Electrocardiography

Chapter 3 Electrocardiography



Larry P. Tilley, Francis W.K. Smith, Jr.



INTRODUCTION


Electrocardiography in clinical practice is the recording at the body surface of electrical fields generated by the heart. Specific waveforms represent stages of myocardial depolarization and repolarization. The electrocardiogram is a basic and valuable diagnostic test in veterinary medicine and is relatively easy to acquire. The electrocardiogram is the initial test of choice in the diagnosis of cardiac arrhythmias and may also yield information regarding chamber dilation and hypertrophy.



INDICATIONS AND ROLE OF THE ELECTROCARDIOGRAM IN CLINICAL PRACTICE



Documentation of Cardiac Arrhythmias


• An electrocardiogram should be recorded when an arrhythmia is detected during physical examination. This may include bradycardia, tachycardia, or irregularity in rhythm that is not secondary to respiratory sinus arrhythmia.

• Animals presenting with a history of syncope or episodic weakness may have cardiac arrhythmias, and an electrocardiogram is indicated in these cases. Arrhythmias in such cases may be transient—a normal electrocardiogram does not rule out transient arrhythmias. In some cases, long-term electrocardiographic monitoring (Holter monitor or cardiac event recorder) is warranted.

• Arrhythmias often accompany significant heart disease and may significantly affect the clinical status of the patient. An electrocardiogram should be recorded in animals with significant heart disease.

• The electrocardiogram is also used to monitor efficacy of antiarrhythmic therapy and to determine whether arrhythmias may have developed secondary to cardiac medications (e.g., digoxin).

• Significant arrhythmias may also occur in animals with systemic disease, including those diseases associated with electrolyte abnormalities (hyperkalemia, hyponatremia, hypercalcemia, and hypocalcemia), neoplasia (particularly splenic neoplasia), gastric dilatation-volvulus, and sepsis.


Assessment of Chamber Enlargement Patterns


• Changes in waveforms may provide indirect evidence of cardiac chamber enlargement. The electrocardiogram may be normal, however, in cases with chamber enlargement. Right ventricular hypertrophy most consistently results in waveform changes.

• As heart disease progresses, waveform changes may indicate progressive chamber enlargement.

• Thoracic radiography and, ideally, echocardiography, should be performed for definitive assessment of chamber enlargement.


Miscellaneous Indications for Electrocardiography


• The electrocardiogram may provide evidence of pericardial effusion (electrical alternans, low-amplitude complexes).

• Electrocardiographic abnormalities are often present with hypothyroidism and hyperthyroidism.

• A pronounced sinus arrhythmia may be present in animals with elevated vagal tone (often seen with diseases affecting the respiratory tract, central nervous system, and gastrointestinal tract).


PRINCIPLES OF ELECTROCARDIOGRAPHY



Surface Electrodes are Placed in a Designated Fashion to Obtain Standard Electrocardiographic Leads


• A lead consists of the electrical activity measured between a positive electrode and a negative electrode.

• Electrical impulses with a net direction toward the positive electrode will generate a positive waveform or deflection. Electrical impulses with a net direction away from the positive electrode will generate a negative waveform or deflection. Electrical impulses with a net direction perpendicular to the positive electrode will not generate a waveform or deflection (isoelectric).

• Standard electrocardiographic lead systems are used to create several angles of assessment. A single lead would provide information on only one dimension of the current (e.g., left vs. right). Two leads would allow two-dimensional information (e.g., left vs. right and cranial vs. caudal). As many as 12 leads may be acquired simultaneously.


Standard Lead Systems


• The standard leads are I, II, III, aVR, aVL, and aVF (Figure 3-1, Box 3-1). Placement of electrodes to generate each lead is illustrated in Figure 3-2.

• Leads I, II, and III are bipolar limb leads. These are termed bipolar because the electrocardiogram is recorded from two specific electrodes.

• Leads aVR, aVL, and aVF are augmented unipolar leads. To generate these, two electrodes are electrically connected (as a negative electrode) and compared with the single electrode (positive).

• Precordial chest leads are obtained using an exploring unipolar positive electrode at specific locations on the chest. These leads may provide additional information or supportive evidence of cardiac chamber enlargement. They are also useful in evaluating for the P wave when limb leads are equivocal.

• The base-apex lead is often used in equine electrocardiography and may also be used in small-animal practice for rhythm assessment. A positive electrode is placed on the left side of the chest, over the heart, and the negative electrode is placed in the area of the right shoulder or neck.

• Esophageal ECG electrode lead for surgical monitoring (Figure 3-3.) This technique is very useful as complexes recorded are increased in size, providing an increased accuracy for diagnosing an arrhythmia during surgery.

• Hand-held, wireless ECG and ECG real time computer display represents new technology for recording electrocardiograms (Figures 3-4 and 3-5.)

image

Figure 3-1 The limb leads (I, II, III, aVR, aVL, aVF) surround the heart in the frontal plane as shown in the top part of the figure (feline). The circled limb lead names indicate the direction of electrical activity if the QRS is positive in that lead. The mean electrical axis in this canine ECG (bottom part of the figure) is +90. Lead I is isoelectric. The lead perpendicular to lead I is aVF (see axis chart on top). Lead aVF is positive, making the axis +90. If lead aVF had been negative, the axis would have been −90.


Rights were not granted to include this figure in electronic media. Please refer to the printed book.


(From Tilley LP: Essentials of canine and feline electrocardiography. ed 3, Malvern, Penn: 1992, Lea & Febiger.)



Box 3-1 Lead Systems Used in Canine and Feline Electrocardiography


Bipolar limb leads



I: Right thoracic limb (−) compared with left thoracic limb (+)



II: Right thoracic limb (−) compared with left pelvic limb (+)



III: Left thoracic limb (−) compared with left pelvic limb (+)


Augmented unipolar limb leads



aVR: Right thoracic limb (+) compared with average voltage of left thoracic limb and left pelvic limb (−)



aVL: Left thoracic limb (+) compared with average voltage of right thoracic limb and left pelvic limb (−)



aVF: Left pelvic limb (+) compared with average voltage of right thoracic limb and left thoracic limb (−)


Unipolar precordial chest leads plus exploring electrode



CV5RL (rV2): Right fifth intercostal space near the sternum



CV6LL (V2): Left sixth intercostal space near the sternum



CV6LU (V4): Left sixth intercostal space near the costochondral junction



V10: Over the dorsal process of the seventh thoracic vertebra


Base-apex bipolar lead


Record in lead I position on ECG machine with leads placed as follows

LA electrode over left sixth intercostal space at costosternal junction



RA electrode over spine of right scapula near the vertebra


image image

Figure 3-2 A, Three bipolar standard leads. By means of a switch incorporated in the instrument, the galvanometer can be connected across any pair of several electrodes. Each pair of electrodes is called a lead. The leads illustrated here are identified as I, II, and III. B, Augmented unipolar limb leads aVR, aVL, and aVF.


Rights were not granted to include this figure in electronic media. Please refer to the printed book.


(From Tilley LP: Essentials of canine and feline electrocardiography: interpretation and treatment, ed 3, Malvern, Penn, 1992, Lea & Febiger.)


image

Figure 3-3 Esophageal ECG electrode and temperature probe positioned for surgical monitoring. This technique is very useful as complexes recorded are increased in size, providing an increased accuracy for diagnosing an arrhythmia during surgery.


image

Figure 3-4 Hand-held, wireless ECG and ECG real time computer display.


(Courtesy Vmed Technology, Redmond, Wash. www.vmedtech.com.)


image

Figure 3-5 Wireless ECG printout report from laptop computer software system in Figure 3-4.


(Courtesy Vmed Technology, Redmond, Wash. www.vmedtech.com.)



RECORDING THE ELECTROCARDIOGRAM


• The electrocardiogram should be recorded in an area as quiet and as free of distraction as possible. Noises from clinical activity and other animals may significantly affect rate and rhythm. Any use of electrically operated equipment, such as clippers, may cause interference and should be minimized during the electrocardiogram. In some cases, fluorescent lighting may result in electrical interference.

• The patient should ideally be placed in right lateral recumbency.
• Electrocardiographic reference values were obtained from animals in right lateral recumbency.

• Limbs should be held perpendicular to the body. Each pair of limbs should be held parallel, and limbs should not be allowed to contact one another.

• The animal should be held as still as possible during the electrocardiogram. When possible, panting should be prevented.

• When dyspnea or other factors prevent standard positioning, the electrocardiogram may be recorded while the animal is standing, or, less ideally, sitting.

• Electrode placement
• Alligator clips or adhesive electrodes may be used. To reduce discomfort, teeth of alligator clips should be blunted and the spring should be relaxed.

• Limb electrodes are placed either distal or proximal to the elbow (caudal surface) and over the stifle. Electrodes placed proximal to the elbow may increase respiratory artifact.

• Each electrode should be wetted with 70 % isopropyl alcohol to ensure electrical contact.

• Recording the electrocardiogram
• Approximately three to four complete complexes should be recorded from each lead at 50mm/s.

• A lead II rhythm strip should then be recorded at 25mm/s or 50mm/s.


Key Point


The 1 mV standardization marker should be recorded at the onset of the electrocardiogram and any time the sensitivity is changed.



CARDIAC CONDUCTION AND GENESIS OF WAVEFORMS


• The function of the cardiac conduction system is to coordinate the contraction and relaxation of the four cardiac chambers (Figures 3-6 and 3-7).

• For each cardiac cycle, the initial impulse originates in the sinoatrial (SA) node located in the wall of the right atrium near the entrance of the cranial vena cava. This impulse is rapidly propagated through the atrial myocardium, resulting in depolarization of the atria. Depolarization of the atria results in the P wave and atrial contraction. The initial SA nodal impulse is small and does not produce an electrocardiographic change on the body’s surface.

• Immediately after atrial depolarization, the impulse travels through the atrioventricular (AV) node, located near the base of the right atrium. Conduction is slow here, which allows atrial contraction to be completed before ventricular depolarization occurs. As the impulse travels through the AV node, there is no electrocardiographic activity on the body’s surface—rather the PR interval is generated.

• Upon leaving the AV node, conduction velocity increases significantly, and the impulse is rapidly spread through the bundle of His, bundle branches, and Purkinje system. This results in rapid and nearly simultaneous depolarization of the ventricles. Depolarization of the ventricles results in the prominent QRS complex and causes ventricular contraction.
• The Q wave represents initial depolarization of the interventricular septum and is defined as the first negative deflection following the P wave and occurring before the R wave. A Q wave may not be identified in all animals.

• The R wave represents depolarization of the ventricular myocardium from the endocardial surface to the epicardial surface. The R wave is the first positive deflection following the P wave and is usually the most prominent waveform.

• The S wave represents depolarization of the basal sections of the ventricular posterior wall and interventricular septum. The S wave is defined as the first negative deflection following the R wave in the QRS complex.

• Ventricular repolarization quickly follows ventricular depolarization and results in the T wave. The delay in repolarization results in the ST segment on the surface electrocardiogram.

image

Figure 3-6 Anatomy of the cardiac conduction system. S-A, Sinoatrial; A-V, atrioventricular.


(Modified from Tilley LP: Essentials of canine and feline electrocardiography: interpretation and treatment, ed 2, Philadelphia, 1985, Lea & Febiger.)


image

Figure 3-7 Sequence of electrical impulse conduction and cardiac chamber activation as it relates to the electrocardiogram.


(Modified from Tilley LP: Essentials of canine and feline electrocardiography: interpretation and treatment, ed 3, Philadelphia, 1992, Lea & Febiger.)



EVALUATION OF THE ELECTROCARDIOGRAM


• The electrocardiogram should be evaluated from left to right.

• Areas of artifact should be identified and avoided in the evaluation.

• Calculate the heart rate.
• Determine the number of R waves (or R-R intervals) within 3 seconds and multiply by 20 to obtain beats per minute (bpm) (for an electrocardiogram recorded at 50mm/s, vertical timing marks above the gridlines occur every 1.5 seconds).

• If the rhythm is regular, the heart rate may be derived by determining the number of small boxes in one R-R interval and dividing 3000 by that number (for paper speed of 25mm/s, use 1500). The method is also useful for determining the rate of paroxysmal ventricular tachycardia and other arrhythmias lasting less than 3 seconds.

• Obtain measurements for the waveforms and intervals (Figure 3-8).
• P wave height and width

• Duration of PR interval

• Duration of QRS complex and height of R wave

• Duration of QT segment

• Determine the approximate mean electrical axis (MEA)
• The MEA refers to the direction of the net ventricular depolarization and refers solely to the QRS complex. If there is significant right ventricular hypertrophy, then the MEA will shift to the right. Because the left ventricle is normally the dominant ventricle, the normal MEA is to the left. A degree system is used—if the MEA is directly to the left, then it is said to be 0 degrees; if the MEA is directly downward, then it is 90 degrees, and if it is directly to the right, then it is 180 degrees. The MEA of the normal dog is 40 to 100 degrees. For the cat, the MEA is more variable, ranging from 0 to 160 degrees.

• The MEA may be determined using the six standard leads and the Bailey axis system (see Figure 3-1). If there is a lead with isoelectric QRS complexes, then the MEA equates to the lead on the Bailey axis perpendicular to the isoelectric lead.

• The MEA may also be determined by plotting the net amplitude of a lead I QRS complex (horizontal axis) and the net amplitude of a lead aVF QRS (vertical axis). The intersection will provide the vector equal to the MEA (see Figure 3-1).

• The MEA may be approximated by inspecting leads I and aVF.
• If the net direction of the lead I QRS is positive, then the MEA is to the left. If the net deflection of the lead I QRS is negative, then the MEA is to the right.

• If the net direction of the lead aVF QRS is positive, then the MEA is downward or caudal. If the net deflection of the lead aVF QRS is negative, then the MEA is upward or cranial.

• The approximate angle can be estimated by examining the relative amplitudes of leads I and aVF.

• Determine the rhythm.

• Compare patient values with reference values (Table 3-1).

image

Figure 3-8 Close-up of a normal feline lead II P-QRS-T complex with labels and intervals. Measurements for amplitude (millivolts) are indicated by positive (+) and negative (−) movement; time intervals (hundredths of a second) are indicated from left to right. Paper speed, 50 mm/s; sensitivity 1 cm = 1 mV.


Rights were not granted to include this figure in electronic media. Please refer to the printed book.


(From Tilley LP: Essentials of canine and feline electrocardiography: interpretation and treatment, ed 2, Philadelphia, 1985, Lea & Febiger.)


Table 3-1 Normal Canine and Feline ECG Values*



























































































































  Canine Feline
Heart rate (HR) Puppy: 70-220 bpm 120-240 bpm
  Toy breeds: 70-180 bpm  
  Standard: 70-160 bpm  
  Giant breeds: 60-140 bpm  
Rhythm Sinus rhythm Sinus rhythm
  Sinus arrhythmia  
  Wandering pacemaker  
P wave    
Height Maximum: 0.4 mV Maximum: 0.2 mV
Width Maximum: 0.04 s (Giant breeds 0.05 s) Maximum: 0.04 s
PR interval 0.06-0.13 s 0.05-0.09 s
QRS    
Height Large breeds: 3.0 mV maximum† Maximum: 0.9 mV
  Small breeds: 2.5 mV maximum  
Width Large breeds: 0.06 s maximum Maximum: 0.04 s
  Small breeds: 0.05 s maximum  
ST segment    
Depression No more than 0.2 mV None
Elevation No more than 0.15 mV None
QT interval 0.15-0.25 s at normal HR 0.12-0.18 s at normal HR
T waves May be positive, negative, or biphasic usually positive and < 0.3 mV
  Amplitude range ± 0.05-1.0 mV in any lead  
  Not more than ¼ of R wave amplitude  
Electrical axis +40 to + 100 0 ± 160
Chest leads    
CV5RL (rV2) T positive; R < 3.0 mV  
CV6LL (v2) S < 0.8 mV; R < 3.0 mV R < 1.0 mV
CV6LU (V4) S < 0.7 mV; R < 3.0 mV R < 1.0 mV
V10 QRS negative; T negative except in Chihuahua T negative. R wave/Q wave < 1

S, Second.


* Measurements are made in lead II unless otherwise stated.


† Not valid for thin, deep-chested dogs under 2 years old.



EVALUATION OF WAVEFORMS



P Wave


• Atrial enlargement patterns (Figure 3-9)
• The P wave is generated by atrial depolarization. Atrial enlargement may result in an increase in width or height of the P waves recorded in lead II.

• Enlargement of the right atrium may result in an increased P wave height. This is referred to as P-pulmonale. The height of the P wave should not exceed 0.4 mV (dog) or 0.2 mV (cat). Chronic pulmonary disease may result in P-pulmonale in the absence of heart disease.

• Enlargement of the left atrium may result in an increased P wave width or duration. This is referred to as P-mitrale. The duration of the P wave should not exceed 0.04 second (dog or cat). Left atrial enlargement may also result in notching of the P wave.

• Presence or absence of P waves
• There is no minimum height or duration for the P wave. In some cases, P waves may be indistinct. In this situation, carefully evaluate all leads for P wave activity. If P waves cannot be discerned in any of the limb leads, evaluation of chest leads is recommended.

• P waves may be absent in several arrhythmias, including atrial fibrillation and atrial standstill. P waves may be superimposed on other waveforms in ventricular tachycardia and supraventricular tachycardia (SVT).

• Variation of P wave height is a normal finding in the dog and a manifestation of alterations in vagal tone.

image

Figure 3-9 A, Biatrial enlargement in a small-breed dog with a collapsed trachea and compensated mitral regurgitation. The P wave is 0.5 mV tall (P pulmonale) and 0.05 sec wide (P mitrale). B, Left atrial enlargement in a geriatric, small-breed dog with chronic acquired valvular disease (mitral insufficiency). P waves are wide (0.075 second), notched, and equivocally tall.


(From Fox PR, Sisson D, Moise NS, eds: Textbook of Canine and Feline Cardiology. Philadelphia, 1999, WB Soundes.)



PR Interval


• The PR interval reflects the slowed conduction through the AV node. The normal PR interval is 0.06 to 0.13 second for dogs and 0.05 to 0.09 second for cats.

• A significantly shortened PR interval may occur when an accessory pathway allows conduction to bypass the AV node.

• Prolongation of the PR interval represents first degree AV block.

• Variation of the PR interval may occur with alterations in vagal tone or secondary to the presence of ectopic beats causing dissociation of atrial and ventricular activity.


QRS Complex


• The QRS complex is generated by ventricular depolarization (left ventricle, interventricular septum, and right ventricle). Ventricular enlargement may result in changes in the QRS complex.

• Left ventricular enlargement pattern
• Increased amplitude of the R wave
• Dog
• Amplitude of R wave greater than 3.0 mV (2.5 mV in small-breed dogs) in leads II, aVF, and the precordial chest leads CV6LU, CV6LL and CV5RL

• Amplitude of R wave greater than 1.5 mV in lead I

• Sum of R wave amplitudes in leads I and aVF greater than 4.0 mV

• Cat

• Amplitude of R wave greater than 0.9 mV in lead II

• Amplitude of R wave greater than 1.0 mV in CV6LU and CV6LL

• R wave/Q wave > 1.0 in lead V10

• QRS duration greater than 0.06 second (large dogs), 0.05 second (small dogs), or 0.04 second (cats)

• ST slurring or coving

• Shift in the MEA to the left (less than 40 degrees for dog, less than 0 degrees for cat)

• Right ventricular enlargement pattern (Figure 3-10)
• Increased amplitude of S waves
• Dog
• Presence of S waves in leads I, II, III, and aVF

• S wave in lead I > 0.05 mV

• S wave in lead II > 0.35 mV

• S wave in lead CV6LL > 0.8 mV

• S wave in lead CV6LU > 0.7 mV

• Cat
• Presence of S waves in leads I, II, III, and aVF

• Prominent S waves in CV6LU and CV6LL

• T wave positive in lead V10 (except Chihuahua)

• W-shaped QRS complex in V10 (dog)

• R:S ratio< 0.87 in CV6LU

• Shift in the MEA to the right (more than 100 degrees for the dog or more than 160 degrees for the cat)

• Widening of the QRS complexes may occur with left ventricular enlargement, right or left bundle branch block (LBBB), and complexes of ventricular origin (ventricular premature complexes [VPCs] or ventricular escape complexes).

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Electrocardiography

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