Calculations in liver disease and renal impairment

CHAPTER 9 Calculations in liver disease and renal impairment



Learning outcomes



• Appreciate the need for dosage reduction in patients with liver disease or renal impairment

• Understand the main principles involved in calculating reduced doses for patients with liver disease or renal impairment


Introduction


Following administration and absorption, drugs are metabolised and excreted. Many factors will influence both the pathways and rate of drug metabolism in the liver and excretion by the kidneys. It follows that if these major organs are in any way damaged by disease or, say, circulatory failure, the metabolism and excretion of drugs is reduced. The reduction will depend on the severity of the condition. It also follows that in order to avoid drug and/or metabolite toxicity (due to an accumulation of the drug in the body), the doses of drugs must be adjusted to take account of liver disease or renal impairment. It is especially important to take particular care when treating older people whose liver and renal functions decline with age.


Nurses will not routinely be involved in the actual calculation of drug doses for patients with liver disease or renal impairment but the principles involved are relevant to many aspects of drug therapy. For paediatric aspects, see Chapter 8.



Liver disease


The liver has a very considerable capacity to metabolise drugs. Damage to the liver has to be severe (due to infection, trauma, cirrhosis or neoplastic disease) before the capacity of the liver to metabolise drugs is significantly reduced. The following exercises illustrate the range of dosage reductions that may be required in patients with liver disease.




Exercises


Answers may be found on page 197.


9.1 The normal adult dose of atomoxetine is 80 mg daily. In patients with severe liver disease, 25% of the usual adult dose should be given. What is the correct dose for this patient?

9.2 A patient is prescribed metronidazole 400 mg every 8 hours. The dose requires to be adjusted for severe liver disease by reducing the total daily dose to one third and given once daily. What dose does the patient take?

9.3 A patient is prescribed rizatriptan 10 mg, but has moderate liver disease and the dose should be halved. What dose should the patient take?

9.4 A 70 kg patient has been prescribed Synercid by intravenous infusion every 8 hours at a dose of 7.5 mg/kg. The patient has moderate hepatic impairment and the recommended dose is 5 mg/kg. What dose should be infused 8-hourly?


Renal impairment (adults)


Patients with reduced renal function are at risk from adverse effects of drugs or their metabolites which are excreted renally.


Other problems such as increased sensitivity and intolerance of side-effects may also be encountered. It follows that great attention needs to be paid to the choice of drug and the dose especially in patients receiving replacement therapy.


Detailed guidance on drugs to be avoided/used with caution is provided in the BNF. Manufacturers’ literature and local protocols should also be consulted.


In deciding on a suitable dose for a patient with renal impairment, it may be necessary to perform a calculation based on physiological parameters which indicate the patient’s degree of renal impairment. The greater the degree of renal impairment, the greater the risk from accumulation of the drug and consequent adverse effects.


Most nurses will not be faced with carrying out calculations to determine renal function although the fundamental principles involved are important to all nurses.



Measuring renal impairment



Glomerular filtration rate (GFR)


This is the volume of fluid filtered by the glomerulus in millilitres per minute and is a measure of the efficacy of the renal excretory capacity. (Most of the fluid filtered is reabsorbed and so GFR must not be confused with urinary output.)

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Jun 18, 2016 | Posted by in MIDWIFERY | Comments Off on Calculations in liver disease and renal impairment

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