It is a sinus rhythm (see above) with an increased sinus nodal rate, and therefore ventricular rate.
Example of Sinus Tachycardia in a Dog (HR 214 bpm)
Examples of etiologies include:
Treatment: Treat the underlying cause.
Example of Supraventricular Premature Beats in a Dog
Example of Supraventricular Tachycardia in a Dog (first third of ECG)
Etiology: The same as for supraventricular premature beats (see above)
Consequences: Paroxysms of supraventricular tachycardia can result in weakness, syncope, and heart failure when sustained and untreated
Atrial fibrillation is a supraventricular arrhythmia associated with many microfoci of atrial myocardium acting as ectopic foci or serving as small reentrant pathways and activating the adjacent atrial myocardium. Only the ectopic foci that manage to depolarize close to the A-V node successfully penetrate the A-V node to activate the ventricles. There is no coordinated atrial contraction, and the ventricles are activated at a fast rate and at irregular intervals.
Example of Atrial Fibrillation in a Dog
Another Example of Atrial Fibrillation in the Dog - Place cursor on the image to see examples of the ECG Findings
The hallmark ECG findings are the presence of an irregular rhythm and absence of P waves. As the heart rate becomes very rapid, the rhythm approaches a regular rhythm; however, a slight irregularity remains present. Although the heart rate is usually rapid, it can be slow if:
The target heart rate in hospital for most canine patients is 120-150 bpm, though this is very patient dependent (some patients require higher heart rate than others to maintain adequate cardiac output). Note that both calcium-channel blockers and beta blockers are negative inotropes as well as negative chronotropes. Beta blockers in particular require starting at low doses and up-titrating gradually.
Ventricular premature contractions (VPCs) are depolarizations from an ectopic focus in the ventricular myocardium.
Example of Ventricular Premature Contractions in a Dog
The intravenous drug of choice for treatment of ventricular tachycardia is typically lidocaine. The first choice for oral therapy of VPCs is often sotalol, however this depends on the case and underlying disease process.
This decision must be approached individually for each case, however some general guidelines include:
There is often little difficulty in determining if condition #1 is present. Determining if condition #2 or 3 is present is more challenging. In determining the significance of ventricular arrhythmias, heart rate is likely one of the more important criteria. If VPCs occur at a fast heart rate (i.e. >170 bpm), this potentially represents a more clinically significant condition, whereas if they occur at a more normal heart rate, this situation may not represent as significant of a risk. The definitive importance of heart rate in the presence of VPCs remains to be determined.
Why not institute anti-arrhythmic therapy in any case just to be safe? The issue is that all anti-arrhythmics have the potential to be pro-arrhythmic, some more than others. Therefore, follow-up to assess efficacy is critical in any patient in which anti-arrhythmic therapy is initiated.
Ventricular tachycardia (VT) refers to runs of more than 3 VPCs in sequence. As above for VPCs, QRS complexes are wide and bizarre, and there is lack of association with P waves (A-V dissociation). VT may markedly reduce cardiac output (through dyssynergy of contraction and high heart rate). Etiology is as for premature ventricular contractions (see above). VT is treated as described above under premature ventricular contractions. Again, the presence of clinical signs, the presence of an underlying heart disease known to be associated with sudden death, and high heart rate are likely the most important criteria for treatment. Slow VT (VT rate from 80 - 140 bpm in dogs) frequently has minimal hemodynamic consequences and therefore warrants observation but not necessarily anti-arrhythmic therapy.
Example of Ventricular Tachycardia in a Dog
While the differentiation of SVPCs and VPCs may often be straightforward, it can also be a challenge in that SVPCs may conduct with aberrancy (abnormally) resulting in bizarre and wide QRS morphology. Since clinical significance, therapy, and prognosis for one may be very different than for the other, it is important to attempt to differentiate these abnormalities.
|Supraventricular Premature Beats (SVPBs)||Ventricular Premature Beats (VPBs)|
|1. Morphology of QRS is usually similar to the sinus beats||1. Morphology of QRS is bizarre vs the sinus beats|
|2. QRS is usually narrow||2. QRS is usually wide|
|3. SVPBs can't occur as fusion beats||3. VPBs may occur as fusion beats|
|So if the beat in question occurs as a fusion beat it must be a VPB|
|4. SVPBs can't occur as interpolated beats||4. VPBs may occur as interpolated beats|
|So if the beat in question occurs as an interpolated beat it must be a VPB|
|5. Atrial (P wave) and ventricular (QRS) activation are associated||5. A-V dissociation is usually present|
|In either case, often the P waves cannot be visualized in the presence of tachycardia because they are buried in the QRS or T waves.|
|6. Non-fully compensatory pause is present||6. Fully compensated pause is present|
|If the background rhythm is irregular (as with sinus arrhythmia or atrial fibrillation) this criterion can’t be used because the duration of 2 normal R to R intervals cannot be predicted with precision.|
|7. When supraventricular tachycardia occurs it tends to be very regular||7.When ventricular tachycardia occurs it may be slightly irregular|
|8. A premature beat that follows a premature P wave is always a SVPC||8. A premature beat that follows a premature P is never a VPC|
|The absence of a premature P does not discriminate between an SVPC and a VPC|
A refers to the time between two normal sinus beats close to the premature beat. B refers to the time between the sinus beat just before the premature beat to the sinus beat just after the premature beat.
If A > B (this is called a non-compensatory pause), indicates the premature beat is a supraventricular beat.
If A < B (this is called a compensatory pause), indicates the premature beat is a ventricular beat.
Example of an Interpolated VPC