DYNAMIC ASSESSMENT OF THE ELECTROCARDIOGRAPHIC QT INTERVAL DURING CITRATE INFUSION IN HEALTHY-VOLUNTEERS

Objective-To investigate changes in the electrocardiographic QT interv al during rapidly induced, sustained hypocalcaemia in healthy voluntee rs. 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 bas eline for 120 min. Subjects-12 healthy teetotallers aged 19-36 years w ho were not receiving medication known to influence calcium homoeostas is. Main outcome measures-Whole blood ionised calcium concentration an d QaTc intervals (onset of the Q wave to T wave apex divided by the sq uare root of the RR interval). Results-Mean (SD) ionised calcium conce ntration decreased from 1 . 18 (0 . 03) mmol/l preinfusion to values c lose to target (0 . 98 mmol/l) between 10 and 120 min. The QaTc interv al lengthened fr om a baseline of 0 . 309 (0 . 021) to a maximum 0 . 3 43 (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) respecti vely). The change from baseline of both variables expressed as a ratio (Delta QaTc/Delta[Ca2+]) was greater during rapid induction of hypoca lcaemia (at 5 and 10 min) than at other times during and after the inf usion (P < 0 . 02). Conclusions-The disproportionate prolongation of Q aTc interval during prompt induction of hypocalcaemia suggests rate de pendency which can be represented by a hysteresis relation between (io nised calcium, QaTc) coordinates. This finding may have clinical impli cations.