Cardiovascular System

1. Observe at rest: Examine external vascular system Venous congestion (localized or generalized?) Jugular distension? Jugular pulse: Normal in distal third of neck with head upright Abnormal if at higher level, especially if head is upright Abnormal if present in ventral segment when you occlude both jugulars (sign of RA problems) Abducted elbows Ventral edema Enlarged abdomen- ascites (rare in horse) Auscultation: Cardiac silhouette located from 2nd-6th ICS between points of shoulder & the elbow Cranial to caudal axis of heart Heart Sounds: Normally 2-4 sounds “ba-lub—dub-ahh” S1 “lub”= long, loud, low pitch Closure of the AV valves S2 “dub”= shorter, softer, high pitch Closure of semilunar valves S3 “ahh”= end of rapid filling S4 “ba”= atrial contraction Cardiovascular System

2. Cardiac Auscultation • Normal sounds are generated by turbulence • Described by intensity, PMI, timing (phase of cycle), duration…etc • PMI for cardiac valves: • PAM 345 R4 • Diminished/absent cardiac sounds: • Thick muscled (Draft horse), pericarditis, diaphragmatic hernia • Radiating cardiac sounds- pleural effusion or consolidation

3. Rhythm • Heart Rate- normally 30-44 BPM • Rate at rest depends on stress, health..etc • The more fit the horse- the lower the HR • Abnormal rhythms

4. Methods of evaluating the heart • Ultrasonography- echocardiography • Preferred method of evaluating equine heart • Blood evaluation: • CBC for determination of infectious/inflammatory components • Chemistries for general metabolic status & electrolytes • Cardiac enzymes- not usually evaluated in hose (ex: LDH) • Radiography: limited use in adult horse • Can’t tell you about chamber enlargement • Heart is too large to fit on a single view • Mostly used for evaluation of pulmonary involvement in suspected cardiac cases • Can be used as adjunct technique in neonatal foals

5. Evaluating the heart cont. • Exercise testing: combination of echo, EKG, blood analysis & treadmill testing • Percussion: variable results, can help you determine size of the heart

6. Dysrhythmias/Arrhythmias • Physiologic arrhythmias must be distinguished from pathologic arrhythmias • Differentials for non-pathologic arrhythmias: • Pathologic arrhythmias of primary or secondary causes (ex: drug induced) • Primary cardiac disease • Metabolic abnormalities: • Electrolytes- especially K+ and Ca++ • Dehydration • Acid-base imbalance

7. Mechanisms of Cardiac Dysrhythmias • Dysrhythmias: abnormalities of impulse generation, conduction, or a combination of both • Abnormal impulse generation: due to changes in ionic currents that flow across cells • Automaticity= ability to initiate AP’s spontaneously • Enhanced automaticity may be responsible for APC’s and VPC’s

8. Mechanisms of Cardiac Dysrhythmias • Abnormal conduction: propagating impulse is allowed to persist and re-excite atria or ventricles after the refractory period • Sympathetic and Parasympathetic influence on the heart: • Sympathetic tone increases HR & contractility • Parasympathetic tone (vagus nerve): • Discharge results in decreased HR • Vagal tone predominates at rest in the horse

9. Electrocardiography • Technique: • No single lead is ideal in LA’s • Most commonly used is the bipolar lead: Base-Apex • Positive on left thorax (LA lead)in 5th ICS at elbow • Negative (RA lead) is attached at right jugular furrow • Ground (RL lead) is attached at any point remote from the heart- many place it near the negative lead in the right jugular furrow • ECG is used primarily for the detection of dysrhythmias- NOT for measuring chamber size

10. Typical ECG findings • P wave is usually positive & is often bifid in the horse • Many horse T waves (atrial repolarization) are negative

11. Sinus arrhythmia: Manifestation of high vagal tone cyclic, repeatable pattern Usually associated with HR<28 bpm Rate increases with inspiration & decreases with expiration ECG changes: R-R intervals vary in a cyclic fashion Wandering pacemaker is often detected Arrhythmias Associated with Bradycardia

12. AV blocks: First degree: can be seen in the normal horse at rest Second degree: Most frequent manifestation of high vagal tone in the normal horse Should go away if you exercise the horse Types of Second degree: Mobitz type I (Wenkenbach): gradual increase between S4 & S1 before beat is dropped Is the most common type of second degree block Mobitz type II: fixed interval between S4 & S1 ECG changes: P waves not followed by QRS Type I: gradual prolongation of PR before one is dropped Type II: fixed PR interval Arrhythmia with regular variations in rhythm

13. AV blocks continued • Third degree AV block: • Complete AV dissociation • Pathologic condition • Exercise intolerance w/rates of 10-20 bpm • No impulse conduction from atria to ventricle through AV node • Rapid atrial rhythm w/a slower independent ventricular rhythm • Fainting is commonly seen when pauses of 60 seconds or longer occur between beats • ECG changes: • Rapid regular P waves with normal configuration • QRS complexes usually wide and bizarre with T waves oriented in the opposite direction • Must be able to distinguish from a VPC in order to tx! • Prognosis is GRAVE • Treatment: usually requires electical pacing (pacemaker)

14. Irregularities Affecting Performance: Irregularly Irregular Arrhythmias • Atrial Fibrillation (AF): is the most common pathologic arrhythmia of the horse • Is also the most common arrhythmia affecting performance in the horse • Predisposed to development of AF in NORMAL atrial tissue due to: • Large syncytium of atrial cells (large atrial mass)

15. Atrial Fibrillation • Signalment/History/Risk factors: • Race horses • Prior episodes of AF • Transient K+ depletion predisposes to AF • Ex: administration of Lasix, sweating • Presenting complaints: • Exercise intolerance- quitting at the ¾ post • Tachypnea • EIPH, CHF, Collapse, Myopathy, Colic

16. Atrial Fibrillation • Clinical signs: • Exercise intolerance • Can be an incidental finding in evaluation of a resting horse • Can be associated w/EIPH, resp distress, CHF, ataxia and collapse • Only a small portion of these horses have significant underlying cardiac disease • Ex: Grade II or louder murmur with HR>60 bpm

17. Atrial Fibrillation • Two forms of AF: • Paroxysmal: • <24-48hrs duration • Occurs during race & can disappear with deceleration of the HR • Can be associated with transient K+ depletion • Sustained: easier to diagnose b/c arrhythmia is sustained for a longer time

18. Atrial Fibrillation • Diagnosis: • PE and history • Auscultation of an irregularly irregular rhythm with an absent S4 • Pulse deficits may be present- usually have a variable pulse strength • ECG: Baseline fibrillation (F-waves) • No P waves seen • Irregular speed, but normally shaped QRST • Echocardiography: • Always perform to evaluate potential underlying dz

19. Treatment of AF • Choice of therapy depends on duration of arrhythmia, resting HR & findings on echo Heart Rate- Cutoff is usually 60 bpm • If HR<60 can start Quinidine therapy to convert • Most horses fall into this category • If echo is normal, this is considered to be an uncomplicated A-fib • If HR >60: • Even if no underlying cardiac dz, you must Digitalize first • Digitalis is needed to slow the HR prior to administering Quinidine b/c Quinidine causes tachyarrhythmias • If HR>60 and is NO underlying dz Digitalize, then when HR has decreased you can convert with Quinidine • If HR>60 and there IS underlying cardiac dz treat for heart disease first (ex: Lasix & Digoxin), then convert with Quinidine

20. Treatment of AF continued • Duration of arrhythmia: • If recent (w/in 72hrs): can use IV Quinidine • If >72hrs: use oral formulation • Quinidine: • Is highly protein bound- therefore can displace Digoxin & increase incidence of Digoxin toxicity (Bute also does this) • Idiosyncratic & side effects: • Diarrhea, depression, colic • Hypotension, collapse • Ataxia, tachycardia, urticaria (hives), laminitis • Can cause rapid supraventricular tachycardia (HR>200)

21. General Recommendations and Prognosis • Be sure that K+ levels are normal before initiating therapy • Monitor via ECG before or during each dose of Quinidine • Prognosis: • <4mths duration and no other cardiac disease= all converted and many had no incidence of recurrence • 80% of horses w/longer durations converted, but had a high rate of recurrence • >95% conversion in horses with HR<60 bpm, with a duration of <4mths and a murmur of <III/VI

22. Ventricular Tachycardia (VT) • Defined as a rapid rhythm originating from the ventricles • Typically caused by disorders of formation of impulses, conduction or a combination of both • Late coupled ventricular complexes or VPC’s are usually needed to initiate VT • Etiologies: • Myocarditis, bacterial endocarditis, electrolyte/metabolic abnormalities, anesthesia, drugs, sepsis, endotoxemia…etc

23. Ventricular Tachycardia • Clinical signs: • Exercise intolerance • Syncope • Sustained VT rapid HR with regular or irregular rhythm (may have HR of 300) • Diagnosis: • CBC, Chemistries (electrolyte evaluation) • ECG: • Needed to make diagnosis • 4 or more VPC’s are diagnostic of VT • QRS is wide and bizarre • Epidemiology: • Seen in all large animals but is most common in the horse • VT leading to VF is considered to be the leading cause of sudden death if no other findings are seen on post-mortem

24. Ventricular tachycardia Treatment is indicated if: • Clinical signs are seen at rest and HR >120 bpm • Rhythm is multiform (irregular) • Irregular R-R intervals • R on T complexes are detected • Depends on cause- often improves w/correction of electrolyte/metabolic problem • Myocarditis- may improve with REST & steroids • Minimum of 4-8wks of rest before returning to work • Initial therapy & most frequently used: LIDOCAINE • Rapid action, administered IV • Given in small doses and always slowly as a bolus • CNS side effects in horses= Hyperexcitability and seizures • Can also use Propafenone in refractory cases and Bretylium tosylate in life threatening V-tach or V-fib cases

25. Cardiac Arrest • Resuscitation is typically unsuccessful in the adult horse b/c external cardiac message is rarely successful & internal message is rarely undertaken • Successful chemical defibrillation of an adult horse with antiarrhythmic drugs had NOT been performed • Consider the primary dz prior to resuscitation • Use the ABC’s of CPR like in other species