| A wave | The initial negative deflection in the electroretinogram, presumably reflecting retinal photoreceptor activity, an atrial deflection in an electrocardiogram recorded from within the atrium of the heart, the first positive deflection of the atrial and venous pulses due to atrial systole. (05 Mar 2000) |
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| a' haemolysis | Haemolysis observed infrequently in blood agar cultures of occasional strains of streptococci; the zone of haemolysis about the colony is not as clear, or wide, or distinctly outlined as it is in b haemolysis; there are a few apparently intact erythrocytes throughout the zone, but they are more numerous in the immediate vicinity of the colony, and there is no discoloration as there is in a haemolysis; the unique feature is that the zone becomes wider, i.e., the process is stimulated, when the culture is incubated at refrigerator temperatures (not true for b haemolysis); some strains of streptococci that are a'-haemolytic on horse blood agar cause typical a haemolysis on rabbit blood agar. (05 Mar 2000) |
| A-DNA | A form of DNA in which the helix is right-handed and the overall appearance is short and broad. (05 Mar 2000) |
| A-E amputation | Acronym for above-the-elbow amputation. (05 Mar 2000) |
| A-esotropia | Convergent strabismus greater in upward than in downward gaze. (05 Mar 2000) |
| A-exotropia | Divergent strabismus greater in downward than in upward gaze. (05 Mar 2000) |
| a-factor | <molecular biology> A protein which is found in the bacterial genus Streptomyces that helps start the production of streptomycin and the process of morphological differentiation. It is used in biotechnology to induce these functions in mutant strains of Streptomyces that cannot produce it themselves. (09 Feb 1998) |
| a-form DNA | <molecular biology> One of several forms that can be assumed by a double helix. A-DNA is stable in dehydrated conditions. This form is less common than the dominant form found under physiological conditions -- beta-DNA. This form is also assumed by DNA-RNA hybrid helices and by regions of double-stranded RNA. It is a right-handed helix and is a more compact form than beta-DNA. (09 Oct 1997) |
| a-good | In earnest; heartily. "I made her weep agood." Origin: Pref. A- + good. Source: Websters Dictionary (01 Mar 1998) |
| A-H conduction time | Forward conduction of the cardiac impulse from atria to ventricles via the A-V node or any bypass tract, represented in the electrocardiogram by the P-R interval. P-H conduction time is from the onset of the P wave to the first high frequency component of the His bundle electrogram (normally 119 ± 38 msec); A-H conduction time is from the onset of the first high frequency component of the atrial electrogram to the first high frequency component of the His bundle electrogram (normally 92 ± 38 msec); P-A conduction time is from the onset of the P wave to the onset of the atrial electrogram (normally 27 ± 18 msec). (05 Mar 2000) |
| A-H interval | The time from the initial rapid deflection of the atrial wave to the initial rapid deflection of the His bundle (H) potential; it approximates the conduction time through the A-V node (normally 50-120 msec). (05 Mar 2000) |
| A-K amputation | Acronym for above-the-knee amputation. (05 Mar 2000) |
| A-mode | In diagnostic ultrasound, a one-dimensional presentation of a reflected sound wave in which echo amplitude (A) is displayed along the vertical axis and time of rebound (depth) along the horizontal axis; the echo information is presented from interfaces along a single line in the direction of the sound beam. (05 Mar 2000) |
| a-mornings | In the morning; every morning. "And have such pleasant walks into the woods A-mornings." (J. Fletcher) See: Amorwe. The -s is a genitival ending. See -wards. Source: Websters Dictionary (01 Mar 1998) |
| A-N interval | The time between onset of the atrial deflection and the nodal potential (normally 40-100 msec). (05 Mar 2000) |