Heart block: Failure of the heart's normal rhythm controls leading to arrhythmia. Heart block: Impairment of conduction in heart excitation. It is often applied specifically to atrioventricular heart block. Heart block: recurrent sudden attacks of unconsciousness caused by impaired conduction of the impulse that regulates the heartbeat.
For pet rabbits and show rabbits, pine is completely safe for rabbit litter, especially kiln-dried pine. Kiln-dried pine is also the cheapest rabbit litter on the market. And since it does a comparable job to all the other litters, pine shavings as rabbit litter and rabbit bedding gets our top vote. See Rabbit Bedding for comparisons with several other rabbit bedding materials, and more recommendations.
Certain trees can be used as forage for rabbits and dried for winter use as well: willow and poplar are two that are excellent and easy to find. Their leaves are quite high in protein and the rabbits will eat the bark from small twigs and branches as well. Here ia a list of trees that I know are safe.