CALCULATING SOLUTIONS AND DRUG DOSE RATES. % solution = (weight (g) 100) volume of solution (ml)

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Kevin Ford
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1 APPENDIX 2 CALCULATIONS For Percentages of solutions Anaesthetic gas flow rates Fluid therapy rates Calorie requirements Radiography calculations CALCULATING SOLUTIONS AND DRUG DOSE RATES % solution = (weight (g) 100) volume of solution (ml) volume of solution (ml) = (weight (g) 100) % solution weight (g) = (volume of solution (ml) % solution) 100 Remember that: 1 ml of solution weighs 1 g 1 g = 1000 mg A 1% solution contains 1 g in 100 ml A 2% solution contains 2 g in 100 ml, and so on A 10% solution contains 10 mg/ml If you know that an animal needs a certain amount of mg of a drug, but do not know how many ml or tablets that is, you can use this formula to work it out: dose prescribed (mg)/dose per ml or concentration

2 356 Appendices A dog needs 25 mg of a drug to be given by injection, and the drug is in a strength of 50 mg/ml: 25 mg 50 mg = 0.5 ml CALCULATING ANAESTHETIC GAS FLOW RATES Tidal volume = the amount of gas passing into and out of the lungs in one breath. This is estimated at ml/kg. Minute volume = the volume of air inhaled or exhaled in one minute. FORMULA FOR WORKING OUT FLOW RATE The animal s tidal volume must be calculated first: Tidal volume = ml/kg Cats/small dogs = 15 ml/kg Medium/large dogs = 10 ml/kg To find out the minute volume, multiply the tidal volume by the respiration rate per minute. Then multiply this by the circuit factor: Ayres T-Piece & Bain = Magill & Lack = The whole formula is: body weight respiration rate circuit factor What would the flow rate be for a 30 kg dog with a respiration rate of 20/min? 30 (body weight) 10 (tidal volume) 20 (respiration rate) 1.5 (circuit factor) = 9000 ml (9 litres)

3 Appendix CALCULATING FLUID THERAPY DRIP RATES The maintenance fluid requirement is the amount of fluid normally required by the patient over a certain period. This is calculated at approximately 50 ml/kg/24 hours WORKING OUT THE NUMBER OF DROPS PER SECOND The drip rate must be worked out. Different giving sets administer different amounts of drops per ml. Paediatric giving set = 60 drops/ml General giving sets = 20 drops/ml So, the amount required: Total amount in 24 hours 24 = hourly rate hourly rate 60 = minute rate minute rate giving set drip factor = number of drops per min 60 drops per min = the second rate A 15 kg dog needs 750 ml over 24 hours. A giving set is used that delivers 20 drops/ml = = = = 5.7 So the giving set should be set at one drop approximately every 6 seconds.

4 358 Appendices CALCULATING CALORIE REQUIREMENTS Basal energy requirement (BER) for dogs over 5 kg = 30 (body weight kg) + 70 BER for small dogs and cats under 5 kg = 60 (body weight kg) Then multiply by the disease factor : Cage rest 1.2 Surgery/trauma 1.3 Multiple surgery/trauma 1.5 Sepsis/neoplasia 1.7 Burns 2.0 Growth 2.0 This total figure = the amount of Kcal required over 24 hours. A 25 kg dog following surgery would require: ( ) 1.3 = 1066 Kcal/day CALCULATING RADIOGRAPHIC EXPOSURES FILM FOCAL DISTANCE AND THE INVERSE SQUARE LAW If you alter the FFD, the new exposure can be worked out using this calculation: new mas = new FFD 2 old FFD 2 old mas Having been using an exposure of 10 mas and FFD of 50 cm, what exposure is required if the FFD is changed to 100 cm? Answer: = 40 mas.

5 Appendix THE 1O kv RULE Increase the kv by 10 and you can halve the mas. Decrease the kv by 10 and you can double the mas. If you need to keep the actual image the same, but need to alter the exposure, the 10 kv rule works like this: 50 kv at 32 mas will create the same exposure as: 60 kv at 16 mas and 70 kv at 8 mas So, if an exposure at 20 mas at 80 kv is adequate, what is the kv required if the mas are changed to 10? Answer: The mas are halved therefore increase kv by 10 = 90 kv. MILLIAMPERAGE (ma) AND TIME (s) mas = ma s If ma is doubled, the time may be halved (thereby decreasing exposure time and reducing the possibility of movement blur). GRID FACTOR mas grid factor (GF) = exposure required using grid For example: the exposure factors required for a lateral radiograph of a dog s abdomen might be 3.2 mas at 80 kv. If a grid with a grid factor of 3 is used, the new exposure will be: = 9.6 mas at 80 kv. This may be rather too much time and so the 10 kv rule can be used. By increasing the kv by 10 it is possible to halve the mas, giving a setting of 90 kv at 4.8 mas.

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