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 Horses have a wide HR range.A resting rate of 24–50 beats/min to a high of 220–240 beats/min at exercise.

 Increased HR will generally result in increased cardiac output if SV is constant.In horses, the greatest changes in cardiac output generally occur due to a change in HR (rather than in SV).Table 3.2 Summary of formulas for cardiovascular variables.NameFormulaCardiac output (Q)Heart rate × Stroke volumeMean arterial blood pressureCardiac output × Systemic vascular resistanceSystemic vascular resistance (SVR)Pulmonary vascular resistance (PVR)Arterial oxygen content (CaO2)Oxygen delivery (DO2)CaO2 = ([Hb] × SaO2% × 1.36) + (PaO2 × 0.003)Cardiac output × arterial oxygen content

  Extreme tachycardia may actually decrease cardiac output because diastolic filling time is decreased, resulting in decreased SV.

 Extreme tachycardia can cause arrhythmias and worsen cardiac disease since the rapidly beating heart spends less time in diastole, the period of the cardiac cycle in which the myocardium is perfused.

 During tachycardia, myocardial oxygen delivery is decreased at a time when myocardial oxygen consumption is increased, resulting in inadequate oxygen delivery to the myocardium (i.e. oxygen debt).