• Ischemia
• Myocardial Ischemic Damage
• Action Potential and Ischemia
• Ischemic Diastolic Vector
• Ischemic Systolic Vector
• ECG and the Isoelectric Line
• Subendocardial Ischemia (ST Depression)
• ST Depression and Ischemia
• Flat Ascending ST Depression and Ischemia
• ST Depression - Differential Diagnosis
• Subepicardial Ischemia (ST Elevations)
• ST Elevations and Ischemia
• ST Elevations - Differential Diagnosis
• ST Segment and Acute Coronary Syndrome

Ischemia

• The heart has the highest oxygen consumption per unit volume compared to all organs
• Oxygen extraction from the coronary circulation is 70-80%
  • Other organs have an extraction rate of approximately 25%
• The only way the heart protects itself from ischemia is by increasing blood flow through the coronary arteries
• At rest, the coronary flow is about 250 ml/min.
  • During exertion, the flow can increase up to 5 times
• Ischemia is the reduction of blood flow to the tissue
  • It secondarily causes hypoxia (lack of oxygen)
  • The most common causes of ischemia are changes in the coronary arteries:
  • Atherosclerosis
  • Spasm
  • Thrombosis

Action Potential and Ischemia

Ischemia During Diastole and Systole

• In ischemia, ion channels are disrupted, altering ion distribution and secondary membrane polarity
• Ventricular Diastole
• It is the phase of the cardiac cycle when the ventricles are relaxed and fill with blood
• Action potential is in Phase 4
• On the ECG, it is the TQ segment
• Ischemic myocardium is during diastole
• More depolarized (less negative)
• Ventricular Systole
• It is the phase of the cardiac cycle when the ventricles contract and pump blood
• Action potential progresses through Phases 1, 2, and 3
• On the ECG, it is the ST segment and T wave
• Ischemic myocardium is during systole
• More repolarized (more negative)
• Depolarizes more slowly (Phase 0 of the action potential is less steep)
• Repolarizes sooner (there is a shorter QT interval)

Ischemic Systolic Vector

• Ischemic myocardium is more negative (-15mV)
• compared to normal myocardium (0mV)
• Extracellular zone of ischemia is more positive
• compared to normal myocardium
• The ischemic vector during systole points:
• From normal myocardium towards ischemic myocardium
• The vector always points towards the more positive area of the myocardium
• ECG electrode over ischemic myocardium
• will show elevated ST segment and T wave during systole

Subendocardial Ischemia (ST Depression)

Subendocardial Ischemia

• Subendocardial ischemia is more common than subepicardial
• Because the main branches of coronary arteries are on the epicardium and then spread through the myocardium to the endocardium
• The endocardium is supplied by the terminal branches of the coronary arteries
• Rarely progresses to transmural ischemia
• Diastolic Vector
• Points towards normal myocardium (relative to the corresponding ECG lead)
• QT segment shifts upward
• Systolic Vector
• Points towards ischemic myocardium (away from the corresponding ECG lead)
• ST segment shifts downward
• A new isoelectric line is formed, which is higher
• On the ECG, we observe ST depressions (in leads that view the ischemia)

Flat Ascending ST Depression and Ischemia

• Flat ascending ST segment (Slow upsloping ST segment)
• Has an ascent of < 1.5mV/1s
• Steep ascending ST segment (Rapid upsloping ST segment)
• Has an ascent of > 1.5mV/1s

Flat Ascending ST Depression

• Flat ascending ST segment < 1.5mV/1s
• Ascending ST depression 1.5mm
• ST depression is measured 80ms after the J point
• This ST depression is ischemic

Steep Ascending ST Depression

• Steep ascending ST segment > 1.5mV/1s
• Ascending ST depression 1.5mm
• ST depression is measured 80ms after the J point
• Steep ST depression is never ischemic

Subepicardial Ischemia (ST Elevations)

Subepicardial Ischemia

• Subepicardial ischemia is less common than subendocardial ischemia
• Because the main branches of the coronary arteries are on the epicardium
• The epicardium is thus more richly supplied with blood than the endocardium
• Often progresses to transmural ischemia
• Diastolic vector
• Sends to normal myocardium (away from the corresponding ECG lead)
• QT segment shifts lower
• Systolic vector
• Sends to ischemic myocardium (toward the corresponding ECG lead)
• ST segment shifts higher
• A new baseline emerges, which is lower
• On the ECG, we observe ST elevations (in leads that "view" the ischemia)

ST Elevations (STE) in Ischemia

• STEMI is characterized by
• Convex STE
• Horizontal STE
• Rarely, ST elevations (STE) may be:
• Descending
• Ascending
• STEMI (subepicardial or transmural ischemia)
• Never produces concave (smiling) ST elevations

ST Segment and Acute Coronary Syndrome

• Acute Coronary Syndrome (ACS) is an acute ischemic injury to the myocardium.
• It most commonly occurs due to rupture of an atheroma (fibrous cap)
• A thrombus forms at the site of rupture, leading to acute
• stenosis (narrowing)
• occlusion (blockage)
• In the case of occlusion, approximately 20 minutes later, myocardial necrosis (infarction) occurs
• Necrotic cardiomyocytes break down and release troponin (ischemic cell death)
• ST segment is the primary ECG marker of ACS
• Troponin is the main laboratory marker for infarction
• Released from necrotic cardiomyocytes during infarction
• Not released from ischemic myocardium

ST Segment and Acute Coronary Syndrome

• Acute Stenosis (narrowing)
• Blood flow through the artery is reduced but maintained
• Causes subendocardial ischemia
• EKG shows ST depressions
• Rarely, ST segment may be normal
• Acute Occlusion (blockage)
• Blood flow through the artery is stopped
• Leads to subepicardial ischemia, and later transmurale ischemia
• EKG shows ST elevations
• 20 minutes after occlusion, necrosis (infarction) develops
• 4-9 hours after occlusion, transmural necrosis occurs
• During necrosis, cardiomyocytes break down and release troponin

Unstable Angina Pectoris

• Sinus Rhythm
• ST Depressions (I, II, V5-V6)
• Possible ST Elevations in V1-V2
  • Consider right-sided EKG leads (V4R)
  • Suspect right ventricular infarction
• Patient had approximately 10 minutes of angina and troponin was later normal
• No ST depressions on previous EKG records
• Indicates unstable angina pectoris
• If the patient had later elevated troponin without ST elevations
  • It would indicate an NSTEMI infarction