ELECTROPHYSIOLOGY

Depolarization of the heart is the initiating event for cardiac contraction. The electric currents that spread through the heart are produced by three components: cardiac pacemaker cells, specialized conduction tissue, and the heart muscle itself.The ECG,however, records only the depolarization (stimulation) and repolarization (recovery) potentials generated by the atrial and ventricular myocardium.

The depolarization stimulus for the normal heartbeat originates in the sinoatrial (SA) node, or sinus node, a collection of pacemaker cells.These cells fire spontaneously; that is, they exhibit automaticity. The first phase of cardiac electrical activation is the spread of the depolarization wave through the right and left atria, followed by atrial contraction. Next, the impulse stimulates pacemaker and specialized conduction tissues in the atrioventricular (AV) nodal and His-bundle areas; together, these two regions constitute the AV junction. The bundle of His bifurcates into two main branches, the right and left bundles, which rapidly transmit depolarization wavefronts to the right and left ventricular myocardium by way of Purkinje fibers. The main left bundle bifurcates into two primary subdivisions, a left anterior fascicle and a left posterior fascicle. The depolarization wavefronts then spread through the ventricular wall, from endocardium to epicardium, triggering ventricular contraction.

Since the cardiac depolarization and repolarization waves have direction and magnitude, they can be represented by vectors. Vectorcardiograms that measure and display these instantaneous potentials are no longer used much in clinical practice.However, the general principles of vector analysis remain fundamental to understanding the genesis of normal and pathologic ECG waveforms.Vector analysis illustrates a central concept of electrocardiography—that the ECG records the complex spatial and temporal summation of electrical potentials from multiple myocardial fibers conducted to the surface of the body.This principle accounts for inherent limitations in both ECG sensitivity (activity from certain cardiac regions may be canceled out or may be too weak to be recorded) and specificity (the same vectorial sum can result from either a selective gain or a loss of forces in opposite directions).

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Care Plan For Decreased Cardiac Output

Nursing diagnosis for decreased cardiac output may be related to altered myocardial contractility, inotropic changes; alterations in rate, rhythm, electrical conduction; or structural changes, such as valvular defects and ventricular aneurysm.

It is possibly evidenced by increased heart rate (tachycardia), dysrhythmias, ECG changes; changes in BP (hypotension, hypertension); extra heart sounds (S3, S4); decreased urine output; diminished peripheral pulses; cool, ashen skin and diaphoresis; orthopnea, crackles, JVD, liver engorgement, edema; or chest pain

Desired outcomes for this nursing diagnosis are, client will have Cardiac Pump Effectiveness-NOC by evaluation criteria

• Display vital signs within acceptable limits, dysrhythmias absent or controlled, and no symptoms of failure, for example, hemodynamic parameters within acceptable limits and urinary output adequate.

• Report decreased episodes of dyspnea and angina.

Client also will have Cardiac Disease Self-Management-NOC by evaluation criteria Participate in activities that reduce cardiac workload.

Possible intervention : Hemodynamic Regulation-NIC by action such as

• Auscultate apical pulse; assess heart rate, rhythm, and document dysrhythmia if telemetry available. Tachycardia is usually present, even at rest, to compensate for decreased ventricular contractility. Premature atrial contractions (PACs), paroxysmal atrial tachycardia (PAT), PVCs, multifocal atrial tachycardia (MAT), and AF are common dysrhythmias associated with HF, although others may also occur. Note: Intractable ventricular dysrhythmias unresponsive to medication suggest ventricular aneurysm.

• Note the heart sounds. S1 and S2 may be weak because of diminished pumping action. Gallop rhythms are common (S3 and S4), produced as blood flows into noncompliant, distended chambers.

• Palpate peripheral pulses. Decreased cardiac output may be reflected in diminished radial, popliteal, dorsalis pedis, and post-tibial pulses. Pulses may be fleeting or irregular to palpation, and pulsus alternans may be present.

• Inspect skin for pallor and cyanosis. Pallor is indicative of diminished peripheral perfusion secondary to inadequate cardiac output, vasoconstriction, and anemia. Cyanosis may develop in refractory HF. Dependent
  areas are often blue or mottled as venous congestion increases.

• Monitor urine output, noting decreasing output and dark or concentrated urine. Kidneys respond to reduced cardiac output by retaining water and sodium. Urine output is usually decreased during the day because of fluid shifts into tissues, but may be increased at night because fluid returns to circulation when client is recumbent.

• etc.

 

 

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Electrocardiography: Equipment Preparation

One of the most valuable and frequently used diagnostic tools, electrocardiography (ECG) measures the heart's electrical activity as waveforms. Impulses moving through the heart's conduction system create electric currents that can be monitored on the body's surface. Electrodes attached to the skin can detect these electric currents and transmit them to an instrument that produces a record (the electrocardiogram) of cardiac activity.

ECG can be used to identify myocardial ischemia and infarction, rhythm and conduction disturbances, chamber enlargement, electrolyte imbalances, and drug toxicity.
The standard 12-lead ECG uses a series of electrodes placed on the extremities and the chest wall to assess the heart from 12 different views (leads). The 12 leads consist of three standard bipolar limb leads (designated I, II, III), three unipolar augmented leads (aVR, aVL, aVF), and six unipolar precordial leads (V1 to V6). The limb leads and augmented leads show the heart from the frontal plane. The precordial leads show the heart from the horizontal plane.
The ECG device measures and averages the differences between the electrical potential of the electrode sites for each lead and graphs them over time. This creates the standard ECG complex, called PQRST. The P wave represents atrial depolarization; the QRS complex, ventricular depolarization; and the T wave, ventricular repolarization. (See Reviewing ECG waveforms and components.)

Variations of standard ECG include exercise ECG (stress ECG) and ambulatory ECG (Holter monitoring). Exercise ECG monitors heart rate, blood pressure, and ECG waveforms as the patient walks on a treadmill or pedals a stationary bicycle. For ambulatory ECG, the patient wears a portable Holter monitor to record heart activity continually over 24 hours.
Today, ECG is typically accomplished using a multichannel method. All electrodes are attached to the patient at once, and the machine prints a simultaneous view of all leads.

Equipment
ECG machine ; recording paper ; disposable pregelled electrodes ; 4″ × 4″ gauze pads ; optional: clippers, marking pen.

Preparation of equipment
Place the ECG machine close to the patient's bed, and plug the power cord into the wall outlet. If the patient is already connected to a cardiac monitor, remove the electrodes to accommodate the precordial leads and minimize electrical interference on the ECG tracing. Keep the patient away from objects that might cause electrical interference, such as equipment, fixtures, and power cords.

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Cardiovascular Disorders: The Leading Cause of Death

The responsibility of caring for patients with these disorders pervades nearly every area of nursing practice. As a result, cardiovascular care ranks as one of the most rapidly growing areas of nursing. In addition, it's one of the most rapidly changing fields, with the continuing proliferation of new diagnostic tests, new drugs and other treatments, and sophisticated monitoring equipment. Consequently, nurses face a constant challenge to keep up with the latest developments.

Today, nurses assume much of the responsibility for preparing patients physically and psychologically for their hospitalization and ongoing care. Specifically, they play a pivotal role in teaching patients and their families about test and procedure preparation and follow-up care, drugs and other treatments, disease prevention, and lifestyle modification. Through patient teaching, nurses can help patients reduce stress and comply with prescribed therapy.

Cardiac and hemodynamic monitoring represent critical cardiovascular care responsibilities. Cardiac monitoring involves either hardwire or telemetric systems that continuously record the patient's cardiac activity. This makes monitoring useful not only for assessing cardiac rhythm, but also for gauging a patient's response to drug therapy and for preventing complications associated with diagnostic and therapeutic procedures. Once used only in critical care areas, cardiac monitoring is now performed in high-risk obstetric, general medical, pediatric, and transplantation departments.

Similarly, hemodynamic monitoring has become more widely used since its inception in the 1970s. It uses invasive techniques to measure pressure, flow, and resistance within the cardiovascular system. Made with a pulmonary artery (PA) catheter, these measurements are used to guide therapy. Hemodynamic monitoring includes pulmonary artery pressure monitoring, cardiac output measurement, right ventricular ejection fraction and volume measurement, temporary pacing through the PA catheter, and continuous evaluation of mixed venous oxygen saturation.

In cardiovascular emergencies, nurses may perform or assist with cardiopulmonary resuscitation, defibrillation, cardioversion, and temporary pacing. Carrying out these life-saving procedures calls for in-depth knowledge of cardiovascular anatomy, physiology, and equipment as well as sound assessment and intervention techniques. Only nurses with up-to-date information and sharpened skills can provide safe, effective patient care.

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