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[Gr. megas, large] 1. SEE: megalo-. 2. In the International System of Units (SI), a prefix meaning 1 million (106).


(megʹă-kōdʺ) [mega- + code] A simulated cardiac arrest used for educational purposes by health care professionals during advanced cardiac life support training. It is intended to sharpen the technical and interpersonal skills needed during the cardiopulmonary resuscitation.


(mĕgʹă-kōʺlŏn) [ʺ + kolon, colon] Massive dilation of the colon, which, if left untreated, may result in perforation and peritonitis.

aganglionic m. Hirschsprung disease.

toxic m. Marked enlargement of the colon, esp. the transverse colon. Clinically, tachycardia, fever, and leukocytosis occur. There may be abdominal tenderness, a palpable abdominal mass, confusion, cramping, and change in number of bowel movements per day. SYN: toxic dilatation of colon.

 ETIOLOGY: The most common causes of toxic megacolon in adults are ulcerative colitis, pseudomembranous colitis, Crohn’s disease, drugs that slow intestinal motility (such as narcotics), and severe electrolyte disturbances. Megacolon in children may result from Hirschsprung disease.

 TREATMENT: Patients with toxic megacolon are treated by withholding oral intake, providing nasogastric suction, giving broad-spectrum antibiotics (and corticosteroids, in inflammatory bowel disease), and carefully resuscitating fluids and electrolytes. Surgery is required if the patient fails to improve or deteriorates.


(mĕgʺă-kūʹrē) [ʺ + curie] ABBR: MCi. A unit of radioactivity equal to 106 curies.


(mĕgʺă-dŏnʹshē-ă) [ʺ + odous, odont-, tooth] Macrodontia.


(mĕʹgă-dōsʺ) A dose of a nutrient, such as a vitamin supplement, that is 10 times greater than the recommended daily allowance for that nutrient.


(mĕgʹă-dīn) A unit equal to 1 million dynes. SEE: dyne.


(mĕgʺă-ĕ-sŏfʹă-gŭs) [ʺ + oisophagos, esophagus] A grossly dilated esophagus usually associated with achalasia. SYN: megaloesophagus.


(mĕgʹă-hĕrtz) ABBR: MHz. One million cycles per second, or 106 hertz.


(mĕgʺă-kărʹē-ō-blăst) An immature megakaryocyte.


(mĕgʺă-kărʹē-ō-sītʺ) [ʺ + karyon, nucleus, + kytos, cell] Large bone marrow cell with large or multiple nuclei from which platelets are derived. SEE: illus.; platelet.




(Orig. mag. ×640)



(mĕgʺă-kărʺē-ō-sī-tōʹsĭs) [ʺ + ʺ + ʺ + osis, condition] An increased number of megakaryocytes in the bone marrow; presence of megakaryocytes in the blood.

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