kronotrope

Kronotrope is a term used in physiology and pharmacology to describe a substance or factor that changes the rate at which the heart beats. In common usage, chronotropic effects are described as positive when the heart rate increases and negative when it decreases. Chronotropy is distinct from inotropy, which refers to changes in the force of cardiac contraction rather than the rate.

The word derives from the Greek roots for time and turning, and in many scientific contexts the same concept is labeled chronotropic or chronotropic agent. In some languages or contexts, kronotrope is used as an alternative spelling.

Mechanisms of action involve the autonomic nervous system and direct effects on the sinoatrial node, the heart’s natural pacemaker. Positive chronotropes typically enhance sympathetic activity or directly stimulate beta-adrenergic receptors, increasing the slope of the pacemaker’s rhythm and speeding impulse generation. Negative chronotropes often increase parasympathetic (vagal) activity or block sympathetic input, slowing the heart rate. Some drugs can have mixed or dose-dependent effects.

Common examples include positive chronotropes such as adrenaline (epinephrine), norepinephrine, isoproterenol, and dobutamine, which raise heart rate and can support cardiac output in certain clinical settings. Negative chronotropes include beta-blockers (for example propranolol and metoprolol) and certain calcium channel blockers (such as verapamil and diltiazem), which slow the heart rate. Thyroid hormones and certain stimulants can also influence chronotropy, usually increasing the heart rate.

Chronotropic effects are clinically important for managing conditions like bradycardia, tachycardia, heart failure, and other cardiac illnesses, and are routinely considered alongside inotropic and lusitropic (relaxation) effects in cardiovascular pharmacology.