Special Diagnostic, Special Specimen Collection, and Postmortem Studies



Special Diagnostic, Special Specimen Collection, and Postmortem Studies






OVERVIEW OF SPECIAL DIAGNOSTIC STUDIES

These special studies have been selected for discussion because of their great diagnostic value in identifying diseases and disorders of certain organs and systems. Tests after death serve to identify previously undiagnosed disease; evaluate accuracy of predeath diagnosis; provide information about sudden, suspicious, or unexplained deaths; assist in organ donation and postmortem legal investigations; and promote quality control in health care settings.


THE EYE


Visual Field Testing

This procedure is used in conjunction with basic eye tests to evaluate and rule out glaucoma. The visual field exam may detect diseases that affect the eye, optic nerve, or brain. Small blind spots in the visual field begin to appear early in glaucoma.


Reference Values


Normal

Negative for blind spots






Retinal Imaging

Retinal imaging (with the scanning laser ophthalmoscope [SLO]) is performed by an optometrist and is used to evaluate the back of the eye. The retina, or the inside layer at the back of the eye, is responsible for the majority of vision (Fig. 16.3).

Cause of vision changes and general health can be diagnosed by viewing the retina. SLO technology uses red and green lasers to detect eye disease and monitor treatment. Green laser (532 mm) scans the sensory retina through the pigment epithelium layers of the retina. Red laser (633 mm) scans the deeper structures of the retina, from the pigment epithelium deep into the choroid. Unlike conventional imaging, Optomap retinal images are made at varying depths, providing additional diagnostic information. This provides up to a 200-degree internal view of the retina and is captured within 0.25 seconds. Even though the patient may not be aware that vision is affected, signs of systemic disease such as diabetes, hypertension, and retinal disease, including macular degeneration, may be seen.


Reference Values


Normal

Retinal scan: healthy eye with no diseases noted





Retinal Nerve Fiber Analysis

This procedure evaluates glaucoma by use of microscopic laser technology to precisely measure the thickness of the retinal nerve fiber of the eye and is recorded in computerized data for analysis. It is this nerve layer that receives and transmits images and gives us vision.


Reference Values


Normal

No abnormalities of retinal nerve fiber

Normal thickness of retinal nerve layer






Fluorescein Angiography (FA)

The purpose of this test is to detect vascular disorders of the retina that may be the cause of poor vision. Fluorescein, a yellow-red contrast substance, is injected intravenously over a 10- to 15-second period. Under ideal conditions, retinal capillaries 5 to 10 µm in diameter can be visualized using FA. Images of the eye, taken by a special camera, are studied to detect the presence of retinal disorders. Choroidal circulation is not seen with color photographs.


Reference Values


Normal

Normal retinal vessels, retina, and circulation






Electro-oculography (EOG)

This test of retinal function is used in the study of suspected hereditary and acquired degeneration of the retina. As a measurement of retinal function, electro-oculography (EOG) serves primarily to complement electroretinography (ERG) by determining the functional state of retinal pigment epithelium, as in retinitis pigmentosa. EOG determines the electrical potential of the eye at rest in both darkness and light. Normally, the potential difference between the front and back of the eye should increase as light intensity increases.


Reference Values


Normal

>1.85 ratio (Arden ratio: maximum height of the retinal potential in light divided by the minimum height of the potential in the dark)






Electroretinography (ERG)

The electroretinography (ERG) is used to study hereditary and acquired disorders of the retina, including partial and total color blindness (achromatopia), night blindness, retinal degeneration, and detachment of the retina in cases in which the ophthalmoscopic view of the retina is prohibited by some opacity, such as vitreous hemorrhage, cataracts, or corneal opacity. When these disorders exclusively involve either the rod system or the cone system to a significant degree, the ERG shows corresponding abnormalities.

In this test, an electrode is placed on the eye to obtain the electrical response to light. When the eye is stimulated with a flash of light, the electrode will record potential (electric) change that can be displayed and recorded on an oscilloscope. The ERG is indicated when surgery is considered in cases of questionable retinal viability.


Reference Values


Normal

Normal A and B waves






Eye and Orbit Sonograms

Ultrasound can be used to describe both normal and abnormal tissues of the eyes when no alternative visualization is possible because of opacities caused by inflammation, hemorrhage, or dense cataracts. This information is valuable in the management of eyes with large corneal leukomas or conjunctival flaps and in the evaluation of the eyes for keratoprosthesis. Orbital lesions can be detected and distinguished from inflammatory and congestive causes of exophthalmus with a high degree of reliability. An extensive preoperative evaluation before vitrectomy or surgery for vitreous hemorrhages is also done. In this case, the vitreous cavity is examined to rule out retinal and choroidal detachments and to detect and localize vitreoretinal adhesions, choroidal lesions, and intraocular foreign bodies. It can also be used to detect optic nerve drusen. Persons who are to have intraocular lens implants after removal of cataracts must be measured for the length of the eye (within 0.1 mm).


Reference Values


Normal

Normal image pattern indicating normal soft tissue of eye, retrobulbar orbital areas, retina, choroid, and orbital fat





Interfering Factors

If, at some time, the vitreous humor in a particular patient was replaced by gas or silicone oil, no result may be obtained.




BRAIN AND NERVOUS SYSTEM


Electroencephalography (EEG) and Epilepsy/Seizure Monitoring

The EEG measures and records electrical impulses from the brain cortex. It is used to investigate causes of seizures, to diagnose epilepsy, and to evaluate brain tumors, brain abscesses, subdural hematomas, cerebral infarcts, and intracranial hemorrhages, among other conditions. It can be a tool for diagnosing narcolepsy, Parkinson’s disease, Alzheimer’s disease, and certain psychoses. It is common practice to consider the EEG pattern, along with other clinical procedures, drug levels, body temperature, and thorough neurologic examinations, to establish electrocerebral silence, otherwise known as “brain death.” The American Electroneurodiagnostic Society sets guidelines for obtaining these recordings. When an electrocerebral silence pattern is recorded in the absence of any hope for neurologic recovery, the patient may be declared brain dead despite cardiovascular and respiratory support.

Epilepsy/seizure monitoring using simultaneous video and EEG recordings (online computer) is done to verify a diagnosis of epilepsy, when seizures begin, and how they appear. The results differentiate and define seizure type, localize region of seizure onset, quantify seizure frequency, and identify candidates for medical implantation of vagus nerve stimulator or surgical treatment of seizures. Hospital admission is required.


Reference Values


Normal



  • Normal, symmetric patterns of electrical brain activity


  • Range of alpha: 8-11 Hertz (cycles per second)


  • Seizure monitoring: expected outcome of at least three typical recorded seizures that may be different from what the patient usually experiences because medications have been reduced; also, onset area and type of seizures


  • No cross-circulation of internal carotid arteries


  • Evidence of hemispheres to support language and memory


Procedure for EEG



  • Scalp hair should be recently washed.


  • Fasten electrodes containing conduction gel to the scalp with a special skin glue or paste. Seventeen to 21 electrodes are used according to an internationally accepted measurement known as the 10-20 System. This system correlates electrode placement with anatomic brain structure.


  • Place the patient in a recumbent position, instruct to keep the eyes closed, and encourage patient to sleep during the test (resting EEG). (Seizure activating procedure [see numbers 4 to 6]).


  • Before beginning the EEG, some patients may be instructed to breathe deeply through the mouth 20 times per minute for 3 minutes. This hyperventilation may cause dizziness or numbness in the
    hands or feet but is nothing to be alarmed about. This activating breathing procedure induces alkalosis, which causes vasoconstriction, which in turn may activate a seizure pattern.


  • Place a light flashing at frequencies of 1 to 30 times per second close to the face. This technique, called photic stimulation, may cause an abnormal EEG pattern not normally recorded.


  • Be aware that certain persons may be intentionally sleep deprived before the test to promote sleep during the test. Administer an oral medication to promote sleep (e.g., Valium chloral hydrate). The sleep state is valuable for revealing abnormalities, especially different forms of epilepsy. Make recordings while the patient is falling asleep, during sleep, and while the patient is waking.


  • Remove electrodes, glue, and paste after the EEG. The patient may then wash the hair.


  • Follow guidelines in Chapter 1 for safe, effective, informed intratest care.


Procedure for Seizure Monitoring



  • Apply electrodes, take EEG, and explain video and EEG monitoring (for up to 6 days). An electrode panel is applied and must be covered when patient eats. Patient remains in bed except to use the bathroom; a helmet is worn when out of bed.


  • Perform neuropsychological testing to evaluate memory (remember objects), language (circles, squares), and problem solving (4 to 6 hours of testing).


  • A cerebral angiogram to assess cross-circulation in carotids is followed by a Wada test to determine the dominant hemisphere for language and whether opposite hemisphere can support memory. An intravenous line is started and a catheter is threaded through the femoral artery to the internal carotid to inject sodium amobarbital to “put the brain to sleep” for 5 minutes in each half of the brain. The Wada test is also known as the amobarbital study or intracarotid amytal test or the Brevital (when sodium methohexital is used) test.


  • Perform a functional brain magnetic resonance imaging (MRI) study. Procedure time is about 90 minutes. Patient wears earphones and is asked to respond to questions, sounds, and pictures by pressing a special button.


  • A combined positron emission tomography/computed tomography (PET/CT) scan is often done to provide further information about brain hemispheres.




Interfering Factors



  • Sedative drugs, mild hypoglycemia, or stimulants can alter normal EEG tracings.


  • Oily hair, hair spray, and other hair care products interfere with the placement of EEG patches and the procurement of accurate EEG tracings.


  • Artifacts can appear in technically well-performed EEGs. Eye and body movements cause changes in brain wave patterns and must be noted so that they are not interpreted as abnormal brain waves.



Evoked Responses/Potentials: Brainstem Auditory Evoked Response (BAER); Visual Evoked Response (VER); Somatosensory Evoked Response (SSER)

These tests use conventional EEG recording techniques with specific electrode site placement for each procedure and include computer data processing to evaluate electrophysiologic integrity of the auditory, visual, and sensory pathways. These are brain responses “time-locked” to some event. See Chart 16.1 for wave and standard deviation (SD) measurements.



Brainstem auditory evoked response. This study allows evaluation of suspected peripheral hearing loss, cerebellopontine angle lesions, brainstem tumors, infarcts, multiple sclerosis, and comatose states. Special stimulating techniques permit recording of signals generated by subcortical structures in the auditory pathway. Stimulation of either ear evokes potentials that can reveal lesions in the brainstem involving the auditory pathway without affecting hearing. Evoked potentials of this type are also used to evaluate hearing in newborns, infants, children, and adults through electrical response audiometry.

Visual evoked response. This test of visual pathway function is valuable for diagnosing lesions involving the optic nerves and optic tracts, multiple sclerosis, and other disorders. Visual stimulation excites retinal pathways and initiates impulses that are conducted through the central visual path to the primary visual cortex. Fibers from this area project to the secondary visual cortical areas on the brain’s occipital convexity. Through this path, a visual stimulus to the eyes causes an electrical response in the occipital regions, which can be recorded with electrodes placed along the vertex and the occipital lobes. It is also used to assess development of blue-yellow pathway in infants.


Somatosensory evoked response. This test assesses spinal cord lesions, stroke, and numbness and weakness of the extremities. It studies impulse conduction through the somatosensory pathway. Electrical stimuli are applied to the median nerve in the wrist or peroneal nerve near the knee at a level near that which produces thumb or foot twitches. The milliseconds it takes for the current to travel along the nerve to the cortex of the brain is then measured. Somatosensory evoked responses can also be used to monitor sensory pathway conduction during surgery for scoliosis or spinal cord decompression and/or ischemia. Loss of the sensory potential can signal impending cord damage.




Interfering Factors



  • Some difficulty in interpreting brainstem evoked potentials may arise in persons with peripheral hearing defects that alter evoked potential results (i.e., subthreshold stimulation of peripheral nerves and inadequate skin preparation).


  • Maximum depolarization stimulation is divided into two protocols:



    • Brachial plexus (BP) protocol involves stimulation of the median, ulnar, and superficial sensory radial nerves just proximal to the wrist.


    • Lumbosacral (LS) protocol involves stimulating the posterior tibial and common peroneal nerves, which are the primary divisions of the lumbosacral plexus forming the sciatic nerve.




Cognitive Tests: Event-Related Potentials (ERPs)

Event-related potentials are used as objective measures of mental function in neurologic diseases that produce cognitive defects. These measurements use the method of auditory evoked response testing in which sound stimuli are transmitted through earphones. A rare tone is associated with a prominent endogenous P3 component that reflects the differential cognitive processing of that tone. Although a systematic neurologic increase in P3 component latency occurs as a function of increasing age in normal persons, in many instances of neurologic diseases associated with dementia, the latency of the P3 component has been reported to exceed substantially the normal age-matched value.

This test is useful in evaluating persons with dementia or decreased mental functioning. It is also helpful in differentiating persons with real organic brain defects affecting cognitive function from those who are unable to interact with the examiner because of motor or language defects or those unwilling to cooperate because of problems such as depression or schizophrenia.


Reference Values


Normal

No shift of P3 components to longer latencies

ERP: absolute latency of P3 waveform

P3 wave mean and SD 294 ± 21 milliseconds




Interfering Factors

Latency of P3 component normally increases with age.




Brain Mapping: Computed Tomography

Brain mapping uses transitional EEG data and specialized computer digitization to display the diagnostic information as a topographic map of the brain and spinal cord. The computer analyzes EEG signals for amplitude and distribution of alpha, beta, theta, and delta frequencies and displays the analysis as a color map. Specific or minute abnormalities are enhanced and allow comparison with normal data. This methodology is used for assessing cognitive function and for evaluating patients with migraine headaches, trauma, or episodes of vertigo or dizziness. Persons who lose periods of time and select patients with generalized seizures, dementia of organic origin, ischemic abnormalities, or certain psychiatric disorders are also candidates for this testing. With this procedure, it is possible to localize a specific area of the brain that may otherwise show up as a generalized area of deficit in the conventional EEG. Children or adults who demonstrate hyperactivity, dyslexia, dementia, or Alzheimer’s disease may benefit from evaluation through brain mapping.


Reference Values


Normal

Normal frequency signals and evoked responses presented as a color-coded map of electrical brain activity




Interfering Factors



  • Tranquilizers may alter results.


  • Unwashed hair or the use of hair preparations can interfere with electrode placement.


  • Eye and body movements cause changes in signals and wave patterns.




Electromyography (EMG); Electroneurography; Electromyoneurogram (EMNG)

Electromyoneurography combines electromyography and electroneurography. These studies, done to detect neuromuscular abnormalities, measure nerve conduction and electrical properties of skeletal muscles. Together with evaluation of range of motion, motor power, sensory defects, and reflexes, these tests can differentiate between neuropathy and myopathy. The electromyogram can define the site and cause of muscle disorders such as myasthenia gravis, muscular dystrophy, and myotonia; inflammatory muscle disorders such as polymyositis; and lesions that involve the motor neurons in the anterior horn of the spinal cord. EMG can also localize the site of peripheral nerve disorders such as radiculopathy and axonopathy. Skin and needle electrodes measure and record electrical activity. Electrical sound equivalents are amplified and recorded on tape for later studies.


Reference Values


Normal

Normal EMG and EMNG




Interfering Factors



  • Conduction can vary with age and normally decreases with increasing age.


  • Pain can yield false results.


  • Electrical activity from extraneous persons and objects can produce false results as a result of movement.


  • The test is ineffective in the presence of edema, hemorrhage, or thick subcutaneous fat.



Electronystagmogram (ENG)

This study aids in the differential diagnosis of lesions in the brainstem and cerebellum. It can confirm the causes of unilateral hearing loss of unknown origin, vertigo, or ringing in the ears. Evaluation of the vestibular system and the muscles controlling eye movement is based on measurements of the nystagmus cycle. In health, the vestibular system maintains visual fixation during head movements by means of nystagmus, the involuntary back-and-forth eye movement caused by initiation of the vestibular-ocular reflex.


Reference Values


Normal

Vestibular-ocular reflex: Normal nystagmus accompanying head turning is expected.




Interfering Factors



  • Test results are altered by the inability of the patient to cooperate, poor eyesight, blinking of the eyes, or poorly applied electrodes.


  • The patient’s anxiety or medications such as central nervous system depressants, stimulants, or antivertigo agents can cause false-positive test results.




HEART


Electrocardiography (ECG or EKG) and Vectorcardiogram

An ECG records the electrical impulses that stimulate the heart to contract. It also records dysfunctions that influence the conduction ability of the myocardium. The ECG is helpful in diagnosing and monitoring the origins of pathologic rhythms; myocardial ischemia; myocardial infarction; atrial and ventricular hypertrophy; atrial, atrioventricular, and ventricular conduction delays; and pericarditis. It can be helpful in diagnosing systemic diseases that affect the heart, determining cardiac drug effects (especially digitalis and antiarrhythmic agents), evaluating disturbances in electrolyte balance (especially potassium and calcium), and analyzing cardiac pacemaker or implanted defibrillator functions.

An ECG provides a continuous picture of electrical activity during a complete cycle. Heart cells are charged or polarized in the resting state, but they depolarize and contract when electrically stimulated. The intracellular body fluids are excellent conductors of electrical current and are an important component of this process. When the depolarization (stimulation) process sweeps in a wave across the cells of the myocardium, the electrical current generated is conducted to the body’s surface, where it is detected by special electrodes placed on the patient’s limbs and chest. An ECG tracing shows the voltage of the waves and the time duration of waves and intervals. By studying the amplitude of the waves and measuring the duration of the waves and intervals, disorders of impulse formation and conduction can be diagnosed.




Reference Values


Normal

Normal positive and negative deflections in an ECG recording

Normal cardiac cycle components (one normal cardiac cycle is represented by the P wave, QRS complex, and T wave; additionally, a U wave may be observed). This cycle is repeated continuously and rhythmically (Fig. 16.4).

The P wave indicates atrial depolarization; QRS complex indicates ventricular depolarization; T wave indicates ventricular repolarization/resting stage between beats; and U wave indicates nonspecific recovery after potentials.


Normal Waves



  • The P wave is normally upright; it represents atrial depolarization and indicates electrical activity associated with the original impulse that travels from the sinus node through the atrial sinus. If P waves are present; are of normal size, shape, and deflection; have normal conduction intervals to the ventricles; and demonstrate rhythmic timing variances between cardiac cycles, it can be assumed that they began in the sinoatrial node.


  • The Ta or Tp designation is used to differentiate atrial repolarization, which ordinarily is obscured by the QRS complex, from the more conventional T wave, which signifies ventricular repolarization (see number 8 below).


  • The Q(q) wave is the first downward/negative deflection in the QRS complex; it results from ventricular depolarization. The Q(q) wave may not always be apparent.


  • The R(r′) wave is the first upright/positive deflection after the P wave (or in the QRS complex); it results from ventricular depolarization.


  • The S(s′) wave is the downward/negative deflection that follows the R wave.


  • The Q and S waves are negative deflections that do not normally rise above the baseline.


  • The T wave is a deflection produced by ventricular repolarization. There is a pause after the QRS complex, and then a T wave appears. The T wave is a period of no cardiac activity before the ventricles are again stimulated. It represents the recovery phase after the ventricular contraction.






    FIGURE 16.4. Commonly measured complex components. (From Smeltzer SC, Bare BG: Brunner and Suddarth’s Textbook of Medical-Surgical Nursing, 8th ed. Philadelphia, Lippincott-Raven, 1996.)



  • The U wave is a deflection (usually positive) following the T wave. It represents late ventricular repolarization of Purkinje’s fibers or the intraventricular papillary muscles. This wave may or may not be present on an ECG. If it appears, it may be abnormal, depending on its configuration.



Normal Intervals



  • The R-R interval (normally, 0.83 second at a heart rate of 72 beats/minute) is the distance between successive R waves. In normal rhythms, the interval, in seconds or fractions of seconds, between two successive R waves divided into 60 seconds provides the heart rate per minute.


  • The P-P interval (normally, 0.83 second at a heart rate of 72 beats/minute) will be the same as the R-R interval in normal sinus rhythm. The responsiveness of the sinus node to physiologic activity (e.g., exercise, rest, respiratory cycling) produces a rhythmic variance in P-P intervals.


  • The PR interval (˜0.16 second) measures conduction tone and includes the time it takes for atrial depolarization and normal conduction delay in the atrioventricular node to occur. It terminates with the onset of ventricular depolarization. It is the period from the start of the P wave to the beginning of the QRS complex. This interval represents the time it takes for the impulse to traverse the atria, proceed through the atrioventricular node, and reach the ventricles and initiate ventricular depolarization.


  • The QRS interval (normally, 0.12 second) represents ventricular depolarization time and tracks the electrical impulse as it travels from the atrioventricular node through the bundle branches to Purkinje’s fibers and into the myocardial cells. Normal waves consist of an initial downward deflection (Q wave), a large upward deflection (R wave), and a second downward deflection (S wave). It is measured from the onset of the Q wave (or R if no Q is visible) to the termination of the S wave.


  • QT interval measures the duration of ventricular activation and recovery. It is measured from the beginning of the QRS complex to the end of the T wave. The QT interval varies with the heart rate, gender, and time of day. Normal QT interval is 350 to 430 milliseconds.


Normal Segments and Junctions



  • The PR segment is normally isoelectric and is the portion of the ECG tracing from the end of the P wave to the onset of the QRS complex.


  • The J junction (or J point) is the point at which the QRS complex ends and the ST segment begins.


  • The ST segment is that part of the ECG from the J point to the onset of the T wave. Elevation or depression is determined by comparing its location with the portion of the baseline between the end of the T wave and the beginning of the P wave or relating it to the PR segment. This segment represents the period between the completion of depolarization and onset of repolarization (i.e., recovery) of the ventricular muscles.


  • The TP segment (˜0.25 second) is the portion of the ECG record between the end of the T wave and the beginning of the next P wave. It is usually isoelectric.


Normal Voltage Measurements



  • Voltage from the top of the R wave to the bottom of the S wave is 1 mV. Voltage of the P wave is ˜0.1 to 0.3 mV. Voltage of the T wave is ˜0.2 to 0.3 mV. Upright deflection voltage is measured from the upper part of the baseline to the peak of the wave.


  • Negative deflection voltage is measured from the lower portion of the baseline to the nadir of the wave.



Recording the Electrical Impulses



  • Because cardiac electrical forces extend in several directions at the same time, a comprehensive view of heart activity is possible only if the flow of current in several different planes is recorded.


  • For a 12-lead ECG, 12 leads are simultaneously used to present this comprehensive picture:



    • Limb leads (I, II, III, AVL, AVF, AVR) record events in the frontal plane of the heart.


    • Chest leads (V1, V2, V3, V4, V5, and V6) record a horizontal view of the heart’s electrical activity.


  • Occasionally, an esophageal lead, which is swallowed or placed in the esophagus, can supply additional information. This type of lead is frequently used during surgical procedures.


  • His bundle electrography is a very specialized procedure that requires placement of an intravenous catheter, which is then advanced into the heart. An ECG is simultaneously being recorded while the electrical activity of the bundle of His is measured by a sensor at the end of the catheter. This test measures the electrical activity between contractions (Fig. 16.5).


ECG Versus Vectorcardiogram

The vectorcardiogram, like the ECG, records the electrical forces of the heart. The major difference between these two methods is the way in which these forces are displayed (Table 16.1). A vectorcardiogram records a three-dimensional display of the heart’s electrical activity, whereas the ECG is a single-plane representation. The following are the three planes of the vectorcardiogram:



  • Frontal plane (combines the Y and X axes)


  • Sagittal plane (combines the Y and Z axes)


  • Horizontal plane (combines the X and Z axes)






FIGURE 16.5. His bundle electrogram. Note that electrophysiologic events are presented in relation to the surface electrocardiogram. (From Phillips RE, Feeney MK: The Cardiac Rhythms, 3rd ed. Philadelphia, WB Saunders, 1990.)









TABLE 16.1 Comparison of the ECG and Vectorcardiogram














ECG


Vectorcardiogram


Records electrical forces as positive or negative deflections on a scale


Depicts electrical forces as vector* loops, which show the direction of electrical flow


Records activity in the frontal and horizontal planes


Records activity in the frontal, horizontal, and sagittal planes


* The term vector indicates the directional flow of electrical activity.





Interfering Factors



  • Race: ST elevation with T-wave inversion is more common in African Americans but disappears with maximal exercise effort.


  • Food intake: High carbohydrate content is especially associated with an intracellular shift of potassium in association with intracellular glucose metabolism. Nondiagnostic ST depression and T-wave inversion are evident with hypokalemia.



  • Anxiety: Episodic anxiety and hyperventilation are associated with prolonged PR interval, sinus tachycardia, and ST depression with or without T-wave inversion. This may be due to autonomic nervous system imbalances.


  • Deep respiration: The position of the heart in the chest shifts more vertically with deep inspiration and more horizontally with deep expiration.


  • Exercise/movement: Strenuous exercise before the test can produce misleading results. Muscle twitching can also alter the tracing.


  • Position of heart within the thoracic cage: There may be an anatomic cardiac rotation in both horizontal and frontal planes.


  • Position of precordial leads: Inaccurate placement of the bipolar chest leads and the transposition of right and left arm and left leg electrodes will affect test results. In normal persons, lead reversal produces the typical ECG findings of dextrocardia (congenital anomaly resulting in the heart being on the right side of the chest) in frontal plane leads and can mimic a myocardial infarction pattern.


  • A leftward shift in the QRS axis occurs with excess body weight, ascites, and pregnancy.


  • Age: At birth and during infancy, the right ventricle is hypertrophied because the fetal right ventricle performs more work than the left ventricle. T-wave inversion in leads V1 to V3 persists into the second decade of life and into the third decade in black persons.


  • Gender: Women exhibit slight ST-segment depression.


  • Chest configuration and dextrocardia: In this congenital anomaly in which the heart is transposed to the right side of the chest, the precordial leads must also be placed over the right side of the chest.


  • Severe drug overdose, especially with barbiturates, and many other medications can influence ECG configuration. Antiarrhythmics, antihistamines, and antibiotics can widen QT intervals.


  • The serious effects of electrolyte imbalances show up on the ECG as follows:



    • Increased Ca++: shortened QT; less frequently, prolonged PR interval and QRS complex


    • Decreased Ca++: prolonged QT


    • Alterations in K+ may produce cardiac arrhythmias.



Signal-Averaged Electrocardiogram (SAE)

The signal-averaged ECG (SAE) is a noninvasive tool for identifying patients at risk for malignant ventricular dysrhythmias, particularly after myocardial infarction. During the later phase of the QRS complex and ST segment, the myocardium produces high-frequency, low-amplitude signals termed late potentials. These late potentials correlate with delayed activation of certain areas within the myocardium, a condition that predisposes to reentrant forms of ventricular tachycardia.


Indications

SAEs are performed to evaluate the etiology of ventricular dysrhythmias or as a precursor to electrophysiologic studies. Disorders that may produce regions of delayed myocardial conduction include myocardial infarction, nonischemic dilated cardiomyopathy, left ventricular aneurysm, and some forms of healed ventricular incisions (e.g., scar from tetralogy of Fallot surgical intervention).


Reference Values


Normal

Normal QRS complexes and ST segments




Interfering Factors



  • Increased time is required for recording beats in the presence of slow heart rates or frequent ventricular ectopics. Patient movement, talking, and restlessness also delay data procurement.


  • Bundle branch block can interfere with impulse averaging.


  • SAE does not provide information about antiarrhythmic drug effectiveness.


  • Late potentials do not occur in every patient with ventricular tachycardia.


  • Ventricular pacing prolongs ventricular activation time and obscures late potentials. Conversely, atrial pacing, even at rapid rates, does not alter ventricular late potentials.

Jun 11, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Special Diagnostic, Special Specimen Collection, and Postmortem Studies
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