Background Hyperventilation is known to cause ST segment changes and QT variability in adults, but this has not been systematically studied in children.

Aim To investigate the effect of hyperventilation on rate corrected QT interval (QTc) in children.

Methods and results 25 children (male=10) with a median age of 14 (range 8.3–17.6) years were asked to hyperventilate for 1 min before exercise testing using the modified Bruce protocol. Mean QTc at rest, after hyperventilation, at peak exercise and at 1 min of recovery was 425(±31), 460(±30), 446(±38) and 420(±32) ms, respectively. Mean increase (95% CI) in QTc after hyperventilation was 35(19 to 51) ms (p<0.001), while there was minimal difference between QT interval at rest and after hyperventilation (mean QT 352(±41) vs 357(±44) ms). In six children, there were abnormalities in T wave morphology following hyperventilation. The QTc increment following hyperventilation was more pronounced in children with resting QTc <440 ms (n=14, mean increment (95% CI): 55 (33 to 78) ms) compared to children with QTc ≥440 ms (n=11, mean increment (95% CI): 9 (−4 to 22) ms) (p=0.001). QTc prolongation following hyperventilation was seen in children with both low and intermediate probability of long QT syndrome (LQTS). Peak exercise and early recovery did not cause a statistically significant change in QTc in either of these groups.

Conclusions Hyperventilation produces repolarisation abnormalities, including prolongation of QTc and T wave abnormalities in children with low probability of LQTS. The likely mechanism is delayed adaptation of QT interval with increased heart rate. Thus, a hyperventilation episode can be misdiagnosed as LQTS, especially in an emergency department.