AN OVERVIEW OF CARDIOLOGY FOR THE BOARD EXAM
Darcy Adin, DVM
Supplemental Notes
Drug therapy
A.
Antiarrhythmic therapy
Classified according to the Vaughn-Williams scheme based on mechanism of action
Class I - inhibits the fast sodium channel – further broken down into 3 subclasses
Class IA examples include quinidine, procainamide.
These are used to treat ventricular arrhythmias. Quinidine is used for conversion of AF in
horses to sinus rhythm.
Class IB – e.g. lidocaine – the treatment of choice for ventricular arrhythmias (can only
be given parenterally). The main toxic effect of lidocaine is neurotoxicity.
Class IC– (e.g. flecainide) these drugs are not used
Class II – beta blockers – propranolol is the prototype. Used for ventricular and supraventricular arrhythmias. Low toxicity, inexpensive, effective. Anything ending in –olol is
probably a beta blocker.
Class III – inhibit the K channel to prolong repolarization. Examples include bretylium,
amiodarone and sotalol.
Class IV – calcium channel blockers. Examples include diltiazem, verapamil and
nifedipine. Used for supraventricular arrhythmias. They slow conduction thru the AV
node. Amlodipine is a newer calcium channel blocker with only peripheral vascular
effects so it is effective for hypertension.
B.
Heart failure therapy
1 Diuretics - for treatment of fluid retentive states
a) Loop diuretics – e.g. furosemide. These are potent diuretics that inhibit the Na/K/Cl
cotransporter at the ascending loop of Henle. Side effects with high doses include
electrolyte depletion and volume depletion.
b) Thiazide diuretics – inhibit sodium and chloride reabsorption in the distal part of the
nephron. e.g. hydrochlorthiazide. Similar side effects to loop diuretics. Less important for
boards.
c) K+ sparing diuretics – act on distal tubule to inihibit Na/K exchange. Example is
spironolactone. Also less important for boards.
Do not use osmotic diuretics (e.g. mannitol) for treatment of CHF!*
2 Angiotensin converting enzyme inhibitors
Most commonly used ACE-I in veterinary medicine is enalapril.
These drugs inhibit ACE to prevent the conversion of angiotensin I to angiotensin
II.
They have a mild vasodilating effect and inhibit Na/H2O retention by inhibiting
aldosterone release.
These drugs are used in conjunction with diuretics to treat CHF.
Side effects can include mild hypotension and increases in BUN & creatinine in
dogs that are dependent on angiotensin II for maintenance of GFR. We recommend
monitoring of renal function in dogs started on an ACE-I
3 Positive Inotropes
a) Digoxin – a weak positive inotrope via inhibition of the Na/K/ATPase pump. This
promotes increased calcium concentration in the myocyte and enhanced contractiliity.
Digoxin is also a negative chronotrope meaning it slows HR via its vagal activation. It
has a long half-life (30-40 hrs) with a narrow therapeutic range. Toxic manifestations
include GI signs and bradyarrhythmias (although tachyarrhythmias can also occur).
Treatment usually is supportive with total discontinuation of digoxin. Atropine can be
used to treat the bradyarrhythmias as it abolishes vagal tone.
b) Dobutamine – a beta agonist. Used for IV treatment of low output failure
c) Isoproterenol - a beta agonist
d) Epinephrine / Norepinephrine
4 Vasodilators - used to reduce preload and afterload in heart failure
a) Nitroprusside – arterial and venous vasodilation. IV CRI used in the acute heart failure
situation. Must monitor BP.
b) Hydralazine – arterial dilation only. Mainly used for hypertension or to reduce
afterload in MR and thus reduce regurgitant fraction.
5 Antithrombotic agents
a) Aspirin – cyclo-oxygenase inhibitor. Mainly used in an attempt to prevent
thromboembolism in cats with cardiomyopathy. Although its efficacy is not great, it is
well tolerated when given every 3 days.
b) Warfarin – inhibits the formation of Vit K dependent clotting factors. This can also be
used in cats, however clotting times need to be closely monitored and spontaneous
hemorrhage is not uncommon.
c) Heparin – binds to ATIII and thrombin for anticoagulation.
6 Oxygen
Common Arrhythmias
1 Ventricular tachyarrhythmias
With a few exceptions (not important for the boards), ventricular arrhythmias can be
identified by their wide, bizarre complexes that are very dissimilar to sinus beats. They
may be in singles (PVCs), couplets, triplets or as VT. Determination to treat depends on
rate, polymorphism, underlying cardiac disease and hemodynamics of the patient. As
stated above, lidocaine is the treatment of choice ( IV only). Some common oral therapies
include procainamide, sotalol and propranolol.
2 Supraventricular tachyarrhythmias
- usually appear approx 90% like the sinus beats since they take the same conduction
through the ventricles. Supraventricular rhythms include sinus, atrial and AV junctional
rhythms. Examples include SVT and AF. The usual therapeutic goal is to slow
conduction through the AV node via digoxin, beta blockers or calcium channel blockers.
Two determinations for susceptibility to atrial fibrillation are vagal tone and atrial mass.
Therefore, it is not uncommon for normal horses to go into AF and thus conversion with
quinidine is reasonable and possible.
Most dogs and certainly cats that go into AF have serious underlying heart disease and
thus the goal is to slow the ventricular response rate – 1st try digoxin and then add a beta
blocker or calcium blocker if necessary.
KNOW WHAT AF LOOKS LIKE – a rapid irregularly irregular supraventricular rhythm
with no p waves.
3 Bradyarrhythmias
a) sinus bradycardia
b) 1st degree AV block – prolonged PR interval
c) 2nd degree AV block – Mobitz I (PR prolongs and then drops a beat) – usually vagally
mediated.
Mobitz II (constant PR, then drops) – usually not vagally mediated.
d) 3rd degree AV block – there is no association between the P waves and the QRS
complexes. The supporting rhythm may be a junctional or ventricular escape rhythm. It is
usually not responsive to atropine and usually requires permanent pacemaker
implantation.
e) Sick sinus syndrome – esp minature schnauzers. An intrinsic conduction disorder of
the sinus node. Subsidiery pacemakers are also abnormal. Often requires pacemaker
implantation.
Hyperkalemia classically produces first, tall spiked T waves, then bradycardia, loss of P
waves and a sinoventricular rhythm as it becomes more severe.
Wandering pacemaker (changes in p wave morphology) and respiratory sinus arrhythmia
are normal findings in dogs due to changes in vagal tone.
Second degree AV block can be normal in athletic horses.
Basic ECG Analysis
The ECG is the test of choice for rhythm disturbances but is not very sensitive for
chamber enlargement detection whereas echocardiography is quite sensitive.
tall p waves – RAE
wide p waves – LAE
prolonged PR interval – 1st degree AV block
wide QRS – LVE
tall QRS – LVE
T waves can be + or – and can vary greatly in dogs
ST segment elevation or depression – myocardial ischemia/hypoxia
Rt axis shift (deep s waves in I, II, III and aVF) – right heart enlargement or RBBB
(RBBB is when the QRS complex is >80 ms duration)
Small complexes can signify pericardial effusion
Electrical alternans can signify pericardial effusion
Feline Cardiac Disease
1 HCM – most common. LV hypertrophy can be due to hyperthyroidism, hypertension
or primary HCM. The classic breed is the Maine coon, however it is seen in many other
breeds and DSHs. Treatment involves heart failure therapy if appropriate and slowing of
the HR to decrease systolic anterior motion (SAM) of the mitral valve with beta blockers
or calcium channel blockers. Diltiazem is commonly used. Arterial thromboembolism is
a common complication.
2 Restrictive / Unclassified Cardiomyopathy – normal wall thicknesses with large
atria. Treatment and clinical outcome is similar to HCM
3 Dilated cardiomyopathy – uncommon today as most cases were caused by taurine
deficiency.
4 Heartworm disease – uncommon in cats. Signs include respiratory (can look like
asthma), GI (vomiting), and neurologic signs. Sudden death is common. Don't treat
unless symptomatic.
Canine Acquired Cardiac Diseases
1 Valvular disease - By far the most common cardiac disease seen by thegeneral
practicioner in dogs. Older small breed dogs mostly. Uncommon in cats. The mitral valve
is most often affected, sometimes the tricuspid as well. Due to myxomatous degeneration
of the valve with age. Also called endocardiosis by some.
2 Dilated cardiomyopathy - a primary myocardial disease characterized by systolic
dysfunction. Poor contractility is the primary abnormality followed by compensatory
dilation. Doberman pinschers are the classic breed. Atrial fibrillation is a common
arrhythmia with this disease. Can also occur in other large breed dogs. Boxers also get
DCM but may present with ventricular arrhythmias and collapse only or initially.
3 Pericardial disease - Pericardial effusion typically occurs in large breed dogs. Causes
include benign idiopathic effusion, heart base tumor, RA hemangiosarcoma,
mesothelioma and infection. Findings include ascites, jugular pulses, weak femoral
pulses, pulsus paradoxus, muffled heart sounds and tachycardia, globoid cardiomegaly on
radiographs and small QRS complexes and electrical alternans on ECG.
Lasix is contraindicated with pericardial effusion as it decreases preload and diastolic
filling is already compromised.
Pericardiocentesis is performed from the right side so as to avoid puncturing the coronary
arteries. Traumatic pericarditis in cows is caused by penetration of a foreign object from
the reticulum to the pericardium.
4 Heartworm disease - Know the life cycle! L1 are the microfilaria and are taken up by
a mosquito during a blood meal. L1 matures to L3 in the mosquito. L3 migrates through
the mosquito mouthparts after a blood meal and penetrates the dogs skin. L3 matures to
L5 in the SQ tissues and L5 migrates to the pulmonary arteries where they mature into
adult heartworms. The whole process takes approx 6.5 mos.
- Know the various classes including caval syndrome.
- Caval syndrome is when the adults who normally live in the pulmonary arteries migrate
into the right heart producing hemoglobinemia, hemoglobinuria, hepatic and renal
dysfunction and right heart failure. Most heartworm cases do not have caval syndrome
and typical signs include coughing and exercise intolerance.
- Pulmonary hypertension can occur and a split S2 may be audible due to delayed closure
of the pulmonic valve.
- Radiographic signs include enlarged, tortuous, and truncated
pulmonary arteries and possibly right heart enlargement.
- Occult heartworm disease implies a positive antigen test and a negative microfilaria test.
- The antigen test detects adult female worms.
- Treatment against the adults is with an arsenical. Caparsalate (thiacetarsemide) used to
be used most commonly. Side effects include hepatotoxicity and phlebitis if injected
perivascularly. Immiticide (melarsamine) is now available and is used more now as it
involves only 2 IM injections, is effective and has few side effects.
- Prevention can be obtained either monthly with ivermectin or milbemycin or daily with
diethylcarbamazine.
5 Infective endocarditis - usually bacterial. A predisposing cause is usually not
identified, however subaortic stenosis is a known risk factor. Many bacteria have been
isolated, however Strep, Staph and E. coli are the classic. Findings include the presence
of new heart murmur, signs of systemic embolism, cyclic fevers and lameness. IE occurs
on the mitral and aortic valves in small animals and on the pulmonic and tricuspid valves
in ruminants.
Congenital Heart Disease
1. Stenotic lesions
Subaortic stenosis – breed predispositions include G. Ret., Boxers, G. Shep, Rott.,
Newfies. This is a pressure
overload so concentric LV hypertrophy occurs
Pulmonic stenosis – Beagles and other small breed dogs especially. Also a pressure
overload
2 Dysplastic lesions – Mitral valve dysplasia (esp bull terr) & Tricuspid dysplasia (esp
labs)
3 Shunts
PDA – most common defect in dogs. Continuous murmur.
Typically small breeds (esp poodles) but G.Sheps also. Radiographs classically show a
“ductal bump” on the aorta, enlarged PA, pulmonary overcirculation, and left sided
enlargement, sometimes pulmonary edema. To determine what enlarges with a shunt,
simply trace where the extra blood flows. Those chambers or vessels that accommodate
the shunted blood will enlarge.
VSD – another common congenital defect. This is the most common
congenital defect in the horse. VSDs have a right sided systolic murmur associated with
them.
ASD – have a soft left basilar systolic murmur due to “relative PS” from the increased
volume crossing the pulmonic valve.
Tetralogy of Fallot – sounds like pulmonic stenosis with clinical signs of right to left
shunting. The 4 essentials are pulmonic stenosis, overriding aorta, large VSD and RV
concentric hypertrophy.
Toxicities
Oleander – the primary toxin is a cardiac glycoside so signs will be similar to dig
toxicity
Adriamycin (doxorubicin) – can cause myocardial failure and arrhythmias
Selenium deficiency