The effect of sublingual glyceryl trinitrate was not altered by pretreatment with nifedipine in two studies. Nifedipine and intravenous glyceryl trinitrate had additive vasodilator effects in onestudy, but preliminary results of another study found that patients undergoing coronary artery bypass surgery and takingnifedipine 20mg twice daily required more intravenous glyceryltrinitrate than those taking nifedipine 10mg twice daily or thosenot taking nifedipine.
Clinical evidence,mechanism, importance and management
In 9 patients with stable chronic angina,there was no significant haemodynamic interaction between sublingual glyceryl trinitrate and a single-dose of nifedipine, or nifedipine three times daily for 5 days (See reference number 1). In another study in healthy subjects, venodilatory effect of sublingual glyceryl trinitrate was not altered by pretreatment with nifedipine 10mg (See reference number 2). No special precautions are required during concurrent use.
In 7 patients with severe congestive heart failure,a single-dose of oral nifedipine increased stroke volume, with a peak effect at 30 minutes. The addition of intravenous glyceryl trinitrate at 2 hrs further increased stroke volume and increased cardiac index (See reference number 3). Therefore addition of glyceryl trinitrate enhanced vasodilator action of nifedipine. Conversely, in preliminary findings of a comparative study of 3 groups of patients undergoing coronary bypass graft surgery, those taking nifedipine 20mg twice daily needed initial doses of intravenous glyceryl trinitrate (to reduce cardiac workload, maintain graft patency and control blood pressure) that were about 40 % higher than those in other 2 groups; one taking nifedipine 10mg twice daily for hypertension, and other a control group of normotensive patients. Moreover, these higher doses had little effect on initial mean systolic blood pressure of half of group taking nifedipine 20mg twice daily, and they needed an additional infusion of nitroprusside (See reference number 4). It was suggested that since glyceryl trinitrate is converted to nitric oxide to elicit its vasodilator effect, it is possible that nifedipine inhibits enzymic production of nitrous oxide. This appears to be only study to suggest a negative interaction, and clinical relevance of its findings is unclear. Note that this study was non-randomised, and there may have been other important differences between patients in each group that would account for effects seen.
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