N. Sydney Moise, DVM, MS, DACVIM (Cardiology, Internal Medicine)
Frequently the routine ECG is inadequate to determine the need for and the response to antiarrhythmic therapy. The 24-hour ambulatory ECG or Holter monitor is better suited for these determinations. The technology for Holter monitoring has made advances relative to the digitalization of the recordings, yet the accuracy of the analysis is still lacking for many of the arrhythmias that dogs and cats display. Despite the latter difficulty, it is still evident that to make the best judgments with regards to the management of arrhythmias, there is no replacement for this diagnostic ability.
During this brief communication the use of the Holter monitor will be demonstrated. An actual analysis will be demonstrated. Also, the results from patients will be used to illustrate the utility of this important diagnostic tool.
Reduction of Heart Rate as an Indicator of Oxygen Consumption
In both reported major studies above, as well as in previously published canine studies and in humans, heart failure therapy with Pimobendan is not associated with an increase in heart rate. In contrary, heart rate usually drops a bit with the general improvement of the patient and the reduction of signs of heart failure.
Sensitized to proarrhythmic effects of the PDE III-inhibitor Milrinone found in the Promise trial (Packer et al 1991), Pimobendan therapy was carefully observed for proarrhythmia in several studies (the Pico trial 1996, Remme et al 1994, the Epoch study 2002). Convincing evidence about increased ventricular tachyarrhythmias with clinical significance could not be found. A veterinary study (Rosenthal 2006) revealed increased VPC's in 6/8 dogs with DCM investigated with Holter-ECG's approx. 3 weeks after initiation of Pimobendan-therapy.
In the major multicenter trials cited above (PiTCH, VetScope) however, no proarrhythmic effect was found.
Comparison to Another Calcium Sensitizer Drug Levosimendan
Levosimendan is a very similar compound that has similar cardiovascular effects, but more calcium sensitizing and less vasodilating effects through PDE III-inhibition than Pimobendan. Levosimendan is used by intravenous infusions, and prolonged lasting effects up to several weeks after discontinuation of the drip are attributed to one of its metabolites OR-1896 in man (Kivikko et al 2003). This drug is being studied and prepared for licensing for the veterinary market as well, but results haven't been published yet. Apparently, dogs metabolize the drug differently without the presence of this effective metabolite.
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