block

block

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. atrioventricular b [Fr. bloquer]
alveolo-capillary 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.
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. , A-V 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 A-V b., there is prolongation of A-V conduction time (P-R interval); in second degree A-V b., some but not all atrial impulses fail to reach the ventricles, thus some ventricular beats are dropped; in complete A-V b., complete atrioventricular dissociation (2) occurs; no impulses can reach the ventricles despite even a slow ventricular rate (under 45 per minute); atria and ventricles beat independently.block (3), heart b;
bone b. a surgical procedure in which the bone adjacent to the joint is modified to limit the motion of the joint mechanically; e.g., at the ankle joint to correct foot-drop by preventing extension below 90°, but allowing flexion within 90°.
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 A-V b. complete atrioventricular dissociation (2), complete A-V 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 caused by 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.
entrance b. 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 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 A-V node; block may occur in any or all fascicles, all three together producing complete A-V block. See also hemiblock.
field b. regional anesthesia produced by infiltration of local anesthetic solution into tissues surrounding an operative field.
first degree A-V b. See atrioventricular b.
heart b. atrioventricular b
incomplete atrioventricular b. partial heart b
intra-atrial b. impaired conduction through the atria, manifested by widened and often notched P waves in the electrocardiogram.
intraventricular b. (IVB) , I-V b. delayed conduction within the ventricular conducting system or myocardium, including bundle-branch, peri-infarction b.'s, the fascicular b.'s, excitation, and the W-P-W (pre-expectation) 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 mucle 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.incomplete atrioventricular b;
peri-infarction b. an electrocardiographic abnormality associated with an old 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.entrance b., protection;
retrograde b. impaired conduction backward from the ventricles or A-V node into the atria.
second degree A-V b. See atrioventricular b.
sinoatrial b. , S-A b. , sinus b. blockade of the impulse leaving the sinus node before it can activate atrial muscle.sinoauricular b;
sinoauricular b. sinoatrial b
spinal b. an obstruction to the flow of cerebrospinal fluid in the spinal subarachnoid space; used inaccurately to refer to spinal anesthesia.
stellate b. injection of local anesthetic solution in the vicinity of the stellate ganglion.
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 A-V 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 until the beat is dropped.
Wilson b. the commonest form of right bundle-branch b., characterized in lead I by a tall slender R wave followed by a wider S wave of lower voltage.
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.Wolff-Chaikoff effect;

 

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