In 1897 Engelmann introduced four Greek terms to describe key physiological properties of the heart: inotropy; the ability to contract, chronotropy; 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, was introduced in 1982 when relaxation was recognized to be an active process, and not simply dissipationof the contractile event . Bathmotropic (derived from the Greek word "bathmos", meaning step or threshold) refers to modification of the degree of excitability (threshold of excitation), of musculature in general, and of heart musculature specifically. It is used especially 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. Bathmotropic is one of five adjectives used to describe various qualities of the cardiac cycle; the other four are: inotropic, chronotropic, dromotropic and lusitropic. In an article in the American Journal of Medical Sciences, these five terms were described as the five fundamental properties of the heart. While bathmotropic, as used herein, has been defined as pertaining to modification of the excitability of the heart, it can equally well refer to modification of the irritability of heart muscle, and the two terms are frequently used interchangeably.
The bathmotropic effect modifies the heart muscle membrane excitability, and thus the ease of generating an action potential. The easiness of generating an action potential is related both to the magnitude of the resting potential and to the activation state of membrane sodium channels. (For a complete description of the membrane action potential please see the related Wikipedia article on action potentials.) During stage 4 of the action potential, the inside of the 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 fast gated sodium channels are opened, and an action potential is generated. The electrical potential at which this occurs is called the threshold potential. As various drugs and other factors act on the resting potential and bring it closer to the threshold potential, the 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 that have a positive bathmotropic effect
- Hypocalcemia - calcium blocks sodium channels which prevents depolarization, so decreases in calcium allow increased sodium passage and which lowers the threshold for depolarization.
- Mild to moderate hyperkalemia - causes a partial depolarization of the resting membrane potential
- Norepinephrine and sympathetic stimulation in general - raises the resting membrane potential
- Digitalis - Converts the normal Purkinje action potential of heart muscle to the automaticity type, which increases myocardial irritability
- 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
Drugs and conditions that have a negative bathmotropic effect
- Hypercalcemia - decreases permeability to sodium, hyperpolarizes membrane.
- Quinidine and other Class A Antiarrhythmic agents - 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 - decreases external sodium concentration
- Hypokalemia - hyper polarization of the resting membrane potential
- Acetylcholine - same as parasympathetic stimulation
- Marked hypoxia - causes a marked depolarization of the resting membrane potential
- Engelmann (1897). Pflugers Archiv 65: 535–578.
- Katz AM; Smith VE (1982). Eur Heart J. 3 (Suppl D): 11–18.
- Miriam Webster's Medical Dictionary and Online Medical Dictionary
- The Kanji Foundry Press - b
- The American Journal of the Medical Sciences. J.B. Lippincott, Company. 1908. pp. 46–.
- Scientific American Medical; Dale and Federman Vol 1; 2003 Edition p. 1907 chapter 160; Disorders of Acid-Base and Potassium Balance
- Armstrong, C.M., Cota, Gabriel. (1999). "Calcium block of Na+ channels and its effect on closing rate". Proceedings of the National Academy of Sciences of the United States of America 96 (7): 4154–4157. PMC 22436. PMID 10097179.
- Kahloon, Mansha U.; Aslam, Ahmad K.; Aslam, Ahmed F.; Wilbur, Sabrina L.; Vasavada, Balendu C.; Khan, Ijaz A. (November 2005). "Hyperkalemia induced failure of atrial and ventricular pacemaker capture". International Journal of Cardiology 105 (2): 224–226. doi:10.1016/j.ijcard.2004.11.028. PMID 16243117.
- Veldkamp, M (1 June 2001). "Norepinephrine induces action potential prolongation and early afterdepolarizations in ventricular myocytes isolated from human end-stage failing hearts". European Heart Journal 22 (11): 955–963. doi:10.1053/euhj.2000.2499. PMID 11428819.
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