Catecholamines, like adrenaline, are released during stress, and will cause an increase in heart rate. Drugs are another common cause of change in the heart rate and most anaesthetic drugs can do this. Halothane affects the SA node and will also depress the force of contraction of the heart. Isoflurane, by contrast has little direct affect on the heart, but causes peripheral vasodilation of the blood vessels. This will then decrease the blood pressure, and hence produce a reflex tachycardia as explained above.
Ketamine causes stimulation of the sympathetic nervous system, and therefore produces a tachycardia.
Other circulating substances may also affect the heart rate, acting indirectly through the autonomic nervous system. For example increased blood concentrations of carbon dioxide will cause stimulation of the sympathetic nervous system and tachycardia, and is an important sign of respiratory failure.