- 1. To obstruct; to arrest passage through. 2. A condition in which the passage of an electrical impulse is arrested, wholly or in part, temporarily or permanently. 3. SYN: atrioventricular b.. [Fr. bloquer]- alveolocapillary b. the presence of material that impairs the diffusion of gases between the air in the alveolar spaces and the blood in alveolar capillaries; b. can be caused by edema, cellular infiltration, fibrosis, or tumor, and results in undersaturation of peripheral arterial blood with oxygen.- anterograde b. conduction b. of an impulse traveling anywhere in its ordinary direction, for example, from the sinoatrial node toward the ventricular myocardium. SYN: antegrade b..- arborization b. intraventricular b. supposedly due to widespread blockage in the Purkinje ramifications and manifested in the electrocardiogram by a pattern similar to bundle-branch b. but with complexes of low amplitude.- atrioventricular b., AV b. partial or complete b. of electric impulses originating in the atrium or sinus node preventing them from reaching the atrioventricular node and ventricles. In first degree AV b., there is prolongation of AV conduction time (PR interval); in second degree AV b., some but not all atrial impulses fail to reach the ventricles, thus some ventricular beats are dropped; in complete AV b. (third degree), complete atrioventricular dissociation (2) occurs; no impulses can reach the ventricles despite even a slow ventricular rate (under 45/min); atria and ventricles beat independently. SYN: b. (3), heart b..- bone b. surgical procedure in which a bone graft is placed adjacent to a joint to limit motion of the joint mechanically or to improve the stability of the joint, e.g., at the ankle joint to correct foot-drop by preventing plantarflexion past 0°, but allowing dorsiflexion beyond 0°, e.g., at the glenohumeral joint to prevent posterior instability.- bundle-branch b. intraventricular b. due to interruption of conduction in one of the two main branches of the bundle of His and manifested in the electrocardiogram by marked prolongation of the QRS complex; b. of each branch has distinctive QRS morphology.- complete AV b. 1. See atrioventricular b.. 2. SYN: complete atrioventricular dissociation. See atrioventricular b..- conduction b. failure of impulse transmission at some point along a nerve, although conduction along the segments proximal and distal to it are unaffected; clinically, most often the result of an area of focal demyelination; when caused by focal trauma, called neurapraxia.- congenital heart b. atrioventricular b. present in utero or at birth and usually of advanced or complete degree.- depolarizing b. skeletal muscle paralysis associated with loss of polarity of the motor endplate, as occurs following administration of succinylcholine.- divisional b. arrest of the impulse in one of the assumed two main divisions of the left branch of the bundle of His; i.e., in either the anterior (superior)_ division or the posterior (inferior) division. SYN: hemiblock.- entrance b. SYN: protective b..- epidural b. an obstruction in the epidural space; used inaccurately to refer to epidural anesthesia.- exit b. inability of an impulse to leave its point of origin, the mechanism for which is conceived as an encircling zone of refractory tissue denying passage to the emerging impulse.- fascicular b. a condition based on the disputed concept that the left branch of the bundle of His provides two of three major fascicles of a system of conduction, of which the right bundle branch constitutes the third, for the transmission of the cardiac impulse from the atrium above to the ventricles below the AV node; b. may occur in any or all fascicles, all three together producing complete AV b.. SEE ALSO: hemiblock.- field b. regional anesthesia produced by infiltration of local anesthetic solution into tissues surrounding an operative field.- interatrial b. SYN: intraatrial b..- intraatrial b. impaired conduction through the atria, manifested by widened and often notched P waves in the electrocardiogram. SYN: interatrial b..- intraventricular b. (IVB), IV b. delayed conduction within the ventricular conducting system or myocardium, including bundle-branch, periinfarction blocks, fascicular blocks, nonspecific IV b.and Wolff-Parkinson-White (preexcitation) syndrome.- Mobitz b. second degree atrioventricular b. in which there is a ratio of two or more atrial deflections (P waves) to ventricular responses.- Mobitz types of atrioventricular b. type I, the dropped beat of the Wenckebach phenomenon; type II, a dropped cardiac cycle that occurs without alteration in the conduction of the preceding intervals.- nerve b. interruption of conduction of impulses in peripheral nerves or nerve trunks by injection of local anesthetic solution.- nondepolarizing b. skeletal muscle paralysis unaccompanied by changes in polarity of the motor endplate, as occurs following administration of tubocurarine.- partial heart b. impulses penetrate the atrioventricular junction in some relation to the ventricular rate. SYN: incomplete atrioventricular b..- periinfarction b. an electrocardiographic abnormality associated with a myocardial infarct and caused by delayed activation of the myocardium in the region of the infarct; characterized by an initial vector directed away from the infarcted region with the terminal vector directed toward it.- phase I b. inhibition of nerve impulse transmission across the myoneural junction associated with depolarization of the motor endplate, as in the muscle paralysis produced by succinylcholine.- phase II b. inhibition of nerve impulse transmission across the myoneural junction unaccompanied by depolarization of the motor endplate, as in the muscle paralysis produced by tubocurarine.- protective b. an incompletely understood mechanism whereby a pacemaker is protected from being discharged by the impulse from another center; the mechanism, usually conceived as an encircling zone of unidirectionally refractory tissue permitting egress of impulses from the center but preventing access to the center, is seen in operation in ventricular parasystole where the parasystolic center is protected from discharge by the sinus pacemaker and so is able to maintain its intrinsic rhythm undisturbed. SYN: entrance b., protection.- pupillary b. increased resistance to flow of aqueous humor through the pupil from the posterior chamber to the anterior chamber, leading to anterior bowing of the peripheral iris over the trabecular meshwork and to angle-closure glaucoma.- reverse pupillary b. increased resistance to flow of aqueous humor through the pupil from the anterior chamber to the posterior chamber, leading to posterior bowing of the peripheral iris against the zonules; a possible mechanism for pigmentary glaucoma.- sinoatrial b., S-A b., sinus b. blockade of the impulse leaving the sinus node before it can activate atrial muscle. SYN: sinoauricular b..- sinoauricular b. SYN: sinoatrial b..- spinal b. an obstruction to the flow of cerebrospinal fluid in the spinal subarachnoid space; used inaccurately to refer to spinal anesthesia.- suprahisian b. atrioventricular conduction delay occurring above, or cephalad to, the bundle of His.- unidirectional b. b. that prevents passage of an impulse when it approaches from one direction but not from the other, as when b. in the AV node prevents anterograde conduction to the ventricles while retrograde conduction to the atria remains intact.- Wenckebach b. a form of b. in any cardiac tissue (most often the atrioventricular junction) in which there is progressive lengthening of conduction (decremental conduction) until the beat is dropped.- Wolff-Chaikoff b. blocking of the organic binding of iodine and its incorporation into hormone caused by large doses of iodine; usually a transient effect, but in large doses in susceptible individuals it can be prolonged and cause iodine myxedema. SYN: Wolff-Chaikoff effect.
* * *block 'bläk n, often attrib1) interruption of normal physiological function of a tissue or organ <respiratory \block due to carbon monoxide> esp HEART BLOCK2 a) BLOCK ANESTHESIAb) NERVE BLOCK (1)3) interruption of a train of thought by competing thoughts or psychological suppressionblock vt1) to prevent normal functioning of (a bodily element) <\block a nerve with novocaine>2) to obstruct the effect of <a carboxyl group \blocked by esterification> vi to experience or exhibit psychological blocking or blockageblock·er -ər n
* * *n.any interruption of physiological or mental function, brought about intentionally (as part of a therapeutic procedure) or by disease. See also heart block, nerve block.
* * *(blok) 1. obstruction. 2. to obstruct. 3. regional anesthesia; see under anesthesia.
Medical dictionary. 2011.