Electrocardiography in toxicology

Electrocardiography

In this topic:

QRS widening

QT prolongation

QRS widening

QRS widening on the ECG is a sodium channel effect, most commonly seen with tricyclic antidepressants, but also with a number of other medications. A QRS duration greater than 120 msec (0.12 seconds) is generally regarded as abnormal. A list of drugs associated with QRS widening is provided in Table 14.29.

Drugs associated with QRS widening and sodium channel blockade (Table 14.29)

Antidepressants	tricyclic antidepressants venlafaxine
Antihistamines	diphenhydramine
Antipsychotics	thioridazine
Cardiovascular drugs	flecainide propranolol quinidine
Local anaesthetics	bupivacaine ropivacaine
Others	bupropion chloroquine cocaine dextropropoxyphene dolasetron hydroxychloroquine quinine

QT prolongation

QT prolongation on ECG is a potassium channel effect associated with torsades de pointes. The assessment of the QT interval remains problematic, but a QT nomogram, see Figure 14.21, has been suggested as a useful way to determine if the QT/HR pair is abnormal (at risk of torsades de pointes). To use the nomogram (Figure 14.21), the QT interval should be measured manually on a 12-lead ECG from the beginning of the Q wave to the end of the T wave in multiple leads (namely, six leads including limb and chest leads) and the median QT calculated. The median QT interval is plotted on the nomogram against the heart rate recorded on the ECG. If the point is above the line, then the QT/HR is regarded as 'at risk'.

Drugs known to cause this effect are listed in Table 14.30.

Drugs associated with QT prolongation and torsades de pointes (Table 14.30)

Antiarrhythmics	amiodarone disopyramide dofetilide procainamide quinidine sotalol
Other cardiovascular drugs	bepridil enalapril propranolol
Antidepressants	citalopram escitalopram fluoxetine moclobemide tricyclic antidepressants [NB1]
Antihistamines	loratadine
Antimicrobials	ciprofloxacin clarithromycin erythromycin fluconazole moxifloxacin pentamidine sparfloxacin voriconazole
Antipsychotics	amisulpride droperidol haloperidol pimozide thioridazine ziprasidone
Chemotherapeutic agents	arsenic
Other drugs	caesium chlorpromazine cisapride cocaine methadone
NB1: QT prolongation is usually due to QRS widening and there is no true lengthening of the JT interval A full list with continuing updates can be found at http://www.qtdrugs.org/medical-pros/drug-lists/drug-lists.htm

QT interval nomogram to assess risk of torsades de pointes (Figure 14.21)

The QT interval should be measured manually on a 12-lead ECG from the beginning of the Q wave to the end of the T wave in multiple leads (six leads including limb and chest leads) and the median QT calculated. The median QT interval is plotted on the nomogram against the heart rate recorded on the ECG. If the point is above the line, then the QT-HR combination implies a risk of torsades de pointes.

Chan A, Isbister GK, Kirkpatrick CM, Dufful SB. Drug-induced QT prolongation and torsades de pointes: evaluation of a QT nomogram. QJM 2007;100(10):609-15 by permission of Oxford University Press.