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The most sensitive leads for ST segment monitoring are V3, V4, and V5.

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Sensitivity of Various Lead Combinations for Detecting Ischemia
London et al.Landesberg et al.
II + V2 + V3 + V4 + V5100%
V4 + V590%V3 + V597%
II + V580%V4 + V592%
II + V4 + V596%V3 + V4100%
Single leads V5, V4, and V375%, 61%, and 24%, respectivelySingle leads V5, V4, and V375%, 83%, and 75%, respectively

  • Cardiac events are the most common cause of death in the perioperative period
  • Unstable angina, myocardial infarction, and heart failure are estimated to occur in 1–10% depending on the risk of the surgical procedure and the severity of patient’s disease
  • Overall, 4% of patients undergoing noncardiac surgery have significant cardiac events


May occur in 84% of patients during surgery.


Etiology: ischemia, anesthetics, electrolyte abnormalities, endotracheal intubation, reflexes, vagal stimulation, central nervous stimulation, intracranial hemorrhage, autonomic nervous system dysfunction, central venous cannulation, surgical manipulation of cardiac structures, pericardial effusion, hypothermia, acute pericarditis, pulmonary embolus.


  • Electrodes are placed on the right arm (RA), left arm (LA), and left leg (LL)
  • For bipolar leads (I, II, and III), one pair is selected for monitoring and the other one is used as a ground (i.e., the electrodes are positive, negative, and ground)
  • While the three-lead system is useful for arrhythmia detection, it is of limited utility when monitoring for myocardial ischemia

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  • The modified three-electrode system includes modified chest leads (MCL), CS5, CM5, CB5, and CC5
  • P waves are maximized for atrial dysrhythmia monitoring and there is increased sensitivity for detecting anterior wall ischemia

Figure 39-1. Modified Three-Lead Systems to Detect Ischemia
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MCL1: monitor lead III with LL electrode placed on right fifth intercostal space, on the medioclavicular line; CS5, monitor lead I with LA electrode placed on left fifth intercostal space on the anterior axillary line; monitor lead II with LL electrode in its normal position (for arrhythmia); CM5, monitor lead I with RA electrode on sternal manubrium and LA electrode on left fifth intercostal space on the anterior axillary line; CB5, monitor lead I with RA electrode over right scapula and LA electrode on left fifth intercostal space on the anterior axillary line; CC5, monitor lead I with RA electrode on right fifth intercostal space on the anterior axillary line and LA electrode on left fifth ...

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