Ischemia occurs when part of the
heartmuscle, the myocardium, is deprived from oxygen and nutrients.
Common causes of ischemia are:
* Narrowing or obstruction of a coronary artery.
* A rapid arrhythmia, causing
a disbalance in supply and demand of energy.
A short period of ischemia causes ''reversible'' effects: The
heartcells will be able to recover. When the episode of ischemia lasts for a longer period of time, heartmuscle cells will die. This is called a '''heart attack''' or '''myocardial infarction'''. That is why it is critical to recognize ischemia on the ECG in an early stage.
Severe ischemia results in ECG changes within minutes. While the ischemia lasts, several ECG changes will occur and disappear again. Therefore, it may be difficult to estimate the duration of the ischemia on the ECG, which is crucial for adequate treatment.
* Shock (manifesting as paleness, low blood pressure, fast weak pulse) shock
dysturbances (in particular increasing prevalnce of ventricular ectopia, ventricular tachycardia, AV block)
===Risk assessment of Cardiovascular disease===
The narrowing of the coronary artery leading to a myocardial infarction, usually develops over several years. An increased risk of cardiovascular disease, which may lead to a myocardial infarction or stroke, can be estimated using [http://www.escardio.org/initiatives/prevention/prevention-tools/SCORE-Risk-Charts.htm SCORE system] which is developed by the European Society of cardiology (ESC).
As shown in the figure, the most important risk factors for myocardial infarction are:
===Risk assessment of ischemia===
An [[Exercise Testing|exercise test]] such as a bicycle or
treadmilltest, may be usefull in detecting myocardial ischemia after exercise.<cite>accexercise</cite> In such a test, a continuous ECG registration is performed during exercise. The ST-segment, blood pressure asnd clinical status of the patient (i.e. chest complaints) are monitorered during and after the test.
An [[Exercise Testing|exercise test]] is positive for myocardial ischemia when the following criteria are met:
* Horizontal or downsloping ST-depression of > 1mm, 60 or 80ms after the J-point
* ST elevation of > 1.0 mm
* Elevated blood levels of cardiac enzymes ([[w:Creatine_kinase|CKMB]] or [[w:Troponin|Troponin T]]) AND
* One of the following criteria are met:
** The patient has typical complaints** The ECG shows ST elevation or depression
** [[Pathologic_Q_Waves|pathological Q waves]] develop on the ECG
** A coronary intervention had been performed (such as stent placement)
So detection of elevated serum heartenzymes is more important than ECG changes. However, the
heartenzymes can only be detected in the serum 5-7 hours after the onset of the myocardial infarction. So especially in the first few hours after the myocardial infarction the ECG can be crucial.
ECG Manifestations of Acute Myocardial Ischaemia (in Absence of LVH and [[MI Diagnosis in LBBB|LBBB]])are <cite>Thygesen</cite>:
:New ST elevation at the J-point in two contiguous leads with the cut-off points:
≥ 0.2 mV in men or ≥ 0.15 mV in women in leads V2–V3 and/or ≥ 0.1 mV in other leads;ST depression and T-wave changes:New horizontal or down-sloping ST depression > 0.05 mV in two contiguous leads; and/or T inversion ≥ 0.1 mVin two contiguous leads with prominent R-wave or R/S ratio ≥ 1
A study using MRI to diagnose myocardial infarction has shown that more emphasis on ST depression could greatly improve the yield of the ECG in the diagnosis of myocardial infarction (sensitivity increase from 50% to 84%).<cite>martin</cite>
Myocardial infarction diagnosis in left or right bundle branch block can be difficult, but is explained in these seperate chapters:
*[[MI Diagnosis in LBBB]]
*[[MI Diagnosis in RBBB]]
Image:coronary_anatomy.png| An overview of the coronary arteries. LM = 'Left Main' = mainstem; LAD = 'Left Anterior Descending' artery; RCX = Ramus Circumflexus; RCA = 'Right Coronary Artery'.
Image:lead_overview.png|Overview of the
seperate ECG leads. The lead with ST elevation 'highlights' the infarct. An infarction of the inferior wall will result in ST elevation in leads II, III and AVF. A lateral wall infarct results in ST elevation in leads I and AVL. An Anterior wall infarct results in ST -elevation in the precordial leads.Image:MI_colours_en.png|The coloured figure shows contiguous leads in matching coloursImage:MIregions.jpg|The ST elevation points at the infarct location. Inferior MI = ST elevation in red regions (lead II,III and AVF). Lateral MI = ST elevation in blue leads (lead I, AVL, V5-V6). Anterio MI: ST elevation in yellow region (V1-V4). Left main stenosis: ST elevation in gray area (AVR)
Image:conduction_blood_supply.png|The coronary blockade can cause conduction block, on AV nodal, His or bundle branch level.