Much ado is made about the risk of QT prolongation and the development of malignant arrhythmias, particularly Torsades de Pointes – but how frequently does TdP actually occur in these patients who QT prolongation? Should we be worried about every EKG that crosses our paths with a prolonged QT?
It seems, like so many things, the answer is yes and no. This is a prospective observational study from a single institution that installed cardiac monitoring that enabled minute-by-minute measurement and recording of QT intervals in their monitored inpatient population. They evaluated 1,039 inpatients for 67,648 hours worth of time, and found these patients spent 24% of their monitored time with a prolonged QTc (>500ms). One single patient had a cardiac arrest event where TdP was evident on the monitoring strip – a comorbidly ill heart failure patient whose QTc ranged as high as 691ms.
The authors then went back to attempt to determine whether the prolonged QT was associated with all-cause mortality with the 41 patients who died during their study period, and they found that 8.7% had QT prolongation versus 2.6% who did not. However, as you can imagine, there are massive baseline differences between the QT prolonged population and the non-QT prolonged population, many of which contribute greater effects to in-hospital all-cause mortality. The authors attempt logistic regression and finally come up with an OR of 2.99 for QT prolongation for all-cause mortality – which is lower in effects than CVA, obesity, pro-arrhythmic drug administration, and high serum BUN.
It’s reasonable to say that patients with a prolonged QT are at higher risk for death – but it’s also reasonable to say that sick patients at a higher risk of death are more likely to have a prolonged QT. Torsades was rare, even with the thousands of hours of QT prolongation noted. I would not get over-excited about QT prolongation in isolation, but, rather, only in the context of multiple risk factors for mortality in acute illness.
“High prevalence of corrected QT interval prolongation in acutely ill patients is associated with mortality: Results of the QT in Practice (QTIP) Study”
http://www.ncbi.nlm.nih.gov/pubmed/22001585
We seem to have an indicator that is similar to the post-MI patient with PVCs. More bad things happen to these patients, but what can we do about it?
More importantly, as we learned from the Cardiac Arrest Suppression Trial, what should we do about it?
Is there any prophylaxis that has been shown to reliably improve the safety of giving QT-prolonging medications either for patients with pre-existing long QT or for patients with normal QT segments?
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I'm not entirely certain – but I think QT might be akin to a "vital sign" for the heart. Simply treating the QT itself may only be of minimal benefit if the underlying comorbidities are not addressed – sort of like arbitrarily giving a tachycardic patient a beta blocker without addressing the cause of the tachycardia.