Calcium channel blockers dosage

The nurse takes Hie patient s vital signs before die drug is administered and frequentiy during administration of die antiaiiginals or die calcium channel blockers. If die heart rate is below 50 bpm or die systolic blood pressure is below 90 mm Hg, the drug is widiheld and die primary health care provider notified. A dosage adjusdnent may be necessary. 

Calcium channel blockers minimally interfere with stimulus-secretion coupling in glands and nerve endings because of differences between calcium channel type and sensitivity in different tissues. Verapamil has been shown to inhibit insulin release in humans, but the dosages required are greater than those used in management of angina. 

It has been suggested that calcium channel blockers can be used to treat cocaine dependence, and some studies have shown reductions in cocaine-induced subjective and cardiovascular responses with nifedipine and diltiazem. The cardiovascular and subjective responses to cocaine have been evaluated in a double-blind, placebo-controlled, crossover study in five subjects pretreated with two dosage of nimodipine (393). Nimodipine 60 mg attenuated the rise in systolic, but not diastolic, blood pressure after cocaine. In three subjects nimodipine 90 mg produced greater attenuation than 60 mg. The subjective effects of cocaine were not altered by either dose of nimodipine. 

There are approximately a dozen calcium channel antagonists marketed in the United States for the treatment of hypertension, certain dysrhythmias, and some forms of angina (see Chaps. 13,15, and 17). The calcium channel blockers are classified by their chemical structure as phenylalkylamines (e.g., verapamil), benzothiapines (e.g., diltiazem), and dihydropyridines (e.g., amlodipine, felodipine, nicardipine, and nifedipine). Several of these agents, namely, diltiazem, nicardipine, nifedipine, and verapamil, are formulated as sustained-release oral dosage forms or have a slow onset of action and longer half-life (e.g., amlodipine " ), allowing once-daily administration. 

All anticonvulsants (except gabapentin), atypical antipsychotics, benzodiazepines, and calcium channel blockers require liver metabolism, and dosage adjustments may be needed (e.g., 25-50% reduction of normal doses) Carbamazepine or oxcarbazepine Alternative lamotrigine Acute mania or mixed episode first choice lithium... 

Lithium is frequently combined with both traditional and atypical antipsychotics in euphoric acute mania with psychotic features. Case reports of neurotoxicity (e.g., delirium, cerebellar dysfunction, extrapyramidal symptoms, and severe tremors) have been reported in elderly patients receiving lithium and traditional antipsychotics. Combining lithium with calcium channel blockers is not recommended because of reports of neurotoxicity and severe bradycardia with verapamil and diltiazem. Acute neurotoxicity and delirium have been reported in patients receiving ECT with lithinm (even at reduced dosages) therefore lithium should be withdrawn and discontinued at least 2 days before ECT and should not be resumed until 2 to 3 days after the last treatment. 

Calcium chaimel blockers traditionally have been the first-line agents to treat hypertension after transplantation. In addition to their ability to control blood pressure, calcium channel blockers may ameliorate the nephrotoxic effects of CSA, improve renal hemodynamics, decrease the incidence of delayed graft function and development of allograft atherosclerosis, and provide some immunosuppression. Calcium channel blockers, however, also may contribute to gingival hyperplasia that is often associated with CSA-based immunosuppression. CYP 3A4 interactions with CSA and TAC are of concern with this class of medications, particularly with dil-tiazem, verapamil, and nicardipine, and CSA or TAC concentrations must be monitored to ensure proper dosage adjustments. 

Except for alpha-blockers, any sympathoplegic can, in sufficient dosage, cause bradycardia. Conversely, any vasodilator may induce tachycardia and, unless it is also sympathoplegic or a calcium channel blocker, will never slow the heart rate. The answer is (C). 

Information about the effects of calcium-channel blockers on phenytoin is limited, but what is known indicates that if diltiazem is given with phenytoin, the dosage of phenytoin may possibly need to be reduced to avoid toxicity. The case report of phenytoin toxicity with nifedipine is isolated, and of unknown importance. Phenytoin markedly reduces felodipine, verapamil and possibly nifedipine levels. Although not all calcium-channel blockers have been studied, most would be expected to interact with phenytoin similarly, as they are metabolised by the same isoenzymes (see Calcium-channel blockers , (p.860)). A considerable increase in the dosage of any calcium-channel blocker will probably be needed in the presence of phenytoin. Note that the manufacturers of nimodipine and nisoldipine contraindieate the eoneurrent use of phenytoin because of the possibility of a large reduction in their levels. 

The interaction between felodipine and itraconazole would appear to be established and elinieally important. It also seems that isradipine, lercanidipine, nifedipine and nisoldipine ean interaet similarly with fluconazole, itraconazole or ketoconazole and, because they are metabolised by CYP3A4, it is likely that other calcium-channel blockers will behave in the same way. If itraconazole, ketoconazole, or fluconazole is given to a patient on established treatment with any calcium-channel blocker be alert for the need to lower the dosage of the calcium-channel blocker. However, some manufaeturers (e.g. felodipine, lercanidipine ) actually contraindicate concurrent use of itraconazole or ketoconazole, and others (e.g. nisoldipine ) additionally contraindicate fluconazole. In the US the guidance differs slightly and only caution is considered necessary with felodipine. The manufacturers of nimodipine predict that fluconazole, itraconazole and ketoconazole will substantially raise nimodipine levels. They say that concurrent use should be avoided, but, if this is not possible then the patient s blood pressure should be carefully monitored."... 

A patient taking chlorpromazine who was given nifedipine [dosage not stated] for 2 days before snidery, developed marked hypotension during surgery, which was eventually controlled with noradrenaline (norepinephrine). Other phenothiazines and calcium-channel blockers may interact similarly, see Antihypertensives + Other drugs that affect blood pressure , p.880. 

The manufacturers say that amlodipine has been safely given with thiazide diuretics and no dosage adjustment of amlodipine is required. Additive antihypertensive effects are expected when diuretics such as hydrochlorothiazide are used in combination with calcium-channel blockers, and such combinations are used clinically. 

The plasma levels of diltiazem, isradipine and nifedipine are increased by cimetidine and it may possibly be necessary to reduce the dosages of these calcium-channel blockers. High doses of cimetidine may increase the bioavailability of lercanidipine. Although studies surest no important interactions occur between nicardipine or nisoldipine and cimetidine, the manufacturers advise caution. Plasma felodipine, lacidipine, nimodipine, and... 

The interactions of cimetidine with diltiazem and nifedipine are established. Concurrent use need not be avoided but the increase in the calcium-channel blocker effects should be taken into account. It has been suggested that the dosage of diltiazem should be reduced by 30 to 50% " and that of nifedipine by 40 to 50%. " The interaction between verapamil and cimetidine is not well established, but monitor the effects until more is known. It has been suggested that the verapamil dose may need to be reduced by 50%. Monitoring is advised if isradipine is given with cimetidine and a reduction in isradipine dose may be required. ... 

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