Cardiac output plateaus then falls at heart rates above 180-200 because there is insufficient time for diastolic filling. So there is no physiological benefit of heart rates increasing much above this in response to hypovolaemia. In fact, if the heart rate is more than around 240 then the rhythm is very likely to be an actual tachyarrhythmia rather than a sinus tachycardia in response to hypovolaemia. In euvolaemic dogs, as the heart rate increases the pulses will get narrower purely because there are more beats per unit time. Your job is to work out whether the reduction is pulse width is the amount that you would expect from the heart rate alone or is there reduced intravascular volume as well ie is the pulse narrower than you would expect from the heart rate alone. Slower heart rates will give you proportionally taller and wider pulses due to the increased stroke volume per beat.Heart sounds can be quieter when there is moderate to severe hypovolaemia presumable due to a lesser amount of blood in the heart to generate the heart sounds. You should cross reference this with your other perfusion parameters. For example, if the dog has only mild hypovolaemia but very quiet heart sounds then this is likely NOT just due to hypovolaemia and you should look for other reasons. A final tip for you regarding counting fast heart rates: count only every 2nd or 4th beat. Much easier!