Appendix B: Constant Rate Infusions Example Calculations For Syringe Pump 1. A 20 lb patient needs a 5 µg/kg fentanyl loading dose before being placed on a fentanyl constant rate infusion (CRI) at 3 µg/kg/h to equal 1 ml/h for 12 h. Fentanyl comes as 50 µg/ml. How many milliliters of fentanyl do you need as a loading dose, and how many milliliters of fentanyl do you need to add with saline in a 12 cm 3 syringe to run at 1 l/h? Step 1: Convert the patients weight into kilograms 20 lb/2.2 lb/kg = 9.09 kg Step 2: Figure out loading dose of fentanyl 9.09 kg 5 µg/kg = 45 µg 45 µg/50 µg/ml = 0.9 ml Loading dose = 0.9 ml fentanyl Step 3: Calculate how many µg/h will be equal to 1 ml/h 3 µg/kg/h = 1 ml/h 3 µg 9.09 kg/h = 1 ml/h 27.27 µg/h = 1 ml/h Step 4: Calculate how many micrograms you will need for 12 h 27.27 µg/h 12 h = 327.24 µg Step 5: Divide by the concentration of fentanyl to figure out how many milliliters you will put in a 12 cm 3 syringe 327.24 µg/50 µg/ml = 6.5 ml fentanyl Step 6: Subtract to find out how much saline you will need to dilute it with for 3 µg/kg/h to = 1 ml/h 12 ml (12 cm 3 syringe) 6.5 ml fentanyl = 5.5 ml NaCl Place 6.5 ml fentanyl and 5.5 ml NaCl into a 12 cm 3 syringe and run at 1 ml/h to equal 3 µg/kg/h. 2. A 3.5 kg postop cat that just had a leg amputation needs 0.2 mg/kg methadone q 4 h, and a lidocaine and ketamine CRI combined. The lidocaine dose 20 µg/kg/min = 1 ml/h, and the ketamine dose 0.2 mg/kg/h will equal 1 ml/h, both combined in a 20 cm 3 syringe to run for 20 h, diluted with NaCl. Methadone = 10 mg/ml Lidocaine = 20 mg/ml Ketamine = 100 mg/ml Step 1: Figure out how much methadone the patient will need q 4 h Pain Management for Veterinary Technicians and Nurses, First Edition. Edited by Mary Ellen Goldberg and Nancy Shaffran. 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc. Companion Website: www.wiley.com/go/goldbergpainmanagement 384
Appendix B: Constant Rate Infusions Example Calculations 385 3.5 kg 0.2 mg/kg = 0.7 mg 0.7 mg/10 mg/ml = 0.07 ml The patient will receive intermittent IV boluses of 0.07 ml of methadone. Step 2: Figure out how much lidocaine to place in a 20 cm 3 syringe 3.5 kg 20 µg/kg/min = 70 µg/min Convert minutes into hours 70 µg/min 60 min/h = 4200 µg/h Convert micrograms into milligrams 4200 µg / h = 4.2 mg / h 1000 µg / mg Divide by the concentration of lidocaine 42. mg / h 20 mg / ml 021. ml / h Find out how much you will need for 20 h 0.21 ml/h 20 h = 4.2 ml Lidocaine Step 3: Figure out how much ketamine you will need to place in a 20 cm 3 syringe 3.5 kg 0.2 mg/kg/h = 0.7 mg/h Divide by the concentration of ketamine 0.7 mg/h/100 mg/ml = 0.007 ml/h Find out how much you will need for 20 h 0.007 ml/h 20 h = 0.14 ml ketamine Step 4: Calculate how much NaCl you will need in the syringe to dilute the agents for their prescribed doses to equal 1 ml/h 20 ml 4.2 ml lidocaine 0.14 ml ketamine = 15.66 ml NaCl Combine 4.2 ml of lidocaine, 0.14 ml of ketamine, and 15.66 ml of NaCl in a 20 cm 3 syringe, and run at 1 ml/h for lidocaine to equal 20 µg/kg/min and ketamine to equal 0.2 mg/kg/h. The following is from Dr. Tamara Grubb s lecture Analgesia Drop by Drop: Constant Rate Infusions Made Easy (with permission). Calculations of CRI dosages Generally, dosing tables or individualized spread sheets (e.g., there are very useful spreadsheets available at multiple websites, including one of my favorites at www.vasg.org) should be used for CRIs. These sheets greatly improve the speed at which CRIs can be initiated and greatly decrease the chance of mathematical errors. However, CRI dosages can also be easily calculated using the formula: B = body wt in kg C = Diluent volume in mls D = Desired fluid rate in mls/hr E = Drug concentration in mg/ml A B C 60 / D E 1000 = mls of drug to add to diluent
Dosages for CRIs used in cats. Drug Loading dose CRI dose Quick calculation Comments Morphine (M) a 0.10 mg/kg IM 0.03 mg/kg/h (0.5 µg/kg/min) Add 15 mg to 500 ml fluid and Cat may need light sedation; can be combined with K and/or L Hydromorphone (H) 0.025 mg/kg IV 0.01 mg/kg/h Add 5 mg to 500 ml fluid and May cause hyperthermia; can be combined with K and/or L Fentanyl (F) 0.001 0.003 mg/kg IM or IV (1 3 µg/kg IV) 2 5 µg/kg/h (0.03 0.08 µg/kg/m) postop 5 20 µg/kg/h (0.08 0.3 µg/kg/m intra-op For 5 µg/kg/h, add 2.5 mg to 500 ml fluid and 2.5 mg = 50 ml F; remove 50 ml lactated ringers solution (LRS) before adding F; can be combined with K and/or L Methadone 0.1 0.2 mg/kg IV 0.12 mg/kg/h Add 60 mg to 500 ml fluid and May cause sedation; can be combined with K and/or L Butorphanol 0.1 mg/kg IV 0.1 0.2 mg/kg/h Add 50 mg to 500 ml fluid and for 0.1 mg/kg/h Only moderately potent and has ceiling effect use as part of multimodal protocol Ketamine (K) a 0.25 mg/kg IV 0.12 0.6 mg/kg/h (2 10 µg/kg/min) Add 60 mg to 500 ml fluid and for 0.12 mg/kg/h Generally combined with opioids; may cause dysphoria Lidocaine (L) 0.25 mg/kg IV 1.5 mg/kg/h (25 µg/kg/min) Add 750 mg to 500 ml fluid and 750 mg = 37.5 ml; remove 37.5 ml LRS before adding L; can be combined with opioid and/or K; Some sources recommend no more than 10 µg/kg/min in cats 10 µg/kg/min would be 300 mg lidocaine in 500 ml fluid with a rate of 1 ml/kg/h Lidocaine MAY be contraindicated in the cat due to cardiovascular effects Medetomidine (Med) or 1 5 µg/kg Med 0.001 0.004 mg/kg/h Med Add 500 µg Med or 250 µg D (0.5 ml of either) to 500 ml fluid and run 1 4 ml/kg/h Provides analgesia and light sedation. Excellent addition to opioid CRI, or can be administered as solo drug CRI Dexmedetomidine (D) 1 2 µg/kg D Can be IV or IM; may not be necessary (1 4 µg/kg/h) 0.0005 0.002 mg/kg/h D (Continued )
Dosages for CRIs used in cats. (Continued ) Drug Loading dose CRI dose Quick calculation Comments Morphine a / M: 0.10 mg/kg IM Ketamine a K: 0.25 mg/kg IV 0.03 mg/kg/h M; 0.12 mg/kg/h K Add 15 mg M and 60 mg K to 500 ml fluid and Can be administered up to 3 ml/kg/h but dysphoria MAY occur. Can substitute F or methadone for M Morphine/ Ketamine/ Lidocaine (MLK) M: 0.10 mg/kg IM K: 0.25 mg/kg IV L: 0.25 mg/kg IV 0.03 mg/kg/h M; 0.12 mg/kg/h K; 1.5 mg/kg/h L Add 15 mg of M, 60 mg K and 750 mg (or 300 mg) L to 500 ml fluid and Can substitute H, F, or methadone for M a Any of the drug amounts in the bag of fluids can be decreased and the fluids administered at a higher rate if necessary. For example, for morphine, ketamine, and morphine/ketamine infusions, 7.5 mg of morphine and 30 mg of ketamine can be used and the CRI administered at 2 ml/kg/h if more fluids are needed.
Dosages for CRIs used in dogs. Drug Loading dose CRI dose Quick calculation Comments Morphine (M) a 0.5 mg/kg IM (or 0.25 mg/kg SLOWLY IV) 0.12 0.3 mg/kg/h (2.0 3.3 µg/kg/min) Add 60 mg to 500 ml fluid and for 0.12 mg/kg/h MAY cause sedation; can be combined with K and/or L Hydromorphone (H) 0.05 0.1 mg/kg IV 0.01 0.05 mg/kg/h Add 5 24 mg to 500 ml fluid and MAY cause sedation; can be combined with K and/or L Fentanyl (F) 0.001 0.003 mg/kg IM or IV (1 3 µg/kg IV) 2 10 µg/kg/h (0.03 0.2 µg/ kg/m) postop 3 40 µg/kg/h (0.05 0.7 µg/ kg/m) intra-op For 5 µg/kg/h, add 2.5 mg to 500 ml fluid and 2.5 mg = 50 ml F, remove 50 ml LRS before adding F; can be combined with K and/or L; intra-op dose can be up to 20 40 µg/kg/h Methadone 0.1 0.2 mg/kg IV 0.12 mg/kg/h Add 60 mg to 500 ml fluid and MAY cause sedation; can be combined with K and/or L Butorphanol 0.1 mg/kg IV 0.1 0.2 mg/kg/h Add 50 mg to 500 ml fluid and for 0.1 mg/kg/h Only moderately potent and has ceiling effect use as part of multimodal protocol Ketamine (K) a 0.25 mg/kg IV 0.12 0.6 mg/kg/h (2 10 µg/ kg/min) Add 60 mg to 500 ml fluid and for 0.12 mg/kg/h Generally combined with opioids; may cause dysphoria; postop dose may be higher Lidocaine (L) 0.5 1.0 mg/kg IV 1.5 3.0 mg/kg/h (25 50 µg/ kg/min) Add 750 mg to 500 ml fluid and for 25 µg/kg/min 750 mg = 37.5 ml, remove 37.5 ml LRS before adding L; can be combined with opioid and/or K Medetomidine (Med) or dexmedetomidine (D) 1 5 µg/kg Med; 1 2 µg/kg D; can be IV or IM; may not be necessary 0.001 0.004 mg/kg/h Med (1 4 µg/kg/h); 0.0005 0.002 mg/kg/h D Add 500 µg Med or 250 µg D (0.5 ml of either) to 500 ml fluid and run 1 4 ml/kg/h Provides analgesia and light sedation. Excellent addition to opioid CRI or can be administered as solo drug CRI Morphine 7 /Ketamine a M: 0.5 mg/kg IM; K: 0.25 mg/kg IV 0.12 mg/kg/h M and 0.12 mg/kg/h K Add 60 mg M and 60 mg K to 500 ml fluid and Can be administered up to 3 ml/kg/h but sedation or dysphoria MAY occur. Can substitute H, F, or methadone for M Morphine/Ketamine/ Lidocaine (MLK) M: 0.5 mg/kg IM; K: 0.25 mg/kg IV; L: 0.5 mg/kg IV 0.12 mg/kg/h M; 0.12 mg/ kg/h K; 1.5 mg/kg/h L Add 60 mg of M, 60 mg K and 750 mg L to 500 ml fluid and Can substitute H, F, or methadone for M. Dr. Muir s dose is 3.3 µg/kg/min M, 50 µg/kg/min L; 10 µg/kg/min K a Any of the drug amounts in the bag of fluids can be decreased and the fluids administered at a higher rate if necessary. For example, for morphine, ketamine, and morphine/ketamine infusions, 30 mg of morphine and 30 mg of ketamine can be used and the CRI administered at 2 ml/kg/h if more fluids are needed.
Appendix B: Constant Rate Infusions Example Calculations 389 Chart for adding analgesic drugs to IV fluids (appropriate if you do not think that you will need to change the IV fluid rate). Amount of lidocaine (20 mg/ml) to add to a 1-L fluid bag: Ketamine CRI: Add 60 mg (0.6 ml of 100 mg/ml) ketamine to a 1-L bag and run at 2 ml/kg/h to provide 2 µg/kg/min or at surgical fluid rate (10 ml/kg/h) to provide 10 µg/kg/min (intra-op dose). Fluid rate a Maintenance (90 ml/kg/24 h) 1/2 Maintenance 2 Maintenance Surgical (10 ml/kg/h) Lidocaine dose 25 µg/kg/min 21 ml 42 ml 11 ml 7.5 ml 50 µg/kg/min 42 ml 83 ml 22 ml 15 ml 75 µg/kg/min 63 ml 125 ml 33 ml 22.5 ml a Most clinicians consider maintenance to be 45 50 ml/kg/24 h so maintenance as defined as 90 ml/kg/24 h may be too high for normal patients. Before adding the lidocaine, remove the same volume of LRS as you will be adding of lidocaine. Lower dosages (25 50 µg/kg/min) are used for analgesia while all three dosages are used for anti-arrhythmic therapy. Quick calculation for 36 µg/kg/min: add 50 ml of 2% lidocaine to 1-L of LRS and run at 1 ml/lb/h. Amount of morphine (15 mg/ml) to add to a 1-l fluid bag: Fentanyl CRI dose: Dog: Loading dose 2 5 µg/kg, 5 20 µg/kg/h intra-op, 2 5 µg/kg/h postop Cat: Loading dose 1 2 µg/kg, 5 10 µg/kg/h µg intra-op, 1 2 µg/kg/h post op. Hydromorphone CRI dose: 0.01 0.05 mg/kg/h (dog and cat). Fluid rate a Maintenance (90 ml/kg/24 h) 1/2 Maintenance 2 Maintenance Surgical (10 ml/kg/h) Morphine dose 0.5 µg/kg/min (cat dose) 0.5 ml 1.0 ml 0.25 ml 0.20 ml 1 µg/kg/min 1.0 ml 2.0 ml 0.5 ml 0.40 ml 2 µg/kg/min 2.0 ml 4.0 ml 1.0 ml 0.80 ml a Most clinicians consider maintenance to be 45 50 ml/kg/24 h so maintenance as defined as 90 ml/kg/24 h may be too high for normal patients.
390 Appendix B: Constant Rate Infusions Example Calculations Morphine/Lidocaine/Ketamine (MLK) Source: from Dr. Muir my doses are a little lower): To a 500 ml bag of LRS add: Administer at 10 ml/ kg/h to provide 10 mg morphine (0.66 cm 3 ) Morphine 0.2 mg/kg/h 120 mg lidocaine (6 cm 3 2%) Lidocaine 2.5 mg/kg/h 100 mg ketamine (1 cm 3 ) Ketamine 2 mg/kg/h Calculating Constant Rate Infusions (for people who really want to do the math) For drugs that do not need to be diluted ( generally used for drugs in an infusion pump): How many total milligrams or micrograms do you need? Dose of infusion in mg/kg/h or µg/kg/min or whatever (lets use mg/kg/h for now) body weight in kg = total mg/h needed. Now divide this by the concentration of the drug in mg/ml and now you have the ml/h. If you divide this by 60, you get the ml/min; divide by 60 again and you get the ml/s and most drip sets are either 10 drops/ml or 60 drops/ml, and you can figure out how many drops/s the patient needs by multiplying the number of drops/ml by the number of ml/s. So, a dog weighs 40 kg and needs a CRI of 2 mg/kg/h and the drug is 0.2%. 5 mg/kg/h 40 kg = 200 mg/h divided by 2 mg/ ml = 100 ml/h. 100 ml/h divided by 60 = 1.666; divide by 60 again = 0.027 10 drops/ml = 0.2 drops/s or (to make counting easier) 1 drops per every 5 s For drugs that need to be diluted: Again calculate out the number of mg/h that you need. Now decide what fluid rate that you would like to deliver in ml/kg/h. Multiply this by the body weight to get the ml/h. We need to deliver x mg/h of a drug in x ml/h of a drug so all we need to do is figure out how many milligrams that we need in each milliliter. Do this by dividing mg/h by ml/h and that gives us mg/ml. Now just multiply the total number of milliliters you plan to deliver and that gives you the number of milligrams that you need to put in the fluids. Finally, divide these milligrams by the concentration of the drug and you have the number of milliliters to add to the fluid. So, a dog weighs 20 kg and needs a CRI of 5 mg/kg/h and a fluid rate of 2 ml/kg/h, and we have 1 l of fluids and a drug that is 0.5%. 5 mg/kg/hr 20 kg = 100 mg/hour. 2 ml/kg/ hour 20 kg = 40 ml/hour. 100 mg/hour / 40 ml/ hour = 2.5 mg/ml. 2.5 mg/ml x 1000 mls = 2500 mg. 2500 mg /5 mg/ml = 500 mls of the drug to add to the 1-L of fluids. (For all you math whizzes, you probably see that the kilograms could be deleted from this formula and it would still work! Regardless of the weight, you would add 250 ml of the drug to 1000 ml of the fluid and deliver at 2 ml/kg/h to get 5 mg/kg/h.) CRI Practice What you need to know to begin: Dose of drug to be delivered (e.g. 3 µg/kg/min or 0.18 mg/kg/h) Patient s body weight (kg) Fluid rate (ml/h) Fluid bag size Drug concentration
Appendix B: Constant Rate Infusions Example Calculations 391 For dosages given in mg/kg/h: Step 1 Set up equation based on dosage Step 2 Replace hash marks with time signs Step 3 Enter known information mg/kg/h = mg to add to bag mg kg h = mg to add to bag Dose and weight Fluid bag size Step 4 Solve for hours = No.of hours the bag will last Hourly rate Step 5 Solve equation mg kg h = mgs to add to bag Desireddrug ( mg) Step 6 Calculate drug volume and add to bag = No. of ml that is to be added Concentration ( m g / ml) For dosages given in µg/kg/min Note: two extra steps are required and are shown in bold Step 1 Set up equation based on dosage Step 2 Replace hash marks with time signs Step 3 Enter known information µg/kg/min = µg to add to bag µg kg min = µg to add to bag Dose and weight Fluid bag size Step 4 Solve for hours = No.of hours the bag will last Hourly rate Step 5 Solve for minutes Step 6 Solve equation No. of hours above 60 min/h µg kg min = µg to add to bag Step 7 Convert µg to mg Divide answer by 1000 Step 8 Calculate drug volume and add to bag Desired drug ( mg) = No. of ml that is to be added Concentration ( m g/ml)
392 Appendix B: Constant Rate Infusions Example Calculations Examples 1. A 60 lb dog needs a hydromorphone drip started. How much 2 mg/ml hydro will you add to a 500 ml bag of NaCl to deliver 0.05 mg/kg/h? The drip rate is 25 ml/h. Solution: 60 lbs = 27 kg Equation is: 0.05 mg/kg/h 27 kg 500 ml /25 ml/h 0.05 27 20 = 27 mg 27 mg/2 mg/ml = 13.4 ml added to 500 ml bag of NaCl. 2. You want to put a 20 kg dog on a 30 µg/kg/ min infusion of lidocaine. Use a 1 l bag of fluids and run at 40 ml per hour. How much 2% lidocaine will you add to the bag? Solution: Equation is: 30 µg 20 kg 1000 ml/40 ml/h 30 20 25 h 25 h 60 min/h = 1500 min 30 20 1500 = 900,000 µg 900,000 µg/1000 µg/mg = 900 mg 2% Lidocaine = 20 mg/ml 900/20 = 45 ml of lidocaine added to 1 l