ____ a multiple of the basic P-P interval.
Does not equal
The most common example of ventriculophasic sinus arrhythmia occurs during _____, when P-P cycles that contain a QRS complex are shorter than P-P cycles without a QRS complex. Similar lengthening can be present in the P-P cycle that follows a premature ventricular complex (PVC) with a compensatory pause. Alterations in the P-P interval are probably caused by the influence of the autonomic nervous system responding to changes in ____.
Complete AV block and a slow ventricular rate
Ventricular stroke volume
Failure of sinus nodal discharge results in the _____ and can also result in _____ if escape beats initiated by latent pacemakers do not occur. Involvement of the sinus node by acute MI, degenerative fibrotic changes, digitalis toxicity, stroke, or excessive vagal tone can produce sinus arrest.
Absence of atrial depolarization
Ventricular asystole
As a general rule, chronic pacing for sinus bradycardia is indicated only in _____ patients.
Symptomatic
_____ is an arrhythmia that is recognized electrocardiographically by a pause resulting from absence of the normally expected P wave. The duration of the pause is a multiple of the basic P-P interval.
This block is caused by a conduction disturbance during which an impulse formed within the sinus node fails to depolarize the atria or does so with delay
SA exit block
An interval without P waves that equals approximately two, three, or four times the normal P-P cycle characterizes _____-degree SA exit block.
Type II second-degree SA exit block
During _____ second-degree SA exit block, the P-P interval progressively shortens before the pause, and the duration of the pause is less than two P-P cycles
Type I (Wenckebach) second-degree SA exit block
_____ SA exit block cannot be recognized on the electrocardiogram (ECG) because SA nodal discharge is not recorded.
First-degree
_____ SA exit block can be manifested as a complete absence of P waves and is difficult to diagnose with certainty without sinus node electrograms.
Third-degree
_____, can produce SA exit block. SA exit block is usually transient. It may be of no clinical importance except to prompt a search for the underlying cause. On occasion, syncope can result if the SA block is prolonged and unaccompanied by an escape rhythm. SA exit block can occur in well-trained athletes.
Excessive vagal stimulation
Acute myocarditis
MI
Fibrosis involving the atrium
Drugs such as quinidine, procainamide, flecainide, and digitalis
Sick sinus syndrome is a term applied to a syndrome encompassing several sinus nodal abnormalities, including:
(1) Persistent spontaneous sinus bradycardia inappropriate for the physiologic circumstance
(2) Sinus arrest or exit block
(3) Combinations of SA and AV conduction disturbances
(4) Alternation of paroxysms of rapid regular or irregular atrial tachyarrhythmias and periods of slow atrial and ventricular rates (bradycardia-tachycardia syndrome)
The anatomic basis of sick sinus syndrome can involve _____.
Fibrosis and fatty infiltration occur, and the sclerodegenerative processes generally involve the sinus node and the AV node or the bundle of His and its branches or distal subdivisions. Occlusion of the sinus node artery can cause sinus node dysfunction.
(1) Total or subtotal destruction of the sinus node
(2) Areas of nodal-atrial discontinuity
(3) Iflammatory or degenerative changes in the nerves and ganglia surrounding the node
(4) Pathologic changes in the atrial wall
_____ is diagnosed when the heart rate does not increase appropriately in the setting of increased physiologic demand
Chronotropic incompetence (CI)
Although many studies for chronotropic incompetence use a definition of failure to obtain _____% of either maximal expected heart rate, or of inadequate heart rate reserve (the difference between resting heart rate and age predicted maximal heart rate), the variance in individual heart rate range can require meticulous clinical assessment
80% or 85%
_____ is the principal determinant of rate of oxygen consumption (VO2) and exercise capacity.The fourfold increase in VO2 during exercise is largely due to a 2.2-fold increase in heart rate. Genome-wide association studies have confirmed heritability of heart rate increase with exercise and heart rate recovery.
Increase in heart rate due to physiologic demand
_____ alone can confer CI; the age-related decline in heart rate response to exercise is inevitable even in healthy older athletes.
Aging
_____ occurs when a patient has tachyarrhythmias and bradyarrhythmias closely associated in time. That can occur when a tachyarrhythmia, typically atrial fibrillation or atrial flutter terminates, with a resultant excessive post-conversion pause.
Tachycardia-bradycardia syndrome (TBS)
For patients with sick sinus syndrome, treatment depends on the basic rhythm problem but usually involves permanent pacemaker implantation when symptoms are manifest. ____ for bradycardia, combined with drug therapy to treat the tachycardia, is required in those with bradycardia-tachycardia syndrome.
Pacing for bradycardia + Drug therapy for tachycardia for TBS
TBS can also occur during atrial fibrillation when periods of atrial fibrillation with rapid ventricular rates alternate with periods of excessive bradycardia (due to high-grade AV block) during atrial fibrillation.
While TBS can occur without medication, it typically occurs as a result of treatment with _____.
Beta blockers or calcium channel blockers.
_____ is a disturbance of impulse conduction that can be permanent or transient, depending on the anatomic or functional impairment. It must be distinguished from interference, a normal phenomenon that is a disturbance of impulse conduction caused by physiologic refractoriness resulting from inexcitability secondary to a preceding impulse.
Heart Block
AV block is classified by severity into three categories. _____.The degree of block may depend in part on the direction of impulse propagation.For unknown reasons, retrograde conduction can still occur in the presence of advanced anterograde AV block.
During first degree heart block, conduction time is prolonged but all impulses are conducted.
Second-degree heart block occurs in two forms, Mobitz type I (Wenckebach) and type II.Type I heart block is characterized by gressive lengthening of the conduction time until an impulse is not conducted.Type II heart block denotes an occasional or repetitive sudden block of conduction of an impulse, without prior measurable lengthening of conduction time.
When no impulses are conducted, complete or third-degree block is present
Some electrocardiographers use the term _____ to indicate blockage of two or more consecutive impulses.
Advanced or high-grade heart block
During first-degree AV block, every atrial impulse is conducted to the ventricles and a regular ventricular rate is produced, but the PR interval exceeds _____ second in adults.
PR intervals as long as 1.0 second have been noted and can at times exceed the P-P interval, a phenomenon known as _____
0.20
Skipped P waves
Acceleration of the atrial rate or enhancement of vagal tone by carotid massage can cause first-degree AV nodal block to progress to _____ AV block. Conversely, type I second-degree AV nodal block can revert to a _____ block with deceleration of the sinus rate.
Type I second-degree AVB
first-degree
Blocking of some atrial impulses conducted to the ventricle at a time when physiologic interference is not involved defines _____.
Second- degree AV block
Electrocardiographically, typical type I second- degree AV block is characterized by _____, whereas in type II second-degree AV block, the ______.
Type I 2nd degree AVB: progressive PR prolongation culminating in a nonconducted P wave
Type II 2nd degree AVB: PR interval remains constant before the blocked P wave
During a typical type I block, the increment in conduction time is greatest in the second beat of the Wenckebach group, and the absolute increase in conduction time decreases progressively over sub- sequent beats. These two features serve to establish the characteristics of classic Wenckebach group beats: ______.
Although much emphasis has been placed on this characteristic grouping of cycles, primarily to be able to diagnose a Wenckebach exit block, this typical grouping occurs in fewer than 50% of patients with a type I Wenckebach AV nodal block.
(1) the interval between successive beats progressively decreases, although the conduction time increases (but by a decreasing function)
(2) the duration of the pause produced by the nonconducted impulse is less than twice the interval preceding the blocked impulse (which is usually the shortest interval)
(3) the cycle that follows the nonconducted beat (beginning the Wenckebach group) is longer than the cycle preceding the blocked impulse.
Type II AV block often antedates the development of _____, whereas type I AV block with a normal QRS complex is generally more benign and does not progress to more advanced forms of AV conduction disturbance.
Adams-Stokes syncope and complete AV block
In a patient with an acute MI, type I AV block usually accompanies _____ infarction (perhaps more often if a right ventricular infarction also occurs),is transient, and does not require temporary pacing, whereas type II AV block occurs in the setting of acute ____ MI, can require temporary or permanent pacing, and is associated with high mortality, generally as a result of pump failure
Type I AVB: inferior infarction (transient, does not require pacing)
Type II AVB: anterior MI (Can require pacing, associated with high mortality)
A _____ of AV block can occur in patients with acute inferior MI and is associated with more myocardial damage and a higher mortality rate than in those without AV block.
High degree
First-degree and type I second-degree AV block can occur in _____, as noted earlier, probably related to an increase in resting vagal tone
Normal healthy children
Wenckebach AV block can be a normal phenomenon in well-trained athletes
In patients who have chronic second-degree AV nodal block (proximal to the His bundle) without structural heart disease, the course is generally benign (except in older age groups),whereas in those with _____ the prognosis is poor and related to the type and severity of the underlying heart disease.
Structural heart disease,
High-grade, or “advanced,” AV block is differentiated from complete AV block by an _____, yet conduction that is more impaired than in second-degree AV block
Intermittent relationship between atrial and ventricular activity
The ventricular rhythm will not be regular since the diagnosis of high-grade AV block requires demonstration of _____. Commonly, ____ consecutive non-conducted P waves are noted on ECG
Intermittent AV conduction
2 or more
Etiologies of high grade AV block include _____. The clinical presentation, symptoms, and outcomes are indistinguishable from third-degree AV block.
Acute coronary syndromes
Rheumatic heart disease
Autoimmune disorders
Myocarditis
Infiltrative cardiomyopathies
Third-degree or complete AV block occurs when _____ and therefore the atria and ventricles are controlled by independent pacemakers. Thus, complete AV block is one type of complete AV dissociation.
No atrial activity is conducted to the ventricles
Sites of ventricular pacemaker activity that are in or closer to the His bundle appear to be more _____ rate than those located more distally in the ventricular conduction system.
Stable and can produce a faster escape
The ventricular rate in acquired complete heart block is less than _____ beats/min but can be faster with congenital complete AV block
Acquired CHB: <40 bpm
Congenital: Faster
Complete AV block can result from a block at the level of the AV node, within the bundle of His, or distal to the His in the Purkinje system. Block proximal to the His bundle generally exhibits normal QRS complexes and rates of _____ beats/min because the escape focus that controls the ventricle arises in or near the His bundle.
40 to 60 bpm
In patients with AV nodal block, _____ generally speeds both the atrial and the ventricular rate.
Exercise can reduce the extent of AV nodal block.
Atropine
Acquired complete AV block occurs most often _____ to the bundle of His because of trifascicular conduction disturbance. Each P wave is followed by a His deflection, and the ventricular escape complexes are not preceded by a His deflection. The QRS complex is abnormal, and the ventricular rate is generally less than _____ beats/min.
Distal
<40 bpm
Paroxysmal AV block in some cases can be caused by exaggerated responsiveness of the AV node to _____.
Surgery, electrolyte disturbances, myoendocarditis, tumors, Chagas disease, rheumatoid nodules, calcific aortic stenosis, myxedema, polymyositis, infiltrative processes (e.g., amyloidosis, sarcoidosis, scleroderma), and an almost endless assortment of common and unusual conditions can produce complete AV block
Vagotonic reflexes
In adults, rapid rates may be followed by block (called _____ AV block), which is thought to result from phase _____ block (block caused by incomplete action potential recovery), postrepolarization refractoriness, and concealed conduction in the AV node
Tachycardia-dependent AVB
Phase 3
Less common than tachycardia-dependent AV block, _____ AV block can also occur; it results in AV block after a pause or during relative bradycardia and thus can be difficult to distinguish from vagal AV block. This form of AV block is often referred to as a phase _____ block because it is thought that spontaneous depolarizations during the resting phase of the action potential result in an inability to depolarize, although other mechanisms may also play a role.
Pause-dependent paroxysmal AVB
Phase 4
In children, the most common cause of AV block is _____. In such circumstances, the AV block can be an iso- lated finding or associated with other lesions. Neonatal autoimmune disease, from maternal antibodies crossing the placenta, accounts for most cases of heart block in utero or in the immediate neonatal period but only for rare cases of congenital heart block occurring after this period
Congenital
Many of the signs of AV block are evident at the bedside.
First-degree AV block can be recognized by a _____ a to c wave interval in the jugular venous pulse and by diminished intensity of the _____ as the PR interval lengthens
Long a to c wave interval
Diminished intensitiy of the first heart sound (S1)
In type I second- degree AV block, the heart rate may increase imperceptibly with gradually diminishing intensity of S1; widening of the a to c interval, terminated by a pause; and an a wave not followed by a v wave.
(1) heart rate may increase imperceptibly with gradually diminishing intensity of S1;
(2) widening of the a to c interval, terminated by a pause;
(3) a wave not followed by a v wave
_____ characterize type II AV block. S1 maintains a constant intensity.
In complete AV block, the findings are the same as those in AV dissociation.
Intermittent ventricular pauses and a waves in the neck not followed by v waves
Significant clinical manifestations of first- and second-degree AV block usually consist of palpitations or subjective feelings of the heart _____.
“missing a beat.”
Complete AV block can be accompanied by signs and symptoms of _____
Reduced cardiac output
Syncope or presyncope
Angina
Palpitations from ventricular tachyarrhythmias
For patients with transient or paroxysmal AV block and presyncope or syncope, the diagnosis can be elusive.
_____ can be useful, but monitoring for longer periods may be necessary, with extended (>3 weeks) Holter or external loop recorders being required.
Longer periods of recording require an _____ to establish the diagnosis.
In patients with presyncope or syncope, one should suspect intermittent infrahisian block in those with BBB or an intraventricular conduction defect. An _____ to evaluate AV conduction thoroughly (including infusion of isoproterenol and/or procainamide) may be warranted to make the diagnosis, particularly in those with severe symptoms
Ambulatory monitoring (Holter or external loop recorders)
Implantable loop recorder
EPS
Drugs cannot be relied on to increase the heart rate for more than several hours to several days in patients with symptomatic heart block without producing significant side effects. Therefore, _____ is indicated for patients with symptomatic bradyarrhythmias.
Temporary or permanent pacemaker insertion
For short-term therapy, when the block is likely to be evanescent but still requires treatment or until adequate pacing therapy can be established, vagolytic agents such as _____ are useful for patients who have AV nodal disturbances, whereas _____ such as isoproterenol can be used transiently to treat patients who have heart block at any site.
Isoproterenol should be used with extreme caution or not at all in patients with acute MI.
Atropine
Catecholamines
For symptomatic AV block or high-grade AV block (e.g., infrahisian, type II AV block, third-degree heart block not caused by congenital AV block), _____ is the treatment of choice.
There is growing evidence that some patients with AV block, especially those with preexisting left ventricle dysfunction, may benefit from _____ pacing rather than right ventricle– only pacing to prevent the development or progression of symptoms caused by heart failure
Permanent pacemaker placement
Biventricular
As the term indicates, _____ defines AV dissociation. AV dissociation is never a primary disturbance of rhythm but rather is a “symptom” of an underlying rhythm disturbance produced by one of three causes or a combination of causes that prevents the normal transmission of impulses from atrium to ventricle
Dissociated or independent beating of the atria and ventricles
The ECG in AV dissociation demonstrates the _____. P wave morphology depends on the rhythm controlling the atria—sinus, atrial tachycardia, junctional, flutter, or fibrillation.
During complete AV dissociation, both the QRS complex and the P waves appear to be regularly spaced without a fixed temporal relationship to each other.
Independence of P waves and QRS complexes
Physical findings in AV dissociation include _____
_____ waves may be seen in the jugular venous pulse when atrial and ventricular contractions occur simultaneously.
The second heart sound can split normally or paradoxically, depending on the manner of ventricular activation.
A premature beat representing ventricular capture can interrupt a regular heart rhythm. When the ventricular rate exceeds the atrial rate, a cyclic increase in intensity of the first heart sound is produced as the PR interval shortens, climaxed by a very loud sound (bruit de canon).
This intense sound is followed by a sudden reduction in intensity of the first heart sound and the appearance of giant a waves as the PR interval shortens and P waves “march through” the cardiac cycle.
(1) Variable intensity of the first heart sound as the PR interval changes
(2) Atrial sounds
(3) a waves in the jugular venous pulse lacking a consistent relationship to ventricular contraction
Intermittent large (cannon) a waves
Since the sinus node and AV node are richly innervated by the autonomic nervous system, both sinus bradyarrhythmias (sinus pauses,sinus arrest, and sinus arrhythmia) and AV block (typically type I AV block, or intermittent complete block) can be caused by _____ influences without any underlying conduction system disease.
Autonomic
Increases in parasympathetic (vagal) tone can be triggered by a variety of events that can result in bradyarrhythmias and include _____ and others
Hypersensitive carotid sinus syndrome
Vasovagal syncope
Cough syncope
Stimulation of the Bezold-Jarisch receptors during an inferior MI
Autonomic dysfunction
Neurally-mediated bradyarrhythmias are characterized most frequently by _____caused by cessation of atrial activity as a result of sinus arrest or SA exit block
Ventricular asystole
In symptomatic patients, AV junctional or ventricular escapes generally do not occur or are present at very slow rates, suggesting that heightened vagal tone and sympathetic withdrawal can _____ subsidiary pacemakers located in the ventricles, as well as in supraventricular structures.
Suppress
There are generally two types of neutrally mediated responses, and frequently both occur in the same patient to varying degrees.
A _____ response is generally defined as ventricular asystole exceeding 3 seconds, although normal limits have not been definitively established. In fact, asystole exceeding 3 seconds during carotid sinus massage is not common but can occur in asymptomatic subjects.
A _____ response is usually defined as a decrease in systolic blood pressure (SBP) of 50 mm Hg or more without associated cardiac slowing or a decrease in SBP exceeding 30 mm Hg when the patient’s symptoms are reproduced.
Cardioinhibitory
Vasodepressor
_____ acutely abolishes cardioinhibitory responses to neurally mediated bradyarrhythmias. However, symptomatic patients with a cardioinhibitory response may benefit from pacemaker implantation. Because AV block can occur with a cardioinhibitory response, some form of _____ is generally required.
Atropine
Ventricular pacing, with or without atrial pacing,
Atropine and pacing do not prevent the SBP decrease in the vasodepressor response, which may result from _____.
Inhibition of sympathetic vasoconstrictor nerves and possibly from activation of cholinergic sympathetic vasodilator fibers.
Patients who have neurally mediated bradyarrhythmias that do not cause symptoms require ___ treatment.
Drugs such as _____ can enhance neurally mediated bradyarrhythmias and be responsible for symptoms in some patients.
No
dDigitalis, methyldopa, clonidine, and propranolol
_____ may be helpful in patients with vasodepressor responses.
Elastic support hose and sodium-retaining drugs
