Unstable angina ischaracterizedbyincreasinglyfrequentpain,precipitatedbyprogressivelylessexertionorevenoccurringatrest.Itresultsfromanerosionorruptureofatheroscleroticplaquetriggeringplateletaggregation,vasoconstriction,andformationofamuralthrombusthatneednotnecessarilybeocclusive.
Acute myocardial infarction typicallyresultsfromacutethrombosisafterplaquedisruption;amajorityoccurinplaquesthatdidnotpreviouslyexhibitcriticalstenosis.
Sudden cardiac death usuallyresultsfromafatalarrhythmia,typicallywithoutsignificantacutemyocardialdamage.
Ischemic cardiomyopathy isprogressiveheartfailureduetoischemicinjury,eitherfrompreviousinfarction(s)orchronicischemia.
Myocardialischemialeadstolossofmyocytefunctionwithin1to2minutesbutcausesdeathafteronly30to40minutes.Myocardialinfarctionisdiagnosedonthebasisofsymptoms,electrocardiographicchanges,andmeasurementofserumbiomarkerssuchascardiac-specifictroponins.Grossandhistologicchangesofinfarctionrequirehourstodaystodevelop.
Infarctioncanbemodifiedbytherapeuticintervention(e.g.,thrombolysisorstenting),whichsalvagesmyocardiumatriskbutmayalsoinducereperfusion-relatedinjury.
Complicationsofinfarctionincludeventricularrupture,papillarymusclerupture,aneurysmformation,muralthrombus,arrhythmia,pericarditis,andCHF.
Aberrant rhythms can be initiated anywhere in the conduction system, from the sinoatrial (SA) node down to the level of an individual myocyte; they are typically designated as originating from the atrium (supraventricular) or within the ventricular myocardium. Abnormalities in myocardial conduction can be sustained or sporadic http://ebooksmedicine.net (paroxysmal). They can manifest as tachycardia (fast heart rate), bradycardia (slow heart rate), an irregular rhythm with normal ventricular contraction, chaotic depolarization without functional ventricular contraction (ventricular fibrillation), or no electrical activity at all (asystole). Patients may be unaware of a rhythm disorder, or may note a “racing heart” or palpitations (irregular rhythm); loss of adequate cardiac output due to sustained arrhythmia can produce lightheadedness (near syncope), loss of consciousness (syncope), or sudden cardiac death (see later).
Ischemic injury is the most common cause of rhythm disorders, either through direct damage or through the dilation of heart chambers that alters signal conduction.
If the SA node is damaged (e.g., sick sinus syndrome), other fibers or even the atrioventricular (AV) node can take over pacemaker function, albeit at a much slower intrinsic rate (causing bradycardia).
If the atrial myocytes become “irritable” and depolarize independently and sporadically (as occurs with atrial dilation), the signals are variably transmitted through the AV node leading to the random “irregularly irregular” heart rate of atrial fibrillation.
If the AV node is dysfunctional, varying degrees of heart block occur, ranging from simple prolongation of the P-R interval on the ECG (first-degree heart block), to intermittent transmission of the signal (second-degree heart block), to complete failure (third-degree heart block).