Q. False Positive ST Changes?
1) The slope of the PQ-segment can help predict the magnitude of the influence of P-wave depolarization and thus help predict which patient would have false positive ST- segments. This sign should be worked for in leads II and III and aVF and is most commonly seen when the P-wave amplitude is increased. It is believed that this is the reason that ST depression found only in these leads is more likely to be a false positive response.
2) Digitalis: It has long been recognized that digitalis may cause ST depression and that it is exaggerated by exercise.
3) Hypokalemia: There are many causes of hypokalemia that may induce ST depression in patients with normal coronary arteries.
4) Hyperventilation: Changes in T-wave with hyperventilation or standing are relatively common and are thought to be mediated through the autonomic nervous system. When they are associated with ST-segment depression the prevalence decreases, it has been reported to be less than 1 per cent to 2 per cent. The mechanism is somewhat obscure and has been attributed to pH changes, electrolyte changes (especially potassium), and changes in heart position, coronary arteriolar vasospasm and excessive catecholamines. Because T-waves can be reduced by beta blockers and accentuated by intravenous epinephrine, the changes are probably mediated through the sympathetic pathway. When exercise induced ST-depression is found in patients who are likely to have non-coronary cause, it can be identified if the changes are abolished by a beta blocker. It has been found that the drug did not eliminate the ST depression in any patients with significant coronary artery disease, but did correct the ST in those with normal coronary angiogram.