premature ventricular contraction (redirected from Ectopic beats)

premature ventricular contraction

n. Abbr. PVC

An extrasystole involving the ventricles of the heart, sometimes producing accompanying palpitations.

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contraction (kntrak´shn),

n 1. a shortening, shrinkage, or reduction in length or size.
2. a condition in which teeth or other maxillary and mandibular structures, such as the dental arch, are nearer than normal to the median plane.

contraction, concentric muscle,

n an unresisted ordinary shortening of muscle.

contraction, eccentric muscle,

n an increase in muscle tonus during lengthening of the muscle. Eccentric contraction occurs when muscles are used to oppose movement but not to stop it (e.g., the action of the biceps in lowering the forearm gradually and in a controlled manner). Eccentric contractions are called
isotonic, because the muscle changes length.

contraction, isometric muscle

n an increase in muscular tension without a change in muscle length, as in clenching the teeth.

contraction, isotonic muscle

n an increase in muscular tension during movement without resistance (either lengthening or shortening), as in free opening and closing of the jaws.

contraction, metal,

n the shrinkage associated with the congealing of a metal from its molten state to a solid after having been cast. See also expansion, thermal.

contraction, muscle,

n the development of tension in a muscle in response to a nerve stimulus.

contraction, muscle, changes in striation bands,

n.pl alterations in bands of striated muscle during contraction. Striated muscle is composed of a darker A band and a lighter I band. Both these alternating bands develop tension during contraction but not to the same degree. In isometric contraction (clenched teeth), the sarcomere muscle unit remains unchanged in length, whereas the A band (the darker band) actually shortens and the I band (the lighter band) lengthens. When a muscle is passively stretched, such as when the mandible is opened by gravity, the A band lengthens relatively more than the I band, and during isotonic contraction, almost all the shortening is in the A segment. It is thus concluded that the contractile properties are not the same throughout the sarcomere, which is the unit of contractility. It is suggested that the darker A band has a greater concentration of contractile substance than the I band and that, in addition to contractile elements, the I band contains elastic noncontractile elements that constitute a series of elastic components throughout the fibril. Thus there is, throughout a fiber, an arrangement of dark, contractile components alternating with lighter, elastic components.

contraction, muscle, chemical factors in,

n.pl the chemical constituents and action involved in the contraction of muscle fibers. Muscle is a structure with working units built up largely from two proteins, actin and myosin, which appear to be organized into separate filaments running longitudinally through the muscle fibers. Neither type of filament runs continuously along the length of the fiber, although the effect is that of a continuous structure. The filaments are organized into a succession of groupings of one type of fiber. Each group is arranged in a regular palisade to overlap the next group of fibers, which are similarly arranged in palisades. This gives a banded appearance to the fiber. The thicker filaments contain myosin and are restricted to the A bands, where they give rise to a higher density and double refraction. The thinner filaments contain actin and extend to either side of the Z band, which is at the center of the I band. When the muscle contracts or is stretched, the two groups of filaments slide past each other like the alternating units of a sliding gate. The controlled sliding motion is presumably brought about through the mediation of oblique cross links between the filaments. These cross links are the structural expression of the biochemical interaction between actin and myosin. The chemical substance that initiates the interaction between these fibrils is adenosine triphosphate (ATP). The final effect of the interaction between ATP, myosin, and actin is to enable the two types of filaments to crawl past each other to create the shortened state of the muscle.

contraction, postural muscle

n the maintenance of muscular tension (usually isometric muscular contraction) sufficient to maintain posture.

contraction, premature ventricular (PVC),

n an extra heartbeat caused by premature contractions of the heart's ventricles resulting in palpitations, or a skipped beat, followed by a more pronounced beat.

contraction, smooth muscle, mechanism of,

n the mechanisms that regulate the functions of smooth muscle fibers. These regulatory mechanisms vary and are affected principally by two methods. First, the parasympathetic and sympathetic nerve fiber endings of the autonomic nervous system form a reticulum around the muscle cells before entering them. The action of these fibers is antagonistic; they act directly on the muscle cell, not on each other. Examples of the structures principally under the control of the autonomic nerve mechanism are the blood vessels and the pilomotor fibers. Second, the selection response to rhythmic activity associated with the automaticity of a viscus or other organ depends on local or hormonal factors. An example of this mechanism is the function of the uterus under the control of the estrogenic hormone.

contraction, static muscle,

n the contraction in which opposing muscles contract against each other and prevent movement. Fixation action of a muscle in a static contraction is termed
isometric, because it develops tension without changing length.

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premature ventricular contraction

Ventricular premature beat Cardiology A ventricular beat initiated by an ectopic focus, which occurs before the usual sinuatrial beat, characterized by premature, widened, bizarre QRS complexes, not preceded by a P wave; PVCs are extremely common, occur in¹⁄₂ of normal adults monitored by EKG for 24 hrs, and are of no significance Etiology Anxiety, fever, stimulants, associated with drug toxicity–digitalis, quinidine, tricyclic antidepressants and an indication for discontinuing therapy Treatment Lidocaine, quinidine, propranolol, amiodarone