EKG

Junctional rhythm

A junctional rhythm originates in the AV junction and has a rate of 40 to 60 beats/minute. Because the P wave occurs simultaneously with the QRS complex, or more commonly is retrograde (seen after the QRS complex located in the ST segment or T wave), it appears to be absent or inverted. Sinus bradycardia has a normal P wave for every QRS complex, a PR interval of 0.10 seconds, and a heart rate of 40 to 60 beats/minute. A normal sinus rhythm has one upright P wave for every QRS complex, a PR interval of 0.12 seconds, and a heart rate of 60 to 100 beats/minute.

An 85-year-old man is admitted to the telemetry unit with shortness of breath. He has a history of paroxysmal atrial fibrillation as well as recurrent ventricular arrhythmias, for which he takes amiodarone. You note that a prolonged QT interval has developed on his ECG rhythm strip. Which arrhythmia must you closely monitor the patient for now?

Amiodarone can prolong the QT interval because it lengthens the action potential and prolongs the effective refractory period. This can lead to a torsades de pointes rhythm, a lethal form of rapid, polymorphic ventricular tachycardia. A prolonged QT interval isn't associated with developing an atrial tachycardia rhythm or an idioventricular rhythm

What sometimes results from an abnormally prolonged QT interval?

An abnormally prolonged QT interval indicates a prolonged relative refractory period, which places the ventricles at risk for premature contractions that can lead to torsades de pointes, a lethal rhythm. Junctional escape beats result from significant bradycardia or block rhythms. Escape atrial beats occur after a long sinus pause, usually from an SA node block.

What is the best way to determine the ventricular rate in an irregular rhythm?

Because 10 seconds x 6 = 60 seconds, it's easy and accurate to count the number of R waves in a 6-second rhythm strip and multiply by 10. This method works for regular and irregular rhythms. For a regular rhythm, you may count the number of large blocks between two consecutive R waves and divide by 300

What is automaticity?

Cardiac cells, especially pacemaker cells, have the ability to initiate an impulse; this ability is called automaticity. Excitability is the ability of cardiac cells to respond to an impulse. Conductivity is the ability of cardiac cells to transmit an impulse.

On which factor does contractility depend?

Contractility depends on preload, which is the amount of blood volume remaining in the left ventricle at the end of diastole. The greater the blood volume in the ventricle, the more the myocardial fibers are stretched. The Starling law states that the greater the myocardial fiber stretch, the more forceful the contraction. Afterload is the amount of pressure the left ventricle must overcome to pump blood out into the circulatory system. Heart rate does not affect contractility.

Initial treatment of ventricular fibrillation includes successful cardiopulmonary resuscitation and early defibrillation for successful conversion to a normal rhythm. What is the first-line drug to treat ventricular fibrillation?

Epinephrine is the first drug used to treat ventricular fibrillation once cardiopulmonary resuscitation and at least one defibrillation event has occurred. Epinephrine can be administered every 3 to 5 minutes during cardiopulmonary resuscitation. Lidocaine is rarely used owing to adverse CNS effects, and has primarily been replaced with amiodarone. Adenosine is administered for atrial, not ventricular, arrhythmias.

Which condition most commonly causes peaked, notched, or enlarged P waves on an ECG rhythm strip?

Hypertrophied atria cause P waves to appear peaked, notched, or enlarged because the impulse must travel through more tissue or damaged tissue. Early depolarization of the atria produces a "bumpy" T wave, in which the P wave is buried in the T wave. A wandering atrial pacemaker causes P waves that vary in appearance, indicating that the impulse originates from somewhere other than the sinoatrial node.

In a junctional rhythm, what happens in the heart's conduction system when the P waves are absent?

In a junctional rhythm, the impulse originates in the lower part of the atria, producing retrograde and antegrade conduction. When the impulse reaches the atria and ventricles at the same time, the atria and ventricles depolarize simultaneously, and the P wave is buried in the QRS complex. If the ventricles did not depolarize, the QRS complex would not occur. The presence of the QRS complex confirms ventricular depolarization.

Which statement accurately characterizes the PR interval in a wandering atrial pacemaker?

In a wandering atrial pacemaker, electrical impulses originate from different areas of the atria, from the SA node, or from the AV junction. Therefore, the PR interval is measurable and variable (not regular), but is always less than 0.20 seconds. Normally, the PR interval is between 0.12 to 2.0 seconds

Failure to pace

In failure to pace, as shown in the first rhythm strip, the pacemaker does not fire when it should; no pacemaker spike is evident, thus no myocardial stimulus occurs. This may result from a faulty pacing lead, a poor connection between the lead and the pacemaker, or an improperly placed pacemaker lead. An X-ray can confirm pacemaker lead placement. In failure to sense, the pacemaker fails to recognize the patient's native myocardial activity, or it senses myocardial activity inappropriately and incorrectly generates an impulse. In failure to capture, a pacemaker spike occurs, but doesn't result in myocardial activity.

Which ECG rhythm strip shows that impulses generated by the SA node are completely blocked from reaching the ventricles?

In third-degree or complete AV block, the SA node generates impulses at 60 to 100 beats/minute, but none of these impulses reach the ventricles. The atria and ventricles act independently of each other, and the P waves seem to march through the QRS complexes. In first-degree AV block, the electrical impulse is delayed as it travels from the SA node to the Purkinje fibers, which results in a prolonged PR interval. In type II second-degree AV block, the PR interval is constant, but impulses occasionally don't reach the ventricles, resulting in some P waves without QRS complexes

A man is admitted to the cardiac intermediate care unit at 2000 hours with the rhythm shown below. He has no complaints, and his vital signs are stable. At 2300, the patient calls for assistance. He reports that he is light-headed and having trouble breathing. You assess his vital signs and find that his blood pressure is 88/56 mm Hg. Which complication is related to these signs and symptoms and the rhythm shown below?

Initially, a type II second-degree atrioventricular (AV) block may cause no signs or symptoms. As the number of dropped beats increases and the ventricular rate slows, cardiac output decreases, which can produce dyspnea, hypotension, and light-headedness. An acute myocardial infarction may cause—not result from—type II second-degree AV block. Type II second-degree AV block does not cause hypertension.

In a patient with normal sinus rhythm, which lead has an upright P wave?

Lead II looks at the heart's inferior wall with the electrical activity traveling from the right arm (which is a negative lead) to the left leg (which is a positive lead). Electrical activity that travels toward the positive pole produces a positive deflection. Leads V1 and aVR, have P waves that are biphasic or have a negative (inverted) deflection.

Which criterion correctly establishes an abnormally elevated ST segment?

Measure ST segment deflection just 0.04 seconds past the J point. Consider the ST segment abnormal if it is elevated 1 mm or more from the isoelectric line. ST-segment elevation of 0.5 mm doesn't meet the criterion. Elevation of 2 mm or more exceeds the minimum criterion for ST-segment elevation.

On an ECG rhythm strip, the P wave represents:

On an ECG rhythm strip, the P wave represents atrial depolarization. The QRS complex represents ventricular depolarization. Atrial repolarization occurs during ventricular depolarization (when the QRS complex occurs).

Sick sinus syndrome may result from a sinus node dysfunction that involves which cardiac cell characteristic?

Sick sinus syndrome is caused by a disturbance in the way impulses are generated. It may result from a dysfunction of the sinus node's automaticity (ability to initiate an impulse spontaneously). Conductivity is the cardiac cells' ability to transmit an impulse from one cardiac cell to another. Contractility is the cardiac cells' ability to contract after receiving an impulse.

SInus arrhythmia

Sinus arrhythmia is a mildly irregular rhythm that corresponds to the respiratory cycle. During inspiration, blood flow increases, which reduces vagal tone and increases heart rate. Venous return decreases during expiration, which increases vagal tone and slows the heart rate. The PR interval, QRS complex, and T wave are normal. Sinus arrest occurs when the sinus node fails to fire. The atria do not depolarize, and the heart pauses for at least three beats; no complexes appear on the rhythm strip. Atrial fibrillation is usually an irregular rhythm and has fibrillatory waves instead of P waves. The ventricular rate can vary from 100 to 150 beats/minute and isn't influenced by breathing.

The PR interval represents the time it takes for:

The PR interval represents the time it takes for the SA node to generate an electrical impulse, and for this impulse to travel down the conduction system to the ventricles. The QRS complex represents ventricular depolarization. Atrial repolarization occurs during ventricular depolarization (when the QRS complex occurs).

On an ECG rhythm strip, the QRS complex represents:

The QRS complex represents ventricular depolarization. The P wave represents atrial depolarization. Atrial repolarization occurs during ventricular depolarization and isn't visible on the rhythm strip.

On an ECG rhythm strip, the ST segment represents:

The ST segment corresponds to the end of ventricular depolarization and the start of ventricular repolarization. The T wave represents ventricular repolarization; no ECG waveforms represent atrial repolarization. The QRS complex represents ventricular depolarization.

R on T

The T wave corresponds to the period when ventricular repolarization occurs. This vulnerable period is also known as the relative refractory period. When an electrical stimulus, such as a PVC, occurs on the preceding T wave (R-on-T phenomenon), it can stimulate repetitive ventricular contractions and chaotic rhythms, such as ventricular tachycardia or ventricular fibrillation. Although a pair of PVCs can cause ventricular tachycardia, this strip has only one PVC. In ventricular trigeminy, every third beat is a PVC.

Which function does the T wave represent?

The T wave represents ventricular repolarization. The P wave represents atrial depolarization. The PR interval is the time the electrical impulse takes to travel from the atria to the ventricles.

Which electrolyte imbalance is linked to the formation of a prominent U wave?

The U wave is not always visible on an ECG rhythm strip. If present, the U wave follows the T wave and may be linked to hypokalemia. The U wave's appearance may be related to delayed repolarization of the Purkinje fibers. U waves may also occur in hypercalcemia, but not in hypocalcemia, hypernatremia, or hypermagnesemia

Which statement most accurately describes the U wave?

The U wave may not appear on an ECG rhythm strip. A prominent U wave occurs in hypercalcemia, hypokalemia, or digoxin toxicity or bradycardia, or may be due to Class I antiarrhythmic drugs. The U wave's origin and significance are not completely understood, but may be related to repolarization in the ventricular conduction fibers. The U wave does not represent recovery of the bundle branch fibers. If present, the U wave appears after the T wave.

A patient with a history of heart failure is admitted to the telemetry unit with shortness of breath, a weight gain of 3 lb (1.4 kg) in 2 days, and a sensation of "butterflies in my chest." The patient's medications include furosemide 40 mg P.O. daily, digoxin 0.125 mg P.O. daily, and carvedilol 25 mg P.O. twice daily. After connecting the patient to the telemetry monitor, you observe the rhythm shown below. Which laboratory tests should you expect to be ordered?

The patient has a normal sinus rhythm with frequent premature junctional contractions (PJCs). The cause of the PJCs should be determined. A common cause of PJCs is digoxin toxicity, which can be detected by the digoxin level. Other common causes of PJCs include inferior wall myocardial infarction, hypoxia, and caffeine use. A complete blood count cannot reveal the cause of PJCs. An increased or decreased calcium level may cause shortening or lengthening of the QT interval.

Which statement best describes the relative refractory period?

The relative refractory period is the vulnerable period in which a strong impulse can be conducted before the cardiac cells have completely repolarized. The refractory period is the period between the repolarization and depolarization of cardiac cells. The absolute refractory period is the period in which the cardiac cells have not repolarized enough to contract, no matter how strong the impulse.

A patient with the rhythm shown below is admitted to the telemetry unit. What structure is acting as the pacemaker in this patient's heart? (No P waves, wide QRS)

The rhythm is an accelerated junctional rhythm. In this type of rhythm, the pacemaker is the AV junction, which produces a rate of more than 60 beats/minute but less than 100 beats/minute and a narrow QRS. The P wave may be inverted, absent, or after the QRS complex. In a sinus rhythm, the pacemaker is in the SA node, as indicated by a normal P wave and PR interval. If the ventricles assume the pacemaker role, no P waves occur, the rate is less than 40 beats/minute, and the QRS complexes are wide and bizarre.

MAT

The rhythm is multifocal atrial tachycardia (MAT), which is characterized by P waves with at least three different shapes. Impulses do not originate from the sinus node, but are produced from different foci of the atria. In MAT, the rhythm is irregular, and the ventricular rate varies from 101 to 250 beats/minute. MAT occurs in patients with COPD who are experiencing an exacerbation and hypoxia.

Afib

The rhythm strip reflects atrial fibrillation, which results in a loss of atrial kick with decreased ventricular filling time. This causes decreased cardiac output, especially if the ventricular rate is high. Atrial fibrillation is a rapid rhythm that originates in the atria, not from the AV node. Acute myocardial ischemia typically causes changes in the ST segment, which is not present in the rhythm strip. Cardiac tamponade, which results from the pericardium filling with blood or fluid, can cause low QRS voltage

How does the sympathetic nervous system affect the heart?

The sympathetic nervous system releases the neurotransmitters norepinephrine and epinephrine. These neurotransmitters speed the heart rate and increase automaticity, contractility, and conduction of impulses in cardiac cells

Which ECG rhythm is responsible for most out-of-hospital, sudden cardiac deaths?

Untreated ventricular fibrillation is responsible for most out-of-hospital, sudden cardiac deaths. In ventricular fibrillation, different locations in the ventricles generate electrical impulses, resulting in no effective ventricular contractions. Ventricular tachycardia can occur in short bursts or may be sustained, and can degenerate into ventricular fibrillation. Torsades de pointes may degenerate into ventricular fibrillation, but this is less common than ventricular fibrillation.

What is the proper method of placing ECG electrodes on a patient?

When placing ECG electrodes on a patient, rub the skin with a gauze pad or a washcloth to remove dead cells and improve electrical contact between the skin and the electrodes. To reduce the risk of infection, clip—don't shave—the hair on the chest. If the patient has oily skin, you may use alcohol, but the skin must dry before you apply the electrodes to ensure adhesion and reduce irritation.

How should you measure the ST segment?

You should measure the ST segment from the J point (the point on the strip where the S wave returns to the isoelectric line) to the beginning of the T wave. Measuring the ST segment from the beginning or end of the S or R wave to the beginning or end of the T wave results in an inaccurate measurement.