Dynamic assessment of the electrocardiographic QT interval during citrate infusion in healthy volunteers.

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 min.Subjects-12 healthy teetotallers aged 19-36 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) mmol/l preinfusion to values close to target (0 . 98 mmol/l) between 10 and 120 min. The QaTc interval lengthened fr om a baseline of 0 . 309 (0 . 021) to a maximum 0 . 343 (0 . 024) s(0.5) at 10 min before returning to a stable level from 15 to 120 min (0 . 334 (0 . 023) and 0 . 330 (0 . 023) s(0.5) respectively). The change from baseline of both variables expressed as a ratio (Delta QaTc/Delta[Ca2+]) 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.