Cards (41)

  • The SA node generates action potential and is hence called the natural pacemaker. All normal impulses start from SA node; if not working properly other parts of the conduction system can take over. Impulses originate from the SA node and spread across atria to cause atrial contraction. The impulse can only pass through the AVN to reach the ventricular myocardium.
    • AVN delays impulse to allow for ventricular filling
    The impulse spreads rapidly through the bundle of His, bundle branches and Purkinje network to cause
    ventricular contraction
  • An electrocardiogram = the recording of cardiac electrical activity by measuring the amplitude and direction of the flow of electricity between a positive and negative electrode.
  • To perform an electrocardiogram…
    • The patient is placed in right lateral recumbency with electrodes attached to limbs
    • Use one electrode as a positive pole and another one as a negative pole to measure the overall direction and magnitude of the electrical current. Most commonly a hex axial system is used which uses 3 electrodes to produce 6 ‘leads’ by using different electrodes as the positive and negative poles
    In practice, we mostly concentrate on lead II as this is most useful for assessing the rhythm.
  • P waves are positive in lead II as the wave of depolarisation is heading from the SAN towards the positive electrode - if not then likely not originating from the SAN; wandering pacemaker at times of high vagal tone.
  • Sinus arrhythmia is similar to normal sinus rhythm except there is a greater variation in P-P interval. Shortened R-R interval on inspiration and the rhythm may be regularly irregular.
  • Respiratory sinus arrhythmia = normal rhythm in the dog, abnormal in the cat in a clinic setting. This is mediated by fluctuations in vagal tone, abolished by atropine and accentuated by vagal manoeuvres.
    • Can have a non-respiratory sinus arrhythmia as well
  • in respiratory sinus arrhythmia HR increases on inspiration and decreases on expiration
  • dysrhythmia = An abnormal heart rhythm caused by a disturbance in the heart’s electrical conduction system
  • The history with a dysrhythmia tends to include…
    • Syncope
    • Lethargy/weakness
    • Exercise Intolerance
    • ‘Funny turns’
    • Known cardiac disease
    • No relevant history (incidental finding)
  • On CE with dysrhythmia cases you tend to find: abnormal heart rate (either too high or too slow), audible irregular rhythm, pulse deficits and evidence of underlying cardiac disease (e.g. murmur). Causes include…
    • Structural cardiac disease
    • Especially if there is cardiomegaly, fibrotic myocardium….
    • Drugs (ironically anti-arrhthymic drygs)
    • Toxins
    • Metabolic diseases/electrolyte imbalance (commonly potassium imbalance in blocked gats)
    • Systemic disease –sepsis, neoplasia
    • Especially if there is splenic neoplasia and GDV
    • The primary issue with the heart’s inherent conduction system
  • assessing ECGs…
    • Count the heart rate
    • Count number of beats in 3 seconds (check paper speed) and multiply by 20
    • Assess different parts of the ECG;
    • Is the rhythm regular or irregular?
    • Do the ECG waves appear normal? (ensure t wave is there but they can be erratic so ignore them in most cases)
    • Is each P wave followed by a QRS?
    • Is there a P wave before each QRS?
  • There are two types of dysrhythmia…
    • Bradyarrhythmia (Leads to a reduction in heart rate)
    • Tachyarrhythmia (Leads to an elevation in heart rate when present)
    • Supraventricular/ atrial - Originating from above the ventricles
    • Ventricular - Originates from the ventricles
  • supraventricular includes the av node and atria
  • Some home HRs can be naturally very low - if concerned give them atropine and see if it goes up - if it does then good
  • Causes of bradyarrhythmia include
    • Markedly increased vagal tone – sinus bradycardia
    • Consider giving atropine (parasympatholytic) and check for resolution
    • Abnormal generation of an impulse at the Sino-atrial node
    • Abnormal conduction of the impulse at the AV node
    • Underlying primary causes
    • Electrolyte imbalances (especially hyperkalaemia)
    • Primary cardiomyopathy/valvular disease
    • Drug toxicity/effect
  • Sick Sinus Syndrome = a term commonly used to describe sinus node dysfunction with clinical signs
  • In cases of persistent atrial standstill, the SAN is not working at all so there is a complete absence of p-waves. HR is usually slower but regular. The next fastest pacemaker takes over in most cases and this is the AVN
    • QRST usually appears normal if AVN takes over
    • Wide/bizarre QRS if ventricular myocardial cells take over
  • there are different types of abnormal conduction of the impulse at the AV node
    • Type 1 = delay in the transmission of the impulse; prolonged P-R interval; p-waves always eventually conducted
    • Type 2 = occasional block; p-wave not conducted
    • Type 3= complete block – p-waves and QRS complexes not related to each other
  • What types of abnormal impulse conduction at the AV node can be seen under GA?
    type 1 and 2
  • Bradyarrhythmia can be a challenge to identify, when considering if an animal has high vagal tone, consider if it’s HR fit with its general appearance. For treatment, be sure to treat any underlying cause e.g. correction of hyperkalaemia. If there are no or limited clinical signs treatment is likely not required (sudden death is very rare). In cases of sick sinus syndrome, advanced type 2 AV block and type 3 AV block an artificial pacemaker is often the only effective treatment.
    • Change pacemakers every 3-4 years
  • Premature beats can indicate disease as it occurs from damaged cells and hence these beats can be a clue to something else going on!
  • tachyarrhythmia can be split into…
    • Ventricular Tachycardia (VT)
    • All rhythm disturbances originating from within the ventricles cause abnormal ventricular conduction leading to wide and bizarre QRS complexes.
    • Supraventricular Tachycardia (SVT)
    • Originating from above the ventricles this means that the ventricular impulse conduction is normal and QRS normal in appearance (tall and narrow).
  • types of ventricular tachycardia include…
    • Ventricular Premature Beat: same causes as in premature beats, they cause wide and bizarre QRS complexes that can occur in couplets and triplets.
    • Ventricular Tachycardia (VTAC) is a sequence of 4 or more ventricular beats with a rate higher than 160bpm –often a fast and unstable rhythm
    • Ventricular Flutter is a very rapid VTAC in which T waves and QRS are no longer distinguishable. DANGER This often precedes death
  • pQRST QRS PQRST = premature ventricular beat
  • what dysrhythmia is shown below?
    ventricular premature beat
  • what dysrhythmia is shown below?
    ventricular tachycardia
  • what dysrhythmia is shown below?
    ventricular flutter
  • types of Supraventricular Tachycardia include…
    • Atrial tachycardia –occurs when there is an ectopic pacemaker in the atria that is able to fire at a high rate
    • Accessory pathway (AP) mediated tachycardia (rare) very high heart rates with a gap in insulation between atria and ventricles
    • Impulse can bypass the AVN or a ventricular impulse can retro-conduct back into the atrium
    • Atrial flutter (rare)
    • Atrial Fibrillation -common especially in conditions that cause significant left atrial enlargement
  • Atrial fibrillation causes a chaotic rhythm as a result of concurrent activation of different areas of the atrial myocardium; likely atrial enlargement
    • ECG characteristics include a normal heart rate to tachycardia with an irregular rhythm. There is a lack of p waves but instead there are fluctuations of baseline (f-waves). The QRS is normal as ventricular activation via normal pathways (can have concurrent ventricular issues in some cases).
    • When looking at an ECG concentrate on lead 2, if unsure if there is atrial fibrillation, measure the R-R interval as there should be no regularity
  • what dysrhythmia is shown below?
    atrial fibrillation
  • Dogs and cats don’t tolerate quinidine well but it is the 1st choice in horses
  • Treatment for atrial fibrillation = if high rate then slow conduction through AVN; combination treatment with Diltiazem and Digoxin often effective
  • Premature beats are any beat that occurs before it is expected. These can occur when normal myocardial cells develop the ability to become a pacemaker cell (ischaemic damage) or a short-circuit is created in the myocardium. If these beats occur alone then they are very unlikely to require treatment but can give clues to myocardial damage/remodelling/other systemic illnesses. They can lead to sustained firing from this ectopic focus resulting in tachyarrhythmias.
    • Will differ in appearance if originating from atrial/AVN or ventricular myocardium
  • fill in the blanks
    A) lidocaine
    B) atenolol
    C) sotalol
    D) diltiazem
    E) digoxin
  • Antiarrhythmic Drugs act by altering ion flow (Class I, III and IV) will affect different areas of the Action Potential reducing the risk of an ectopic impulse being generated or slowing impulse generation/conduction
    • Class II drugs (+ sotalol) impact the sympathetic innervation to the SA and AV nodes slowing heart rate
  • Class I and III acting primarily on myocardial cells while Class II and IV primarily affect SAN and AVN
  • CLASS 1B
    • Lidocaine:
    • Blocks Na channels reducing rate of ventricular firing
    • Rapid acting intravenous medication –initial bolus followed by CRI
    • No oral version so only used in hospital setting
    • Used in an emergency setting for haemodynamically significant ventricular arrhythmias
    • Adverse reactions: depression, seizures, bradycardia, hypotension (care if severe underlying cardiac disease)
    • Care with cats more prone to neuro side effects and cardiorespiratory depression
    • Care with use with other medications reducing cardiac output
  • CLASS II
    Atenolol…
    • Beta-blocker: reduces rate and contractility
    • Can be used for both SVT and VT
    • Reduces myocardial O2 demand
    • Beware if underlying myocardial disease as will reduce cardiac output –generally contraindicated if CHF present
    • Side-effects: bradycardia, AV block, myocardial depression, hypotension
    • Beware use alongside other drugs that will lead to cardiovascular depression
  • CLASS III
    Sotalol…
    • Prolongs action potential duration and refractory period by inhibiting K+ channels; also weak Beta-blocker
    • Used to treat VTAC  -can use for SVT as well but less common
    • Less cardiac depression compared to class II so often used if concurrent myocardial disease
    • Side effects: decrease cardiac output, hypotension, bradyarrhythmia, depression, vomiting/diarrhoea
  • CLASS IV
    Diltiazem…
    • Ca-channel blocker: slows AV node conduction, some reduction in myocardial contractility
    • Primary use to control SVTs
    • Used to control AF either as monotherapy or in combination with digoxin
    • Not stopping AF just reducing ventricular rate by delaying conduction across AVN –(you will still hear an irregularly irregular rhythm just hopefully at a more acceptable rate)
    • Side effects: bradycardia, vomiting in cats, lethargy