Principles of Electrocardiography: Relevant Physiology

Question 1

describe ECG

Answer 1

1. records heart’s electrical activity from the body surface
   -records EXTRAcellular signals producedby movement of depolarization/repolarization waves through cardiac myocytes
   -graph of voltage (mV, y axis), over time (seconds, x-axis)
2. changes in voltage are recorded as waves/complexes, named by letters: P QSR T
3. ECG eval can give insight into:
   -HR
   -disturbances of heart rhythm and conduction
   -relative size of heart chambers (SA only)
4. NOTE: ECG does NOT record mechanical activity so cannot give insight into:
   -whether heart is contracting
   -strength of cardiac contractions
   -presence/absence of heart failure

Question 2

describe how cardiac tissue is excitable

Answer 2

1. at rest, myocytes are POLARIZED: membrane is negatively charged (inside relative to outside)
2. when stimulated, resting myocyte depolarizes (membrane polarity reverses, now OUTSIDE is negative relative to outside)
3. depolarized cell stimulates adjacent cell to depolarize
   -depolarization impulse spreads as a wave
   -extracellular currents associated with wave of depolarization are detected by the ECG!
4. cells must REpolarize so this process can happen again and again

Question 3

describe the basics of electrocardiography as relates to physiology

Answer 3

1. when ECG electrodes are placed on either side of a wave of depolarization, the electrical field can be measured
2. VERY IMPORTANT:
   -by convention, if a wavefront of negative extracellular charges moves TOWARD the POSITIVE electrode, an UPWARD deflection is recorded on the ECG
   -wave of positive charges (repolarization) = downward deflection
3. when a wavefront moves directly toward an electrode, in PARALLEL with the lead axis, the largest possible deflection will be recorded
   -lead = a pair of electrodes, axis is the straight line that connects them
   -if wave is perfectly perpendicular to the lead axis, the wave is not seen by either lead and therefore there is no deflection

Question 4

describe the normal activation sequence of the heart

Answer 4

1. sinus node
   -dominant pacemaker; 60-250 impulses per minute
2. spread cell by cell through atrial muscle until activates entire atrial syncytium
3. pass through AV node in the floor of the intra-atrial septum/top of intraventricular septum
   -SLOWEST conduction, to allow ventricular filling from atrial squeezing before contract together
   -can act as a rescue pacemaker, but only gives 40-60 impulses per minute
4. bundle of His-purkinje system
   -FASTEST conduction to allow coordinated contraction of the ventricles
   -in super emergency, can act as a rescue pacemaker, but only 20-40 impulses per minute
5. ventricular myocytes

Question 5

describe the atrial internodal tracts, the bundle of His/bundle branches, and terminal purkinje fibers

Answer 5

1. rapidly-conducting tissue connecting SA and AV nodes
   -relatively resistant to effects of hyperkalemia!!
2. Bundle of His/bundle branches
   -divides into right and left bundle branches (supply right and left ventricle respectively)
   -rapidly conduct impulse to terminal purkinje fibers
3. terminal purkinje fibers
   -rapidly conducting, subendocardial
   -penetrate inner 1/3 of myocardium in dogs and cats, so depolarization proceeds endocardium to epidcardium
   -penetrate near-complete thickness in horses, cattle, birds

Question 6

describe the method for recording a 6-lead surface ECG in small animals

Answer 6

1. patient in right lateral recumbency
   -limbs parallel to one another and perpendicular to the trunk
2. electrode placement
   -forelimb electrodes: white/black; over elbows
   -hindlimb electrodes: red/green over stifles
   -avoid contact with trunk and with each other!
3. several standard leads used clinically
   -lead = electrode pair (1 positive and 1 negative)
   -lead axis = orientation of lead relative to heart
   -lead II used most frequently in the clinic:
   –negative electrode on right forelimb (white)
   –positive electrode on left hindlimb (red)
   –lead axis oriented cranial-caudal, right-left

Question 7

describe ECG lead placement in horses and ruminants

Answer 7

1, no lead system is universally accepted in LA!

2. electrode placement for base-to-apex lead
   -set machine to LEAD I (RA- to LA+)
   -place white electrode (RA) over right jugular furrow or top of right scapular spine (base: white on right)
   -place black electrode (LA) over left apex beat (black on heart)
   -lead axis oriented cranial-to-caudal, right-to-left

RA=right arm
LA=left arm

Question 8

describe the normal (lead II) ECG tracing

Answer 8

negative electrode on left arm!! positive on right arm!!

P wave: cell-by-cell atrial depolarization
-positive in lead II and base-apex lead
-frequently bifid (M-shaped_ in horses

PR (PQ) interval:
-includes depolarization of atria, AV node, and His-Bundle
-PR interval approximates signal transmission through AV node
-normal <0.13 seconds (dog), or <0.09 seconds (cat)!!!!!!!!
-tells us a lot about the function of the AV node; as HR increases, this PR interval will get shorter, can also be prolonged or lost all together in some cases

QRS complex:
-ventricular depolarization
-should be tall, skinny, and upright in lead II (small animals)
-normal: <0.06 sec (dog), <0.04 sec (cat)!!!!
-in horses and ruminants, normal QRS complex is NEGATIVELY deflected (full thickness His-purkinje penetration, explosive depolarization wave moves up and away)
-abnormal prolonged: enlargement of muscle, block in normal His-purkinje system, random cell depolarizing

ST segment:
-isoelectric (flat) line connecting S and T waves
-all ventricular cells depolarized, no current flowing

T wave: ventricular repolarization:
-complicated
-may be negative, positive, or biphasic but MUST be there!! if not = death