Normal resting heart rates range from 60â100 bpm. Bradycardia is defined as a resting heart rate below 60 bpm. However, heart rates from 50 to 60 bpm are common among healthy people and do not necessarily require special attention.achycardia is a fast heart rate, defined as above 100 bpm at rest. Bradycardia is a slow heart rate, defined as below 60 bpm at rest. During sleep a slow heartbeat with rates around 40â50 BPM is common and is considered normal.
The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines, especially norepinephrine (noradrenaline) and epinephrine (adrenaline).
There are two main groups of adrenergic receptors, Î± and Î², with several subtypes. Î± receptors have the subtypes Î±1 (a Gq coupled receptor) and Î±2 (a Gi coupled receptor). Phenylephrine is a selective agonist of the Î± receptor.