Rabbit Trails: ep 1; Ondansetron (Zofran)

Rabbit Trails because there is no defined endpoint or goal, but everything is connected and circles back again and again.

Ondansetron (Zofran) is classified as an antiemetic. It blocks specific receptor sites (selective 5-HT3 Antagonist), which are associated with nausea and vomiting in the chemoreceptor trigger zone, centrally and at specific sites peripherally.

Zofran can be given IV, IM, and PO. The onset for PO is 30min, while the onset for IV is immediate. IV Zofran can be diluted in either 0.9 NS or in Dextrose 5%, it should be infused slowly over at least 30 seconds, and preferable 2—5min.

Adverse Effects: headache, diarrhea, and constipation. A serious complication is EKG changes, specifically prolonged QT interval. (the 32mg dose of Zofran was voluntarily removed from the market for this reason) EKG monitoring is recommended if the patient is suspected to have electrolyte abnormality, bradycardia, or taking other drugs that can cause QT prolongation such as Erythromycin, Methadone, and Haldol (there are many more). Out of respect for potential cardiac arrhythmias, infuse Zofran slowly and keep an eye on the EKG monitor watching for rhythm changes.  (Karch, 2011; Medscape, 2013).

Zofran is most effective if given prophylactically, once the N/V chemoreceptors have been triggered the medication is not nearly as effective. Consider promethazine (Phenergan) which is more effective at curbing N/V once triggered.

Prolonged QT interval

The QT interval represents the time in between ventricular depolarization, QRS wave (ventricular systole) and ventricular repolarization, the T wave. This time period should be no longer than 0.43—0.45 seconds, (two big boxes on the EKG strip).

http://cdn.lifeinthefastlane.com/wp-content/uploads/2011/01/waves-of-the-ecg.gif

*Because the QT interval is rate dependent an accurate value must be calculated through the Bazett formula: QT _corrected = QT/square root of RR interval. It is also possible to get close by considering the QT interval to be 0.4 seconds at a HR of 70, and then subtracting 0.02 seconds for every 10 beat increase in HR, or adding 0.02 seconds for every 10 beat decrease in HR.

During the QT interval the ventricles are actively contracting and do not relax until the end of the T wave when enough potassium (K⁺) ions have left the myocytes; this means that the QT interval can be used as a gauge of the length ventricular systole. Because of K⁺ crucial role in repolarization, a long QT interval is often caused by an electrolyte imbalance, (hypokalemia, hypomagnesemia), or a genetic abnormality in the ion pump, or medication that effects the ion pumps or electrolyte levels. The discovery of an individual with long QT syndrome is almost always only found upon review of an EKG which usually doesn’t happen until the individual presents with a cardiac event such as syncope, aborted cardiac arrest, or sudden death (Medscape, 2013).

A prolonged QT interval, whether it is congenital, drug induced, or pathologic in nature are vulnerable to dangerous, perhaps deadly, rapid ventricular rhythms (Dubin, 2000).

(Gupta et al., 2007)

Torsades de Pointes (Polymorphic Ventricular Tachycardia due to prolonged QT interval)

Torsades de Pointes (TdP) means the twisting of the points, it is one of the most common abnormal rhythms that is a result of a QT prolongation. Treatment for (TdP) that does not spontaneously revert is non synchronized defibrillation for a hemodynamic unstable patient, IV MgSO₄, and the discontinuation of any QT prolonging agent. If the patient is not hemodynamically unstable then a 2mg bolus of MgSO₄ is recommended regardless of serum level; K⁺ levels need to be returned to or remain in the high end of the normal range 4.5-5mmol/L). Short term management can be achieved with HR pacing >70 due to the prolonging effect that bradycardia has on the QT interval, long term management involves avoidance of offending agents, tight electrolyte control, and possibly a pacemaker.  (Gupta et al., 2007)

If someone accidently gives way too much mag sulfate, what should be done?

Reference:

Dubin, D. (2000). Rapid interpretation of EKG's: an interactive course. Cover Publishing Company.

Gupta, A., Lawrence, A. T., Krishnan, K., Kavinsky, C. J., & Trohman, R. G. (2007). Current concepts in the mechanisms and management of drug-induced QT prolongation and torsade de pointes. American heart journal, 153(6), 891-899.

Karch, A. M. (Ed.). (2011). 2011 Lippincott's nursing drug guide. Lippincott Williams & Wilkins.

Medscape. (2013) Reference section accessed for Ondansetron and Long QT Syndrome. Retrived 11/04/13 from: http://reference.medscape.com/nurses