Duration of the Q-T interval during the anoxemia test

1. 1. The Levy anoxemia test has been studied in forty normal men and ninety-five male patients with reference to alterations in the duration of electrical systole, as measured by the corrected Q-T interval (Q-T c). 2. 2. Initial Q-T c durations were significantly higher in patients with angina pectoris as a group than in controls, confirming previous studies. The maximal increase in Q-T c during the anoxemia test in patients with true angina pectoris was compared with the maximal increase in Q-T c in normal subjects and patients with chest pain of noncardiac origin. The prolongation of Q-T c was much greater in the patients with angina, the mean values showing highly significant differences. However, the wide range of response among the subjects and the patients precludes the use of this measurement alone to distinguish between normal and abnormal. 3. 4. When the maximal increase in Q-T c during the anoxemia test is plotted against the control (initial) Q-T c value for all subjects and patients, it is found that an oblique line may be drawn which separates nearly all of the patients with true angina pectoris from nearly all of those without it. The equation representing this line of separation is: 0.480 sec. = (initial Q-T c) + 2.6 (maximal increase in Q-T c). For simplicity in clinical application it can be stated that 0.480 is the “upper limit of normal” for men of a value computed by adding 2.6 times the maximal increase of Q-T c during anoxia to the initial Q-T c duration. The value so determined is termed the “Q-T Anoxemia Index”. 4. 5. Of two observed “false-positive” Levy anoxemia tests, one case in which hyperventilation was causative also demonstrated an abnormal Q-T c response. The other, in whom the RS-T, T-wave abnormalities are considered a residuum of recent pericarditis, showed a normal “Q-T Anoxemia Index”. 5. 6. These observations suggest that the alteration of Q-T c duration during the course of the anoxemia test may be an additional aid in distinguishing a normal from an abnormal response since 89.5 per cent of patients considered to have true angina pectoris exhibited positive tests by the Q-T Anoxemia Index criterion, in contrast to only 55.2 per cent positive by the Levy criteria, while the incidence of false positives was not significantly increased.