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Graphic representation of summed electrical activity of the heart.
EKG/ECG
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******What is the standardization for voltage measurement (calibration) with EKG paper?
small squares= 1mm; (((1cm)))= 1mV; therefore, 10 small squares or (((2cm/2 large squares)))= 1mV
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What is the baseline on an EKG?
line immediately prior to P wave
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What is the most important/most common information that can be obtained from an EKG?
cardiac rhythm and conduction
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What information can be obtained from an EKG? (5)
heart rate, cardiac rhythm and conduction, info about heart muscle (hypertrophy, ischemia), pericardial diseases, effects of extracardiac diseases on the heart
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Describe the electrocardiographic system as a function of the body.
- heart- current generator as cardiac cell depolarize
- torso- volume conductor of current with resistance
- Current flowing across the heart in sheets of dipoles creates depolarization boundaries with an instantaneous electrical field measured as potential difference
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________ form leads.
Connected electrodes
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The potential for current flow between electrodes is measured by a _____________, and _____________ is displayed as the EKG.
galvanometer (electrocardiograph); potenital difference
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The QRS complex represents the summed _____________ across the ___________.
phase 0 depolarizations; ventricular myocytes
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The T wave represents the summed ___________ of the ___________.
phase 3 repolarizations; ventricular myocytes
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As far as action potential, the ST interval is the __________.
phase 2 plateau
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The EKG is the test of choice for...
the identification of heart rhythm and conduction disturbances (arrhythmias).
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The EKG CANNOT measure...
mechanical activity or contractility of the heart...you can be nearly dead with a normal EKG tracing.
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What are the x and y-axes on EKG paper?
- x-axis: time in seconds or milliseconds
- y-axis: potential difference in millivolts
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Determine the time duration for a canine P-wave measuring 3.5mm boxes wide, with an EKG recording speed of 50mm/sec.
 - 0.07sec
- Normal P wave duration for a dog is <0.04s, but it is often longer in giant breeds.
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Determine the amplitude of a feline R-wave in lead II, when the distance from the top pf the baseline to the tip of the R-wave measures 14.5mm and the calibration is "standard".
- 2cm=1mV<--is this the right conversion??
 - Normal feline R wave amplitude for lead II or aVF is <0.7mV. R wave amplitude suggestive of LV dilation or hypertrophy is 1.0mV in cats; therefore, this value is normal.
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Conduction current across the heart is _________.
dromotropic
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Areas of lower electrical resistance, through which current flows between most heart muscle cells; allow one cell to initiate depolarization in adjacent cells.
gap junctions
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Potential at which the cell membrane has opened voltage-gated channels to undergo depolarization (phase 0).
threshold
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What are the 3 major exceptions to the rule of rapid cell-to-cell conduction speed?
SA node, AV node, and ischemic tissues
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Describe the pattern by which current spread through the heart.
initiated in SA node (pacemaker)--> across atria and internodal pathways--> down AV node and Bundle of His--> traverses the ventricular Purkinje system to reach ventricular myocardium
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Describe the Purkinje penetration of dogs/cats/rodents versus large animals/birds.
dogs/cats/rodents penetrate ~1/4-1/2 of the distance into the myocardium; large animals/birds penetrate completely across the wall (transmural penetration)
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What cells discharge to rescue the heart when the sinus node fails or if there is a block of conduction, creating "escape" discharges?
AV nodal/junctional region and His-Purkinje system
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What process leads to the P-wave?
atrial depolarization starting in the SA node--> right atrium and atrial septum--> left atrium
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The ventricular activation process of dogs and cats can be divided into what 4 distinct phases?
- 1- initial activation of IVS, Q wave
- 2- mutually cancelling boundaries spread around the heart and return deflection back to baseline
- 3- depolarization boundary spreads across LVFW, R wave
- 4- tracing returns to baseline with a smaller basilar oriented boundary
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How is the ventricular activation of horses and ruminants different from dogs to cats and why?
due to greater penetration of purkinje cells, ventricular activation is dominated by a prominent depolarization and a deep S wave
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What is the PR interval?
slow conduction across cells of theAV node
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What is the P wave?
depolarization started by the SA node discharge but cause by activation of fast sodium channels (phase 0) in atrial myocytes
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What is the QRS complex?
ventricular depolarization dictated by the bundle branches and Purkinje system
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What is the J point?
the junction between the QRS complex and the ST segment
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What are the ST segment and T wave?
ventricular repolarization
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What is the QT interval?
time required for ventricular depolarization and repolarization
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What is "wandering pacemaker"?
atrial arrhythmia that occurs when the natural cardiac pacemaker site shifts between the sinoatrial node (SA node), the atria, and/or the atrioventricular node (AV node) that occurs when vagal tone is high--> normal in dogs and horses
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Whether the P wave is positive or negative depends on...
where the electrode is oriented (looking at the same thing from different aspects).
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What are the 4 wavefronts/boundaries of depolarization?
- Boundary 1- ventricular septum (left to right, dorsal to ventral)
- Boundary 2- mutually cancelling boundary (endocardium to epicardium)
- Boundary 3- left ventricular free wall (left and caudal)
- Boundary 4- base of the heart (dorsally)
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What do the 4 wavefronts/boundaries of depolarization correspond to on the EKG?
- Boundary 1- Q wave
- Boundary 2- back to baseline
- Boundary 3- R wave
- Boundary 4- S wave
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What is the difference in EKG between small animals and horses/ruminants?
In small animals, boundary 3 is the predominant boundary (left ventricular free wall); in large animals, boundary 4 is dominant (base of heart)
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The biggest boundaries of repolarization occur during the _________.
T wave
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QT interval is inversely related to _________, but the relationship is not ___________; QT also depends somewhat on the ___________.
HR; linear; size of the animal
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What species is it normal to have a positive and quite notched P wave?
horses
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What boundaries are picked up by a given probe and which are not?
boundaries traveling perpendicular to the lead are NOT recorded; waveforms going parallel to the lead are recording [if traveling at an angle, partially picked up]
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What is the difference between a unipolar and bipolar lead?
Unipolar- mainly recording the potential at one electrode compared to the average of the other two or to zero; Bipolar- compares potential at one electrode to another.
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How do you determine the lead axis of a unipolar lead versus a bipolar lead?
- With a unipolar lead, the lead axis can be estimated by drawing a line b/w the surface electrode (the + end of the lead) and the center of the heart.
- With a bipolar lead, the lead axis is determined by connecting the 2 surface electrodes.
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When determining the effects of boundaries on the EKG deflection, you must consider... (4)
size of boundary, direction, lead axis, and cancellation from other boundaries.
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What are the frontal plane leads used in clinical EKG?
- bipolar leads I, II, III
- augmented unipolar leads aVR, aVL, aVFprecordial/ chest leads- V leads
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What lead is typically used to monitor heart rhythm in small animals?
lead II
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What lead is typically used to monitor heart rhythm in large animals?
base-apex lead [+ left apex to right jugular furrow]
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Describe the orientation of Lead I according to Einthoven's triangle.
left arm minus right arm: potential difference b/w the + LA and -RA
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What is a lead axis?
a straight line b/w the 2 electrodes
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Describe the orientation of Lead II according to Einthoven's triangle.
Left foot minus right arm: potential difference b/w the +LF and -RA
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Describe the orientation of Lead II according to Einthoven's triangle.
Left foot minus left arm: potential difference b/w +LF and -LA
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Describe the orientation of Lead aVR according to Einthoven's triangle.
right arm minus the average of the left arm+left foot: potential difference b/w +RA and -avg of LA & LF
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Describe the orientation of Lead aVL according to Einthoven's triangle.
Left arm minus the average of the right arm and left foot; potential difference b/w +RA and the -avg of RA & LF
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Describe the orientation of Lead aVF according to Einthoven's triangle.
Left foot minus the average of left arm and right arm; potential difference b/w the +LF and -avg of LA & RA
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Describe the lead axis of chest leads. What are they comparing?
lead axis: electrode --> center of the heart; comparing electrode potential to Wilson's central terminal (zero)
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What is the base to apex lead used for in large animal medicine?
monitoring heart rhythm in large animals
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Describe the orientation of the base to apex lead in large animals and what it "sees"?
"sees" ventral to dorsal basilar activation; electrode going dorsally, so normal QRS is negative deflection
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What is the purpose of a Holter EKG?
heart rhythm data for 24 hours (ambulatory EKG that is worn like a sweater)--> find short periods of abnormal heart rhythm
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What is the purpose of an "event monitor"?
like Holter EKG, it is worm and record for 24 hours, BUT it only save data that is a recording of an "event" (fainting spells, collapsing)
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What is the purpose of telemetry EKG monitoring?
intensive care settings as well as for exercise EKG recordings
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What is the standard position of the patient for recording an EKG in small animals?
right lateral recumbancy
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What is the standard position of the patients for recording an EKG in large animals?
standing with left forelimb advanced
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What colored lead cables are attached to the hindlimbs?
red and green (green is ground)
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What colored lead cables are attached to the forelimbs?
black and white
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What color are chest leads?
usually brown
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What does a sine wave at 60 cycles/second indicate on EKG tracing?
artifact: electrical interference from the outlet
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What does a non-rhythmic undulation of the baseline indicate? What can be done?
muscle tremoring; filtering cleans up the trace but reduces the overall voltage displayed
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What happens to the heartbeat if the heart becomes denervated?
A denervated heart fills with blood and pumps as any other normal heart because the SA node has intrinsic pacemaker activity. However, a denervated heart takes longer to increase HR with exertion and slows more gradually with rest because it is not under the influence of the CNS.
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What are normal HR ranges for horses, cows, dogs, and cats?
- Horses- 44-50bpm
- Cows- 75-80bpm
- Dogs- 60-180bpm
- Cats- 140-240bpm
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Does the size of the QRS complex relate directly to myocardial contractility?
NO. The EKG cannot measure mechanical activity or contractility of the heart; you can be nearly dead and have a normal EKG tracing!
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Calculate the HR for a regular heart rhythm based on the R-R interval of 20mm; the paper speed is 25mm/s.
If the heartbeat is regular, count the number of 1mm boxes b/w two consecutive R waves and divide it by the paper speed to get the R-R interval; then divide 60 by the R-R interval to get bpm.
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How is HR calculated with heart rhythm is irregular?
count the number f QRS complexes per 3s (or 6s) then multiply by 20 (or 10).
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What does isoelectric mean?
flat or nearly at baseline
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What is a normal sinus rhythm?
inspiration decreases vagal activity and causes HR to increase; expiration decreases HR as it causes vagal activity to resume. [Vagal tone decreases HR by inhibiting firing of the SA node].
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