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How to use nitrates

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VOL: 97, ISSUE: 27, PAGE NO: 43

Nigel Davies, MSc, BSc, RN, CertEd, is deputy director of nursing, South Buckinghamshire NHS Trust, and visiting lecturer, South Bank University

Nitrates have been a mainstay in the treatment of angina for years and many different preparations have been developed to enable them to be used for treatment and prophylaxis.

They are used intravenously for unstable angina when the sublingual form is ineffective and also in the treatment of acute left-ventricular failure. They may also be used to counteract hypertension, for example after cardiac surgery, and to promote vasodilation. Two drugs are available:

- Glyceryl trinitrate - GTN

- Isosorbide dinitrate - Isoket(R)

How do they work?

Nitrates cause the walls of blood vessels to relax by releasing nitric oxide, which acts as a vasodilator. This action can have two effects:

- Arteriolar dilation, which reduces peripheral vascular resistance and left-ventricular systolic pressure, and therefore increases cardiac output;

- Venous dilation leading to venous pooling and a reduction of venous return, which reduces left-ventricular end-diastolic pressure.

There is likely to be some coronary vasodilation, but the main benefit to patients is not from this but from the reduction in venous return, which reduces the heart’s work and therefore the demand for oxygen. The dilation of the coronary arteries may be helpful if there is coronary spasm.

Administration

Intravenous nitrates (Box 1) are given to patients with unstable angina, where the aim is to provide support and pain relief during an acute attack and to prevent myocardial infarction and death. Patients receive nitrates with other agents, such as heparin or beta-blockers. Revascularisation procedures will probably be considered.

Side-effects

Flushing, headaches and postural hypotension may limit therapy when nitrates are given sublingually, orally or via a transdermal patch. These effects can be particularly evident when the drug is given continuously by intravenous infusion. Blood pressure (BP), therefore, should be monitored. Tachycardia and paradoxical bradycardia, which can lead to syncope and collapse, can also occur, making continuous electrocardiogram (ECG) monitoring necessary.

Tolerance

A reduction in blood nitrate concentrations to low levels for four to eight hours each day usually prevents tolerance and maintains effectiveness. This can often be carried out during sleep periods if nocturnal angina does not occur. It is preferable for intravenous infusions not to be administered for more than 36 hours without a break.

Other nursing implications

Intravenous nitrates can be supplied and administered in different ways (Box 2), so care should be taken when patients are transferred between units or hospitals.

The administration of intravenous nitrates needs to be controlled by an infusion pump and non-PVC giving sets should be used. The patient’s heart rate (ECG) and BP should be monitored for signs of adverse effects (Box 3).

Health warning

Every effort has been made to ensure that the information and drug dosages are correct at the time of publication. Where there is any doubt, this information should be checked against the manufacturer’s data or another authoritative source, such as the British National Formulary.

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