7/28/2019 Pharmacology and Drug Doses an Introduction

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An intrductn t Parmacy and Dru Dses

Lauren BarkerCorrespondence Email: Lauren.Barker@rdeft.nhs.uk

Lauren Barker

Consultant Anaesthetist

Royal Devon and Exeter

NHS Foundation Trust

Barrack Road

Exeter EX2 5DW

UK

DoSES oF DRUgS iN ANAESThESiA

Drugs in anaesthesia are commonly expressed ingrams, milligrams or micrograms which reer to theirmass.

Abbreviations are often used:

Mass Abbrevation

kilogram kg

gram g

milligram mg

microgram mcg

nanogram ng

Examples:1g fucloxacillin, 500mg thiopentone, 600mcgatropine. Te words milli-, micro- and nano- whichappear in ront o gram reer to how many multipleso 10 are present.

It is possible to convert rom grams to nanogramsas ollows:

1g = 1 000 mg = 1 000 000mcg = 1 000 000 000ng

o convert, or example, rom micrograms tomilligrams:

600mcg atropine = 0.6mg atropine

RECoMMENDED oR ThERAPEUTiC DRUg

DoSES

Recommended or therapeutic doses enable one tocalculate the correct dose o drug or the patient

undergoing anaesthesia. Tese doses were previouslyexpressed as mg/kg or mcg/kg but should nowormally be written as mg.kg-1 or mcg.kg-1 and arecalculated as ollows:

Example:Te dose o atropine is 20mcg.kg-1

o calculate the correct dose o drug or a patient,multiply the drug dose by the patients weight. Ina 20kg patient we would give:

20mcg.kg-1 = 20mcg.kg-1 x 20kg =400mcg = 0.4mg

o calculate the correct dose o atracurium(0.5mg.kg-1) or a 70kg adult.

0.5mg.kg-1 x 70kg = 35mg

MAXiMUM DoSESMaximum doses may reer to local anaesthetic drugslike lignocaine or bupivacaine and indicate themaximum dose o drug that may be given to thepatient saely without causing toxicity. In the case olocal anaesthetics cardiac arrhythmias or convulsionsmay result i the maximum dose is exceeded.

oThER UNiTS FoR DESCRiBiNg DRUgSMost commonly we describe the amount o drugpresent by reerence to the mass o drug (see above).However drug preparations may also be described by

how many particles they contain. Tis gives an ideao the amount o drug present rather than the masso the drug.

By convention the amount o a substance is measuredin moles (abbreviation: mol). A mole has been denedby the Systme International as the quantity o asubstance that contains the same number o particlesas there are atoms in 12g o carbon-12. Tere are6.022 x 1023 atoms present in 12g carbon-12 and thisequals one mole. (6.022 x 1023 is the abbreviated wayo writing a large number written in ull you wouldhave to move the decimal point to the right 23 timesi.e. 6022 ollowed by 20 zeros).

Tis dose method is oten used or substances suchas potassium (K+) or sodium (Na+) in the orm omillimole (1mole = 1000 millimole - oten writtenas mmol). Tis method is useul as sodium orpotassium are oten prepared with chloride and whenadministered it is helpul to consider only the amounto Na+ or K+ that is given. Tereore it is usuallydescribed in mmol.

Example:A solution o normal saline contains 154mmolNa+ and 154mmol C1- in each litre.

AnaesthesiaUpdate in

Summary

This article explains the

internationally recognised

ormats or describing

drugs, their dosage and

dilutions. Commonlyencountered dilutions o

drugs such as epinephrine

(adrenaline) and local

anaesthetic agents are

clarifed using examples.

The recommended

international non-propriety

names (rINN) o some

o the more commonly

encountered anaesthetic

and critical care drugs are

listed.

Originally published in Update in Anaesthesia, edition 13 (2001)

page 80Update in Anaesthesia | www.worldanaesthesia.org

7/28/2019 Pharmacology and Drug Doses an Introduction

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DRUgS iN CoNCENTRATioNS

When a substance is dissolved in a liquid it orms a solution.Te volume o a solution is expressed in litres or millilitres.1litre = 1000ml.

Te substance dissolved is known as the solute. Te amount o solutein a solution is expressed as a concentration. Te amount o solutemay be described by its mass (grams or milligrams per litre) or by itsamount (moles per litre or millimoles per litre).

I the solute has a known chemical ormula (e.g. salt - NaCl), thenit is preerable to use mol.l-1 or mmol.l-1. I the solute does not havea dened chemical composition (such as a protein), then mg.l-1or g.l-1 is used.

Some solutions such as local anaesthetics and thiopentone that areused in anaesthesia on a daily basis are expressed as a percentage e.g.lignocaine 2% and thiopentone 2.5%.

When using drugs prepared in this way it is necessary to calculate thenumber o mg in ml o solution. Tis is easiest done by multiplyingthe percentage o the solution by 10:

2% lignocaine x 10 = 20mg.ml-1

0.5% bupivacaine x 10 = 5mg.ml-1

2.5% thiopentone x 10 = 25 mg.ml-1

Te maths behind this calculation is as ollows: A 2.5% solution means that there is 2.5g o thiopentone in100ml:

2.5g in 100ml

= 2500mg in 100ml= 25mg in 1ml (25mg.ml-1)

Some solutions such as epinephrine (adrenaline) may be expressedas 1:1000 or 1:10 000 or 1:100 000. Tis means that in a 1:1000epinephrine ampoule there is one part epinephrine to 1000 partssolution.

o work out how many milligrams o epinephrine are present:

1:1000 solution epinephrine

= 1g epinephrine in 1000ml solution

= 1000mg epinephrine per 1000ml solution

= 1mg per ml

1:10 000 solution epinephrine

= 1g epinephrine in 10,000ml solution

= 1000mg per 10,000ml

= 1mg per 10ml which can also be expressed as 100mcg per ml

PREPARiNg loCAl ANAESThETiCS WiTh EPiNEPhRiNEPre-mixed ampoules o lignocaine 1% and 2% with adrenaline aresometimes not available and it is may be necessary to prepare thesesolutions locally. A 1:200 000 solution means that there is 1 partadrenaline to 200 000 parts o solution (lignocaine in this instance).

In order to produce a 1:200 000 adrenaline solution, add 0.1mladrenaline 1:1000 to 20ml lignocaine. Te method is as ollows:

ake 1ml o adrenaline 1:1000 - dilute to 10mls with saline. ake 1ml o this mix which is now 1ml o 1:10 000 adrenaline.

Add 19ml lignocaine.

otal solution is now 20mls and the original adrenaline has beendiluted 200 times = 1:200 000 solution

oThER UNiTS - MilliEQUiVAlENTS (mEq)Sometimes the term milliequivalent is used in textbooks, althoughmillimoles is the more correct expression. When substances aredissolved in a liquid, they may develop a charge. For example,when salt (NaCl) is dissolved in water, it separates into its twocomponents: Na+ and Cl- (these are known as ions). Sodium ions

have one positive charge and chloride ions have one negative charge.Te word milliequivalent reers to how many ions are present. In thiscase the number o milliequivalents will be the same as the numbero millimoles in solution.

Example:A plasma sodium concentration o 140mEq.l-1 equals140mmol.l-1.

However, in a magnesium chloride solution (MgCl2) the magnesium

ion has two positive charges (Mg2+) so there will be two milliequivalentso magnesium per litre o solution. In this case the number omilliequivalents will not equal the number o millimoles.

ChANgES iN DRUg NAMESNew regulations rom the EEC now require the use o theRecommended International Non-proprietary Name (rINN) ordrugs. In drugs where there is concern that a name change may posea serious risk to patients, both the British Approved Name (BAN)and the rINN name will appear on the drug ampoule or at least thenext 5 years. In other cases, the new name will appear alone.

Some examples o name changes that will aect anaesthetists are:

UK name rINN name

adrenaline epinephrine

noradrenaline norepinephrine

rusemide urosemide

lignocaine lidocaine

thiopentone thiopental

phenobarbitone phenobarbital

page 81 Update in Anaesthesia | www.anaesthesiologists.org