QT interval during citrate infusion in healthy volunteers

Accepted for publication 20 January 1995 Abstract Objective-To investigate changes in the electrocardiographic QT interval during rapidly induced, sustained hypocalcaemia in healthy volunteers. Design-Serial rate corrected QT measurements were made during and after a variable rate trisodium citrate infusion designed to "clamp" the whole blood ionised calcium concentration 0-20 mmol/l below baseline for 120 mn. Subjects-12 healthy teetotallers aged 1936 years who were not receiving medication known to influence calcium homoeostasis. Main outcome measures-Whole blood ionised calcium concentration and QaTc intervals (onset of the Q wave to T wave apex divided by the square root of the RR interval). Results-Mean (SD) ionised calcium concentration decreased from 1-18 (0.03) mmolMl preinfusion to values close to target (0.98 mmol/1) between 10 and 120 min. The QaTc interval lengthened from a baseline of 0 309 (0.021) to a maximum 0 343 (0 024) s05 at 10 min before returning to a stable level from 15 to 120 min (0 334 (0.023) and 0 330 (0.023) s05 respectively). The change from baseline of both variables expressed as a ratio (AQaTc/A[CaJ+]) was greater during rapid induction of hypocalcaemia (at 5 and 10 min) than at other times during and after the infusion (P < 0.02). Conclusions-The disproportionate prolongation of QaTc interval during prompt induction of hypocalcaemia suggests rate dependency which can be represented by a hysteresis relation between (ionised calcium, QaTc) coordinates. This finding may have clinical implications.