Bathmotropic often refers to modifying the degree of excitability specifically of the heart; in general, it refers to modification of the degree of excitability (threshold of excitation) of musculature in general, including the heart. It especially is used to describe the effects of the cardiac nerves on cardiac excitability. Positive bathmotropic effects increase the response of muscle to stimulation, whereas negative bathmotropic effects decrease the response of muscle to stimulation. In a whole, it is the heart's reaction to catecholamines (

norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad' ...

, epinephrine, dopamine). Conditions that decrease bathmotropy (i.e. hypercarbia) cause the heart to be less responsive to

catecholaminergic Catecholaminergic means "related to catecholamines". The catecholamine neurotransmitters include dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline). A catecholaminergic agent (or drug) is a chemical which functions to direc ...

drugs. A substance that has a bathmotropic effect is known as a bathmotrope. While bathmotropic, as used herein, has been defined as pertaining to modification of the ''excitability'' of the heart, it can also refer to modification of the ''irritability'' of heart muscle, and the two terms are frequently used interchangeably.

Etymology

The term "bathmotropic" is derived from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

word ''βαθμός'' (''bathmós''), meaning "step" or "threshold".

History

In 1897 Engelmann introduced four Greek terms to describe key physiological properties of the heart:

inotropy An inotrope is an agent that alters the force or energy of muscular contractions. Negatively inotropic agents weaken the force of muscular contractions. Positively inotropic agents increase the strength of muscular contraction. The term ''inotr ...

, the ability to contract;

chronotropy Chronotropic effects (from ''chrono-'', meaning time, and ''tropos'', "a turn") are those that change the heart rate. Chronotropic drugs may change the heart rate and rhythm by affecting the electrical conduction system of the heart and the ne ...

, the ability to initiate an electrical impulse; dromotropy, the ability to conduct an electrical impulse; and bathmotropy, the ability to respond to direct mechanical stimulation. A fifth term,

lusitropy Lusitropy is the rate of myocardial relaxation. The increase in cytosolic calcium of cardiomyocytes via increased uptake leads to increased myocardial contractility (positive inotropic effect), but the myocardial relaxation, or lusitropy, decrease ...

, was introduced in 1982 when relaxation was recognized to be an active process, and not simply dissipation of the contractile event. In an article in the ''American Journal of the Medical Sciences'', these five terms were described as the five fundamental properties of the heart.

Physiological explanation

The bathmotropic effect modifies the heart muscle membrane excitability, and thus the ease of generating an

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

. The ease of generating an action potential is related both to the magnitude of the

resting potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opp ...

and to the activation state of membrane

sodium channels Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the chann ...

. During stage 4 of an action potential, the inside of a cardiac muscle cell rests at −90 mV. As the inner muscle cell potential rises towards −60 mV, electrochemical changes begin to take place in the voltage-gated rapid sodium channels, which permit the rapid influx of sodium ions. When enough sodium channels are opened, so that the rapid influx of sodium ions is greater than the tonic efflux of potassium ions, then the resting potential becomes progressively less negative, more and more sodium channels are opened, and an action potential is generated. The electrical potential at which this occurs is called the

threshold potential In electrophysiology, the threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neuroscience, threshold potentials are necessary to regulate and propagate signaling in both ...

. As various drugs and other factors act on the resting potential and bring it closer to the threshold potential, an action potential is more easily and rapidly obtained. Likewise, when the sodium channels are in a state of greater activation, then the influx of sodium ions that allows the membrane to reach threshold potential occurs more readily. In both instances, the excitability of the myocardium is increased.

Drugs, ions and conditions

Increasing bathmotropy

Hypocalcemia Hypocalcemia is a medical condition characterized by low calcium levels in the blood serum. The normal range of blood calcium is typically between 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L) while levels less than 2.1 mm ...

- calcium blocks

which prevents depolarization, so decreases in calcium allow increased

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

passage and which lowers the threshold for depolarization. *Mild to moderate

hyperkalemia Hyperkalemia is an elevated level of potassium (K+) in the blood. Normal potassium levels are between 3.5 and 5.0mmol/L (3.5 and 5.0mEq/L) with levels above 5.5mmol/L defined as hyperkalemia. Typically hyperkalemia does not cause symptoms. Occasi ...

- causes a partial depolarization of the resting membrane potential *

Norepinephrine Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad' ...

and sympathetic stimulation in general - raises the resting membrane potential *

Digitalis ''Digitalis'' ( or ) is a genus of about 20 species of herbaceous perennial plants, shrubs, and biennials, commonly called foxgloves. ''Digitalis'' is native to Europe, western Asia, and northwestern Africa. The flowers are tubular in shap ...

- Converts the normal Purkinje action potential of heart muscle to the automaticity type, which increases myocardial irritability * Epinephrine - Also known as adrenaline, effects are similar to sympathetic stimulation *Mild hypoxia - causes a partial depolarization of the muscle membrane *Ischaemia - causes a partial depolarization of the muscle membrane

Decreasing bathmotropy

* Hypercalcemia - decreases permeability to sodium, hyperpolarizes membrane. *

Propranolol Propranolol, sold under the brand name Inderal among others, is a medication of the beta blocker class. It is used to treat high blood pressure, a number of types of irregular heart rate, thyrotoxicosis, capillary hemangiomas, performance an ...

Quinidine Quinidine is a class I antiarrhythmic agent, class IA antiarrhythmic agent used to treat heart rhythm disturbances. It is the enantiomer of Antimalarial medication, antimalarial agent quinine, originally derived from the bark of the cinchona tre ...

and other Class A

Antiarrhythmic agents Antiarrhythmic agents, also known as cardiac dysrhythmia medications, are a group of pharmaceuticals that are used to suppress abnormally fast rhythms ( tachycardias), such as atrial fibrillation, supraventricular tachycardia and ventricular ta ...

- block the voltage gated sodium channels *Calcium channel blockers - in general have negative bathmotropic effects * Parasympathetic stimulation - decreases excitability only of atrial muscle cells *

Hyponatremia Hyponatremia or hyponatraemia is a low concentration of sodium in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be a ...

- decreases external sodium concentration *

Hypokalemia Hypokalemia is a low level of potassium (K+) in the blood serum. Mild low potassium does not typically cause symptoms. Symptoms may include feeling tired, leg cramps, weakness, and constipation. Low potassium also increases the risk of an abno ...

- hyper polarization of the resting membrane potential * Acetylcholine - same as parasympathetic stimulation *Marked hypoxia - causes a marked depolarization of the resting membrane potential

References

{{Cardiovascular physiology Electrophysiology Cardiovascular physiology